ADAFRUIT ADA TRKT PRO 5V Datasheet

Atmel 8-bit Microcontroller with 4/8/16/32KBytes In-

System Programmable Flash

ATmega48A; ATmega48PA; ATmega88A; ATmega88PA;
ATmega168A; ATmega168PA; ATme ga 32 8; AT me ga 328P

Features

• High Performance, Low Power Atmel

• Advanced RISC Architecture

– 131 Powerful Instructions – Most Single Clock Cycle Execution
– 32 x 8 General Purpose Working Registers
– Fully Static Operation
– Up to 20 MIPS Throughput at 20MHz
– On-chip 2-cycle Multiplier

• High Endurance Non-volatile Memory Segments

– 4/8/16/32KBytes of In-System Self-Programmable Flash program memory
– 256/512/512/1KBytes EEPROM
– 512/1K/1K/2KBytes Internal SRAM
– Write/Erase Cycles: 10,000 Flash/100,000 EEPROM
– Data retention: 20 years at 85C/100 years at 25C
– Optional Boot Code Section with Independent Lock Bits

In-System Programming by On-chip Boot Program
True Read-While-Write Operation

– Programming Lock for Software Security

• Atmel

®

QTouch® library support
– Capacitive touch buttons, sliders and wheels
– QTouch and QMatrix
– Up to 64 sense channels

®

acquisition

• Peripheral Features

– Two 8-bit Timer/Counters with Separate Prescaler and Compare Mode
– One 16-bit Timer/Counter with Separate Prescaler, Compare Mode, and Capture Mode
– Real Time Counter with Separate Oscillator
– Six PWM Channels
– 8-channel 10-bit ADC in TQFP and QFN/MLF package

Temperature Measurement

– 6-channel 10-bit ADC in PDIP Package

Temperature Measurement
– Programmable Serial USART
– Master/Slave SPI Serial Interface
– Byte-oriented 2-wire Serial Interface (Philips I
– Programmable Watchdog Timer with Separate On-chip Oscillator
– On-chip Analog Comparator
– Interrupt and Wake-up on Pin Change

• Special Microcontroller Features

– Power-on Reset and Programmable Brown-out Detection
– Internal Calibrated Oscillator
– External and Internal Interrupt Sources
– Six Sleep Modes: Idle, ADC Noise Reduction, Power-save, Power-down, Standby, and

Extended Standby

• I/O and Packages

– 23 Programmable I/O Lines
– 28-pin PDIP, 32-lead TQFP, 28-pad QFN/MLF and 32-pad QFN/MLF

®

AVR® 8-Bit Microcontroller Family

(1)

2

C compatible)

8271E–AVR–07/2012

• Operating Voltage:

1
2
3
4
5
6
7
8

24
23
22
21
20
19
18
17

(PCINT19/OC2B/INT1) PD3

(PCINT20/XCK/T0) PD4

GND
VCC
GND

VCC
(PCINT6/XTAL1/TOSC1) PB6
(PCINT7/XTAL2/TOSC2) PB7

PC1 (ADC1/PCINT9)
PC0 (ADC0/PCINT8)
ADC7
GND
AREF
ADC6
AVCC
PB5 (SCK/PCINT5)

32313029282726

25

9101112131415

16

(PCINT21/OC0B/T1) PD5

(PCINT22/OC0A/AIN0) PD6

(PCINT23/AIN1) PD7

(PCINT0/CLKO/ICP1) PB0

(PCINT1/OC1A) PB1

(PCINT2/SS/OC1B) PB2

(PCINT3/OC2A/MOSI) PB3

(PCINT4/MISO) PB4

PD2 (INT0/PCINT18)

PD1 (TXD/PCINT17)

PD0 (RXD/PCINT16)

PC6 (RESET/PCINT14)

PC5 (ADC5/SCL/PCINT13)

PC4 (ADC4/SDA/PCINT12)

PC3 (ADC3/PCINT11)

PC2 (ADC2/PCINT10)

32 TQFP Top View

1
2
3
4
5
6
7
8
9
10
11
12
13
14

28
27
26
25
24
23
22
21
20
19
18
17
16
15

(PCINT14/RESET) PC6

(PCINT16/RXD) PD0
(PCINT17/TXD) PD1
(PCINT18/INT0) PD2

(PCINT19/OC2B/INT1) PD3

(PCINT20/XCK/T0) PD4

VCC

GND
(PCINT6/XTAL1/TOSC1) PB6
(PCINT7/XTAL2/TOSC2) PB7

(PCINT21/OC0B/T1) PD5

(PCINT22/OC0A/AIN0) PD6

(PCINT23/AIN1) PD7

(PCINT0/CLKO/ICP1) PB0

PC5 (ADC5/SCL/PCINT13)
PC4 (ADC4/SDA/PCINT12)
PC3 (ADC3/PCINT11)
PC2 (ADC2/PCINT10)
PC1 (ADC1/PCINT9)
PC0 (ADC0/PCINT8)
GND
AREF
AVCC
PB5 (SCK/PCINT5)
PB4 (MISO/PCINT4)
PB3 (MOSI/OC2A/PCINT3)
PB2 (SS/OC1B/PCINT2)
PB1 (OC1A/PCINT1)

28 PDIP

1
2
3
4
5
6
7
8

24
23
22
21
20
19
18
17

32313029282726

25

9101112131415

16

32 MLF Top View

(PCINT19/OC2B/INT1) PD3

(PCINT20/XCK/T0) PD4

GND
VCC
GND

VCC
(PCINT6/XTAL1/TOSC1) PB6
(PCINT7/XTAL2/TOSC2) PB7

PC1 (ADC1/PCINT9)
PC0 (ADC0/PCINT8)
ADC7
GND
AREF
ADC6
AVCC
PB5 (SCK/PCINT5)

(PCINT21/OC0B/T1) PD5

(PCINT23/AIN1) PD7

(PCINT0/CLKO/ICP1) PB0

(PCINT1/OC1A) PB1

(PCINT2/SS/OC1B) PB2

(PCINT3/OC2A/MOSI) PB3

(PCINT4/MISO) PB4

PD2 (INT0/PCINT18)

PD1 (TXD/PCINT17)

PD0 (RXD/PCINT16)

PC6 (RESET/PCINT14)

PC5 (ADC5/SCL/PCINT13)

PC4 (ADC4/SDA/PCINT12)

PC3 (ADC3/PCINT11)

PC2 (ADC2/PCINT10)

NOTE: Bottom pad should be soldered to ground.

1
2
3
4
5
6
7

21
20
19
18
17
16
15

28272625242322

891011121314

28 MLF Top View

(PCINT19/OC2B/INT1) PD3

(PCINT20/XCK/T0) PD4

VCC

GND
(PCINT6/XTAL1/TOSC1) PB6
(PCINT7/XTAL2/TOSC2) PB7

(PCINT21/OC0B/T1) PD5

(PCINT22/OC0A/AIN0) PD6

(PCINT23/AIN1) PD7

(PCINT0/CLKO/ICP1) PB0

(PCINT1/OC1A) PB1

(PCINT2/SS/OC1B) PB2

(PCINT3/OC2A/MOSI) PB3

(PCINT4/MISO) PB4

PD2 (INT0/PCINT18)

PD1 (TXD/PCINT17)

PD0 (RXD/PCINT16)

PC6 (RESET/PCINT14)

PC5 (ADC5/SCL/PCINT13)

PC4 (ADC4/SDA/PCINT12)

PC3 (ADC3/PCINT11)

PC2 (ADC2/PCINT10)
PC1 (ADC1/PCINT9)
PC0 (ADC0/PCINT8)
GND
AREF
AVCC
PB5 (SCK/PCINT5)

NOTE: Bottom pad should be soldered to ground.

– 1.8 - 5.5V

• Temperature Range:

C to 85C

–-40

• Speed Grade:

– 0 - 4MHz@1.8 - 5.5V, 0 - 10MHz@2.7 - 5.5.V, 0 - 20MHz @ 4.5 - 5.5V

• Power Consumption at 1MHz, 1.8V, 25C

– Active Mode: 0.2mA
– Power-down Mode: 0.1µA
– Power-save Mode: 0.75µA (Including 32kHz RTC)

1. Pin Configurations

Figure 1-1. Pinout ATmega48A/PA/88A/PA/168A/PA/328/P

ATmega48A/PA/88A/PA/168A/PA/328/P [DATASHEET]

2

8271E–AVR–07/2012

Table 1-1. 32UFBGA - Pinout ATmega48A/48PA/88A/88PA/168A/168PA

123456

A PD2 PD1 PC6 PC4 PC2 PC1

B PD3 PD4 PD0 PC5 PC3 PC0

C GND GND

ADC7 GND

D VDD VDD

E PB6 PD6 PB0 PB2 AVDD PB5

F PB7 PD5 PD7 PB1 PB3 PB4

1.1 Pin Descriptions

1.1.1 VCC

Digital supply voltage.

1.1.2 GND

Ground.

1.1.3 Port B (PB7:0) XTAL1/XTAL2/TOSC1/TOSC2

Port B is an 8-bit bi-directional I/O port with internal pull-up resistors (selected for each bit). The Port B output buf­fers have symmetrical drive characteristics with both high sink and source capability. As inputs, Port B pins that are
externally pulled low will source current if the pull-up resistors are activated. The Port B pins are tri-stated when a
reset condition becomes active, even if the clock is not running.

Depending on the clock selection fuse settings, PB6 can be used as input to the inverting Oscillator amplifier and
input to the internal clock operating circuit.

Depending on the clock selection fuse settings, PB7 can be used as output from the inverting Oscillator amplifier.

If the Internal Calibrated RC Oscillator is used as chip clock source, PB7...6 is used as TOSC2...1 input for the
Asynchronous Timer/Counter2 if the AS2 bit in ASSR is set.

AREF ADC6

The various special features of Port B are elaborated in ”Alternate Functions of Port B” on page 83 and ”System

Clock and Clock Options” on page 26.

1.1.4 Port C (PC5:0)

Port C is a 7-bit bi-directional I/O port with internal pull-up resistors (selected for each bit). The PC5...0 output buf­fers have symmetrical drive characteristics with both high sink and source capability. As inputs, Port C pins that are
externally pulled low will source current if the pull-up resistors are activated. The Port C pins are tri-stated when a
reset condition becomes active, even if the clock is not running.

1.1.5 PC6/RESET

If the RSTDISBL Fuse is programmed, PC6 is used as an I/O pin. Note that the electrical characteristics of PC6 dif­fer from those of the other pins of Port C.

If the RSTDISBL Fuse is unprogrammed, PC6 is used as a Reset input. A low level on this pin for longer than the
minimum pulse length will generate a Reset, even if the clock is not running. The minimum pulse length is given in

Table 29-12 on page 310. Shorter pulses are not guaranteed to generate a Reset.

The various special features of Port C are elaborated in ”Alternate Functions of Port C” on page 86.

ATmega48A/PA/88A/PA/168A/PA/328/P [DATASHEET]

8271E–AVR–07/2012

3

1.1.6 Port D (PD7:0)

Port D is an 8-bit bi-directional I/O port with internal pull-up resistors (selected for each bit). The Port D output buf­fers have symmetrical drive characteristics with both high sink and source capability. As inputs, Port D pins that are
externally pulled low will source current if the pull-up resistors are activated. The Port D pins are tri-stated when a
reset condition becomes active, even if the clock is not running.

The various special features of Port D are elaborated in ”Alternate Functions of Port D” on page 89.

1.1.7 AV

CC

AVCC is the supply voltage pin for the A/D Converter, PC3:0, and ADC7:6. It should be externally connected to VCC,
even if the ADC is not used. If the ADC is used, it should be connected to V
PC6...4 use digital supply voltage, V

CC

.

1.1.8 AREF

AREF is the analog reference pin for the A/D Converter.

1.1.9 ADC7:6 (TQFP and QFN/MLF Package Only)

In the TQFP and QFN/MLF package, ADC7:6 serve as analog inputs to the A/D converter. These pins are powered
from the analog supply and serve as 10-bit ADC channels.

through a low-pass filter. Note that

CC

ATmega48A/PA/88A/PA/168A/PA/328/P [DATASHEET]

8271E–AVR–07/2012

4

2. Overview

The ATmega48A/PA/88A/PA/168A/PA/328/P is a low-power CMOS 8-bit microcontroller based on the AVR
enhanced RISC architecture. By executing powerful instructions in a single clock cycle, the
ATmega48A/PA/88A/PA/168A/PA/328/P achieves throughputs approaching 1 MIPS per MHz allowing the system
designer to optimize power consumption versus processing speed.

2.1 Block Diagram

Figure 2-1. Block Diagram

Comp.

VCC

debugWIRE

PROGRAM

CPU

Internal

Bandgap

LOGIC

SRAMFlash

AVC C

AREF

GND

2

6

GND

Watchdog

Timer

Watchdog

Oscillator

Oscillator

Circuits /

Clock

Generation

EEPROM

8bit T/C 2

DATA B US

Powe r

Supervision

POR / BOD &

RESET

16bit T/C 18bit T/C 0 A/D Conv.

Analog

USART 0

SPI TWI

PORT C (7)PORT B (8)PORT D (8)

RESET

XTAL[1..2]

ADC[6..7]PC[0..6]PB[0..7]PD[0..7]

The AVR core combines a rich instruction set with 32 general purpose working registers. All the 32 registers are
directly connected to the Arithmetic Logic Unit (ALU), allowing two independent registers to be accessed in one
single instruction executed in one clock cycle. The resulting architecture is more code efficient while achieving
throughputs up to ten times faster than conventional CISC microcontrollers.

ATmega48A/PA/88A/PA/168A/PA/328/P [DATASHEET]

8271E–AVR–07/2012

5

The ATmega48A/PA/88A/PA/168A/PA/328/P provides the following features: 4K/8Kbytes of In-System Program­mable Flash with Read-While-Write capabilities, 256/512/512/1Kbytes EEPROM, 512/1K/1K/2Kbytes SRAM,
23 general purpose I/O lines, 32 general purpose working registers, three flexible Timer/Counters with compare
modes, internal and external interrupts, a serial programmable USART, a byte-oriented 2-wire Serial Interface, an
SPI serial port, a 6-channel 10-bit ADC (8 channels in TQFP and QFN/MLF packages), a programmable Watchdog
Timer with internal Oscillator, and five software selectable power saving modes. The Idle mode stops the CPU
while allowing the SRAM, Timer/Counters, USART, 2-wire Serial Interface, SPI port, and interrupt system to con­tinue functioning. The Power-down mode saves the register contents but freezes the Oscillator, disabling all other
chip functions until the next interrupt or hardware reset. In Power-save mode, the asynchronous timer continues to
run, allowing the user to maintain a timer base while the rest of the device is sleeping. The ADC Noise Reduction
mode stops the CPU and all I/O modules except asynchronous timer and ADC, to minimize switching noise during
ADC conversions. In Standby mode, the crystal/resonator Oscillator is running while the rest of the device is sleep­ing. This allows very fast start-up combined with low power consumption.

®

Atmel
AVR
debounced reporting of touch keys and includes Adjacent Key Suppression

offers the QTouch® library for embedding capacitive touch buttons, sliders and wheels functionality into

®

microcontrollers. The patented charge-transfer signal acquisition offers robust sensing and includes fully

®

(AKS™) technology for unambiguous
detection of key events. The easy-to-use QTouch Suite toolchain allows you to explore, develop and debug your
own touch applications.

The device is manufactured using Atmel’s high density non-volatile memory technology. The On-chip ISP Flash
allows the program memory to be reprogrammed In-System through an SPI serial interface, by a conventional non­volatile memory programmer, or by an On-chip Boot program running on the AVR core. The Boot program can use
any interface to download the application program in the Application Flash memory. Software in the Boot Flash
section will continue to run while the Application Flash section is updated, providing true Read-While-Write opera­tion. By combining an 8-bit RISC CPU with In-System Self-Programmable Flash on a monolithic chip, the Atmel
ATmega48A/PA/88A/PA/168A/PA/328/P is a powerful microcontroller that provides a highly flexible and cost effec­tive solution to many embedded control applications.

The ATmega48A/PA/88A/PA/168A/PA/328/P AVR is supported with a full suite of program and system develop­ment tools including: C Compilers, Macro Assemblers, Program Debugger/Simulators, In-Circuit Emulators, and
Evaluation kits.

2.2 Comparison Between Processors

The ATmega48A/PA/88A/PA/168A/PA/328/P differ only in memory sizes, boot loader support, and interrupt vector
sizes. Table 2-1 summarizes the different memory and interrupt vector sizes for the devices.

Table 2-1. Memory Size Summary

Device Flash EEPROM RAM Interrupt Vector Size

ATmega48A 4KBytes 256Bytes 512Bytes 1 instruction word/vector

ATmega48PA 4KBytes 256Bytes 512Bytes 1 instruction word/vector

ATmega88A 8KBytes 512Bytes 1KBytes 1 instruction word/vector

ATmega88PA 8KBytes 512Bytes 1KBytes 1 instruction word/vector

ATmega168A 16KBytes 512Bytes 1KBytes 2 instruction words/vector

ATmega168PA 16KBytes 512Bytes 1KBytes 2 instruction words/vector

ATmega328 32KBytes 1KBytes 2KBytes 2 instruction words/vector

ATmega328P 32KBytes 1KBytes 2KBytes 2 instruction words/vector

ATmega48A/PA/88A/PA/168A/PA/328/P support a real Read-While-Write Self-Programming mechanism. There is
a separate Boot Loader Section, and the SPM instruction can only execute from there. In ATmega 48A/48PA there

ATmega48A/PA/88A/PA/168A/PA/328/P [DATASHEET]

8271E–AVR–07/2012

6

is no Read-While-Write support and no separate Boot Loader Section. The SPM instruction can execute from the
entire Flash

3. Resources

A comprehensive set of development tools, application notes and datasheets are available for download on
http://www.atmel.com/avr.

Note: 1.

4. Data Retention

Reliability Qualification results show that the projected data retention failure rate is much less than 1 PPM over 20
years at 85°C or 100 years at 25°C.

5. About Code Examples

This documentation contains simple code examples that briefly show how to use various parts of the device. These
code examples assume that the part specific header file is included before compilation. Be aware that not all C
compiler vendors include bit definitions in the header files and interrupt handling in C is compiler dependent.
Please confirm with the C compiler documentation for more details.

For I/O Registers located in extended I/O map, “IN”, “OUT”, “SBIS”, “SBIC”, “CBI”, and “SBI” instructions must be
replaced with instructions that allow access to extended I/O. Typically “LDS” and “STS” combined with “SBRS”,
“SBRC”, “SBR”, and “CBR”.

6. Capacitive Touch Sensing

The Atmel® QTouch® Library provides a simple to use solution to realize touch sensitive interfaces on most Atmel

®

AVR

microcontrollers. The QTouch Library includes support for the Atmel QTouch and Atmel QMatrix® acquisition

methods.

Touch sensing can be added to any application by linking the appropriate Atmel QTouch Library for the AVR Micro­controller. This is done by using a simple set of APIs to define the touch channels and sensors, and then calling the
touch sensing API’s to retrieve the channel information and determine the touch sensor states.

The QTouch Library is FREE and downloadable from the Atmel website at the following location:

www.atmel.com/qtouchlibrary. For implementation details and other information, refer to the Atmel QTouch Library
User Guide - also available for download from Atmel website.

ATmega48A/PA/88A/PA/168A/PA/328/P [DATASHEET]

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7

7. AVR CPU Core

Flash

Program

Memory

Instruction

Register

Instruction

Decoder

Program

Counter

Control Lines

32 x 8

General

Purpose

Registrers

ALU

Status

and Control

I/O Lines

EEPROM

Data Bus 8-bit

Data

SRAM

Direct Addressing

Indirect Addressing

Interrupt

Unit

SPI

Unit

Watchdog

Timer

Analog

Comparator

I/O Module 2

I/O Module1

I/O Module n

7.1 Overview

This section discusses the AVR core architecture in general. The main function of the CPU core is to ensure cor­rect program execution. The CPU must therefore be able to access memories, perform calculations, control
peripherals, and handle interrupts.

Figure 7-1. Block Diagram of the AVR Architecture

In order to maximize performance and parallelism, the AVR uses a Harvard architecture – with separate memories
and buses for program and data. Instructions in the program memory are executed with a single level pipelining.
While one instruction is being executed, the next instruction is pre-fetched from the program memory. This concept
enables instructions to be executed in every clock cycle. The program memory is In-System Reprogrammable
Flash memory.

The fast-access Register File contains 32 x 8-bit general purpose working registers with a single clock cycle
access time. This allows single-cycle Arithmetic Logic Unit (ALU) operation. In a typical ALU operation, two oper-

ATmega48A/PA/88A/PA/168A/PA/328/P [DATASHEET]

8271E–AVR–07/2012

8

ands are output from the Register File, the operation is executed, and the result is stored back in the Register File
– in one clock cycle.

Six of the 32 registers can be used as three 16-bit indirect address register pointers for Data Space addressing –
enabling efficient address calculations. One of the these address pointers can also be used as an address pointer
for look up tables in Flash program memory. These added function registers are the 16-bit X-, Y-, and Z-register,
described later in this section.

The ALU supports arithmetic and logic operations between registers or between a constant and a register. Single
register operations can also be executed in the ALU. After an arithmetic operation, the Status Register is updated
to reflect information about the result of the operation.

Program flow is provided by conditional and unconditional jump and call instructions, able to directly address the
whole address space. Most AVR instructions have a single 16-bit word format. Every program memory address
contains a 16- or 32-bit instruction.

Program Flash memory space is divided in two sections, the Boot Program section and the Application Program
section. Both sections have dedicated Lock bits for write and read/write protection. The SPM instruction that writes
into the Application Flash memory section must reside in the Boot Program section.

During interrupts and subroutine calls, the return address Program Counter (PC) is stored on the Stack. The Stack
is effectively allocated in the general data SRAM, and consequently the Stack size is only limited by the total
SRAM size and the usage of the SRAM. All user programs must initialize the SP in the Reset routine (before sub­routines or interrupts are executed). The Stack Pointer (SP) is read/write accessible in the I/O space. The data
SRAM can easily be accessed through the five different addressing modes supported in the AVR architecture.

The memory spaces in the AVR architecture are all linear and regular memory maps.

A flexible interrupt module has its control registers in the I/O space with an additional Global Interrupt Enable bit in
the Status Register. All interrupts have a separate Interrupt Vector in the Interrupt Vector table. The interrupts have
priority in accordance with their Interrupt Vector position. The lower the Interrupt Vector address, the higher the
priority.

The I/O memory space contains 64 addresses for CPU peripheral functions as Control Registers, SPI, and other
I/O functions. The I/O Memory can be accessed directly, or as the Data Space locations following those of the Reg­ister File, 0x20 - 0x5F. In addition, the ATmega48A/PA/88A/PA/168A/PA/328/P has Extended I/O space from 0x60

- 0xFF in SRAM where only the ST/STS/STD and LD/LDS/LDD instructions can be used.

7.2 ALU – Arithmetic Logic Unit

The high-performance AVR ALU operates in direct connection with all the 32 general purpose working registers.
Within a single clock cycle, arithmetic operations between general purpose registers or between a register and an
immediate are executed. The ALU operations are divided into three main categories – arithmetic, logical, and bit­functions. Some implementations of the architecture also provide a powerful multiplier supporting both
signed/unsigned multiplication and fractional format. See the “Instruction Set” section for a detailed description.

7.3 Status Register

The Status Register contains information about the result of the most recently executed arithmetic instruction. This
information can be used for altering program flow in order to perform conditional operations. Note that the Status
Register is updated after all ALU operations, as specified in the Instruction Set Reference. This will in many cases
remove the need for using the dedicated compare instructions, resulting in faster and more compact code.

The Status Register is not automatically stored when entering an interrupt routine and restored when returning
from an interrupt. This must be handled by software.

ATmega48A/PA/88A/PA/168A/PA/328/P [DATASHEET]

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7.3.1 SREG – AVR Status Register

The AVR Status Register – SREG – is defined as:

Bit 76543210

0x3F (0x5F) I T H S V N Z C SREG

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W

Initial Value 0 0 0 0 0 0 0 0

• Bit 7 – I: Global Interrupt Enable

The Global Interrupt Enable bit must be set for the interrupts to be enabled. The individual interrupt enable control
is then performed in separate control registers. If the Global Interrupt Enable Register is cleared, none of the inter­rupts are enabled independent of the individual interrupt enable settings. The I-bit is cleared by hardware after an
interrupt has occurred, and is set by the RETI instruction to enable subsequent interrupts. The I-bit can also be set
and cleared by the application with the SEI and CLI instructions, as described in the instruction set reference.

• Bit 6 – T: Bit Copy Storage

The Bit Copy instructions BLD (Bit LoaD) and BST (Bit STore) use the T-bit as source or destination for the oper­ated bit. A bit from a register in the Register File can be copied into T by the BST instruction, and a bit in T can be
copied into a bit in a register in the Register File by the BLD instruction.

• Bit 5 – H: Half Carry Flag

The Half Carry Flag H indicates a Half Carry in some arithmetic operations. Half Carry Is useful in BCD arithmetic.
See the “Instruction Set Description” for detailed information.

• Bit 4 – S: Sign Bit, S = N

 V

The S-bit is always an exclusive or between the Negative Flag N and the Two’s Complement Overflow Flag V. See
the “Instruction Set Description” for detailed information.

• Bit 3 – V: Two’s Complement Overflow Flag

The Two’s Complement Overflow Flag V supports two’s complement arithmetic. See the “Instruction Set Descrip­tion” for detailed information.

• Bit 2 – N: Negative Flag

The Negative Flag N indicates a negative result in an arithmetic or logic operation. See the “Instruction Set
Description” for detailed information.

• Bit 1 – Z: Zero Flag

The Zero Flag Z indicates a zero result in an arithmetic or logic operation. See the “Instruction Set Description” for
detailed information.

• Bit 0 – C: Carry Flag

The Carry Flag C indicates a carry in an arithmetic or logic operation. See the “Instruction Set Description” for
detailed information.

7.4 General Purpose Register File

The Register File is optimized for the AVR Enhanced RISC instruction set. In order to achieve the required perfor­mance and flexibility, the following input/output schemes are supported by the Register File:

• One 8-bit output operand and one 8-bit result input

• Two 8-bit output operands and one 8-bit result input

• Two 8-bit output operands and one 16-bit result input

ATmega48A/PA/88A/PA/168A/PA/328/P [DATASHEET]

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8271E–AVR–07/2012

• One 16-bit output operand and one 16-bit result input

Figure 7-2 shows the structure of the 32 general purpose working registers in the CPU.

Figure 7-2. AVR CPU General Purpose Working Registers

7 0 Addr.

R0 0x00

R1 0x01

R2 0x02

…

R13 0x0D

General R14 0x0E

Purpose R15 0x0F

Working R16 0x10

Registers R17 0x11

…

R26 0x1A X-register Low Byte

R27 0x1B X-register High Byte

R28 0x1C Y-register Low Byte

R29 0x1D Y-register High Byte

R30 0x1E Z-register Low Byte

R31 0x1F Z-register High Byte

Most of the instructions operating on the Register File have direct access to all registers, and most of them are sin­gle cycle instructions.

As shown in Figure 7-2, each register is also assigned a data memory address, mapping them directly into the first
32 locations of the user Data Space. Although not being physically implemented as SRAM locations, this memory
organization provides great flexibility in access of the registers, as the X-, Y- and Z-pointer registers can be set to
index any register in the file.

7.4.1 The X-register, Y-register, and Z-register

The registers R26...R31 have some added functions to their general purpose usage. These registers are 16-bit
address pointers for indirect addressing of the data space. The three indirect address registers X, Y, and Z are
defined as described in Figure 7-3.

Figure 7-3. The X-, Y-, and Z-registers

15 XH XL 0

X-register 707 0

R27 (0x1B) R26 (0x1A)

15 YH YL 0

Y-register 707 0

R29 (0x1D) R28 (0x1C)

15 ZH ZL 0

Z-register 70 7 0

R31 (0x1F) R30 (0x1E)

In the different addressing modes these address registers have functions as fixed displacement, automatic incre­ment, and automatic decrement (see the instruction set reference for details).

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7.5 Stack Pointer

The Stack is mainly used for storing temporary data, for storing local variables and for storing return addresses
after interrupts and subroutine calls. Note that the Stack is implemented as growing from higher to lower memory
locations. The Stack Pointer Register always points to the top of the Stack. The Stack Pointer points to the data
SRAM Stack area where the Subroutine and Interrupt Stacks are located. A Stack PUSH command will decrease
the Stack Pointer.

The Stack in the data SRAM must be defined by the program before any subroutine calls are executed or interrupts
are enabled. Initial Stack Pointer value equals the last address of the internal SRAM and the Stack Pointer must be
set to point above start of the SRAM, see Table 8-3 on page 18.

See Table 7-1 for Stack Pointer details.

Table 7-1. Stack Pointer instructions

Instruction Stack pointer Description

PUSH Decremented by 1 Data is pushed onto the stack

CALL
ICALL
RCALL

POP Incremented by 1 Data is popped from the stack

Decremented by 2

Return address is pushed onto the stack with a subroutine call or
interrupt

RET
RETI

Incremented by 2 Return address is popped from the stack with return from

subroutine or return from interrupt

The AVR Stack Pointer is implemented as two 8-bit registers in the I/O space. The number of bits actually used is
implementation dependent. Note that the data space in some implementations of the AVR architecture is so small
that only SPL is needed. In this case, the SPH Register will not be present.

7.5.1 SPH and SPL – Stack Pointer High and Stack Pointer Low Register

Bit 151413121110 9 8

0x3E (0x5E) SP15 SP14 SP13 SP12 SP11 SP10 SP9 SP8 SPH

0x3D (0x5D) SP7 SP6 SP5 SP4 SP3 SP2 SP1 SP0 SPL

76543210

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W

R/W R/W R/W R/W R/W R/W R/W R/W

Initial Value

RAMEND RAMEND RAMEND RAMEND RAMEND RAMEND RAMEND RAMEND

RAMEND RAMEND RAMEND RAMEND RAMEND RAMEND RAMEND RAMEND

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7.6 Instruction Execution Timing

clk

1st Instruction Fetch

1st Instruction Execute

2nd Instruction Fetch

2nd Instruction Execute

3rd Instruction Fetch

3rd Instruction Execute

4th Instruction Fetch

T1 T2 T3 T4

CPU

Total Execution Time

Register Operands Fetch

ALU Operation Execute

Result Write Back

T1 T2 T3 T4

clk

CPU

This section describes the general access timing concepts for instruction execution. The AVR CPU is driven by the
CPU clock clk

Figure 7-4 shows the parallel instruction fetches and instruction executions enabled by the Harvard architecture

and the fast-access Register File concept. This is the basic pipelining concept to obtain up to 1 MIPS per MHz with
the corresponding unique results for functions per cost, functions per clocks, and functions per power-unit.

Figure 7-4. The Parallel Instruction Fetches and Instruction Executions

Figure 7-5 shows the internal timing concept for the Register File. In a single clock cycle an ALU operation using

two register operands is executed, and the result is stored back to the destination register.

, directly generated from the selected clock source for the chip. No internal clock division is used.

CPU

Figure 7-5. Single Cycle ALU Operation

7.7 Reset and Interrupt Handling

The AVR provides several different interrupt sources. These interrupts and the separate Reset Vector each have a
separate program vector in the program memory space. All interrupts are assigned individual enable bits which
must be written logic one together with the Global Interrupt Enable bit in the Status Register in order to enable the
interrupt. Depending on the Program Counter value, interrupts may be automatically disabled when Boot Lock bits
BLB02 or BLB12 are programmed. This feature improves software security. See the section ”Memory Program-

ming” on page 285 for details.

The lowest addresses in the program memory space are by default defined as the Reset and Interrupt Vectors.
The complete list of vectors is shown in ”Interrupts” on page 57. The list also determines the priority levels of the
different interrupts. The lower the address the higher is the priority level. RESET has the highest priority, and next
is INT0 – the External Interrupt Request 0. The Interrupt Vectors can be moved to the start of the Boot Flash sec­tion by setting the IVSEL bit in the MCU Control Register (MCUCR). Refer to ”Interrupts” on page 57 for more
information. The Reset Vector can also be moved to the start of the Boot Flash section by programming the
BOOTRST Fuse, see ”Boot Loader Support – Read-While-Write Self-Programming” on page 269.

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When an interrupt occurs, the Global Interrupt Enable I-bit is cleared and all interrupts are disabled. The user soft­ware can write logic one to the I-bit to enable nested interrupts. All enabled interrupts can then interrupt the current
interrupt routine. The I-bit is automatically set when a Return from Interrupt instruction – RETI – is executed.

There are basically two types of interrupts. The first type is triggered by an event that sets the Interrupt Flag. For
these interrupts, the Program Counter is vectored to the actual Interrupt Vector in order to execute the interrupt
handling routine, and hardware clears the corresponding Interrupt Flag. Interrupt Flags can also be cleared by writ­ing a logic one to the flag bit position(s) to be cleared. If an interrupt condition occurs while the corresponding
interrupt enable bit is cleared, the Interrupt Flag will be set and remembered until the interrupt is enabled, or the
flag is cleared by software. Similarly, if one or more interrupt conditions occur while the Global Interrupt Enable bit
is cleared, the corresponding Interrupt Flag(s) will be set and remembered until the Global Interrupt Enable bit is
set, and will then be executed by order of priority.

The second type of interrupts will trigger as long as the interrupt condition is present. These interrupts do not nec­essarily have Interrupt Flags. If the interrupt condition disappears before the interrupt is enabled, the interrupt will
not be triggered.

When the AVR exits from an interrupt, it will always return to the main program and execute one more instruction
before any pending interrupt is served.

Note that the Status Register is not automatically stored when entering an interrupt routine, nor restored when
returning from an interrupt routine. This must be handled by software.

When using the CLI instruction to disable interrupts, the interrupts will be immediately disabled. No interrupt will be
executed after the CLI instruction, even if it occurs simultaneously with the CLI instruction. The following example
shows how this can be used to avoid interrupts during the timed EEPROM write sequence.

Assembly Code Example

in r16, SREG ; store SREG value

cli ; disable interrupts during timed sequence

sbi EECR, EEMPE ; start EEPROM write

sbi EECR, EEPE

out SREG, r16 ; restore SREG value (I-bit)

C Code Example

char cSREG;

cSREG = SREG; /* store SREG value */

/* disable interrupts during timed sequence */

_CLI();

EECR |= (1<<EEMPE); /* start EEPROM write */

EECR |= (1<<EEPE);

SREG = cSREG; /* restore SREG value (I-bit) */

When using the SEI instruction to enable interrupts, the instruction following SEI will be executed before any pend­ing interrupts, as shown in this example.

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Assembly Code Example

sei ; set Global Interrupt Enable

sleep; enter sleep, waiting for interrupt

; note: will enter sleep before any pending interrupt(s)

C Code Example

__enable_interrupt(); /* set Global Interrupt Enable */

__sleep(); /* enter sleep, waiting for interrupt */

/* note: will enter sleep before any pending interrupt(s) */

7.7.1 Interrupt Response Time

The interrupt execution response for all the enabled AVR interrupts is four clock cycles minimum. After four clock
cycles the program vector address for the actual interrupt handling routine is executed. During this four clock cycle
period, the Program Counter is pushed onto the Stack. The vector is normally a jump to the interrupt routine, and
this jump takes three clock cycles. If an interrupt occurs during execution of a multi-cycle instruction, this instruction
is completed before the interrupt is served. If an interrupt occurs when the MCU is in sleep mode, the interrupt exe­cution response time is increased by four clock cycles. This increase comes in addition to the start-up time from the
selected sleep mode.

A return from an interrupt handling routine takes four clock cycles. During these four clock cycles, the Program
Counter (two bytes) is popped back from the Stack, the Stack Pointer is incremented by two, and the I-bit in SREG
is set.

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8. AVR Memories

8.1 Overview

This section describes the different memories in the ATmega48A/PA/88A/PA/168A/PA/328/P. The AVR architec­ture has two main memory spaces, the Data Memory and the Program Memory space. In addition, the
ATmega48A/PA/88A/PA/168A/PA/328/P features an EEPROM Memory for data storage. All three memory spaces
are linear and regular.

8.2 In-System Reprogrammable Flash Program Memory

The ATmega48A/PA/88A/PA/168A/PA/328/P contains 4/8/16/32Kbytes On-chip In-System Reprogrammable
Flash memory for program storage. Since all AVR instructions are 16 or 32 bits wide, the Flash is organized as
2/4/8/16K x 16. For software security, the Flash Program memory space is divided into two sections, Boot Loader
Section and Application Program Section in ATmega88PA and ATmega168PA. See SPMEN description in section

”SPMCSR – Store Program Memory Control and Status Register” on page 283 for more details.

The Flash memory has an endurance of at least 10,000 write/erase cycles. The
ATmega48A/PA/88A/PA/168A/PA/328/P Program Counter (PC) is 11/12/13/14 bits wide, thus addressing the
2/4/8/16K program memory locations. The operation of Boot Program section and associated Boot Lock bits for
software protection are described in detail in ”Self-Programming the Flash, ATmega 48A/48PA” on page 261 and

”Boot Loader Support – Read-While-Write Self-Programming” on page 269. ”Memory Programming” on page 285

contains a detailed description on Flash Programming in SPI- or Parallel Programming mode.

Constant tables can be allocated within the entire program memory address space (see the LPM – Load Program
Memory instruction description).

Timing diagrams for instruction fetch and execution are presented in ”Instruction Execution Timing” on page 13.

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Figure 8-1. Program Memory Map ATmega 48A/48PA

0x0000

0x0FFF/0x1FFF/0x3FFF

Program Memory

Application Flash Section

Boot Flash Section

Program Memory

Application Flash Section

0x0000

Figure 8-2. Program Memory Map ATmega88A, ATmega88PA, ATmega168A, ATmega168PA, ATmega328 and

0x7FF

ATmega328P

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8.3 SRAM Data Memory

Figure 8-3 shows how the ATmega48A/PA/88A/PA/168A/PA/328/P SRAM Memory is organized.

The ATmega48A/PA/88A/PA/168A/PA/328/P is a complex microcontroller with more peripheral units than can be
supported within the 64 locations reserved in the Opcode for the IN and OUT instructions. For the Extended I/O
space from 0x60 - 0xFF in SRAM, only the ST/STS/STD and LD/LDS/LDD instructions can be used.

The lower 768/1280/1280/2303 data memory locations address both the Register File, the I/O memory, Extended
I/O memory, and the internal data SRAM. The first 32 locations address the Register File, the next 64 location the
standard I/O memory, then 160 locations of Extended I/O memory, and the next 512/1024/1024/2048 locations
address the internal data SRAM.

The five different addressing modes for the data memory cover: Direct, Indirect with Displacement, Indirect, Indi­rect with Pre-decrement, and Indirect with Post-increment. In the Register File, registers R26 to R31 feature the
indirect addressing pointer registers.

The direct addressing reaches the entire data space.

The Indirect with Displacement mode reaches 63 address locations from the base address given by the Y- or Z­register.

When using register indirect addressing modes with automatic pre-decrement and post-increment, the address
registers X, Y, and Z are decremented or incremented.

The 32 general purpose working registers, 64 I/O Registers, 160 Extended I/O Registers, and the
512/1024/1024/2048 bytes of internal data SRAM in the ATmega48A/PA/88A/PA/168A/PA/328/P are all accessi­ble through all these addressing modes. The Register File is described in ”General Purpose Register File” on page

10.

Figure 8-3. Data Memory Map

Data Memory

32 Registers
64 I/O Registers
160 Ext I/O Reg.

Internal SRAM

(512/1024/1024/2048 x 8)

0x0000 - 0x001F
0x0020 - 0x005F
0x0060 - 0x00FF

0x0100

0x02FF/0x04FF/0x4FF/0x08FF

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8.3.1 Data Memory Access Times

clk

WR

RD

Data

Data

Address

Address valid

T1 T2 T3

Compute Address

Read

Write

CPU

Memory Access Instruction

Next Instruction

This section describes the general access timing concepts for internal memory access. The internal data SRAM
access is performed in two clk

CPU

Figure 8-4. On-chip Data SRAM Access Cycles

cycles as described in Figure 8-4.

8.4 EEPROM Data Memory

The ATmega48A/PA/88A/PA/168A/PA/328/P contains 256/512/512/1Kbytes of data EEPROM memory. It is orga­nized as a separate data space, in which single bytes can be read and written. The EEPROM has an endurance of
at least 100,000 write/erase cycles. The access between the EEPROM and the CPU is described in the following,
specifying the EEPROM Address Registers, the EEPROM Data Register, and the EEPROM Control Register.

”Memory Programming” on page 285 contains a detailed description on EEPROM Programming in SPI or Parallel

Programming mode.

8.4.1 EEPROM Read/Write Access

The EEPROM Access Registers are accessible in the I/O space.

The write access time for the EEPROM is given in Table 8-2. A self-timing function, however, lets the user software
detect when the next byte can be written. If the user code contains instructions that write the EEPROM, some pre­cautions must be taken. In heavily filtered power supplies, V
causes the device for some period of time to run at a voltage lower than specified as minimum for the clock fre­quency used. See ”Preventing EEPROM Corruption” on page 20 for details on how to avoid problems in these
situations.

In order to prevent unintentional EEPROM writes, a specific write procedure must be followed. Refer to the
description of the EEPROM Control Register for details on this.

When the EEPROM is read, the CPU is halted for four clock cycles before the next instruction is executed. When
the EEPROM is written, the CPU is halted for two clock cycles before the next instruction is executed.

is likely to rise or fall slowly on power-up/down. This

CC

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8.4.2 Preventing EEPROM Corruption

During periods of low V

the EEPROM data can be corrupted because the supply voltage is too low for the CPU

CC,

and the EEPROM to operate properly. These issues are the same as for board level systems using EEPROM, and
the same design solutions should be applied.

An EEPROM data corruption can be caused by two situations when the voltage is too low. First, a regular write
sequence to the EEPROM requires a minimum voltage to operate correctly. Secondly, the CPU itself can execute
instructions incorrectly, if the supply voltage is too low.

EEPROM data corruption can easily be avoided by following this design recommendation:

Keep the AVR RESET active (low) during periods of insufficient power supply voltage. This can be done by
enabling the internal Brown-out Detector (BOD). If the detection level of the internal BOD does not match the
needed detection level, an external low V
operation is in progress, the write operation will be completed provided that the power supply voltage is sufficient.

8.5 I/O Memory

The I/O space definition of the ATmega48A/PA/88A/PA/168A/PA/328/P is shown in ”Register Summary” on page

518.

All ATmega48A/PA/88A/PA/168A/PA/328/P I/Os and peripherals are placed in the I/O space. All I/O locations may
be accessed by the LD/LDS/LDD and ST/STS/STD instructions, transferring data between the 32 general purpose
working registers and the I/O space. I/O Registers within the address range 0x00 - 0x1F are directly bit-accessible
using the SBI and CBI instructions. In these registers, the value of single bits can be checked by using the SBIS
and SBIC instructions. Refer to the instruction set section for more details. When using the I/O specific commands
IN and OUT, the I/O addresses 0x00 - 0x3F must be used. When addressing I/O Registers as data space using LD
and ST instructions, 0x20 must be added to these addresses. The ATmega48A/PA/88A/PA/168A/PA/328/P is a
complex microcontroller with more peripheral units than can be supported within the 64 location reserved in
Opcode for the IN and OUT instructions. For the Extended I/O space from 0x60 - 0xFF in SRAM, only the
ST/STS/STD and LD/LDS/LDD instructions can be used.

reset Protection circuit can be used. If a reset occurs while a write

CC

For compatibility with future devices, reserved bits should be written to zero if accessed. Reserved I/O memory
addresses should never be written.

Some of the Status Flags are cleared by writing a logical one to them. Note that, unlike most other AVRs, the CBI
and SBI instructions will only operate on the specified bit, and can therefore be used on registers containing such
Status Flags. The CBI and SBI instructions work with registers 0x00 to 0x1F only.

The I/O and peripherals control registers are explained in later sections.

8.5.1 General Purpose I/O Registers

The ATmega48A/PA/88A/PA/168A/PA/328/P contains three General Purpose I/O Registers. These registers can
be used for storing any information, and they are particularly useful for storing global variables and Status Flags.
General Purpose I/O Registers within the address range 0x00 - 0x1F are directly bit-accessible using the SBI, CBI,
SBIS, and SBIC instructions.

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8.6 Register Description

8.6.1 EEARH and EEARL – The EEPROM Address Register

Bit 151413121110 9 8

0x22 (0x42) ––––––EEAR9

0x21 (0x41) EEAR7 EEAR6 EEAR5 EEAR4 EEAR3 EEAR2 EEAR1 EEAR0 EEARL

76543210

Read/Write RRRRRRRR/W

R/W R/W R/W R/W R/W R/W R/W R/W

Initial Value0000000X

XXXXXXXX

• Bits [15:10] – Reserved

These bits are reserved bits in the ATmega48A/PA/88A/PA/168A/PA/328/P and will always read as zero.

• Bits 9:0 – EEAR[9:0]: EEPROM Address

The EEPROM Address Registers – EEARH and EEARL specify the EEPROM address in the 256/512/512/1Kbytes
EEPROM space. The EEPROM data bytes are addressed linearly between 0 and 255/511/511/1023. The initial
value of EEAR is undefined. A proper value must be written before the EEPROM may be accessed.

Note: 1. EEAR9 and EEAR8 are unused bits in ATmega 48A/48PA and must always be written to zero.

8.6.2 EEDR – The EEPROM Data Register

Bit 76543210

0x20 (0x40) MSB LSB EEDR

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W

Initial Value 0 0 0 0 0 0 0 0

(1)

EEAR8

(1)

EEARH

• Bits 7:0 – EEDR[7:0]: EEPROM Data

For the EEPROM write operation, the EEDR Register contains the data to be written to the EEPROM in the
address given by the EEAR Register. For the EEPROM read operation, the EEDR contains the data read out from
the EEPROM at the address given by EEAR.

8.6.3 EECR – The EEPROM Control Register

Bit 76543210

0x1F (0x3F) – – EEPM1 EEPM0 EERIE EEMPE EEPE EERE EECR

Read/Write R R R/W R/W R/W R/W R/W R/W

Initial Value 0 0 X X 0 0 X 0

• Bits 7:6 – Reserved

These bits are reserved bits in the ATmega48A/PA/88A/PA/168A/PA/328/P and will always read as zero.

• Bits 5, 4 – EEPM1 and EEPM0: EEPROM Programming Mode Bits

The EEPROM Programming mode bit setting defines which programming action that will be triggered when writing
EEPE. It is possible to program data in one atomic operation (erase the old value and program the new value) or to
split the Erase and Write operations in two different operations. The Programming times for the different modes are
shown in Table 8-1. While EEPE is set, any write to EEPMn will be ignored. During reset, the EEPMn bits will be
reset to 0b00 unless the EEPROM is busy programming.

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Table 8-1. EEPROM Mode Bits

Programming

EEPM1 EEPM0

0 0 3.4ms Erase and Write in one operation (Atomic Operation)

0 1 1.8ms Erase Only

1 0 1.8ms Write Only

1 1 – Reserved for future use

Time Operation

• Bit 3 – EERIE: EEPROM Ready Interrupt Enable

Writing EERIE to one enables the EEPROM Ready Interrupt if the I bit in SREG is set. Writing EERIE to zero dis­ables the interrupt. The EEPROM Ready interrupt generates a constant interrupt when EEPE is cleared. The
interrupt will not be generated during EEPROM write or SPM.

• Bit 2 – EEMPE: EEPROM Master Write Enable

The EEMPE bit determines whether setting EEPE to one causes the EEPROM to be written. When EEMPE is set,
setting EEPE within four clock cycles will write data to the EEPROM at the selected address If EEMPE is zero, set­ting EEPE will have no effect. When EEMPE has been written to one by software, hardware clears the bit to zero
after four clock cycles. See the description of the EEPE bit for an EEPROM write procedure.

• Bit 1 – EEPE: EEPROM Write Enable

The EEPROM Write Enable Signal EEPE is the write strobe to the EEPROM. When address and data are correctly
set up, the EEPE bit must be written to one to write the value into the EEPROM. The EEMPE bit must be written to
one before a logical one is written to EEPE, otherwise no EEPROM write takes place. The following procedure
should be followed when writing the EEPROM (the order of steps 3 and 4 is not essential):

1. Wait until EEPE becomes zero.

2. Wait until SPMEN in SPMCSR becomes zero.

3. Write new EEPROM address to EEAR (optional).

4. Write new EEPROM data to EEDR (optional).

5. Write a logical one to the EEMPE bit while writing a zero to EEPE in EECR.

6. Within four clock cycles after setting EEMPE, write a logical one to EEPE.
The EEPROM can not be programmed during a CPU write to the Flash memory. The software must check that the

Flash programming is completed before initiating a new EEPROM write. Step 2 is only relevant if the software con­tains a Boot Loader allowing the CPU to program the Flash. If the Flash is never being updated by the CPU, step 2
can be omitted. See ”Boot Loader Support – Read-While-Write Self-Programming” on page 269 for details about
Boot programming.

Caution: An interrupt between step 5 and step 6 will make the write cycle fail, since the EEPROM Master Write
Enable will time-out. If an interrupt routine accessing the EEPROM is interrupting another EEPROM access, the
EEAR or EEDR Register will be modified, causing the interrupted EEPROM access to fail. It is recommended to
have the Global Interrupt Flag cleared during all the steps to avoid these problems.

When the write access time has elapsed, the EEPE bit is cleared by hardware. The user software can poll this bit
and wait for a zero before writing the next byte. When EEPE has been set, the CPU is halted for two cycles before
the next instruction is executed.

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• Bit 0 – EERE: EEPROM Read Enable

The EEPROM Read Enable Signal EERE is the read strobe to the EEPROM. When the correct address is set up in
the EEAR Register, the EERE bit must be written to a logic one to trigger the EEPROM read. The EEPROM read
access takes one instruction, and the requested data is available immediately. When the EEPROM is read, the
CPU is halted for four cycles before the next instruction is executed.

The user should poll the EEPE bit before starting the read operation. If a write operation is in progress, it is neither
possible to read the EEPROM, nor to change the EEAR Register.

The calibrated Oscillator is used to time the EEPROM accesses. Table 8-2 lists the typical programming time for
EEPROM access from the CPU.

Table 8-2. EEPROM Programming Time

Symbol Number of Calibrated RC Oscillator Cycles Typ Programming Time

EEPROM write
(from CPU)

26,368 3.3ms

The following code examples show one assembly and one C function for writing to the EEPROM. The examples
assume that interrupts are controlled (e.g. by disabling interrupts globally) so that no interrupts will occur during
execution of these functions. The examples also assume that no Flash Boot Loader is present in the software. If
such code is present, the EEPROM write function must also wait for any ongoing SPM command to finish.

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Assembly Code Example

EEPROM_write:

; Wait for completion of previous write

sbic EECR,EEPE

rjmp EEPROM_write

; Set up address (r18:r17) in address register

out EEARH, r18

out EEARL, r17

; Write data (r16) to Data Register

out EEDR,r16

; Write logical one to EEMPE

sbi EECR,EEMPE

; Start eeprom write by setting EEPE

sbi EECR,EEPE

ret

C Code Example

void EEPROM_write(unsigned int uiAddress, unsigned char ucData)

{

/* Wait for completion of previous write */

while(EECR & (1<<EEPE))

;

/* Set up address and Data Registers */

EEAR = uiAddress;

EEDR = ucData;

/* Write logical one to EEMPE */

EECR |= (1<<EEMPE);

/* Start eeprom write by setting EEPE */

EECR |= (1<<EEPE);

}

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The next code examples show assembly and C functions for reading the EEPROM. The examples assume that
interrupts are controlled so that no interrupts will occur during execution of these functions.

Assembly Code Example

EEPROM_read:

; Wait for completion of previous write

sbic EECR,EEPE

rjmp EEPROM_read

; Set up address (r18:r17) in address register

out EEARH, r18

out EEARL, r17

; Start eeprom read by writing EERE

sbi EECR,EERE

; Read data from Data Register

in r16,EEDR

ret

C Code Example

unsigned char EEPROM_read(unsigned int uiAddress)

{

/* Wait for completion of previous write */

while(EECR & (1<<EEPE))

;

/* Set up address register */

EEAR = uiAddress;

/* Start eeprom read by writing EERE */

EECR |= (1<<EERE);

/* Return data from Data Register */

return EEDR;

}

8.6.4 GPIOR2 – General Purpose I/O Register 2

Bit 76543210

0x2B (0x4B) MSB LSB GPIOR2

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W

Initial Value 0 0 0 0 0 0 0 0

8.6.5 GPIOR1 – General Purpose I/O Register 1

Bit 76543210

0x2A (0x4A) MSB LSB GPIOR1

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W

Initial Value 0 0 0 0 0 0 0 0

8.6.6 GPIOR0 – General Purpose I/O Register 0

Bit 76543210

0x1E (0x3E) MSB LSB GPIOR0

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W

Initial Value 0 0 0 0 0 0 0 0

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9. System Clock and Clock Options

9.1 Clock Systems and their Distribution

Figure 9-1 presents the principal clock systems in the AVR and their distribution. All of the clocks need not be

active at a given time. In order to reduce power consumption, the clocks to modules not being used can be halted
by using different sleep modes, as described in ”Power Management and Sleep Modes” on page 38. The clock
systems are detailed below.

Figure 9-1. Clock Distribution

Asynchronous
Timer/Counter

Timer/Counter

Oscillator

General I/O

Modules

clk

I/O

clk

ASY

External Clock

ADC

clk

AVR Clock

Control Unit

System Clock

Prescaler

Source clock

Clock

Multiplexer

Oscillator

ADC

Crystal

CPU Core RAM

clk

CPU

clk

FLASH

Reset Logic

Watchdog Timer

Watchdog clock

Watchdog

Oscillator

Low-frequency

Crystal Oscillator

Flash and
EEPROM

Calibrated RC

Oscillator

9.1.1 CPU Clock – clk

The CPU clock is routed to parts of the system concerned with operation of the AVR core. Examples of such mod­ules are the General Purpose Register File, the Status Register and the data memory holding the Stack Pointer.
Halting the CPU clock inhibits the core from performing general operations and calculations.

9.1.2 I/O Clock – clk

I/O

The I/O clock is used by the majority of the I/O modules, like Timer/Counters, SPI, and USART. The I/O clock is
also used by the External Interrupt module, but note that start condition detection in the USI module is carried out
asynchronously when clk

Note: Note that if a level triggered interrupt is used for wake-up from Power-down, the required level must be held long

enough for the MCU to complete the wake-up to trigger the level interrupt. If the level disappears before the end of the
Start-up Time, the MCU will still wake up, but no interrupt will be generated. The start-up time is defined by the SUT
and CKSEL Fuses as described in ”System Clock and Clock Options” on page 26.

9.1.3 Flash Clock – clk

The Flash clock controls operation of the Flash interface. The Flash clock is usually active simultaneously with the
CPU clock.

CPU

FLASH

is halted, TWI address recognition in all sleep modes.

I/O

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9.1.4 Asynchronous Timer Clock – clk

The Asynchronous Timer clock allows the Asynchronous Timer/Counter to be clocked directly from an external
clock or an external 32kHz clock crystal. The dedicated clock domain allows using this Timer/Counter as a real­time counter even when the device is in sleep mode.

ASY

9.1.5 ADC Clock – clk

ADC

The ADC is provided with a dedicated clock domain. This allows halting the CPU and I/O clocks in order to reduce
noise generated by digital circuitry. This gives more accurate ADC conversion results.

9.2 Clock Sources

The device has the following clock source options, selectable by Flash Fuse bits as shown below. The clock from
the selected source is input to the AVR clock generator, and routed to the appropriate modules.

Table 9-1. Device Clocking Options Select

Device Clocking Option CKSEL3...0

Low Power Crystal Oscillator 1111 - 1000

Full Swing Crystal Oscillator 0111 - 0110

Low Frequency Crystal Oscillator 0101 - 0100

Internal 128kHz RC Oscillator 0011

Calibrated Internal RC Oscillator 0010

External Clock 0000

Reserved 0001

Note: 1. For all fuses “1” means unprogrammed while “0” means programmed.

9.2.1 Default Clock Source

The device is shipped with internal RC oscillator at 8.0MHz and with the fuse CKDIV8 programmed, resulting in

1.0MHz system clock. The startup time is set to maximum and time-out period enabled. (CKSEL = "0010", SUT =
"10", CKDIV8 = "0"). The default setting ensures that all users can make their desired clock source setting using
any available programming interface.

(1)

9.2.2 Clock Startup Sequence

Any clock source needs a sufficient V
can be considered stable.

To ensure sufficient V

, the device issues an internal reset with a time-out delay (t

CC

released by all other reset sources. ”System Control and Reset” on page 46 describes the start conditions for the
internal reset. The delay (t

TOUT

by the SUTx and CKSELx fuse bits. The selectable delays are shown in Table 9-2. The frequency of the Watchdog
Oscillator is voltage dependent as shown in ”Typical Characteristics” on page 318.

Table 9-2. Number of Watchdog Oscillator Cycles

Typ Time-out (VCC = 5.0V) Typ Time-out (VCC = 3.0V) Number of Cycles

0ms 0ms 0

4.1ms 4.3ms 512

65ms 69ms 8K (8,192)

Main purpose of the delay is to keep the AVR in reset until it is supplied with minimum V
itor the actual voltage and it will be required to select a delay longer than the V

to start oscillating and a minimum number of oscillating cycles before it

CC

) after the device reset is

TOUT

) is timed from the Watchdog Oscillator and the number of cycles in the delay is set

. The delay will not mon-

CC

rise time. If this is not possible, an

CC

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internal or external Brown-Out Detection circuit should be used. A BOD circuit will ensure sufficient VCC before it

XTAL2 (TOSC2)

XTAL1 (TOSC1)

GND

C2

C1

releases the reset, and the time-out delay can be disabled. Disabling the time-out delay without utilizing a Brown­Out Detection circuit is not recommended.

The oscillator is required to oscillate for a minimum number of cycles before the clock is considered stable. An
internal ripple counter monitors the oscillator output clock, and keeps the internal reset active for a given number of
clock cycles. The reset is then released and the device will start to execute. The recommended oscillator start-up
time is dependent on the clock type, and varies from 6 cycles for an externally applied clock to 32K cycles for a low
frequency crystal.

The start-up sequence for the clock includes both the time-out delay and the start-up time when the device starts
up from reset. When starting up from Power-save or Power-down mode, V
and only the start-up time is included.

9.3 Low Power Crystal Oscillator

Pins XTAL1 and XTAL2 are input and output, respectively, of an inverting amplifier which can be configured for use
as an On-chip Oscillator, as shown in Figure 9-2 on page 28. Either a quartz crystal or a ceramic resonator may be
used.

This Crystal Oscillator is a low power oscillator, with reduced voltage swing on the XTAL2 output. It gives the low­est power consumption, but is not capable of driving other clock inputs, and may be more susceptible to noise in
noisy environments. In these cases, refer to the ”Full Swing Crystal Oscillator” on page 29.

C1 and C2 should always be equal for both crystals and resonators. The optimal value of the capacitors depends
on the crystal or resonator in use, the amount of stray capacitance, and the electromagnetic noise of the environ­ment. Some initial guidelines for choosing capacitors for use with crystals are given in Table 9-3 on page 28. For
ceramic resonators, the capacitor values given by the manufacturer should be used.

is assumed to be at a sufficient level

CC

Figure 9-2. Crystal Oscillator Connections

The Low Power Oscillator can operate in three different modes, each optimized for a specific frequency range. The
operating mode is selected by the fuses CKSEL3...1 as shown in Table 9-3 on page 28.

Table 9-3. Low Power Crystal Oscillator Operating Modes

Frequency Range

(MHz)

0.4 - 0.9 – 100

0.9 - 3.0 12 - 22 101

3.0 - 8.0 12 - 22 110

8.0 - 16.0 12 - 22 111

Recommended Range for

Capacitors C1 and C2 (pF) CKSEL3...1

(3)

(1)

(2)

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Notes: 1. This is the recommended CKSEL settings for the difference frequency ranges.

2. This option should not be used with crystals, only with ceramic resonators.

3. If the crystal frequency exceeds the specification of the device (depends on VCC), the CKDIV8 Fuse can be pro­grammed in order to divide the internal frequency by 8. It must be ensured that the resulting divided clock meets
the frequency specification of the device.

The CKSEL0 Fuse together with the SUT1...0 Fuses select the start-up times as shown in Table 9-4.

Table 9-4. Start-up Times for the Low Power Crystal Oscillator Clock Selection

Oscillator Source /
Power Conditions

Ceramic resonator, fast
rising power

Ceramic resonator, slowly
rising power

Ceramic resonator, BOD
enabled

Ceramic resonator, fast
rising power

Ceramic resonator, slowly
rising power

Crystal Oscillator, BOD
enabled

Crystal Oscillator, fast
rising power

Crystal Oscillator, slowly
rising power

Start-up Time from

Power-down and

Power-save

258 CK 14CK + 4.1ms

258 CK 14CK + 65ms

1K CK 14CK

1K CK 14CK + 4.1ms

1K CK 14CK + 65ms

16K CK 14CK 1 01

16K CK 14CK + 4.1ms 1 10

16K CK 14CK + 65ms 1 11

Additional Delay

from Reset

(VCC = 5.0V) CKSEL0 SUT1...0

(1)

(1)

(2)

(2)

(2)

000

001

010

011

100

Notes: 1. These options should only be used when not operating close to the maximum frequency of the device, and only if

frequency stability at start-up is not important for the application. These options are not suitable for crystals.

2. These options are intended for use with ceramic resonators and will ensure frequency stability at start-up. They can
also be used with crystals when not operating close to the maximum frequency of the device, and if frequency sta­bility at start-up is not important for the application.

9.4 Full Swing Crystal Oscillator

Pins XTAL1 and XTAL2 are input and output, respectively, of an inverting amplifier which can be configured for use
as an On-chip Oscillator, as shown in Figure 9-2 on page 28. Either a quartz crystal or a ceramic resonator may be
used.

This Crystal Oscillator is a full swing oscillator, with rail-to-rail swing on the XTAL2 output. This is useful for driving
other clock inputs and in noisy environments. The current consumption is higher than the ”Low Power Crystal

Oscillator” on page 28. Note that the Full Swing Crystal Oscillator will only operate for V

C1 and C2 should always be equal for both crystals and resonators. The optimal value of the capacitors depends
on the crystal or resonator in use, the amount of stray capacitance, and the electromagnetic noise of the environ­ment. Some initial guidelines for choosing capacitors for use with crystals are given in Table 9-6 on page 30. For
ceramic resonators, the capacitor values given by the manufacturer should be used.

= 2.7 - 5.5 volts.

CC

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The operating mode is selected by the fuses CKSEL3...1 as shown in Table 9-5.

XTAL2 (TOSC2)

XTAL1 (TOSC1)

GND

C2

C1

Table 9-5. Full Swing Crystal Oscillator operating modes

Frequency Range
(MHz)

0.4 - 20 12 - 22 011

Notes: 1. If the cryatal frequency exceeds the specification of the device (depends on VCC), the CKDIV8 Fuse can be pro-

grammed in order to divide the internal frequency by 8. It must be ensured that the resulting divided clock meets
the frequency specification of the device.

(1)

Recommended Range for

Capacitors C1 and C2 (pF) CKSEL3...1

Figure 9-3. Crystal Oscillator Connections

Table 9-6. Start-up Times for the Full Swing Crystal Oscillator Clock Selection

Start-up Time from
Oscillator Source /
Power Conditions

Ceramic resonator, fast
rising power

Ceramic resonator, slowly
rising power

Ceramic resonator, BOD
enabled

Ceramic resonator, fast
rising power

Ceramic resonator, slowly
rising power

Crystal Oscillator, BOD
enabled

Crystal Oscillator, fast
rising power

Crystal Oscillator, slowly
rising power

Power-down and

Power-save

258 CK 14CK + 4.1ms

258 CK 14CK + 65ms

1K CK 14CK

1K CK 14CK + 4.1ms

1K CK 14CK + 65ms

16K CK 14CK 1 01

16K CK 14CK + 4.1ms 1 10

16K CK 14CK + 65ms 1 11

Notes: 1. These options should only be used when not operating close to the maximum frequency of the device, and only if

frequency stability at start-up is not important for the application. These options are not suitable for crystals.

2. These options are intended for use with ceramic resonators and will ensure frequency stability at start-up. They can
also be used with crystals when not operating close to the maximum frequency of the device, and if frequency sta­bility at start-up is not important for the application.

Additional Delay

from Reset

= 5.0V) CKSEL0 SUT1...0

(V

CC

(1)

(1)

(2)

(2)

(2)

000

001

010

011

100

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9.5 Low Frequency Crystal Oscillator

C 2 CL C

s

–=

The Low-frequency Crystal Oscillator is optimized for use with a 32.768kHz watch crystal. When selecting crystals,
load capacitance and crystal’s Equivalent Series Resistance, ESR must be taken into consideration. Both values
are specified by the crystal vendor. ATmega48A/PA/88A/PA/168A/PA/328/P oscillator is optimized for very low
power consumption, and thus when selecting crystals, see Table for maximum ESR recommendations on 6.5pF,

9.0pF and 12.5pF crystals

Table 9-7. Maximum ESR Recommendation for 32.768kHz Crystal

Crystal CL (pF) Max ESR [k]

6.5 75

9.0 65

12.5 30

Note: 1. Maximum ESR is typical value based on characterization

The Low-frequency Crystal Oscillator provides an internal load capacitance, see Table 9-8 at each TOSC pin.

Table 9-8. Capacitance for Low-frequency Oscillator

Device 32kHz Osc. Type Cap(Xtal1/Tosc1) Cap(Xtal2/Tosc2)

(1)

ATmega48A/PA/88A/PA/168A/PA/3

28/P

System Osc. 18pF 8pF

Timer Osc. 18pF 8pF

The capacitance (Ce+Ci) needed at each TOSC pin can be calculated by using:

where:

– Ce - is optional external capacitors as described in Figure 9-2 on page 28

– Ci - is the pin capacitance in Table 9-8

– CL - is the load capacitance for a 32.768kHz crystal specified by the crystal vendor

– CS - is the total stray capacitance for one TOSC pin.

Crystals specifying load capacitance (CL) higher than 6 pF, require external capacitors applied as described in Fig-

ure 9-2 on page 28.

The Low-frequency Crystal Oscillator must be selected by setting the CKSEL Fuses to “0110” or “0111”, as shown
in Table 9-10 on page 32. Start-up times are determined by the SUT Fuses as shown in Table 9-9.

Table 9-9. Start-up Times for the Low-frequency Crystal Oscillator Clock Selection

SUT1...0 Additional Delay from Reset (VCC = 5.0V) Recommended Usage

00 4 CK Fast rising power or BOD enabled

01 4 CK + 4.1ms Slowly rising power

10 4 CK + 65ms Stable frequency at start-up

11 Reserved

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Table 9-10. Start-up Times for the Low-frequency Crystal Oscillator Clock Selection

CKSEL3...

0

(1)

0100

0101 32K CK Stable frequency at start-up

Note: 1. This option should only be used if frequency stability at start-up is not important for the application

Start-up Time from

Power-down and Power-save Recommended Usage

1K CK

9.6 Calibrated Internal RC Oscillator

By default, the Internal RC Oscillator provides an approximate 8.0MHz clock. Though voltage and temperature
dependent, this clock can be very accurately calibrated by the user. See Table 29-10 on page 309 for more details.
The device is shipped with the CKDIV8 Fuse programmed. See ”System Clock Prescaler” on page 34 for more
details.

This clock may be selected as the system clock by programming the CKSEL Fuses as shown in Table 9-11. If
selected, it will operate with no external components. During reset, hardware loads the pre-programmed calibration
value into the OSCCAL Register and thereby automatically calibrates the RC Oscillator. The accuracy of this cali­bration is shown as Factory calibration in Table 29-10 on page 309.

By changing the OSCCAL register from SW, see ”OSCCAL – Oscillator Calibration Register” on page 36, it is pos-
sible to get a higher calibration accuracy than by using the factory calibration. The accuracy of this calibration is
shown as User calibration in Table 29-10 on page 309.

When this Oscillator is used as the chip clock, the Watchdog Oscillator will still be used for the Watchdog Timer
and for the Reset Time-out. For more information on the pre-programmed calibration value, see the section ”Cali-

bration Byte” on page 289.

Table 9-11. Internal Calibrated RC Oscillator Operating Modes

Frequency Range

7.3 - 8.1 0010

Notes: 1. The device is shipped with this option selected.

2. If 8MHz frequency exceeds the specification of the device (depends on V
grammed in order to divide the internal frequency by 8.

(2)

(MHz) CKSEL3...0

(1)

), the CKDIV8 Fuse can be pro-

CC

When this Oscillator is selected, start-up times are determined by the SUT Fuses as shown in Table 9-12.

Table 9-12. Start-up times for the internal calibrated RC Oscillator clock selection

Start-up Time from Power-

Power Conditions

BOD enabled 6 CK 14CK

Fast rising power 6 CK 14CK + 4.1ms 01

Slowly rising power 6 CK 14CK + 65ms

Note: 1. If the RSTDISBL fuse is programmed, this start-up time will be increased to

14CK + 4.1ms to ensure programming mode can be entered.

2. The device is shipped with this option selected.

down and Power-save

Reserved 11

Additional Delay from

Reset (VCC = 5.0V) SUT1...0

(1)

(2)

00

10

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9.7 128kHz Internal Oscillator

The 128kHz internal Oscillator is a low power Oscillator providing a clock of 128kHz. The frequency is nominal at
3V and 25C. This clock may be select as the system clock by programming the CKSEL Fuses to “11” as shown in

Table 9-13.

Table 9-13. 128kHz Internal Oscillator Operating Modes

Nominal Frequency

128kHz 0011

Note: 1. Note that the 128kHz oscillator is a very low power clock source, and is not designed for high accuracy.

When this clock source is selected, start-up times are determined by the SUT Fuses as shown in Table 9-14.

Table 9-14. Start-up Times for the 128kHz Internal Oscillator

(1)

CKSEL3...0

Power Conditions

BOD enabled 6 CK 14CK

Fast rising power 6 CK 14CK + 4ms 01

Slowly rising power 6 CK 14CK + 64ms 10

Note: 1. If the RSTDISBL fuse is programmed, this start-up time will be increased to

14CK + 4.1ms to ensure programming mode can be entered.

9.8 External Clock

To drive the device from an external clock source, XTAL1 should be driven as shown in Figure 9-4. To run the
device on an external clock, the CKSEL Fuses must be programmed to “0000” (see Table 9-15).

Table 9-15. Crystal Oscillator Clock Frequency

Figure 9-4. External Clock Drive Configuration

Start-up Time from Power-

down and Power-save

Reserved 11

Frequency CKSEL3...0

0 - 20MHz 0000

PB7

Additional Delay from

Reset SUT1...0

(1)

XTAL2

00

EXTERNAL

CLOCK

SIGNAL

XTAL1

GND

When this clock source is selected, start-up times are determined by the SUT Fuses as shown in Table 9-16.

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Table 9-16. Start-up Times for the External Clock Selection

Power Conditions

BOD enabled 6 CK 14CK 00

Fast rising power 6 CK 14CK + 4.1ms 01

Slowly rising power 6 CK 14CK + 65ms 10

When applying an external clock, it is required to avoid sudden changes in the applied clock frequency to ensure
stable operation of the MCU. A variation in frequency of more than 2% from one clock cycle to the next can lead to
unpredictable behavior. If changes of more than 2% is required, ensure that the MCU is kept in Reset during the
changes.

Note that the System Clock Prescaler can be used to implement run-time changes of the internal clock frequency
while still ensuring stable operation. Refer to ”System Clock Prescaler” on page 34 for details.

9.9 Clock Output Buffer

The device can output the system clock on the CLKO pin. To enable the output, the CKOUT Fuse has to be pro­grammed. This mode is suitable when the chip clock is used to drive other circuits on the system. The clock also
will be output during reset, and the normal operation of I/O pin will be overridden when the fuse is programmed.
Any clock source, including the internal RC Oscillator, can be selected when the clock is output on CLKO. If the
System Clock Prescaler is used, it is the divided system clock that is output.

Start-up Time from Power-

down and Power-save

Reserved 11

Additional Delay from

Reset (VCC = 5.0V) SUT1...0

9.10 Timer/Counter Oscillator

ATmega48A/PA/88A/PA/168A/PA/328/P uses the same crystal oscillator for Low-frequency Oscillator and
Timer/Counter Oscillator. See ”Low Frequency Crystal Oscillator” on page 31 for details on the oscillator and crys­tal requirements.

ATmega48A/PA/88A/PA/168A/PA/328/P share the Timer/Counter Oscillator Pins (TOSC1 and TOSC2) with
XTAL1 and XTAL2. When using the Timer/Counter Oscillator, the system clock needs to be four times the oscilla­tor frequency. Due to this and the pin sharing, the Timer/Counter Oscillator can only be used when the Calibrated
Internal RC Oscillator is selected as system clock source.

Applying an external clock source to TOSC1 can be done if EXTCLK in the ASSR Register is written to logic one.
See ”Asynchronous Operation of Timer/Counter2” on page 153 for further description on selecting external clock
as input instead of a 32.768kHz watch crystal.

9.11 System Clock Prescaler

The ATmega48A/PA/88A/PA/168A/PA/328/P has a system clock prescaler, and the system clock can be divided
by setting the ”CLKPR – Clock Prescale Register” on page 367. This feature can be used to decrease the system
clock frequency and the power consumption when the requirement for processing power is low. This can be used
with all clock source options, and it will affect the clock frequency of the CPU and all synchronous peripherals.
clk

, clk

I/O

When switching between prescaler settings, the System Clock Prescaler ensures that no glitches occurs in the
clock system. It also ensures that no intermediate frequency is higher than neither the clock frequency correspond­ing to the previous setting, nor the clock frequency corresponding to the new setting. The ripple counter that
implements the prescaler runs at the frequency of the undivided clock, which may be faster than the CPU's clock
frequency. Hence, it is not possible to determine the state of the prescaler - even if it were readable, and the exact
time it takes to switch from one clock division to the other cannot be exactly predicted. From the time the CLKPS

ADC

, clk

, and clk

CPU

FLASH

are divided by a factor as shown in Table 29-12 on page 310.

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values are written, it takes between T1 + T2 and T1 + 2 * T2 before the new clock frequency is active. In this inter­val, 2 active clock edges are produced. Here, T1 is the previous clock period, and T2 is the period corresponding to
the new prescaler setting.

To avoid unintentional changes of clock frequency, a special write procedure must be followed to change the
CLKPS bits:

1. Write the Clock Prescaler Change Enable (CLKPCE) bit to one and all other bits in CLKPR to zero.

2. Within four cycles, write the desired value to CLKPS while writing a zero to CLKPCE.
Interrupts must be disabled when changing prescaler setting to make sure the write procedure is not interrupted.

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9.12 Register Description

9.12.1 OSCCAL – Oscillator Calibration Register

Bit 76543210

(0x66) CAL7 CAL6 CAL5 CAL4 CAL3 CAL2 CAL1 CAL0 OSCCAL

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W

Initial Value Device Specific Calibration Value

• Bits 7:0 – CAL[7:0]: Oscillator Calibration Value

The Oscillator Calibration Register is used to trim the Calibrated Internal RC Oscillator to remove process varia­tions from the oscillator frequency. A pre-programmed calibration value is automatically written to this register
during chip reset, giving the Factory calibrated frequency as specified in Table 29-10 on page 309. The application
software can write this register to change the oscillator frequency. The oscillator can be calibrated to frequencies
as specified in Table 29-10 on page 309. Calibration outside that range is not guaranteed.

Note that this oscillator is used to time EEPROM and Flash write accesses, and these write times will be affected
accordingly. If the EEPROM or Flash are written, do not calibrate to more than 8.8MHz. Otherwise, the EEPROM
or Flash write may fail.

The CAL7 bit determines the range of operation for the oscillator. Setting this bit to 0 gives the lowest frequency
range, setting this bit to 1 gives the highest frequency range. The two frequency ranges are overlapping, in other
words a setting of OSCCAL = 0x7F gives a higher frequency than OSCCAL = 0x80.

The CAL6...0 bits are used to tune the frequency within the selected range. A setting of 0x00 gives the lowest fre­quency in that range, and a setting of 0x7F gives the highest frequency in the range.

9.12.2 CLKPR – Clock Prescale Register

Bit 7 6543210

(0x61)

Read/Write R/W R R R R/W R/W R/W R/W

Initial Value 0 0 0 0 See Bit Description

CLKPCE – – – CLKPS3 CLKPS2 CLKPS1 CLKPS0 CLKPR

• Bit 7 – CLKPCE: Clock Prescaler Change Enable

The CLKPCE bit must be written to logic one to enable change of the CLKPS bits. The CLKPCE bit is only updated
when the other bits in CLKPR are simultaneously written to zero. CLKPCE is cleared by hardware four cycles after
it is written or when CLKPS bits are written. Rewriting the CLKPCE bit within this time-out period does neither
extend the time-out period, nor clear the CLKPCE bit.

• Bits 3:0 – CLKPS[3:0]: Clock Prescaler Select Bits 3 - 0

These bits define the division factor between the selected clock source and the internal system clock. These bits
can be written run-time to vary the clock frequency to suit the application requirements. As the divider divides the
master clock input to the MCU, the speed of all synchronous peripherals is reduced when a division factor is used.
The division factors are given in Table 9-17 on page 37.

The CKDIV8 Fuse determines the initial value of the CLKPS bits. If CKDIV8 is unprogrammed, the CLKPS bits will
be reset to “0000”. If CKDIV8 is programmed, CLKPS bits are reset to “0011”, giving a division factor of 8 at start
up. This feature should be used if the selected clock source has a higher frequency than the maximum frequency
of the device at the present operating conditions. Note that any value can be written to the CLKPS bits regardless
of the CKDIV8 Fuse setting. The Application software must ensure that a sufficient division factor is chosen if the
selected clock source has a higher frequency than the maximum frequency of the device at the present operating
conditions. The device is shipped with the CKDIV8 Fuse programmed.

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Table 9-17. Clock Prescaler Select

CLKPS3 CLKPS2 CLKPS1 CLKPS0 Clock Division Factor

0000 1

0001 2

0010 4

0011 8

0100 16

0101 32

0110 64

0111 128

1000 256

1001 Reserved

1010 Reserved

1011 Reserved

1100 Reserved

1101 Reserved

1110 Reserved

1111 Reserved

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10. Power Management and Sleep Modes

Sleep modes enable the application to shut down unused modules in the MCU, thereby saving power. The AVR
provides various sleep modes allowing the user to tailor the power consumption to the application’s requirements.

When enabled, the Brown-out Detector (BOD) actively monitors the power supply voltage during the sleep periods.
To further save power, it is possible to disable the BOD in some sleep modes. See ”BOD Disable(1)” on page 39
for more details.

10.1 Sleep Modes

Figure 9-1 on page 26 presents the different clock systems in the ATmega48A/PA/88A/PA/168A/PA/328/P, and

their distribution. The figure is helpful in selecting an appropriate sleep mode. Table 10-1 shows the different sleep
modes, their wake up sources BOD disable ability.

Note: 1. BOD disable is only available for ATmega48PA/88PA/168PA/328P.

Table 10-1. Active Clock Domains and Wake-up Sources in the Different Sleep Modes.

Active Clock Domains Oscillators Wake-up Sources

CPU

FLASH

clk

clk

Sleep Mode

clkIOclk

Idle XXX X X

ADC Noise
Reduction

Power-down X

Power-save X X

Standby

(1)

Extended
Standby

Notes: 1. Only recommended with external crystal or resonator selected as clock source.

2. If Timer/Counter2 is running in asynchronous mode.

3. For INT1 and INT0, only level interrupt.

ADC

ASY

clk

XX X X

(2)

X

(1)

Main Clock

Source Enabled

Timer Oscillator

Enabled

INT1, INT0 and

Pin Change

TWI Address

Match

(2)

(2)

(2)

XX

XX

(2)

X X X X XXX

(3)

X

(3)

(3)

X

(3)

(3)

X

XX

XXX

XX X X

XXX

XX X X

(2)

Timer2

SPM/EEPROM

Ready

ADC

WDT

XXX

Other I/O

Software

BOD Disable

To enter any of the six sleep modes, the SE bit in SMCR must be written to logic one and a SLEEP instruction must
be executed. The SM2, SM1, and SM0 bits in the SMCR Register select which sleep mode (Idle, ADC Noise
Reduction, Power-down, Power-save, Standby, or Extended Standby) will be activated by the SLEEP instruction.
See Table 10-2 on page 43 for a summary.

If an enabled interrupt occurs while the MCU is in a sleep mode, the MCU wakes up. The MCU is then halted for
four cycles in addition to the start-up time, executes the interrupt routine, and resumes execution from the instruc­tion following SLEEP. The contents of the Register File and SRAM are unaltered when the device wakes up from
sleep. If a reset occurs during sleep mode, the MCU wakes up and executes from the Reset Vector.

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10.2 BOD Disable

When the Brown-out Detector (BOD) is enabled by BODLEVEL fuses - see Table 28-7 on page 287 and onwards,
the BOD is actively monitoring the power supply voltage during a sleep period. To save power, it is possible to dis­able the BOD by software for some of the sleep modes, see Table 10-1 on page 38. The sleep mode power
consumption will then be at the same level as when BOD is globally disabled by fuses. If BOD is disabled in soft­ware, the BOD function is turned off immediately after entering the sleep mode. Upon wake-up from sleep, BOD is
automatically enabled again. This ensures safe operation in case the V
period.

When the BOD has been disabled, the wake-up time from sleep mode will be approximately 60 µs to ensure that
the BOD is working correctly before the MCU continues executing code.

BOD disable is controlled by bit 6, BODS (BOD Sleep) in the control register MCUCR, see ”MCUCR – MCU Con-

trol Register” on page 44. Writing this bit to one turns off the BOD in relevant sleep modes, while a zero in this bit

keeps BOD active. Default setting keeps BOD active, i.e. BODS set to zero.

Writing to the BODS bit is controlled by a timed sequence and an enable bit, see ”MCUCR – MCU Control Regis-

ter” on page 44.

Note: 1. BOD disable only available in picoPower devices ATmega48PA/88PA/168PA/328P

10.3 Idle Mode

When the SM2...0 bits are written to 000, the SLEEP instruction makes the MCU enter Idle mode, stopping the
CPU but allowing the SPI, USART, Analog Comparator, ADC, 2-wire Serial Interface, Timer/Counters, Watchdog,
and the interrupt system to continue operating. This sleep mode basically halts clk
the other clocks to run.

(1)

level has dropped during the sleep

CC

CPU

and clk

, while allowing

FLASH

Idle mode enables the MCU to wake up from external triggered interrupts as well as internal ones like the Timer
Overflow and USART Transmit Complete interrupts. If wake-up from the Analog Comparator interrupt is not
required, the Analog Comparator can be powered down by setting the ACD bit in the Analog Comparator Control
and Status Register – ACSR. This will reduce power consumption in Idle mode. If the ADC is enabled, a conver­sion starts automatically when this mode is entered.

10.4 ADC Noise Reduction Mode

When the SM2...0 bits are written to 001, the SLEEP instruction makes the MCU enter ADC Noise Reduction
mode, stopping the CPU but allowing the ADC, the external interrupts, the 2-wire Serial Interface address watch,
Timer/Counter2

, and clk

CPU

(1)

, and the Watchdog to continue operating (if enabled). This sleep mode basically halts clk

, while allowing the other clocks to run.

FLASH

This improves the noise environment for the ADC, enabling higher resolution measurements. If the ADC is
enabled, a conversion starts automatically when this mode is entered. Apart from the ADC Conversion Complete
interrupt, only an External Reset, a Watchdog System Reset, a Watchdog Interrupt, a Brown-out Reset, a 2-wire
Serial Interface address match, a Timer/Counter2 interrupt, an SPM/EEPROM ready interrupt, an external level
interrupt on INT0 or INT1 or a pin change interrupt can wake up the MCU from ADC Noise Reduction mode.

Note: 1. Timer/Counter2 will only keep running in asynchronous mode, see ”8-bit Timer/Counter2 with PWM and Asynchro-

nous Operation” on page 142 for details.

10.5 Power-down Mode

When the SM2...0 bits are written to 010, the SLEEP instruction makes the MCU enter Power-down mode. In this
mode, the external Oscillator is stopped, while the external interrupts, the 2-wire Serial Interface address watch,
and the Watchdog continue operating (if enabled). Only an External Reset, a Watchdog System Reset, a Watch­dog Interrupt, a Brown-out Reset, a 2-wire Serial Interface address match, an external level interrupt on INT0 or

I/O

, clk-

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INT1, or a pin change interrupt can wake up the MCU. This sleep mode basically halts all generated clocks, allow­ing operation of asynchronous modules only.

Note: If a level triggered interrupt is used for wake-up from Power-down, the required level must be held long enough for the

MCU to complete the wake-up to trigger the level interrupt. If the level disappears before the end of the Start-up Time,
the MCU will still wake up, but no interrupt will be generated. ”External Interrupts” on page 71. The start-up time is
defined by the SUT and CKSEL Fuses as described in ”System Clock and Clock Options” on page 26.

When waking up from Power-down mode, there is a delay from the wake-up condition occurs until the wake-up
becomes effective. This allows the clock to restart and become stable after having been stopped. The wake-up
period is defined by the same CKSEL Fuses that define the Reset Time-out period, as described in ”Clock

Sources” on page 27.

10.6 Power-save Mode

When the SM2...0 bits are written to 011, the SLEEP instruction makes the MCU enter Power-save mode. This
mode is identical to Power-down, with one exception:

If Timer/Counter2 is enabled, it will keep running during sleep. The device can wake up from either Timer Overflow
or Output Compare event from Timer/Counter2 if the corresponding Timer/Counter2 interrupt enable bits are set in
TIMSK2, and the Global Interrupt Enable bit in SREG is set.

If Timer/Counter2 is not running, Power-down mode is recommended instead of Power-save mode.

The Timer/Counter2 can be clocked both synchronously and asynchronously in Power-save mode. If
Timer/Counter2 is not using the asynchronous clock, the Timer/Counter Oscillator is stopped during sleep. If
Timer/Counter2 is not using the synchronous clock, the clock source is stopped during sleep. Note that even if the
synchronous clock is running in Power-save, this clock is only available for Timer/Counter2.

10.7 Standby Mode

When the SM2...0 bits are 110 and an external crystal/resonator clock option is selected, the SLEEP instruction
makes the MCU enter Standby mode. This mode is identical to Power-down with the exception that the Oscillator is
kept running. From Standby mode, the device wakes up in six clock cycles.

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10.8 Extended Standby Mode

When the SM2...0 bits are 111 and an external crystal/resonator clock option is selected, the SLEEP instruction
makes the MCU enter Extended Standby mode. This mode is identical to Power-save with the exception that the
Oscillator is kept running. From Extended Standby mode, the device wakes up in six clock cycles.

10.9 Power Reduction Register

The Power Reduction Register (PRR), see ”PRR – Power Reduction Register” on page 44, provides a method to
stop the clock to individual peripherals to reduce power consumption. The current state of the peripheral is frozen
and the I/O registers can not be read or written. Resources used by the peripheral when stopping the clock will
remain occupied, hence the peripheral should in most cases be disabled before stopping the clock. Waking up a
module, which is done by clearing the bit in PRR, puts the module in the same state as before shutdown.

Module shutdown can be used in Idle mode and Active mode to significantly reduce the overall power consump­tion. In all other sleep modes, the clock is already stopped.

10.10 Minimizing Power Consumption

There are several possibilities to consider when trying to minimize the power consumption in an AVR controlled
system. In general, sleep modes should be used as much as possible, and the sleep mode should be selected so
that as few as possible of the device’s functions are operating. All functions not needed should be disabled. In par­ticular, the following modules may need special consideration when trying to achieve the lowest possible power
consumption.

10.10.1 Analog to Digital Converter

If enabled, the ADC will be enabled in all sleep modes. To save power, the ADC should be disabled before entering
any sleep mode. When the ADC is turned off and on again, the next conversion will be an extended conversion.
Refer to ”Analog-to-Digital Converter” on page 242 for details on ADC operation.

10.10.2 Analog Comparator

When entering Idle mode, the Analog Comparator should be disabled if not used. When entering ADC Noise
Reduction mode, the Analog Comparator should be disabled. In other sleep modes, the Analog Comparator is
automatically disabled. However, if the Analog Comparator is set up to use the Internal Voltage Reference as
input, the Analog Comparator should be disabled in all sleep modes. Otherwise, the Internal Voltage Reference will
be enabled, independent of sleep mode. Refer to ”Analog Comparator” on page 239 for details on how to configure
the Analog Comparator.

10.10.3 Brown-out Detector

If the Brown-out Detector is not needed by the application, this module should be turned off. If the Brown-out
Detector is enabled by the BODLEVEL Fuses, it will be enabled in all sleep modes, and hence, always consume
power. In the deeper sleep modes, this will contribute significantly to the total current consumption. Refer to

”Brown-out Detection” on page 48 for details on how to configure the Brown-out Detector.

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10.10.4 Internal Voltage Reference

The Internal Voltage Reference will be enabled when needed by the Brown-out Detection, the Analog Comparator
or the ADC. If these modules are disabled as described in the sections above, the internal voltage reference will be
disabled and it will not be consuming power. When turned on again, the user must allow the reference to start up
before the output is used. If the reference is kept on in sleep mode, the output can be used immediately. Refer to

”Internal Voltage Reference” on page 49 for details on the start-up time.

10.10.5 Watchdog Timer

If the Watchdog Timer is not needed in the application, the module should be turned off. If the Watchdog Timer is
enabled, it will be enabled in all sleep modes and hence always consume power. In the deeper sleep modes, this
will contribute significantly to the total current consumption. Refer to ”Watchdog Timer” on page 50 for details on
how to configure the Watchdog Timer.

10.10.6 Port Pins

When entering a sleep mode, all port pins should be configured to use minimum power. The most important is then
to ensure that no pins drive resistive loads. In sleep modes where both the I/O clock (clk
(clk

) are stopped, the input buffers of the device will be disabled. This ensures that no power is consumed by

ADC

the input logic when not needed. In some cases, the input logic is needed for detecting wake-up conditions, and it
will then be enabled. Refer to the section ”Digital Input Enable and Sleep Modes” on page 80 for details on which
pins are enabled. If the input buffer is enabled and the input signal is left floating or have an analog signal level
close to V

/2, the input buffer will use excessive power.

CC

For analog input pins, the digital input buffer should be disabled at all times. An analog signal level close to V
on an input pin can cause significant current even in active mode. Digital input buffers can be disabled by writing to
the Digital Input Disable Registers (DIDR1 and DIDR0). Refer to ”DIDR1 – Digital Input Disable Register 1” on

page 241 and ”DIDR0 – Digital Input Disable Register 0” on page 257 for details.

) and the ADC clock

I/O

CC

/2

10.10.7 On-chip Debug System

If the On-chip debug system is enabled by the DWEN Fuse and the chip enters sleep mode, the main clock source
is enabled and hence always consumes power. In the deeper sleep modes, this will contribute significantly to the
total current consumption.

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10.11 Register Description

10.11.1 SMCR – Sleep Mode Control Register

The Sleep Mode Control Register contains control bits for power management.

Bit 76543210

0x33 (0x53) ––––SM2SM1SM0SESMCR

Read/Write R R R R R/W R/W R/W R/W

Initial Value 0 0 0 0 0 0 0 0

• Bits [7:4]: Reserved

These bits are unused in the ATmega48A/PA/88A/PA/168A/PA/328/P, and will always be read as zero.

• Bits 3:1 – SM[2:0]: Sleep Mode Select Bits 2, 1, and 0

These bits select between the five available sleep modes as shown in Table 10-2.

Table 10-2. Sleep Mode Select

SM2 SM1 SM0 Sleep Mode

000Idle

0 0 1 ADC Noise Reduction

0 1 0 Power-down

0 1 1 Power-save

100Reserved

101Reserved

1 1 0 Standby

1 1 1 External Standby

(1)

(1)

Note: 1. Standby mode is only recommended for use with external crystals or resonators.

• Bit 0 – SE: Sleep Enable

The SE bit must be written to logic one to make the MCU enter the sleep mode when the SLEEP instruction is exe­cuted. To avoid the MCU entering the sleep mode unless it is the programmer’s purpose, it is recommended to
write the Sleep Enable (SE) bit to one just before the execution of the SLEEP instruction and to clear it immediately
after waking up.

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10.11.2 MCUCR – MCU Control Register

Bit 7 6 5 4 3 2 1 0

0x35 (0x55) –BODS

Read/Write R R/W R/W R/W R R R/W R/W

Initial Value 0 0 0 0 0 0 0 0

• Bit 6 – BODS: BOD Sleep

(1)

BODSE

(1)

The BODS bit must be written to logic one in order to turn off BOD during sleep, see Table 10-1 on page 38. Writ­ing to the BODS bit is controlled by a timed sequence and an enable bit, BODSE in MCUCR. To disable BOD in
relevant sleep modes, both BODS and BODSE must first be set to one. Then, to set the BODS bit, BODS must be
set to one and BODSE must be set to zero within four clock cycles.

The BODS bit is active three clock cycles after it is set. A sleep instruction must be executed while BODS is active
in order to turn off the BOD for the actual sleep mode. The BODS bit is automatically cleared after three clock
cycles.

• Bit 5 – BODSE: BOD Sleep Enable

BODSE enables setting of BODS control bit, as explained in BODS bit description. BOD disable is controlled by a
timed sequence.

Note: 1. BODS and BODSE only available for picoPower devices ATmega48PA/88PA/168PA/328P

10.11.3 PRR – Power Reduction Register

Bit 7 6 5 4 3 2 1 0

(0x64) PRTWI PRTIM2 PRTIM0 – PRTIM1 PRSPI PRUSART0 PRADC PRR

Read/Write R/W R/W R/W R R/W R/W R/W R/W

Initial Value 0 0 0 0 0 0 0 0

(1)

PUD – – IVSEL IVCE MCUCR

(1)

• Bit 7 – PRTWI: Power Reduction TWI

Writing a logic one to this bit shuts down the TWI by stopping the clock to the module. When waking up the TWI
again, the TWI should be re initialized to ensure proper operation.

• Bit 6 – PRTIM2: Power Reduction Timer/Counter2

Writing a logic one to this bit shuts down the Timer/Counter2 module in synchronous mode (AS2 is 0). When the
Timer/Counter2 is enabled, operation will continue like before the shutdown.

• Bit 5 – PRTIM0: Power Reduction Timer/Counter0

Writing a logic one to this bit shuts down the Timer/Counter0 module. When the Timer/Counter0 is enabled, opera­tion will continue like before the shutdown.

• Bit 4 – Reserved

This bit is reserved in ATmega48A/PA/88A/PA/168A/PA/328/P and will always read as zero.

• Bit 3 – PRTIM1: Power Reduction Timer/Counter1

Writing a logic one to this bit shuts down the Timer/Counter1 module. When the Timer/Counter1 is enabled, opera­tion will continue like before the shutdown.

• Bit 2 – PRSPI: Power Reduction Serial Peripheral Interface

If using debugWIRE On-chip Debug System, this bit should not be written to one. Writing a logic one to this bit
shuts down the Serial Peripheral Interface by stopping the clock to the module. When waking up the SPI again, the
SPI should be re initialized to ensure proper operation.

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• Bit 1 – PRUSART0: Power Reduction USART0

Writing a logic one to this bit shuts down the USART by stopping the clock to the module. When waking up the
USART again, the USART should be re initialized to ensure proper operation.

• Bit 0 – PRADC: Power Reduction ADC

Writing a logic one to this bit shuts down the ADC. The ADC must be disabled before shut down. The analog com­parator cannot use the ADC input MUX when the ADC is shut down.

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11. System Control and Reset

11.1 Resetting the AVR

During reset, all I/O Registers are set to their initial values, and the program starts execution from the Reset Vec­tor. For ATmega168A/168PA/328/328P the instruction placed at the Reset Vector must be a JMP – Absolute Jump
– instruction to the reset handling routine. For the ATmega 48A/48PA and ATmega88A/88PA, the instruction
placed at the Reset Vector must be an RJMP – Relative Jump – instruction to the reset handling routine. If the pro­gram never enables an interrupt source, the Interrupt Vectors are not used, and regular program code can be
placed at these locations. This is also the case if the Reset Vector is in the Application section while the Interrupt
Vectors are in the Boot section or vice versa (ATmega88A/88PA/168A/168PA/328/328P only). The circuit diagram
in Figure 11-1 on page 47 shows the reset logic. Table 29-12 on page 310 defines the electrical parameters of the
reset circuitry.

The I/O ports of the AVR are immediately reset to their initial state when a reset source goes active. This does not
require any clock source to be running.

After all reset sources have gone inactive, a delay counter is invoked, stretching the internal reset. This allows the
power to reach a stable level before normal operation starts. The time-out period of the delay counter is defined by
the user through the SUT and CKSEL Fuses. The different selections for the delay period are presented in ”Clock

Sources” on page 27.

11.2 Reset Sources

The ATmega48A/PA/88A/PA/168A/PA/328/P has four sources of reset:

• Power-on Reset. The MCU is reset when the supply voltage is below the Power-on Reset threshold (V

• External Reset. The MCU is reset when a low level is present on the RESET

pin for longer than the minimum

POT

).

pulse length.

• Watchdog System Reset. The MCU is reset when the Watchdog Timer period expires and the Watchdog System
Reset mode is enabled.

• Brown-out Reset. The MCU is reset when the supply voltage V

is below the Brown-out Reset threshold (V

CC

BOT

and the Brown-out Detector is enabled.

)

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Figure 11-1. Reset Logic

MCU Status

Register (MCUSR)

Brown-out

Reset Circuit

BODLEVEL [2..0]

Delay Counters

CKSEL[3:0]

CK

TIMEOUT

WDRF

BORF

EXTRF

PORF

DATA BU S

Clock

Generator

SPIKE

FILTER

Pull-up Resistor

Watchdog
Oscillator

SUT[1:0]

Power-on Reset

Circuit

RSTDISBL

V

RESET

TIME-OUT

INTERNAL

RESET

t

TOUT

V

POT

V

RST

CC

11.3 Power-on Reset

A Power-on Reset (POR) pulse is generated by an On-chip detection circuit. The detection level is defined in ”Sys-

tem and Reset Characteristics” on page 310. The POR is activated whenever V

is below the detection level. The

CC

POR circuit can be used to trigger the start-up Reset, as well as to detect a failure in supply voltage.

A Power-on Reset (POR) circuit ensures that the device is reset from Power-on. Reaching the Power-on Reset
threshold voltage invokes the delay counter, which determines how long the device is kept in RESET after V
The RESET signal is activated again, without any delay, when V

Figure 11-2. MCU Start-up, RESET

Tied to V

CC

decreases below the detection level.

CC

CC

rise.

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Figure 11-3. MCU Start-up, RESET Extended Externally

RESET

TIME-OUT

INTERNAL

RESET

t

TOUT

V

POT

V

RST

V

CC

CC

11.4 External Reset

An External Reset is generated by a low level on the RESET pin. Reset pulses longer than the minimum pulse
width (see ”System and Reset Characteristics” on page 310) will generate a reset, even if the clock is not running.
Shorter pulses are not guaranteed to generate a reset. When the applied signal reaches the Reset Threshold Volt­age – V
The External Reset can be disabled by the RSTDISBL fuse, see Table 28-7 on page 287.

Figure 11-4. External Reset During Operation

– on its positive edge, the delay counter starts the MCU after the Time-out period – t

RST

TOUT –

has expired.

11.5 Brown-out Detection

ATmega48A/PA/88A/PA/168A/PA/328/P has an On-chip Brown-out Detection (BOD) circuit for monitoring the V
level during operation by comparing it to a fixed trigger level. The trigger level for the BOD can be selected by the
BODLEVEL Fuses. The trigger level has a hysteresis to ensure spike free Brown-out Detection. The hysteresis on
the detection level should be interpreted as V
enabled, and V
Reset is immediately activated. When V
delay counter starts the MCU after the Time-out period t

The BOD circuit will only detect a drop in V
in ”System and Reset Characteristics” on page 310.

decreases to a value below the trigger level (V

CC

CC

= V

BOT+

increases above the trigger level (V

CC

if the voltage stays below the trigger level for longer than t

CC

ATmega48A/PA/88A/PA/168A/PA/328/P [DATASHEET]

BOT

+ V

TOUT

/2 and V

HYST

in Figure 11-5 on page 49), the Brown-out

BOT-

has expired.

BOT-

= V

BOT+

BOT

- V

/2.When the BOD is

HYST

in Figure 11-5 on page 49), the

8271E–AVR–07/2012

BOD

given

48

Figure 11-5. Brown-out Reset During Operation

V

CC

RESET

TIME-OUT

INTERNAL

RESET

V

BOT-

V

BOT+

t

TOUT

CK

CC

11.6 Watchdog System Reset

When the Watchdog times out, it will generate a short reset pulse of one CK cycle duration. On the falling edge of
this pulse, the delay timer starts counting the Time-out period t
Watchdog Timer.

Figure 11-6. Watchdog System Reset During Operation

. Refer to page 50 for details on operation of the

TOUT

11.7 Internal Voltage Reference

ATmega48A/PA/88A/PA/168A/PA/328/P features an internal bandgap reference. This reference is used for
Brown-out Detection, and it can be used as an input to the Analog Comparator or the ADC.

11.7.1 Voltage Reference Enable Signals and Start-up Time

The voltage reference has a start-up time that may influence the way it should be used. The start-up time is given
in ”System and Reset Characteristics” on page 310. To save power, the reference is not always turned on. The ref-
erence is on during the following situations:

1. When the BOD is enabled (by programming the BODLEVEL [2:0] Fuses).

2. When the bandgap reference is connected to the Analog Comparator (by setting the ACBG bit in ACSR).

3. When the ADC is enabled.

Thus, when the BOD is not enabled, after setting the ACBG bit or enabling the ADC, the user must always allow
the reference to start up before the output from the Analog Comparator or ADC is used. To reduce power con­sumption in Power-down mode, the user can avoid the three conditions above to ensure that the reference is
turned off before entering Power-down mode.

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11.8 Watchdog Timer

128kHz

OSCILLATOR

OSC/2K

OSC/4K

OSC/8K

OSC/16K

OSC/32K

OSC/64K

OSC/128K

OSC/256K

OSC/512K

OSC/1024K

WDP0
WDP1
WDP2
WDP3

WATCHDOG
RESET

WDE

WDIF

WDIE

MCU RESET

INTERRUPT

11.8.1 Features

Clocked from separate On-chip Oscillator

•

• 3 Operating modes

–Interrupt
– System Reset
– Interrupt and System Reset

• Selectable Time-out period from 16ms to 8s

• Possible Hardware fuse Watchdog always on (WDTON) for fail-safe mode

11.8.2 Overview

ATmega48A/PA/88A/PA/168A/PA/328/P has an Enhanced Watchdog Timer (WDT). The WDT is a timer counting
cycles of a separate on-chip 128kHz oscillator. The WDT gives an interrupt or a system reset when the counter
reaches a given time-out value. In normal operation mode, it is required that the system uses the WDR - Watchdog
Timer Reset - instruction to restart the counter before the time-out value is reached. If the system doesn't restart
the counter, an interrupt or system reset will be issued.

Figure 11-7. Watchdog Timer

In Interrupt mode, the WDT gives an interrupt when the timer expires. This interrupt can be used to wake the
device from sleep-modes, and also as a general system timer. One example is to limit the maximum time allowed
for certain operations, giving an interrupt when the operation has run longer than expected. In System Reset mode,
the WDT gives a reset when the timer expires. This is typically used to prevent system hang-up in case of runaway
code. The third mode, Interrupt and System Reset mode, combines the other two modes by first giving an interrupt
and then switch to System Reset mode. This mode will for instance allow a safe shutdown by saving critical param­eters before a system reset.

The Watchdog always on (WDTON) fuse, if programmed, will force the Watchdog Timer to System Reset mode.
With the fuse programmed the System Reset mode bit (WDE) and Interrupt mode bit (WDIE) are locked to 1 and 0
respectively. To further ensure program security, alterations to the Watchdog set-up must follow timed sequences.
The sequence for clearing WDE and changing time-out configuration is as follows:

1. In the same operation, write a logic one to the Watchdog change enable bit (WDCE) and WDE. A logic one

2. Within the next four clock cycles, write the WDE and Watchdog prescaler bits (WDP) as desired, but with

must be written to WDE regardless of the previous value of the WDE bit.

the WDCE bit cleared. This must be done in one operation.

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The following code example shows one assembly and one C function for turning off the Watchdog Timer. The
example assumes that interrupts are controlled (e.g. by disabling interrupts globally) so that no interrupts will occur
during the execution of these functions.

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Assembly Code Example

WDT_off:

; Turn off global interrupt

cli

; Reset Watchdog Timer

wdr

; Clear WDRF in MCUSR

in r16, MCUSR

andi r16, (0xff & (0<<WDRF))

out MCUSR, r16

; Write logical one to WDCE and WDE

; Keep old prescaler setting to prevent unintentional time-out

lds r16, WDTCSR

ori r16, (1<<WDCE) | (1<<WDE)

sts WDTCSR, r16

; Turn off WDT

ldi r16, (0<<WDE)

sts WDTCSR, r16

; Turn on global interrupt

sei

ret

C Code Example

(1)

(1)

void WDT_off(void)

{

__disable_interrupt();

__watchdog_reset();

/* Clear WDRF in MCUSR */

MCUSR &= ~(1<<WDRF);

/* Write logical one to WDCE and WDE */

/* Keep old prescaler setting to prevent unintentional time-out */

WDTCSR |= (1<<WDCE) | (1<<WDE);

/* Turn off WDT */

WDTCSR = 0x00;

__enable_interrupt();

}

Note: 1. See ”About Code Examples” on page 7.

Note: If the Watchdog is accidentally enabled, for example by a runaway pointer or brown-out condition, the device
will be reset and the Watchdog Timer will stay enabled. If the code is not set up to handle the Watchdog, this might
lead to an eternal loop of time-out resets. To avoid this situation, the application software should always clear the
Watchdog System Reset Flag (WDRF) and the WDE control bit in the initialization routine, even if the Watchdog is
not in use.

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The following code example shows one assembly and one C function for changing the time-out value of the Watch­dog Timer.

Assembly Code Example

WDT_Prescaler_Change:

; Turn off global interrupt

cli

; Reset Watchdog Timer

wdr

; Start timed sequence

lds r16, WDTCSR

ori r16, (1<<WDCE) | (1<<WDE)

sts WDTCSR, r16

; -- Got four cycles to set the new values from here -

; Set new prescaler(time-out) value = 64K cycles (~0.5 s)

ldi r16, (1<<WDE) | (1<<WDP2) | (1<<WDP0)

sts WDTCSR, r16

; -- Finished setting new values, used 2 cycles -

; Turn on global interrupt

sei

ret

C Code Example

(1)

(1)

void WDT_Prescaler_Change(void)

{

__disable_interrupt();

__watchdog_reset();

/* Start timed sequence */

WDTCSR |= (1<<WDCE) | (1<<WDE);

/* Set new prescaler(time-out) value = 64K cycles (~0.5 s) */

WDTCSR = (1<<WDE) | (1<<WDP2) | (1<<WDP0);

__enable_interrupt();

}

Note: 1. See ”About Code Examples” on page 7.

Note: The Watchdog Timer should be reset before any change of the WDP bits, since a change in the WDP bits
can result in a time-out when switching to a shorter time-out period.

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11.9 Register Description

11.9.1 MCUSR – MCU Status Register

The MCU Status Register provides information on which reset source caused an MCU reset.

Bit 76543210

0x34 (0x54) ––––WDRFBORFEXTRFPORFMCUSR

Read/Write RRRRR/WR/WR/WR/W

Initial Value 0000 See Bit Description

• Bit 7:4: Reserved

These bits are unused bits in the ATmega48A/PA/88A/PA/168A/PA/328/P, and will always read as zero.

• Bit 3 – WDRF: Watchdog System Reset Flag

This bit is set if a Watchdog System Reset occurs. The bit is reset by a Power-on Reset, or by writing a logic zero
to the flag.

• Bit 2 – BORF: Brown-out Reset Flag

This bit is set if a Brown-out Reset occurs. The bit is reset by a Power-on Reset, or by writing a logic zero to the
flag.

• Bit 1 – EXTRF: External Reset Flag

This bit is set if an External Reset occurs. The bit is reset by a Power-on Reset, or by writing a logic zero to the flag.

• Bit 0 – PORF: Power-on Reset Flag

This bit is set if a Power-on Reset occurs. The bit is reset only by writing a logic zero to the flag.

To make use of the Reset Flags to identify a reset condition, the user should read and then Reset the MCUSR as
early as possible in the program. If the register is cleared before another reset occurs, the source of the reset can
be found by examining the Reset Flags.

11.9.2 WDTCSR – Watchdog Timer Control Register

Bit 76543210

(0x60) WDIF WDIE WDP3 WDCE WDE WDP2 WDP1 WDP0 WDTCSR

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W

Initial Value 0 0 0 0 X 0 0 0

• Bit 7 – WDIF: Watchdog Interrupt Flag

This bit is set when a time-out occurs in the Watchdog Timer and the Watchdog Timer is configured for interrupt.
WDIF is cleared by hardware when executing the corresponding interrupt handling vector. Alternatively, WDIF is
cleared by writing a logic one to the flag. When the I-bit in SREG and WDIE are set, the Watchdog Time-out Inter­rupt is executed.

• Bit 6 – WDIE: Watchdog Interrupt Enable

When this bit is written to one and the I-bit in the Status Register is set, the Watchdog Interrupt is enabled. If WDE
is cleared in combination with this setting, the Watchdog Timer is in Interrupt Mode, and the corresponding inter­rupt is executed if time-out in the Watchdog Timer occurs. If WDE is set, the Watchdog Timer is in Interrupt and
System Reset Mode. The first time-out in the Watchdog Timer will set WDIF. Executing the corresponding interrupt
vector will clear WDIE and WDIF automatically by hardware (the Watchdog goes to System Reset Mode). This is
useful for keeping the Watchdog Timer security while using the interrupt. To stay in Interrupt and System Reset
Mode, WDIE must be set after each interrupt. This should however not be done within the interrupt service routine

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itself, as this might compromise the safety-function of the Watchdog System Reset mode. If the interrupt is not exe­cuted before the next time-out, a System Reset will be applied.

Table 11-1. Watchdog Timer Configuration

WDTON

(1)

1 0 0 Stopped None

1 0 1 Interrupt Mode Interrupt

1 1 0 System Reset Mode Reset

WDE WDIE Mode Action on Time-out

111

0 x x System Reset Mode Reset

Note: 1. WDTON Fuse set to “0” means programmed and “1” means unprogrammed.

Interrupt and System Reset
Mode

Interrupt, then go to System
Reset Mode

• Bit 4 – WDCE: Watchdog Change Enable

This bit is used in timed sequences for changing WDE and prescaler bits. To clear the WDE bit, and/or change the
prescaler bits, WDCE must be set.

Once written to one, hardware will clear WDCE after four clock cycles.

• Bit 3 – WDE: Watchdog System Reset Enable

WDE is overridden by WDRF in MCUSR. This means that WDE is always set when WDRF is set. To clear WDE,
WDRF must be cleared first. This feature ensures multiple resets during conditions causing failure, and a safe
start-up after the failure.

• Bit 5, 2:0 - WDP[3:0]: Watchdog Timer Prescaler 3, 2, 1 and 0

The WDP[3:0] bits determine the Watchdog Timer prescaling when the Watchdog Timer is running. The different
prescaling values and their corresponding time-out periods are shown in Table 11-2 on page 55.

Table 11-2. Watchdog Timer Prescale Select

Number of WDT Oscillator

WDP3 WDP2 WDP1 WDP0

0 0 0 0 2K (2048) cycles 16ms

0 0 0 1 4K (4096) cycles 32ms

Cycles

Typical Time-out at

VCC = 5.0V

0 0 1 0 8K (8192) cycles 64ms

0 0 1 1 16K (16384) cycles 0.125 s

0 1 0 0 32K (32768) cycles 0.25 s

0 1 0 1 64K (65536) cycles 0.5 s

0 1 1 0 128K (131072) cycles 1.0 s

0 1 1 1 256K (262144) cycles 2.0 s

1 0 0 0 512K (524288) cycles 4.0 s

1 0 0 1 1024K (1048576) cycles 8.0 s

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Table 11-2. Watchdog Timer Prescale Select (Continued)

WDP3 WDP2 WDP1 WDP0

1010

1011

1100

1101

1110

1111

Number of WDT Oscillator

Cycles

Reserved

Typical Time-out at

VCC = 5.0V

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12. Interrupts

This section describes the specifics of the interrupt handling as performed in
ATmega48A/PA/88A/PA/168A/PA/328/P. For a general explanation of the AVR interrupt handling, refer to ”Reset

and Interrupt Handling” on page 13.

The interrupt vectors in ATmega 48A/48PA, ATmega88A/88PA, ATmega168A/168PA and ATmega328/328P are
generally the same, with the following differences:

• Each Interrupt Vector occupies two instruction words in ATmega168A/168PA and ATmega328/328P, and one
instruction word in ATmega 48A/48PA and ATmega88A/88PA.

• ATmega 48A/48PA does not have a separate Boot Loader Section. In ATmega88A/88PA, ATmega168A/168PA
and ATmega328/328P, the Reset Vector is affected by the BOOTRST fuse, and the Interrupt Vector start address
is affected by the IVSEL bit in MCUCR.

12.1 Interrupt Vectors in ATmega48A and ATmega48PA

Table 12-1. Reset and Interrupt Vectors in ATmega48A and ATmega48PA

Vector No. Program Address Source Interrupt Definition

1 0x000 RESET

2 0x001 INT0 External Interrupt Request 0

3 0x002 INT1 External Interrupt Request 1

4 0x003 PCINT0 Pin Change Interrupt Request 0

5 0x004 PCINT1 Pin Change Interrupt Request 1

6 0x005 PCINT2 Pin Change Interrupt Request 2

External Pin, Power-on Reset, Brown-out Reset and Watchdog System Reset

7 0x006 WDT Watchdog Time-out Interrupt

8 0x007 TIMER2 COMPA Timer/Counter2 Compare Match A

9 0x008 TIMER2 COMPB Timer/Counter2 Compare Match B

10 0x009 TIMER2 OVF Timer/Counter2 Overflow

11 0x00A TIMER1 CAPT Timer/Counter1 Capture Event

12 0x00B TIMER1 COMPA Timer/Counter1 Compare Match A

13 0x00C TIMER1 COMPB Timer/Coutner1 Compare Match B

14 0x00D TIMER1 OVF Timer/Counter1 Overflow

15 0x00E TIMER0 COMPA Timer/Counter0 Compare Match A

16 0x00F TIMER0 COMPB Timer/Counter0 Compare Match B

17 0x010 TIMER0 OVF Timer/Counter0 Overflow

18 0x011 SPI, STC SPI Serial Transfer Complete

19 0x012 USART, RX USART Rx Complete

20 0x013 USART, UDRE USART, Data Register Empty

21 0x014 USART, TX USART, Tx Complete

22 0x015 ADC ADC Conversion Complete

23 0x016 EE READY EEPROM Ready

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Table 12-1. Reset and Interrupt Vectors in ATmega48A and ATmega48PA (Continued)

Vector No. Program Address Source Interrupt Definition

24 0x017 ANALOG COMP Analog Comparator

25 0x018 TWI 2-wire Serial Interface

26 0x019 SPM READY Store Program Memory Ready

The most typical and general program setup for the Reset and Interrupt Vector Addresses in ATmega 48A/48PA is:

Address Labels Code Comments

0x000 rjmp RESET ; Reset Handler

0x001 rjmp EXT_INT0 ; IRQ0 Handler

0x002 rjmp EXT_INT1 ; IRQ1 Handler

0x003 rjmp PCINT0 ; PCINT0 Handler

0x004 rjmp PCINT1 ; PCINT1 Handler

0x005 rjmp PCINT2 ; PCINT2 Handler

0x006 rjmp WDT ; Watchdog Timer Handler

0x007 rjmp TIM2_COMPA ; Timer2 Compare A Handler

0x008 rjmp TIM2_COMPB ; Timer2 Compare B Handler

0x009 rjmp TIM2_OVF ; Timer2 Overflow Handler

0x00A rjmp TIM1_CAPT ; Timer1 Capture Handler

0x00B rjmp TIM1_COMPA ; Timer1 Compare A Handler

0x00C rjmp TIM1_COMPB ; Timer1 Compare B Handler

0x00D rjmp TIM1_OVF ; Timer1 Overflow Handler

0x00E rjmp TIM0_COMPA ; Timer0 Compare A Handler

0x00F rjmp TIM0_COMPB ; Timer0 Compare B Handler

0x010 rjmp TIM0_OVF ; Timer0 Overflow Handler

0x011 rjmp SPI_STC ; SPI Transfer Complete Handler

0x012 rjmp USART_RXC ; USART, RX Complete Handler

0x013 rjmp USART_UDRE ; USART, UDR Empty Handler

0x014 rjmp USART_TXC ; USART, TX Complete Handler

0x015 rjmp ADC ; ADC Conversion Complete Handler

0x016 rjmp EE_RDY ; EEPROM Ready Handler

0x017 rjmp ANA_COMP ; Analog Comparator Handler

0x018 rjmp TWI ; 2-wire Serial Interface Handler

0x019 rjmp SPM_RDY ; Store Program Memory Ready Handler

;

0x01ARESET: ldi r16, high(RAMEND); Main program start

0x01B out SPH,r16 ; Set Stack Pointer to top of RAM

0x01C ldi r16, low(RAMEND)

0x01D out SPL,r16

0x01E sei ; Enable interrupts

0x01F <instr> xxx

... ... ... ...

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12.2 Interrupt Vectors in ATmega88A and ATmega88PA

Table 12-2. Reset and Interrupt Vectors in ATmega88A and ATmega88PA

Program

Vector No.

1 0x000

2 0x001 INT0 External Interrupt Request 0

3 0x002 INT1 External Interrupt Request 1

4 0x003 PCINT0 Pin Change Interrupt Request 0

5 0x004 PCINT1 Pin Change Interrupt Request 1

6 0x005 PCINT2 Pin Change Interrupt Request 2

7 0x006 WDT Watchdog Time-out Interrupt

8 0x007 TIMER2 COMPA Timer/Counter2 Compare Match A

9 0x008 TIMER2 COMPB Timer/Counter2 Compare Match B

10 0x009 TIMER2 OVF Timer/Counter2 Overflow

11 0x00A TIMER1 CAPT Timer/Counter1 Capture Event

12 0x00B TIMER1 COMPA Timer/Counter1 Compare Match A

13 0x00C TIMER1 COMPB Timer/Coutner1 Compare Match B

14 0x00D TIMER1 OVF Timer/Counter1 Overflow

15 0x00E TIMER0 COMPA Timer/Counter0 Compare Match A

16 0x00F TIMER0 COMPB Timer/Counter0 Compare Match B

Address

(2)

(1)

Source Interrupt Definition

RESET

External Pin, Power-on Reset, Brown-out Reset and Watchdog System Reset

17 0x010 TIMER0 OVF Timer/Counter0 Overflow

18 0x011 SPI, STC SPI Serial Transfer Complete

19 0x012 USART, RX USART Rx Complete

20 0x013 USART, UDRE USART, Data Register Empty

21 0x014 USART, TX USART, Tx Complete

22 0x015 ADC ADC Conversion Complete

23 0x016 EE READY EEPROM Ready

24 0x017 ANALOG COMP Analog Comparator

25 0x018 TWI 2-wire Serial Interface

26 0x019 SPM READY Store Program Memory Ready

Notes: 1. When the BOOTRST Fuse is programmed, the device will jump to the Boot Loader address at reset, see ”Boot Loader Sup-

port – Read-While-Write Self-Programming” on page 269.

2. When the IVSEL bit in MCUCR is set, Interrupt Vectors will be moved to the start of the Boot Flash Section. The address of

each Interrupt Vector will then be the address in this table added to the start address of the Boot Flash Section.

Table 12-3 on page 60 shows reset and Interrupt Vectors placement for the various combinations of BOOTRST

and IVSEL settings. If the program never enables an interrupt source, the Interrupt Vectors are not used, and reg­ular program code can be placed at these locations. This is also the case if the Reset Vector is in the Application
section while the Interrupt Vectors are in the Boot section or vice versa.

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Table 12-3. Reset and Interrupt Vectors Placement in ATmega88A and ATmega88PA

(1)

BOOTRST IVSEL Reset Address Interrupt Vectors Start Address

1 0 0x000 0x001

1 1 0x000 Boot Reset Address + 0x001

0 0 Boot Reset Address 0x001

0 1 Boot Reset Address Boot Reset Address + 0x001

Note: 1. The Boot Reset Address is shown in Table 27-7 on page 280. For the BOOTRST Fuse “1” means unprogrammed

while “0” means programmed.

The most typical and general program setup for the Reset and Interrupt Vector Addresses in ATmega88A/88PA is:

Address Labels Code Comments

0x000 rjmp RESET ; Reset Handler

0x001 rjmp EXT_INT0 ; IRQ0 Handler

0x002 rjmp EXT_INT1 ; IRQ1 Handler

0x003 rjmp PCINT0 ; PCINT0 Handler

0x004 rjmp PCINT1 ; PCINT1 Handler

0x005 rjmp PCINT2 ; PCINT2 Handler

0x006 rjmp WDT ; Watchdog Timer Handler

0x007 rjmp TIM2_COMPA ; Timer2 Compare A Handler

0X008 rjmp TIM2_COMPB ; Timer2 Compare B Handler

0x009 rjmp TIM2_OVF ; Timer2 Overflow Handler

0x00A rjmp TIM1_CAPT ; Timer1 Capture Handler

0x00B rjmp TIM1_COMPA ; Timer1 Compare A Handler

0x00C rjmp TIM1_COMPB ; Timer1 Compare B Handler

0x00D rjmp TIM1_OVF ; Timer1 Overflow Handler

0x00E rjmp TIM0_COMPA ; Timer0 Compare A Handler

0x00F rjmp TIM0_COMPB ; Timer0 Compare B Handler

0x010 rjmp TIM0_OVF ; Timer0 Overflow Handler

0x011 rjmp SPI_STC ; SPI Transfer Complete Handler

0x012 rjmp USART_RXC ; USART, RX Complete Handler

0x013 rjmp USART_UDRE ; USART, UDR Empty Handler

0x014 rjmp USART_TXC ; USART, TX Complete Handler

0x015 rjmp ADC ; ADC Conversion Complete Handler

0x016 rjmp EE_RDY ; EEPROM Ready Handler

0x017 rjmp ANA_COMP ; Analog Comparator Handler

0x018 rjmp TWI ; 2-wire Serial Interface Handler

0x019 rjmp SPM_RDY ; Store Program Memory Ready Handler

;

0x01ARESET: ldi r16, high(RAMEND); Main program start

0x01B out SPH,r16 ; Set Stack Pointer to top of RAM

0x01C ldi r16, low(RAMEND)

0x01D out SPL,r16
0x01E sei ; Enable interrupts

0x01F <instr> xxx

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When the BOOTRST Fuse is unprogrammed, the Boot section size set to 2Kbytes and the IVSEL bit in the
MCUCR Register is set before any interrupts are enabled, the most typical and general program setup for the
Reset and Interrupt Vector Addresses in ATmega88A/88PA is:

Address Labels Code Comments

0x000 RESET: ldi r16,high(RAMEND); Main program start

0x001 out SPH,r16 ; Set Stack Pointer to top of RAM

0x002 ldi r16,low(RAMEND)

0x003 out SPL,r16
0x004 sei ; Enable interrupts

0x005 <instr> xxx

;

.org 0xC01

0xC01 rjmp EXT_INT0 ; IRQ0 Handler

0xC02 rjmp EXT_INT1 ; IRQ1 Handler

... ... ... ;

0xC19 rjmp SPM_RDY ; Store Program Memory Ready Handler

When the BOOTRST Fuse is programmed and the Boot section size set to 2Kbytes, the most typical and general
program setup for the Reset and Interrupt Vector Addresses in ATmega88A/88PA is:

Address Labels Code Comments

.org 0x001

0x001 rjmp EXT_INT0 ; IRQ0 Handler

0x002 rjmp EXT_INT1 ; IRQ1 Handler

... ... ... ;

0x019 rjmp SPM_RDY ; Store Program Memory Ready Handler

;

.org 0xC00
0xC00 RESET: ldi r16,high(RAMEND); Main program start

0xC01 out SPH,r16 ; Set Stack Pointer to top of RAM

0xC02 ldi r16,low(RAMEND)

0xC03 out SPL,r16
0xC04 sei ; Enable interrupts

0xC05 <instr> xxx

When the BOOTRST Fuse is programmed, the Boot section size set to 2Kbytes and the IVSEL bit in the MCUCR
Register is set before any interrupts are enabled, the most typical and general program setup for the Reset and
Interrupt Vector Addresses in ATmega88A/88PA is:

Address Labels Code Comments

;

.org 0xC00
0xC00 rjmp RESET ; Reset handler

0xC01 rjmp EXT_INT0 ; IRQ0 Handler

0xC02 rjmp EXT_INT1 ; IRQ1 Handler

... ... ... ;

0xC19 rjmp SPM_RDY ; Store Program Memory Ready Handler

;

0xC1A RESET: ldi r16,high(RAMEND); Main program start

0xC1B out SPH,r16 ; Set Stack Pointer to top of RAM

0xC1C ldi r16,low(RAMEND)

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0xC1D out SPL,r16
0xC1E sei ; Enable interrupts

0xC1F <instr> xxx

12.3 Interrupt Vectors in ATmega168A and ATmega168PA

Table 12-4. Reset and Interrupt Vectors in ATmega168A and ATmega168PA

VectorNo.

1 0x0000

Program

Address

(2)

(1)

Source Interrupt Definition

RESET

External Pin, Power-on Reset, Brown-out Reset and Watchdog System Reset

2 0x0002 INT0 External Interrupt Request 0

3 0x0004 INT1 External Interrupt Request 1

4 0x0006 PCINT0 Pin Change Interrupt Request 0

5 0x0008 PCINT1 Pin Change Interrupt Request 1

6 0x000A PCINT2 Pin Change Interrupt Request 2

7 0x000C WDT Watchdog Time-out Interrupt

8 0x000E TIMER2 COMPA Timer/Counter2 Compare Match A

9 0x0010 TIMER2 COMPB Timer/Counter2 Compare Match B

10 0x0012 TIMER2 OVF Timer/Counter2 Overflow

11 0x0014 TIMER1 CAPT Timer/Counter1 Capture Event

12 0x0016 TIMER1 COMPA Timer/Counter1 Compare Match A

13 0x0018 TIMER1 COMPB Timer/Coutner1 Compare Match B

14 0x001A TIMER1 OVF Timer/Counter1 Overflow

15 0x001C TIMER0 COMPA Timer/Counter0 Compare Match A

16 0x001E TIMER0 COMPB Timer/Counter0 Compare Match B

17 0x0020 TIMER0 OVF Timer/Counter0 Overflow

18 0x0022 SPI, STC SPI Serial Transfer Complete

19 0x0024 USART, RX USART Rx Complete

20 0x0026 USART, UDRE USART, Data Register Empty

21 0x0028 USART, TX USART, Tx Complete

22 0x002A ADC ADC Conversion Complete

23 0x002C EE READY EEPROM Ready

24 0x002E ANALOG COMP Analog Comparator

25 0x0030 TWI 2-wire Serial Interface

26 0x0032 SPM READY Store Program Memory Ready

Notes: 1. When the BOOTRST Fuse is programmed, the device will jump to the Boot Loader address at reset, see ”Boot Loader Sup-

port – Read-While-Write Self-Programming” on page 269.

2. When the IVSEL bit in MCUCR is set, Interrupt Vectors will be moved to the start of the Boot Flash Section. The address of

each Interrupt Vector will then be the address in this table added to the start address of the Boot Flash Section.

Table 12-5 on page 63 shows reset and Interrupt Vectors placement for the various combinations of BOOTRST

and IVSEL settings. If the program never enables an interrupt source, the Interrupt Vectors are not used, and reg-

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ular program code can be placed at these locations. This is also the case if the Reset Vector is in the Application
section while the Interrupt Vectors are in the Boot section or vice versa.

Table 12-5. Reset and Interrupt Vectors Placement in ATmega168A and ATmega168PA

BOOTRST IVSEL Reset Address Interrupt Vectors Start Address

1 0 0x000 0x002

1 1 0x000 Boot Reset Address + 0x0002

0 0 Boot Reset Address 0x002

0 1 Boot Reset Address Boot Reset Address + 0x0002

Note: 1. The Boot Reset Address is shown in Table 27-7 on page 280. For the BOOTRST Fuse “1” means unprogrammed

while “0” means programmed.

(1)

The most typical and general program setup for the Reset and Interrupt Vector Addresses in ATmega168A/168PA
is:

Address Labels Code Comments

0x0000 jmp RESET ; Reset Handler

0x0002 jmp EXT_INT0 ; IRQ0 Handler

0x0004 jmp EXT_INT1 ; IRQ1 Handler

0x0006 jmp PCINT0 ; PCINT0 Handler

0x0008 jmp PCINT1 ; PCINT1 Handler

0x000A jmp PCINT2 ; PCINT2 Handler

0x000C jmp WDT ; Watchdog Timer Handler

0x000E jmp TIM2_COMPA ; Timer2 Compare A Handler

0x0010 jmp TIM2_COMPB ; Timer2 Compare B Handler

0x0012 jmp TIM2_OVF ; Timer2 Overflow Handler

0x0014 jmp TIM1_CAPT ; Timer1 Capture Handler

0x0016 jmp TIM1_COMPA ; Timer1 Compare A Handler

0x0018 jmp TIM1_COMPB ; Timer1 Compare B Handler

0x001A jmp TIM1_OVF ; Timer1 Overflow Handler

0x001C jmp TIM0_COMPA ; Timer0 Compare A Handler

0x001E jmp TIM0_COMPB ; Timer0 Compare B Handler

0x0020 jmp TIM0_OVF ; Timer0 Overflow Handler

0x0022 jmp SPI_STC ; SPI Transfer Complete Handler

0x0024 jmp USART_RXC ; USART, RX Complete Handler

0x0026 jmp USART_UDRE ; USART, UDR Empty Handler

0x0028 jmp USART_TXC ; USART, TX Complete Handler

0x002A jmp ADC ; ADC Conversion Complete Handler

0x002C jmp EE_RDY ; EEPROM Ready Handler

0x002E jmp ANA_COMP ; Analog Comparator Handler

0x0030 jmp TWI ; 2-wire Serial Interface Handler

0x0032 jmp SPM_RDY ; Store Program Memory Ready Handler

;

0x0033RESET: ldi r16, high(RAMEND); Main program start

0x0034 out SPH,r16 ; Set Stack Pointer to top of RAM

0x0035 ldi r16, low(RAMEND)

0x0036 out SPL,r16

0x0037 sei ; Enable interrupts

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0x0038 <instr> xxx

... ... ... ...

When the BOOTRST Fuse is unprogrammed, the Boot section size set to 2Kbytes and the IVSEL bit in the
MCUCR Register is set before any interrupts are enabled, the most typical and general program setup for the
Reset and Interrupt Vector Addresses in ATmega168A/168PA is:

Address Labels Code Comments

0x0000 RESET: ldi r16,high(RAMEND); Main program start

0x0001 out SPH,r16 ; Set Stack Pointer to top of RAM

0x0002 ldi r16,low(RAMEND)

0x0003 out SPL,r16
0x0004 sei ; Enable interrupts

0x0005 <instr> xxx

;

.org 0x1C02

0x1C02 jmp EXT_INT0 ; IRQ0 Handler

0x1C04 jmp EXT_INT1 ; IRQ1 Handler

... ... ... ;

0x1C32 jmp SPM_RDY ; Store Program Memory Ready Handler

When the BOOTRST Fuse is programmed and the Boot section size set to 2Kbytes, the most typical and general
program setup for the Reset and Interrupt Vector Addresses in ATmega168A/168PA is:

Address Labels Code Comments

.org 0x0002

0x0002 jmp EXT_INT0 ; IRQ0 Handler

0x0004 jmp EXT_INT1 ; IRQ1 Handler

... ... ... ;

0x0032 jmp SPM_RDY ; Store Program Memory Ready Handler

;

.org 0x1C00
0x1C00 RESET: ldi r16,high(RAMEND); Main program start

0x1C01 out SPH,r16 ; Set Stack Pointer to top of RAM

0x1C02 ldi r16,low(RAMEND)

0x1C03 out SPL,r16
0x1C04 sei ; Enable interrupts

0x1C05 <instr> xxx

When the BOOTRST Fuse is programmed, the Boot section size set to 2Kbytes and the IVSEL bit in the MCUCR
Register is set before any interrupts are enabled, the most typical and general program setup for the Reset and
Interrupt Vector Addresses in ATmega168A/168PA is:

Address Labels Code Comments

;

.org 0x1C00
0x1C00 jmp RESET ; Reset handler

0x1C02 jmp EXT_INT0 ; IRQ0 Handler

0x1C04 jmp EXT_INT1 ; IRQ1 Handler

... ... ... ;

0x1C32 jmp SPM_RDY ; Store Program Memory Ready Handler

;

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0x1C33 RESET: ldi r16,high(RAMEND); Main program start

0x1C34 out SPH,r16 ; Set Stack Pointer to top of RAM

0x1C35 ldi r16,low(RAMEND)

0x1C36 out SPL,r16
0x1C37 sei ; Enable interrupts

0x1C38 <instr> xxx

12.4 Interrupt Vectors in ATmega328 and ATmega328P

Table 12-6. Reset and Interrupt Vectors in ATmega328 and ATmega328P

VectorNo.

1 0x0000

Program

Address

(2)

(1)

Source Interrupt Definition

RESET

External Pin, Power-on Reset, Brown-out Reset and Watchdog System Reset

2 0x0002 INT0 External Interrupt Request 0

3 0x0004 INT1 External Interrupt Request 1

4 0x0006 PCINT0 Pin Change Interrupt Request 0

5 0x0008 PCINT1 Pin Change Interrupt Request 1

6 0x000A PCINT2 Pin Change Interrupt Request 2

7 0x000C WDT Watchdog Time-out Interrupt

8 0x000E TIMER2 COMPA Timer/Counter2 Compare Match A

9 0x0010 TIMER2 COMPB Timer/Counter2 Compare Match B

10 0x0012 TIMER2 OVF Timer/Counter2 Overflow

11 0x0014 TIMER1 CAPT Timer/Counter1 Capture Event

12 0x0016 TIMER1 COMPA Timer/Counter1 Compare Match A

13 0x0018 TIMER1 COMPB Timer/Coutner1 Compare Match B

14 0x001A TIMER1 OVF Timer/Counter1 Overflow

15 0x001C TIMER0 COMPA Timer/Counter0 Compare Match A

16 0x001E TIMER0 COMPB Timer/Counter0 Compare Match B

17 0x0020 TIMER0 OVF Timer/Counter0 Overflow

18 0x0022 SPI, STC SPI Serial Transfer Complete

19 0x0024 USART, RX USART Rx Complete

20 0x0026 USART, UDRE USART, Data Register Empty

21 0x0028 USART, TX USART, Tx Complete

22 0x002A ADC ADC Conversion Complete

23 0x002C EE READY EEPROM Ready

24 0x002E ANALOG COMP Analog Comparator

25 0x0030 TWI 2-wire Serial Interface

26 0x0032 SPM READY Store Program Memory Ready

Notes: 1. When the BOOTRST Fuse is programmed, the device will jump to the Boot Loader address at reset, see ”Boot Loader Sup-

port – Read-While-Write Self-Programming” on page 269.

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2. When the IVSEL bit in MCUCR is set, Interrupt Vectors will be moved to the start of the Boot Flash Section. The address of

each Interrupt Vector will then be the address in this table added to the start address of the Boot Flash Section.

Table 12-7 on page 66 shows reset and Interrupt Vectors placement for the various combinations of BOOTRST

and IVSEL settings. If the program never enables an interrupt source, the Interrupt Vectors are not used, and reg­ular program code can be placed at these locations. This is also the case if the Reset Vector is in the Application
section while the Interrupt Vectors are in the Boot section or vice versa.

Table 12-7. Reset and Interrupt Vectors Placement in ATmega328 and ATmega328P

BOOTRST IVSEL Reset Address Interrupt Vectors Start Address

1 0 0x000 0x002

1 1 0x000 Boot Reset Address + 0x0002

0 0 Boot Reset Address 0x002

0 1 Boot Reset Address Boot Reset Address + 0x0002

Note: 1. The Boot Reset Address is shown in Table 27-7 on page 280. For the BOOTRST Fuse “1” means unprogrammed

while “0” means programmed.

(1)

The most typical and general program setup for the Reset and Interrupt Vector Addresses in ATmega328/328P is:

Address Labels Code Comments

0x0000 jmp RESET ; Reset Handler

0x0002 jmp EXT_INT0 ; IRQ0 Handler

0x0004 jmp EXT_INT1 ; IRQ1 Handler

0x0006 jmp PCINT0 ; PCINT0 Handler

0x0008 jmp PCINT1 ; PCINT1 Handler

0x000A jmp PCINT2 ; PCINT2 Handler

0x000C jmp WDT ; Watchdog Timer Handler

0x000E jmp TIM2_COMPA ; Timer2 Compare A Handler

0x0010 jmp TIM2_COMPB ; Timer2 Compare B Handler

0x0012 jmp TIM2_OVF ; Timer2 Overflow Handler

0x0014 jmp TIM1_CAPT ; Timer1 Capture Handler

0x0016 jmp TIM1_COMPA ; Timer1 Compare A Handler

0x0018 jmp TIM1_COMPB ; Timer1 Compare B Handler

0x001A jmp TIM1_OVF ; Timer1 Overflow Handler

0x001C jmp TIM0_COMPA ; Timer0 Compare A Handler

0x001E jmp TIM0_COMPB ; Timer0 Compare B Handler

0x0020 jmp TIM0_OVF ; Timer0 Overflow Handler

0x0022 jmp SPI_STC ; SPI Transfer Complete Handler

0x0024 jmp USART_RXC ; USART, RX Complete Handler

0x0026 jmp USART_UDRE ; USART, UDR Empty Handler

0x0028 jmp USART_TXC ; USART, TX Complete Handler

0x002A jmp ADC ; ADC Conversion Complete Handler

0x002C jmp EE_RDY ; EEPROM Ready Handler

0x002E jmp ANA_COMP ; Analog Comparator Handler

0x0030 jmp TWI ; 2-wire Serial Interface Handler

0x0032 jmp SPM_RDY ; Store Program Memory Ready Handler

;

0x0033RESET: ldi r16, high(RAMEND); Main program start

0x0034 out SPH,r16 ; Set Stack Pointer to top of RAM

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0x0035 ldi r16, low(RAMEND)

0x0036 out SPL,r16

0x0037 sei ; Enable interrupts

0x0038 <instr> xxx

... ... ... ...

When the BOOTRST Fuse is unprogrammed, the Boot section size set to 2Kbytes and the IVSEL bit in the
MCUCR Register is set before any interrupts are enabled, the most typical and general program setup for the
Reset and Interrupt Vector Addresses in ATmega328/328P is:

Address Labels Code Comments

0x0000 RESET: ldi r16,high(RAMEND); Main program start

0x0001 out SPH,r16 ; Set Stack Pointer to top of RAM

0x0002 ldi r16,low(RAMEND)

0x0003 out SPL,r16
0x0004 sei ; Enable interrupts

0x0005 <instr> xxx

;

.org 0x3C02

0x3C02 jmp EXT_INT0 ; IRQ0 Handler

0x3C04 jmp EXT_INT1 ; IRQ1 Handler

... ... ... ;

0x3C32 jmp SPM_RDY ; Store Program Memory Ready Handler

When the BOOTRST Fuse is programmed and the Boot section size set to 2Kbytes, the most typical and general
program setup for the Reset and Interrupt Vector Addresses in ATmega328/328P is:

Address Labels Code Comments

.org 0x0002

0x0002 jmp EXT_INT0 ; IRQ0 Handler

0x0004 jmp EXT_INT1 ; IRQ1 Handler

... ... ... ;

0x0032 jmp SPM_RDY ; Store Program Memory Ready Handler

;

.org 0x3C00
0x3C00 RESET: ldi r16,high(RAMEND); Main program start

0x3C01 out SPH,r16 ; Set Stack Pointer to top of RAM

0x3C02 ldi r16,low(RAMEND)

0x3C03 out SPL,r16
0x3C04 sei ; Enable interrupts

0x3C05 <instr> xxx

When the BOOTRST Fuse is programmed, the Boot section size set to 2Kbytes and the IVSEL bit in the MCUCR
Register is set before any interrupts are enabled, the most typical and general program setup for the Reset and
Interrupt Vector Addresses in ATmega328/328P is:

Address Labels Code Comments

;

.org 0x3C00
0x3C00 jmp RESET ; Reset handler

0x3C02 jmp EXT_INT0 ; IRQ0 Handler

0x3C04 jmp EXT_INT1 ; IRQ1 Handler

... ... ... ;

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0x3C32 jmp SPM_RDY ; Store Program Memory Ready Handler

;

0x3C33 RESET: ldi r16,high(RAMEND); Main program start

0x3C34 out SPH,r16 ; Set Stack Pointer to top of RAM

0x3C35 ldi r16,low(RAMEND)

0x3C36 out SPL,r16
0x3C37 sei ; Enable interrupts

0x3C38 <instr> xxx

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12.5 Register Description

12.5.1 Moving Interrupts Between Application and Boot Space, ATmega88A/88PA, ATmega168A/168PA and ATmega328/328P

The MCU Control Register controls the placement of the Interrupt Vector table.

MCUCR – MCU Control Register

Bit 76 5 43210

0x35 (0x55)

Read/Write R R/W R/W R/W R R R/W R/W

Initial Value00 0 00000

– BODS

(1)

BODSE

(1)

PUD – – IVSEL IVCE MCUCR

Note: 1. BODS and BODSE only available for picoPower devices ATmega48PA/88PA/168PA/328P

• Bit 1 – IVSEL: Interrupt Vector Select

When the IVSEL bit is cleared (zero), the Interrupt Vectors are placed at the start of the Flash memory. When this
bit is set (one), the Interrupt Vectors are moved to the beginning of the Boot Loader section of the Flash. The actual
address of the start of the Boot Flash Section is determined by the BOOTSZ Fuses. Refer to the section ”Boot

Loader Support – Read-While-Write Self-Programming” on page 269 for details. To avoid unintentional changes of

Interrupt Vector tables, a special write procedure must be followed to change the IVSEL bit:

a. Write the Interrupt Vector Change Enable (IVCE) bit to one.
b. Within four cycles, write the desired value to IVSEL while writing a zero to IVCE.

Interrupts will automatically be disabled while this sequence is executed. Interrupts are disabled in the cycle IVCE
is set, and they remain disabled until after the instruction following the write to IVSEL. If IVSEL is not written, inter­rupts remain disabled for four cycles. The I-bit in the Status Register is unaffected by the automatic disabling.

Note: If Interrupt Vectors are placed in the Boot Loader section and Boot Lock bit BLB02 is programmed, interrupts are dis-

abled while executing from the Application section. If Interrupt Vectors are placed in the Application section and Boot
Lock bit BLB12 is programed, interrupts are disabled while executing from the Boot Loader section. Refer to the sec­tion ”Boot Loader Support – Read-While-Write Self-Programming” on page 269 for details on Boot Lock bits.

• Bit 0 – IVCE: Interrupt Vector Change Enable

The IVCE bit must be written to logic one to enable change of the IVSEL bit. IVCE is cleared by hardware four
cycles after it is written or when IVSEL is written. Setting the IVCE bit will disable interrupts, as explained in the
IVSEL description above. See Code Example below.

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Assembly Code Example

Move_interrupts:

; Enable change of Interrupt Vectors

ldi r16, (1<<IVCE)

out MCUCR, r16

; Move interrupts to Boot Flash section

ldi r16, (1<<IVSEL)

out MCUCR, r16

ret

C Code Example

void Move_interrupts(void)

{

/* Enable change of Interrupt Vectors */

MCUCR = (1<<IVCE);

/* Move interrupts to Boot Flash section */

MCUCR = (1<<IVSEL);

}

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13. External Interrupts

The External Interrupts are triggered by the INT0 and INT1 pins or any of the PCINT23...0 pins. Observe that, if
enabled, the interrupts will trigger even if the INT0 and INT1 or PCINT23...0 pins are configured as outputs. This
feature provides a way of generating a software interrupt. The pin change interrupt PCI2 will trigger if any enabled
PCINT[23:16] pin toggles. The pin change interrupt PCI1 will trigger if any enabled PCINT[14:8] pin toggles. The
pin change interrupt PCI0 will trigger if any enabled PCINT[7:0] pin toggles. The PCMSK2, PCMSK1 and PCMSK0
Registers control which pins contribute to the pin change interrupts. Pin change interrupts on PCINT23...0 are
detected asynchronously. This implies that these interrupts can be used for waking the part also from sleep modes
other than Idle mode.

The INT0 and INT1 interrupts can be triggered by a falling or rising edge or a low level. This is set up as indicated
in the specification for the External Interrupt Control Register A – EICRA. When the INT0 or INT1 interrupts are
enabled and are configured as level triggered, the interrupts will trigger as long as the pin is held low. Note that rec­ognition of falling or rising edge interrupts on INT0 or INT1 requires the presence of an I/O clock, described in

”Clock Systems and their Distribution” on page 26. Low level interrupt on INT0 and INT1 is detected asynchro-

nously. This implies that this interrupt can be used for waking the part also from sleep modes other than Idle mode.
The I/O clock is halted in all sleep modes except Idle mode.

Note: Note that if a level triggered interrupt is used for wake-up from Power-down, the required level must be held long

enough for the MCU to complete the wake-up to trigger the level interrupt. If the level disappears before the end of the
Start-up Time, the MCU will still wake up, but no interrupt will be generated. The start-up time is defined by the SUT
and CKSEL Fuses as described in ”System Clock and Clock Options” on page 26.

13.1 Pin Change Interrupt Timing

An example of timing of a pin change interrupt is shown in Figure 13-1.

Figure 13-1. Timing of pin change interrupts

PCINT(0)

clk

clk

PCINT(0)

pin_lat

pin_sync

pcint_in_(0)

pcint_syn

pcint_setflag

PCIF

pin_lat

D Q

LE

pin_sync

PCINT(0) in PCMSK(x)

pcint_in_(0)

0

x

clk

pcint_syn

pcint_setflag

PCIF

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13.2 Register Description

13.2.1 EICRA – External Interrupt Control Register A

The External Interrupt Control Register A contains control bits for interrupt sense control.

Bit 76543210

(0x69) – – – – ISC11 ISC10 ISC01 ISC00 EICRA

Read/Write R R R R R/W R/W R/W R/W

Initial Value 0 0 0 0 0 0 0 0

• Bit 7:4 – Reserved

These bits are unused bits in the ATmega48A/PA/88A/PA/168A/PA/328/P, and will always read as zero.

• Bit 3, 2 – ISC11, ISC10: Interrupt Sense Control 1 Bit 1 and Bit 0

The External Interrupt 1 is activated by the external pin INT1 if the SREG I-flag and the corresponding interrupt
mask are set. The level and edges on the external INT1 pin that activate the interrupt are defined in Table 13-1.
The value on the INT1 pin is sampled before detecting edges. If edge or toggle interrupt is selected, pulses that last
longer than one clock period will generate an interrupt. Shorter pulses are not guaranteed to generate an interrupt.
If low level interrupt is selected, the low level must be held until the completion of the currently executing instruction
to generate an interrupt.

Table 13-1. Interrupt 1 Sense Control

ISC11 ISC10 Description

0 0 The low level of INT1 generates an interrupt request.

0 1 Any logical change on INT1 generates an interrupt request.

1 0 The falling edge of INT1 generates an interrupt request.

1 1 The rising edge of INT1 generates an interrupt request.

• Bit 1, 0 – ISC01, ISC00: Interrupt Sense Control 0 Bit 1 and Bit 0

The External Interrupt 0 is activated by the external pin INT0 if the SREG I-flag and the corresponding interrupt
mask are set. The level and edges on the external INT0 pin that activate the interrupt are defined in Table 13-2.
The value on the INT0 pin is sampled before detecting edges. If edge or toggle interrupt is selected, pulses that last
longer than one clock period will generate an interrupt. Shorter pulses are not guaranteed to generate an interrupt.
If low level interrupt is selected, the low level must be held until the completion of the currently executing instruction
to generate an interrupt.

Table 13-2. Interrupt 0 Sense Control

ISC01 ISC00 Description

0 0 The low level of INT0 generates an interrupt request.

0 1 Any logical change on INT0 generates an interrupt request.

1 0 The falling edge of INT0 generates an interrupt request.

1 1 The rising edge of INT0 generates an interrupt request.

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13.2.2 EIMSK – External Interrupt Mask Register

Bit 76543210

0x1D (0x3D) ––––––INT1INT0EIMSK

Read/Write RRRRRRR/WR/W

Initial Value 00000000

• Bit 7:2 – Reserved

These bits are unused bits in the ATmega48A/PA/88A/PA/168A/PA/328/P, and will always read as zero.

• Bit 1 – INT1: External Interrupt Request 1 Enable

When the INT1 bit is set (one) and the I-bit in the Status Register (SREG) is set (one), the external pin interrupt is
enabled. The Interrupt Sense Control1 bits 1/0 (ISC11 and ISC10) in the External Interrupt Control Register A
(EICRA) define whether the external interrupt is activated on rising and/or falling edge of the INT1 pin or level
sensed. Activity on the pin will cause an interrupt request even if INT1 is configured as an output. The correspond­ing interrupt of External Interrupt Request 1 is executed from the INT1 Interrupt Vector.

• Bit 0 – INT0: External Interrupt Request 0 Enable

When the INT0 bit is set (one) and the I-bit in the Status Register (SREG) is set (one), the external pin interrupt is
enabled. The Interrupt Sense Control0 bits 1/0 (ISC01 and ISC00) in the External Interrupt Control Register A
(EICRA) define whether the external interrupt is activated on rising and/or falling edge of the INT0 pin or level
sensed. Activity on the pin will cause an interrupt request even if INT0 is configured as an output. The correspond­ing interrupt of External Interrupt Request 0 is executed from the INT0 Interrupt Vector.

13.2.3 EIFR – External Interrupt Flag Register

Bit 76543210

0x1C (0x3C) ––––––INTF1INTF0EIFR

Read/Write RRRRRRR/WR/W

Initial Value 00000000

• Bit 7:2 – Reserved

These bits are unused bits in the ATmega48A/PA/88A/PA/168A/PA/328/P, and will always read as zero.

• Bit 1 – INTF1: External Interrupt Flag 1

When an edge or logic change on the INT1 pin triggers an interrupt request, INTF1 becomes set (one). If the I-bit in
SREG and the INT1 bit in EIMSK are set (one), the MCU will jump to the corresponding Interrupt Vector. The flag
is cleared when the interrupt routine is executed. Alternatively, the flag can be cleared by writing a logical one to it.
This flag is always cleared when INT1 is configured as a level interrupt.

• Bit 0 – INTF0: External Interrupt Flag 0

When an edge or logic change on the INT0 pin triggers an interrupt request, INTF0 becomes set (one). If the I-bit in
SREG and the INT0 bit in EIMSK are set (one), the MCU will jump to the corresponding Interrupt Vector. The flag
is cleared when the interrupt routine is executed. Alternatively, the flag can be cleared by writing a logical one to it.
This flag is always cleared when INT0 is configured as a level interrupt.

13.2.4 PCICR – Pin Change Interrupt Control Register

Bit 76543210

(0x68) –––––PCIE2PCIE1PCIE0PCICR

Read/Write RRRRRR/WR/WR/W

Initial Value 00000000

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• Bit 7:3 – Reserved

These bits are unused bits in the ATmega48A/PA/88A/PA/168A/PA/328/P, and will always read as zero.

• Bit 2 – PCIE2: Pin Change Interrupt Enable 2

When the PCIE2 bit is set (one) and the I-bit in the Status Register (SREG) is set (one), pin change interrupt 2 is
enabled. Any change on any enabled PCINT[23:16] pin will cause an interrupt. The corresponding interrupt of Pin
Change Interrupt Request is executed from the PCI2 Interrupt Vector. PCINT[23:16] pins are enabled individually
by the PCMSK2 Register.

• Bit 1 – PCIE1: Pin Change Interrupt Enable 1

When the PCIE1 bit is set (one) and the I-bit in the Status Register (SREG) is set (one), pin change interrupt 1 is
enabled. Any change on any enabled PCINT[14:8] pin will cause an interrupt. The corresponding interrupt of Pin
Change Interrupt Request is executed from the PCI1 Interrupt Vector. PCINT[14:8] pins are enabled individually by
the PCMSK1 Register.

• Bit 0 – PCIE0: Pin Change Interrupt Enable 0

When the PCIE0 bit is set (one) and the I-bit in the Status Register (SREG) is set (one), pin change interrupt 0 is
enabled. Any change on any enabled PCINT[7:0] pin will cause an interrupt. The corresponding interrupt of Pin
Change Interrupt Request is executed from the PCI0 Interrupt Vector. PCINT[7:0] pins are enabled individually by
the PCMSK0 Register.

13.2.5 PCIFR – Pin Change Interrupt Flag Register

Bit 76543210

0x1B (0x3B) –––––PCIF2PCIF1PCIF0PCIFR

Read/Write RRRRRR/WR/WR/W

Initial Value 00000000

• Bit 7:3 – Reserved

These bits are unused bits in the ATmega48A/PA/88A/PA/168A/PA/328/P, and will always read as zero.

• Bit 2 – PCIF2: Pin Change Interrupt Flag 2

When a logic change on any PCINT[23:16] pin triggers an interrupt request, PCIF2 becomes set (one). If the I-bit in
SREG and the PCIE2 bit in PCICR are set (one), the MCU will jump to the corresponding Interrupt Vector. The flag
is cleared when the interrupt routine is executed. Alternatively, the flag can be cleared by writing a logical one to it.

• Bit 1 – PCIF1: Pin Change Interrupt Flag 1

When a logic change on any PCINT[14:8] pin triggers an interrupt request, PCIF1 becomes set (one). If the I-bit in
SREG and the PCIE1 bit in PCICR are set (one), the MCU will jump to the corresponding Interrupt Vector. The flag
is cleared when the interrupt routine is executed. Alternatively, the flag can be cleared by writing a logical one to it.

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• Bit 0 – PCIF0: Pin Change Interrupt Flag 0

When a logic change on any PCINT[7:0] pin triggers an interrupt request, PCIF0 becomes set (one). If the I-bit in
SREG and the PCIE0 bit in PCICR are set (one), the MCU will jump to the corresponding Interrupt Vector. The flag
is cleared when the interrupt routine is executed. Alternatively, the flag can be cleared by writing a logical one to it.

13.2.6 PCMSK2 – Pin Change Mask Register 2

Bit 76543210

(0x6D)

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W

Initial Value 0 0 0 0 0 0 0 0

PCINT23 PCINT22 PCINT21 PCINT20 PCINT19 PCINT18 PCINT17 PCINT16 PCMSK2

• Bit 7:0 – PCINT[23:16]: Pin Change Enable Mask 23...16

Each PCINT[23:16]-bit selects whether pin change interrupt is enabled on the corresponding I/O pin. If
PCINT[23:16] is set and the PCIE2 bit in PCICR is set, pin change interrupt is enabled on the corresponding I/O
pin. If PCINT[23:16] is cleared, pin change interrupt on the corresponding I/O pin is disabled.

13.2.7 PCMSK1 – Pin Change Mask Register 1

Bit 76543210

(0x6C)

Read/Write R R/W R/W R/W R/W R/W R/W R/W

Initial Value 0 0 0 0 0 0 0 0

– PCINT14 PCINT13 PCINT12 PCINT11 PCINT10 PCINT9 PCINT8 PCMSK1

• Bit 7 – Reserved

This bit is an unused bit in the ATmega48A/PA/88A/PA/168A/PA/328/P, and will always read as zero.

• Bit 6:0 – PCINT[14:8]: Pin Change Enable Mask 14...8

Each PCINT[14:8]-bit selects whether pin change interrupt is enabled on the corresponding I/O pin. If PCINT[14:8]
is set and the PCIE1 bit in PCICR is set, pin change interrupt is enabled on the corresponding I/O pin. If
PCINT[14:8] is cleared, pin change interrupt on the corresponding I/O pin is disabled.

13.2.8 PCMSK0 – Pin Change Mask Register 0

Bit 76543210

(0x6B) PCINT7 PCINT6 PCINT5 PCINT4 PCINT3 PCINT2 PCINT1 PCINT0 PCMSK0

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W

Initial Value 0 0 0 0 0 0 0 0

• Bit 7:0 – PCINT[7:0]: Pin Change Enable Mask 7...0

Each PCINT[7:0] bit selects whether pin change interrupt is enabled on the corresponding I/O pin. If PCINT[7:0] is
set and the PCIE0 bit in PCICR is set, pin change interrupt is enabled on the corresponding I/O pin. If PCINT[7:0]
is cleared, pin change interrupt on the corresponding I/O pin is disabled.

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14. I/O-Ports

C

pin

Logic

R

pu

See Figure

"General Digital I/O" for

Details

Pxn

14.1 Overview

All AVR ports have true Read-Modify-Write functionality when used as general digital I/O ports. This means that
the direction of one port pin can be changed without unintentionally changing the direction of any other pin with the
SBI and CBI instructions. The same applies when changing drive value (if configured as output) or enabling/dis­abling of pull-up resistors (if configured as input). Each output buffer has symmetrical drive characteristics with
both high sink and source capability. The pin driver is strong enough to drive LED displays directly. All port pins
have individually selectable pull-up resistors with a supply-voltage invariant resistance. All I/O pins have protection
diodes to both V
complete list of parameters.

Figure 14-1. I/O Pin Equivalent Schematic

and Ground as indicated in Figure 14-1. Refer to ”Electrical Characteristics” on page 303 for a

CC

All registers and bit references in this section are written in general form. A lower case “x” represents the number­ing letter for the port, and a lower case “n” represents the bit number. However, when using the register or bit
defines in a program, the precise form must be used. For example, PORTB3 for bit no. 3 in Port B, here docu­mented generally as PORTxn. The physical I/O Registers and bit locations are listed in ”Register Description” on

page 92.

Three I/O memory address locations are allocated for each port, one each for the Data Register – PORTx, Data
Direction Register – DDRx, and the Port Input Pins – PINx. The Port Input Pins I/O location is read only, while the
Data Register and the Data Direction Register are read/write. However, writing a logic one to a bit in the PINx Reg­ister, will result in a toggle in the corresponding bit in the Data Register. In addition, the Pull-up Disable – PUD bit
in MCUCR disables the pull-up function for all pins in all ports when set.

Using the I/O port as General Digital I/O is described in ”Ports as General Digital I/O” on page 77. Most port pins
are multiplexed with alternate functions for the peripheral features on the device. How each alternate function inter­feres with the port pin is described in ”Alternate Port Functions” on page 81. Refer to the individual module sections
for a full description of the alternate functions.

Note that enabling the alternate function of some of the port pins does not affect the use of the other pins in the port
as general digital I/O.

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14.2 Ports as General Digital I/O

clk

RPx

RRx

RDx

WDx

PUD

SYNCHRONIZER

WDx: WRITE DDRx

WRx: WRITE PORTx
RRx: READ PORTx REGISTER
RPx: READ PORTx PIN

PUD: PULLUP DISABLE

clk

I/O

: I/O CLOCK

RDx: READ DDRx

DLQ

Q

RESET

RESET

Q

Q

D

Q

Q

D

CLR

PORTxn

Q

Q

D

CLR

DDxn

PINxn

DATA B U S

SLEEP

SLEEP: SLEEP CONTROL

Pxn

I/O

WPx

0

1

WRx

WPx: WRITE PINx REGISTER

The ports are bi-directional I/O ports with optional internal pull-ups. Figure 14-2 shows a functional description of
one I/O-port pin, here generically called Pxn.

Figure 14-2. General Digital I/O

(1)

Note: 1. WRx, WPx, WDx, RRx, RPx, and RDx are common to all pins within the same port. clk

common to all ports.

14.2.1 Configuring the Pin

Each port pin consists of three register bits: DDxn, PORTxn, and PINxn. As shown in ”Register Description” on

page 92, the DDxn bits are accessed at the DDRx I/O address, the PORTxn bits at the PORTx I/O address, and

the PINxn bits at the PINx I/O address.

The DDxn bit in the DDRx Register selects the direction of this pin. If DDxn is written logic one, Pxn is configured
as an output pin. If DDxn is written logic zero, Pxn is configured as an input pin.

If PORTxn is written logic one when the pin is configured as an input pin, the pull-up resistor is activated. To switch
the pull-up resistor off, PORTxn has to be written logic zero or the pin has to be configured as an output pin. The
port pins are tri-stated when reset condition becomes active, even if no clocks are running.

If PORTxn is written logic one when the pin is configured as an output pin, the port pin is driven high (one). If
PORTxn is written logic zero when the pin is configured as an output pin, the port pin is driven low (zero).

14.2.2 Toggling the Pin

Writing a logic one to PINxn toggles the value of PORTxn, independent on the value of DDRxn. Note that the SBI
instruction can be used to toggle one single bit in a port.

, SLEEP, and PUD are

I/O

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14.2.3 Switching Between Input and Output

XXX in r17, PINx

0x00 0xFF

INSTRUCTIONS

SYNC LATCH

PINxn

r17

XXX

SYSTEM CLK

t

pd, max

t

pd, min

When switching between tri-state ({DDxn, PORTxn} = 0b00) and output high ({DDxn, PORTxn} = 0b11), an inter­mediate state with either pull-up enabled {DDxn, PORTxn} = 0b01) or output low ({DDxn, PORTxn} = 0b10) must
occur. Normally, the pull-up enabled state is fully acceptable, as a high-impedance environment will not notice the
difference between a strong high driver and a pull-up. If this is not the case, the PUD bit in the MCUCR Register
can be set to disable all pull-ups in all ports.

Switching between input with pull-up and output low generates the same problem. The user must use either the tri­state ({DDxn, PORTxn} = 0b00) or the output high state ({DDxn, PORTxn} = 0b11) as an intermediate step.

Table 14-1 summarizes the control signals for the pin value.

Table 14-1. Port Pin Configurations

PUD

DDxn PORTxn

0 0 X Input No Tri-state (Hi-Z)

0 1 0 Input Yes Pxn will source current if ext. pulled low.

0 1 1 Input No Tri-state (Hi-Z)

1 0 X Output No Output Low (Sink)

1 1 X Output No Output High (Source)

(in MCUCR) I/O Pull-up Comment

14.2.4 Reading the Pin Value

Independent of the setting of Data Direction bit DDxn, the port pin can be read through the PINxn Register bit. As
shown in Figure 14-2, the PINxn Register bit and the preceding latch constitute a synchronizer. This is needed to
avoid metastability if the physical pin changes value near the edge of the internal clock, but it also introduces a
delay. Figure 14-3 shows a timing diagram of the synchronization when reading an externally applied pin value.
The maximum and minimum propagation delays are denoted t

pd,max

and t

respectively.

pd,min

Figure 14-3. Synchronization when Reading an Externally Applied Pin value

Consider the clock period starting shortly after the first falling edge of the system clock. The latch is closed when
the clock is low, and goes transparent when the clock is high, as indicated by the shaded region of the “SYNC
LATCH” signal. The signal value is latched when the system clock goes low. It is clocked into the PINxn Register at
the succeeding positive clock edge. As indicated by the two arrows tpd,max and tpd,min, a single signal transition
on the pin will be delayed between ½ and 1½ system clock period depending upon the time of assertion.

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When reading back a software assigned pin value, a nop instruction must be inserted as indicated in Figure 14-4.

out PORTx, r16 nop in r17, PINx

0xFF

0x00 0xFF

SYSTEM CLK

r16

INSTRUCTIONS

SYNC LATCH

PINxn

r17

t

pd

The out instruction sets the “SYNC LATCH” signal at the positive edge of the clock. In this case, the delay tpd
through the synchronizer is 1 system clock period.

Figure 14-4. Synchronization when Reading a Software Assigned Pin Value

The following code example shows how to set port B pins 0 and 1 high, 2 and 3 low, and define the port pins from
4 to 7 as input with pull-ups assigned to port pins 6 and 7. The resulting pin values are read back again, but as pre­viously discussed, a nop instruction is included to be able to read back the value recently assigned to some of the
pins.

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Assembly Code Example

...

; Define pull-ups and set outputs high

; Define directions for port pins

ldi r16,(1<<PB7)|(1<<PB6)|(1<<PB1)|(1<<PB0)

ldi r17,(1<<DDB3)|(1<<DDB2)|(1<<DDB1)|(1<<DDB0)

out PORTB,r16

out DDRB,r17

; Insert nop for synchronization

nop

; Read port pins

in r16,PINB

...

(1)

C Code Example

unsigned char i;

...

/* Define pull-ups and set outputs high */

/* Define directions for port pins */

PORTB = (1<<PB7)|(1<<PB6)|(1<<PB1)|(1<<PB0);

DDRB = (1<<DDB3)|(1<<DDB2)|(1<<DDB1)|(1<<DDB0);

/* Insert nop for synchronization*/

__no_operation();

/* Read port pins */

i = PINB;

...

Note: 1. For the assembly program, two temporary registers are used to minimize the time from pull-ups are set on pins 0,

1, 6, and 7, until the direction bits are correctly set, defining bit 2 and 3 as low and redefining bits 0 and 1 as strong
high drivers.

14.2.5 Digital Input Enable and Sleep Modes

As shown in Figure 14-2, the digital input signal can be clamped to ground at the input of the Schmitt Trigger. The
signal denoted SLEEP in the figure, is set by the MCU Sleep Controller in Power-down mode, Power-save mode,
and Standby mode to avoid high power consumption if some input signals are left floating, or have an analog signal
level close to V

CC

/2.

SLEEP is overridden for port pins enabled as external interrupt pins. If the external interrupt request is not enabled,
SLEEP is active also for these pins. SLEEP is also overridden by various other alternate functions as described in

”Alternate Port Functions” on page 81.

If a logic high level (“one”) is present on an asynchronous external interrupt pin configured as “Interrupt on Rising
Edge, Falling Edge, or Any Logic Change on Pin” while the external interrupt is not enabled, the corresponding
External Interrupt Flag will be set when resuming from the above mentioned Sleep mode, as the clamping in these
sleep mode produces the requested logic change.

14.2.6 Unconnected Pins

If some pins are unused, it is recommended to ensure that these pins have a defined level. Even though most of
the digital inputs are disabled in the deep sleep modes as described above, floating inputs should be avoided to
reduce current consumption in all other modes where the digital inputs are enabled (Reset, Active mode and Idle
mode).

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The simplest method to ensure a defined level of an unused pin, is to enable the internal pull-up. In this case, the

clk

RPx

RRx

WRx

RDx

WDx

PUD

SYNCHRONIZER

WDx: WRITE DDRx

WRx: WRITE PORTx

RRx: READ PORTx REGISTER

RPx: READ PORTx PIN

PUD: PULLUP DISABLE

clk

I/O

: I/O CLOCK

RDx: READ DDRx

DLQ

Q

SET

CLR

0

1

0

1

0

1

DIxn

AIOxn

DIEOExn

PVOVxn

PVOExn

DDOVxn

DDOExn

PUOExn

PUOVxn

PUOExn: Pxn PULL-UP OVERRIDE ENABLE
PUOVxn: Pxn PULL-UP OVERRIDE VALUE
DDOExn: Pxn DATA DIRECTION OVERRIDE ENABLE
DDOVxn: Pxn DATA DIRECTION OVERRIDE VALUE
PVOExn: Pxn PORT VALUE OVERRIDE ENABLE
PVOVxn: Pxn PORT VALUE OVERRIDE VALUE

DIxn: DIGITAL INPUT PIN n ON PORTx
AIOxn: ANALOG INPUT/OUTPUT PIN n ON PORTx

RESET

RESET

Q

Q

D

CLR

Q

Q

D

CLR

Q

Q

D

CLR

PINxn

PORTxn

DDxn

DATA BU S

0

1

DIEOVxn

SLEEP

DIEOExn: Pxn DIGITAL INPUT-ENABLE OVERRIDE ENABLE
DIEOVxn: Pxn DIGITAL INPUT-ENABLE OVERRIDE VALUE
SLEEP: SLEEP CONTROL

Pxn

I/O

0

1

PTOExn

PTOExn: Pxn, PORT TOGGLE OVERRIDE ENABLE

WPx: WRITE PINx

WPx

pull-up will be disabled during reset. If low power consumption during reset is important, it is recommended to use
an external pull-up or pull-down. Connecting unused pins directly to V
may cause excessive currents if the pin is accidentally configured as an output.

14.3 Alternate Port Functions

Most port pins have alternate functions in addition to being general digital I/Os. Figure 14-5 shows how the port pin
control signals from the simplified Figure 14-2 on page 77 can be overridden by alternate functions. The overriding
signals may not be present in all port pins, but the figure serves as a generic description applicable to all port pins
in the AVR microcontroller family.

or GND is not recommended, since this

CC

Figure 14-5. Alternate Port Functions

(1)

Note: 1. WRx, WPx, WDx, RRx, RPx, and RDx are common to all pins within the same port. clk

common to all ports. All other signals are unique for each pin.

ATmega48A/PA/88A/PA/168A/PA/328/P [DATASHEET]

, SLEEP, and PUD are

I/O

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Table 14-2 summarizes the function of the overriding signals. The pin and port indexes from Figure 14-5 on page
81 are not shown in the succeeding tables. The overriding signals are generated internally in the modules having

the alternate function.

Table 14-2. Generic Description of Overriding Signals for Alternate Functions

Signal Name Full Name Description

If this signal is set, the pull-up enable is controlled by the PUOV
signal. If this signal is cleared, the pull-up is enabled when
{DDxn, PORTxn, PUD} = 0b010.

If PUOE is set, the pull-up is enabled/disabled when PUOV is
set/cleared, regardless of the setting of the DDxn, PORTxn,
and PUD Register bits.

If this signal is set, the Output Driver Enable is controlled by the
DDOV signal. If this signal is cleared, the Output driver is
enabled by the DDxn Register bit.

If DDOE is set, the Output Driver is enabled/disabled when
DDOV is set/cleared, regardless of the setting of the DDxn
Register bit.

If this signal is set and the Output Driver is enabled, the port
value is controlled by the PVOV signal. If PVOE is cleared, and
the Output Driver is enabled, the port Value is controlled by the
PORTxn Register bit.

PUOE

PUOV

DDOE

DDOV

PVOE

Pull-up Override
Enable

Pull-up Override
Val ue

Data Direction
Override Enable

Data Direction
Override Value

Por t Value
Override Enable

PVOV

PTOE

DIEOE

DIEOV

DI Digital Input

AIO

Por t Value
Override Value

Port Toggle
Override Enable

Digital Input
Enable Override
Enable

Digital Input
Enable Override
Val ue

Analog
Input/Output

If PVOE is set, the port value is set to PVOV, regardless of the
setting of the PORTxn Register bit.

If PTOE is set, the PORTxn Register bit is inverted.

If this bit is set, the Digital Input Enable is controlled by the
DIEOV signal. If this signal is cleared, the Digital Input Enable
is determined by MCU state (Normal mode, sleep mode).

If DIEOE is set, the Digital Input is enabled/disabled when
DIEOV is set/cleared, regardless of the MCU state (Normal
mode, sleep mode).

This is the Digital Input to alternate functions. In the figure, the
signal is connected to the output of the Schmitt Trigger but
before the synchronizer. Unless the Digital Input is used as a
clock source, the module with the alternate function will use its
own synchronizer.

This is the Analog Input/output to/from alternate functions. The
signal is connected directly to the pad, and can be used bi­directionally.

The following subsections shortly describe the alternate functions for each port, and relate the overriding signals to
the alternate function. Refer to the alternate function description for further details.

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14.3.1 Alternate Functions of Port B

The Port B pins with alternate functions are shown in Table 14-3.

Table 14-3. Port B Pins Alternate Functions

Port Pin Alternate Functions

PB7

PB6

PB5

PB4

PB3

PB2

PB1

PB0

XTAL2 (
TOSC2 (Timer Oscillator pin 2)
PCINT7 (Pin Change Interrupt 7)

XTAL1 (
TOSC1 (Timer Oscillator pin 1)
PCINT6 (Pin Change Interrupt 6)

SCK (SPI Bus Master clock Input)
PCINT5 (Pin Change Interrupt 5)

MISO (SPI Bus Master Input/Slave Output)
PCINT4 (Pin Change Interrupt 4)

MOSI (SPI Bus Master Output/Slave Input)
OC2A (Timer/Counter2 Output Compare Match A Output)
PCINT3 (Pin Change Interrupt 3)

SS
OC1B (Timer/Counter1 Output Compare Match B Output)
PCINT2 (Pin Change Interrupt 2)

OC1A (Timer/Counter1 Output Compare Match A Output)
PCINT1 (Pin Change Interrupt 1)

ICP1 (Timer/Counter1 Input Capture Input)
CLKO (Divided System Clock Output)
PCINT0 (Pin Change Interrupt 0)

Chip Clock Oscillator pin 2)

Chip Clock Oscillator pin 1 or External clock input)

(SPI Bus Master Slave select)

The alternate pin configuration is as follows:

• XTAL2/TOSC2/PCINT7 – Port B, Bit 7

XTAL2: Chip clock Oscillator pin 2. Used as clock pin for crystal Oscillator or Low-frequency crystal Oscillator.
When used as a clock pin, the pin can not be used as an I/O pin.

TOSC2: Timer Oscillator pin 2. Used only if internal calibrated RC Oscillator is selected as chip clock source, and
the asynchronous timer is enabled by the correct setting in ASSR. When the AS2 bit in ASSR is set (one) and the
EXCLK bit is cleared (zero) to enable asynchronous clocking of Timer/Counter2 using the Crystal Oscillator, pin
PB7 is disconnected from the port, and becomes the inverting output of the Oscillator amplifier. In this mode, a
crystal Oscillator is connected to this pin, and the pin cannot be used as an I/O pin.

PCINT7: Pin Change Interrupt source 7. The PB7 pin can serve as an external interrupt source.

If PB7 is used as a clock pin, DDB7, PORTB7 and PINB7 will all read 0.

• XTAL1/TOSC1/PCINT6 – Port B, Bit 6

XTAL1: Chip clock Oscillator pin 1. Used for all chip clock sources except internal calibrated RC Oscillator. When
used as a clock pin, the pin can not be used as an I/O pin.

TOSC1: Timer Oscillator pin 1. Used only if internal calibrated RC Oscillator is selected as chip clock source, and
the asynchronous timer is enabled by the correct setting in ASSR. When the AS2 bit in ASSR is set (one) to enable
asynchronous clocking of Timer/Counter2, pin PB6 is disconnected from the port, and becomes the input of the

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inverting Oscillator amplifier. In this mode, a crystal Oscillator is connected to this pin, and the pin can not be used
as an I/O pin.

PCINT6: Pin Change Interrupt source 6. The PB6 pin can serve as an external interrupt source.

If PB6 is used as a clock pin, DDB6, PORTB6 and PINB6 will all read 0.

• SCK/PCINT5 – Port B, Bit 5

SCK: Master Clock output, Slave Clock input pin for SPI channel. When the SPI is enabled as a Slave, this pin is
configured as an input regardless of the setting of DDB5. When the SPI is enabled as a Master, the data direction
of this pin is controlled by DDB5. When the pin is forced by the SPI to be an input, the pull-up can still be controlled
by the PORTB5 bit.

PCINT5: Pin Change Interrupt source 5. The PB5 pin can serve as an external interrupt source.

• MISO/PCINT4 – Port B, Bit 4

MISO: Master Data input, Slave Data output pin for SPI channel. When the SPI is enabled as a Master, this pin is
configured as an input regardless of the setting of DDB4. When the SPI is enabled as a Slave, the data direction of
this pin is controlled by DDB4. When the pin is forced by the SPI to be an input, the pull-up can still be controlled by
the PORTB4 bit.

PCINT4: Pin Change Interrupt source 4. The PB4 pin can serve as an external interrupt source.

• MOSI/OC2/PCINT3 – Port B, Bit 3

MOSI: SPI Master Data output, Slave Data input for SPI channel. When the SPI is enabled as a Slave, this pin is
configured as an input regardless of the setting of DDB3. When the SPI is enabled as a Master, the data direction
of this pin is controlled by DDB3. When the pin is forced by the SPI to be an input, the pull-up can still be controlled
by the PORTB3 bit.

OC2, Output Compare Match Output: The PB3 pin can serve as an external output for the Timer/Counter2 Com­pare Match. The PB3 pin has to be configured as an output (DDB3 set (one)) to serve this function. The OC2 pin is
also the output pin for the PWM mode timer function.

PCINT3: Pin Change Interrupt source 3. The PB3 pin can serve as an external interrupt source.

•SS

/OC1B/PCINT2 – Port B, Bit 2

SS

: Slave Select input. When the SPI is enabled as a Slave, this pin is configured as an input regardless of the set­ting of DDB2. As a Slave, the SPI is activated when this pin is driven low. When the SPI is enabled as a Master, the
data direction of this pin is controlled by DDB2. When the pin is forced by the SPI to be an input, the pull-up can still
be controlled by the PORTB2 bit.

OC1B, Output Compare Match output: The PB2 pin can serve as an external output for the Timer/Counter1 Com­pare Match B. The PB2 pin has to be configured as an output (DDB2 set (one)) to serve this function. The OC1B
pin is also the output pin for the PWM mode timer function.

PCINT2: Pin Change Interrupt source 2. The PB2 pin can serve as an external interrupt source.

• OC1A/PCINT1 – Port B, Bit 1

OC1A, Output Compare Match output: The PB1 pin can serve as an external output for the Timer/Counter1 Com­pare Match A. The PB1 pin has to be configured as an output (DDB1 set (one)) to serve this function. The OC1A
pin is also the output pin for the PWM mode timer function.

PCINT1: Pin Change Interrupt source 1. The PB1 pin can serve as an external interrupt source.

• ICP1/CLKO/PCINT0 – Port B, Bit 0

ICP1, Input Capture Pin: The PB0 pin can act as an Input Capture Pin for Timer/Counter1.

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CLKO, Divided System Clock: The divided system clock can be output on the PB0 pin. The divided system clock
will be output if the CKOUT Fuse is programmed, regardless of the PORTB0 and DDB0 settings. It will also be out­put during reset.

PCINT0: Pin Change Interrupt source 0. The PB0 pin can serve as an external interrupt source.

Table 14-4 and Table 14-5 on page 86 relate the alternate functions of Port B to the overriding signals shown in
Figure 14-5 on page 81. SPI MSTR INPUT and SPI SLAVE OUTPUT constitute the MISO signal, while MOSI is

divided into SPI MSTR OUTPUT and SPI SLAVE INPUT.

Table 14-4. Overriding Signals for Alternate Functions in PB7...PB4

Signal
Name

PUOE

PUOV 0 0 PORTB5 • PUD PORTB4 • PUD

DDOE

DDOV0000

PVOE 0 0 SPE • MSTR SPE • MSTR

PVOV 0 0 SCK OUTPUT

DIEOE

DIEOV

DI PCINT7 INPUT PCINT6 INPUT

AIO Oscillator Output

Notes: 1. INTRC means that one of the internal RC Oscillators are selected (by the CKSEL fuses), EXTCK means that exter-

PB7/XTAL2/
TOSC2/PCINT7

INTRC

• EXTCK+

AS2

INTRC

• EXTCK+

AS2

INTRC • EXTCK +
AS2 + PCINT7 •
PCIE0

(INTRC + EXTCK) •
AS2

nal clock is selected (by the CKSEL fuses)

PB6/XTAL1/

(1)

TOSC1/PCINT6

+ AS2 SPE • MSTR SPE • MSTR

INTRC

+ AS2 SPE • MSTR SPE • MSTR

INTRC

+ AS2 +

INTRC
PCINT6 • PCIE0

INTRC • AS2 11

Oscillator/Clock
Input

PB5/SCK/

(1)

PCINT5

PCINT5 • PCIE0 PCINT4 • PCIE0

PCINT5 INPUT
SCK INPUT

––

PB4/MISO/
PCINT4

SPI SLAVE
OUTPUT

PCINT4 INPUT
SPI MSTR INPUT

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Table 14-5. Overriding Signals for Alternate Functions in PB3...PB0

Signal
Name

PUOE SPE • MSTR

PUOV PORTB3 • PUD

DDOE SPE • MSTR

DDOV 0 0 0 0

PVOE

PVOV

DIEOE PCINT3 • PCIE0 PCINT2 • PCIE0 PCINT1 • PCIE0 PCINT0 • PCIE0

DIEOV 1 1 1 1

DI

AIO – – – –

PB3/MOSI/
OC2/PCINT3

SPE • MSTR +
OC2A ENABLE

SPI MSTR OUTPUT
+ OC2A

PCINT3 INPUT
SPI SLAVE INPUT

14.3.2 Alternate Functions of Port C

The Port C pins with alternate functions are shown in Table 14-6.

Table 14-6. Port C Pins Alternate Functions

PB2/SS/
OC1B/PCINT2

SPE • MSTR 00

PORTB2 • PUD 00

SPE • MSTR 00

OC1B ENABLE OC1A ENABLE 0

OC1B OC1A 0

PCINT2 INPUT
SPI SS

PB1/OC1A/
PCINT1

PCINT1 INPUT

PB0/ICP1/
PCINT0

PCINT0 INPUT
ICP1 INPUT

Port Pin Alternate Function

RESET

PC6

PC5

PC4

PC3

PC2

PC1

PC0

(Reset pin)

PCINT14 (Pin Change Interrupt 14)

ADC5 (ADC Input Channel 5)
SCL (2-wire Serial Bus Clock Line)
PCINT13 (Pin Change Interrupt 13)

ADC4 (ADC Input Channel 4)
SDA (2-wire Serial Bus Data Input/Output Line)
PCINT12 (Pin Change Interrupt 12)

ADC3 (ADC Input Channel 3)
PCINT11 (Pin Change Interrupt 11)

ADC2 (ADC Input Channel 2)
PCINT10 (Pin Change Interrupt 10)

ADC1 (ADC Input Channel 1)
PCINT9 (Pin Change Interrupt 9)

ADC0 (ADC Input Channel 0)
PCINT8 (Pin Change Interrupt 8)

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The alternate pin configuration is as follows:

• RESET

RESET

/PCINT14 – Port C, Bit 6

, Reset pin: When the RSTDISBL Fuse is programmed, this pin functions as a normal I/O pin, and the part
will have to rely on Power-on Reset and Brown-out Reset as its reset sources. When the RSTDISBL Fuse is unpro­grammed, the reset circuitry is connected to the pin, and the pin can not be used as an I/O pin.

If PC6 is used as a reset pin, DDC6, PORTC6 and PINC6 will all read 0.

PCINT14: Pin Change Interrupt source 14. The PC6 pin can serve as an external interrupt source.

• SCL/ADC5/PCINT13 – Port C, Bit 5

SCL, 2-wire Serial Interface Clock: When the TWEN bit in TWCR is set (one) to enable the 2-wire Serial Interface,
pin PC5 is disconnected from the port and becomes the Serial Clock I/O pin for the 2-wire Serial Interface. In this
mode, there is a spike filter on the pin to suppress spikes shorter than 50 ns on the input signal, and the pin is
driven by an open drain driver with slew-rate limitation.

PC5 can also be used as ADC input Channel 5. Note that ADC input channel 5 uses digital power.

PCINT13: Pin Change Interrupt source 13. The PC5 pin can serve as an external interrupt source.

• SDA/ADC4/PCINT12 – Port C, Bit 4

SDA, 2-wire Serial Interface Data: When the TWEN bit in TWCR is set (one) to enable the 2-wire Serial Interface,
pin PC4 is disconnected from the port and becomes the Serial Data I/O pin for the 2-wire Serial Interface. In this
mode, there is a spike filter on the pin to suppress spikes shorter than 50 ns on the input signal, and the pin is
driven by an open drain driver with slew-rate limitation.

PC4 can also be used as ADC input Channel 4. Note that ADC input channel 4 uses digital power.

PCINT12: Pin Change Interrupt source 12. The PC4 pin can serve as an external interrupt source.

• ADC3/PCINT11 – Port C, Bit 3

PC3 can also be used as ADC input Channel 3. Note that ADC input channel 3 uses analog power.

PCINT11: Pin Change Interrupt source 11. The PC3 pin can serve as an external interrupt source.

• ADC2/PCINT10 – Port C, Bit 2

PC2 can also be used as ADC input Channel 2. Note that ADC input channel 2 uses analog power.

PCINT10: Pin Change Interrupt source 10. The PC2 pin can serve as an external interrupt source.

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• ADC1/PCINT9 – Port C, Bit 1

PC1 can also be used as ADC input Channel 1. Note that ADC input channel 1 uses analog power.

PCINT9: Pin Change Interrupt source 9. The PC1 pin can serve as an external interrupt source.

• ADC0/PCINT8 – Port C, Bit 0

PC0 can also be used as ADC input Channel 0. Note that ADC input channel 0 uses analog power.

PCINT8: Pin Change Interrupt source 8. The PC0 pin can serve as an external interrupt source.

Table 14-7 and Table 14-8 relate the alternate functions of Port C to the overriding signals shown in Figure 14-5 on
page 81.

Table 14-7. Overriding Signals for Alternate Functions in PC6...PC4

Signal
Name PC6/RESET

PUOE RSTDISBL TWEN TWEN

PUOV 1 PORTC5 • PUD

DDOE RSTDISBL TWEN TWEN

DDOV 0 SCL_OUT SDA_OUT

PVOE 0 TWEN TWEN

PVOV 0 0 0

DIEOE

DIEOV RSTDISBL PCINT13 • PCIE1 PCINT12 • PCIE1

DI PCINT14 INPUT PCINT13 INPUT PCINT12 INPUT

AIO RESET INPUT ADC5 INPUT / SCL INPUT ADC4 INPUT / SDA INPUT

Note: 1. When enabled, the 2-wire Serial Interface enables slew-rate controls on the output pins PC4 and PC5. This is not

RSTDISBL + PCINT14 •
PCIE1

shown in the figure. In addition, spike filters are connected between the AIO outputs shown in the port figure and
the digital logic of the TWI module.

/PCINT14 PC5/SCL/ADC5/PCINT13 PC4/SDA/ADC4/PCINT12

PCINT13 • PCIE1 + ADC5D PCINT12 • PCIE1 + ADC4D

(1)

PORTC4 • PUD

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Table 14-8. Overriding Signals for Alternate Functions in PC3...PC0

Signal
Name

PUOE0000

PUOV0000

DDOE 0 0 0 0

DDOV 0 0 0 0

PVOE0000

PVOV0000

DIEOE

DIEOV PCINT11 • PCIE1 PCINT10 • PCIE1 PCINT9 • PCIE1 PCINT8 • PCIE1

DI PCINT11 INPUT PCINT10 INPUT PCINT9 INPUT PCINT8 INPUT

AIO ADC3 INPUT ADC2 INPUT ADC1 INPUT ADC0 INPUT

PC3/ADC3/
PCINT11

PCINT11 • PCIE1 +
ADC3D

14.3.3 Alternate Functions of Port D

The Port D pins with alternate functions are shown in Table 14-9.

Table 14-9. Port D Pins Alternate Functions

PC2/ADC2/
PCINT10

PCINT10 • PCIE1 +
ADC2D

PC1/ADC1/
PCINT9

PCINT9 • PCIE1 +
ADC1D

PC0/ADC0/
PCINT8

PCINT8 • PCIE1 +
ADC0D

Port Pin Alternate Function

PD7

PD6

PD5

PD4

PD3

PD2

PD1

PD0

AIN1 (Analog Comparator Negative Input)
PCINT23 (Pin Change Interrupt 23)

AIN0 (Analog Comparator Positive Input)
OC0A (Timer/Counter0 Output Compare Match A Output)
PCINT22 (Pin Change Interrupt 22)

T1 (Timer/Counter 1 External Counter Input)
OC0B (Timer/Counter0 Output Compare Match B Output)
PCINT21 (Pin Change Interrupt 21)

XCK (USART External Clock Input/Output)
T0 (Timer/Counter 0 External Counter Input)
PCINT20 (Pin Change Interrupt 20)

INT1 (External Interrupt 1 Input)
OC2B (Timer/Counter2 Output Compare Match B Output)
PCINT19 (Pin Change Interrupt 19)

INT0 (External Interrupt 0 Input)
PCINT18 (Pin Change Interrupt 18)

TXD (USART Output Pin)
PCINT17 (Pin Change Interrupt 17)

RXD (USART Input Pin)
PCINT16 (Pin Change Interrupt 16)

The alternate pin configuration is as follows:

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• AIN1/OC2B/PCINT23 – Port D, Bit 7

AIN1, Analog Comparator Negative Input. Configure the port pin as input with the internal pull-up switched off to
avoid the digital port function from interfering with the function of the Analog Comparator.

PCINT23: Pin Change Interrupt source 23. The PD7 pin can serve as an external interrupt source.

• AIN0/OC0A/PCINT22 – Port D, Bit 6

AIN0, Analog Comparator Positive Input. Configure the port pin as input with the internal pull-up switched off to
avoid the digital port function from interfering with the function of the Analog Comparator.

OC0A, Output Compare Match output: The PD6 pin can serve as an external output for the Timer/Counter0 Com­pare Match A. The PD6 pin has to be configured as an output (DDD6 set (one)) to serve this function. The OC0A
pin is also the output pin for the PWM mode timer function.

PCINT22: Pin Change Interrupt source 22. The PD6 pin can serve as an external interrupt source.

• T1/OC0B/PCINT21 – Port D, Bit 5

T1, Timer/Counter1 counter source.

OC0B, Output Compare Match output: The PD5 pin can serve as an external output for the Timer/Counter0 Com­pare Match B. The PD5 pin has to be configured as an output (DDD5 set (one)) to serve this function. The OC0B
pin is also the output pin for the PWM mode timer function.

PCINT21: Pin Change Interrupt source 21. The PD5 pin can serve as an external interrupt source.

• XCK/T0/PCINT20 – Port D, Bit 4

XCK, USART external clock.

T0, Timer/Counter0 counter source.

PCINT20: Pin Change Interrupt source 20. The PD4 pin can serve as an external interrupt source.

• INT1/OC2B/PCINT19 – Port D, Bit 3

INT1, External Interrupt source 1: The PD3 pin can serve as an external interrupt source.

OC2B, Output Compare Match output: The PD3 pin can serve as an external output for the Timer/Counter0 Com­pare Match B. The PD3 pin has to be configured as an output (DDD3 set (one)) to serve this function. The OC2B
pin is also the output pin for the PWM mode timer function.

PCINT19: Pin Change Interrupt source 19. The PD3 pin can serve as an external interrupt source.

• INT0/PCINT18 – Port D, Bit 2

INT0, External Interrupt source 0: The PD2 pin can serve as an external interrupt source.

PCINT18: Pin Change Interrupt source 18. The PD2 pin can serve as an external interrupt source.

• TXD/PCINT17 – Port D, Bit 1

TXD, Transmit Data (Data output pin for the USART). When the USART Transmitter is enabled, this pin is config­ured as an output regardless of the value of DDD1.

PCINT17: Pin Change Interrupt source 17. The PD1 pin can serve as an external interrupt source.

• RXD/PCINT16 – Port D, Bit 0

RXD, Receive Data (Data input pin for the USART). When the USART Receiver is enabled this pin is configured as
an input regardless of the value of DDD0. When the USART forces this pin to be an input, the pull-up can still be
controlled by the PORTD0 bit.

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PCINT16: Pin Change Interrupt source 16. The PD0 pin can serve as an external interrupt source.

Table 14-10 and Table 14-11 relate the alternate functions of Port D to the overriding signals shown in Figure 14-5
on page 81.

Table 14-10. Overriding Signals for Alternate Functions PD7...PD4

Signal
Name

PUOE0000

PUO0000

DDOE 0 0 0 0

DDOV 0 0 0 0

PVOE 0 OC0A ENABLE OC0B ENABLE UMSEL

PVOV 0 OC0A OC0B XCK OUTPUT

DIEOE PCINT23 • PCIE2 PCINT22 • PCIE2 PCINT21 • PCIE2 PCINT20 • PCIE2

DIEOV1111

DI PCINT23 INPUT PCINT22 INPUT

AIO AIN1 INPUT AIN0 INPUT – –

PD7/AIN1
/PCINT23

PD6/AIN0/
OC0A/PCINT22

PD5/T1/OC0B/
PCINT21

PCINT21 INPUT
T1 INPUT

PD4/XCK/
T0/PCINT20

PCINT20 INPUT
XCK INPUT
T0 INPUT

Table 14-11. Overriding Signals for Alternate Functions in PD3...PD0

Signal
Name

PUOE 0 0 TXEN RXEN

PD3/OC2B/INT1/
PCINT19

PD2/INT0/
PCINT18

PD1/TXD/
PCINT17

PD0/RXD/
PCINT16

PUO000PORTD0 • PUD

DDOE 0 0 TXEN RXEN

DDOV 0 0 1 0

PVOE OC2B ENABLE 0 TXEN 0

PVOV OC2B 0 TXD 0

DIEOE

DIEOV1111

DI

AIO––––

INT1 ENABLE +
PCINT19 • PCIE2

PCINT19 INPUT
INT1 INPUT

INT0 ENABLE +
PCINT18 • PCIE1

PCINT18 INPUT
INT0 INPUT

PCINT17 • PCIE2 PCINT16 • PCIE2

PCINT17 INPUT

PCINT16 INPUT
RXD

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14.4 Register Description

14.4.1 MCUCR – MCU Control Register

Bit 7 6 5 4 3 2 1 0

0x35 (0x55)

Read/Write R R/W R/W R/W R R R/W R/W

Initial Value 0 0 0 0 0 0 0 0

– BODS

Notes: 1. BODS and BODSE only available for picoPower devices ATmega48PA/88PA/168PA/328P

• Bit 4 – PUD: Pull-up Disable

When this bit is written to one, the pull-ups in the I/O ports are disabled even if the DDxn and PORTxn Registers
are configured to enable the pull-ups ({DDxn, PORTxn} = 0b01). See ”Configuring the Pin” on page 77 for more
details about this feature.

14.4.2 PORTB – The Port B Data Register

Bit 76543210

0x05 (0x25)

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W

Initial Value00000000

PORTB7 PORTB6 PORTB5 PORTB4 PORTB3 PORTB2 PORTB1 PORTB0 PORTB

14.4.3 DDRB – The Port B Data Direction Register

Bit 76543210

0x04 (0x24) DDB7 DDB6 DDB5 DDB4 DDB3 DDB2 DDB1 DDB0 DDRB

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W

Initial Value00000000

(1)

BODSE

(1)

PUD – – IVSEL IVCE MCUCR

14.4.4 PINB – The Port B Input Pins Address

Bit 76543210

0x03 (0x23) PINB7 PINB6 PINB5 PINB4 PINB3 PINB2 PINB1 PINB0 PINB

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W

Initial Value N/A N/A N/A N/A N/A N/A N/A N/A

(1)

14.4.5 PORTC – The Port C Data Register

Bit 76543210

0x08 (0x28)

Read/Write R R/W R/W R/W R/W R/W R/W R/W

Initial Value00000000

– PORTC6 PORTC5 PORTC4 PORTC3 PORTC2 PORTC1 PORTC0 PORTC

14.4.6 DDRC – The Port C Data Direction Register

Bit 76543210

0x07 (0x27) – DDC6 DDC5 DDC4 DDC3 DDC2 DDC1 DDC0 DDRC

Read/Write R R/W R/W R/W R/W R/W R/W R/W

Initial Value00000000

14.4.7 PINC – The Port C Input Pins Address

Bit 76543210

0x06 (0x26) – PINC6 PINC5 PINC4 PINC3 PINC2 PINC1 PINC0 PINC

Read/Write R R/W R/W R/W R/W R/W R/W R/W

Initial Value 0 N/A N/A N/A N/A N/A N/A N/A

(1)

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14.4.8 PORTD – The Port D Data Register

Bit 76543210

0x0B (0x2B)

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W

Initial Value00000000

PORTD7 PORTD6 PORTD5 PORTD4 PORTD3 PORTD2 PORTD1 PORTD0 PORTD

14.4.9 DDRD – The Port D Data Direction Register

Bit 76543210

0x0A (0x2A) DDD7 DDD6 DDD5 DDD4 DDD3 DDD2 DDD1 DDD0 DDRD

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W

Initial Value00000000

14.4.10 PIND – The Port D Input Pins Address

Bit 76543210

0x09 (0x29) PIND7 PIND6 PIND5 PIND4 PIND3 PIND2 PIND1 PIND0 PIND

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W

Initial Value N/A N/A N/A N/A N/A N/A N/A N/A

Note: 1. Writting to the pin register provides toggle functionality for IO (see ”Toggling the Pin” on page 77)

(1)

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15. 8-bit Timer/Counter0 with PWM

15.1 Features

• Two Independent Output Compare Units

• Double Buffered Output Compare Registers

• Clear Timer on Compare Match (Auto Reload)

• Glitch Free, Phase Correct Pulse Width Modulator (PWM)

• Variable PWM Period

• Frequency Generator

• Three Independent Interrupt Sources (TOV0, OCF0A, and OCF0B)

15.2 Overview

Timer/Counter0 is a general purpose 8-bit Timer/Counter module, with two independent Output Compare Units,
and with PWM support. It allows accurate program execution timing (event management) and wave generation.

A simplified block diagram of the 8-bit Timer/Counter is shown in Figure 15-1. For the actual placement of I/O pins,
refer to ”Pinout ATmega48A/PA/88A/PA/168A/PA/328/P” on page 2. CPU accessible I/O Registers, including I/O
bits and I/O pins, are shown in bold. The device-specific I/O Register and bit locations are listed in the ”Register

Description” on page 105.

The PRTIM0 bit in ”Minimizing Power Consumption” on page 41 must be written to zero to enable Timer/Counter0
module.

Figure 15-1. 8-bit Timer/Counter Block Diagram

Count

Clear

Direction

Timer/Counter

TCNTn

=

OCRnA

=

DATA BUS

OCRnB

TCCRnA TCCRnB

Control Logic

TOP BOTTOM

=

Fixed

TOP

Val ue

TOVn

(Int.Req.)

clk

Tn

=

0

Clock Select

Edge

Detector

( From Prescaler )

OCnA

(Int.Req.)

Wavefo rm

Generation

OCnB

(Int.Req.)

Wavefo rm

Generation

Tn

OCnA

OCnB

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15.2.1 Definitions

Many register and bit references in this section are written in general form. A lower case “n” replaces the
Timer/Counter number, in this case 0. A lower case “x” replaces the Output Compare Unit, in this case Compare
Unit A or Compare Unit B. However, when using the register or bit defines in a program, the precise form must be
used, i.e., TCNT0 for accessing Timer/Counter0 counter value and so on.

The definitions in Table 15-1 are also used extensively throughout the document.

Table 15-1. Definitions

BOTTOM The counter reaches the BOTTOM when it becomes 0x00.

MAX The counter reaches its MAXimum when it becomes 0xFF (decimal 255).

TOP The counter reaches the TOP when it becomes equal to the highest value in the

15.2.2 Registers

The Timer/Counter (TCNT0) and Output Compare Registers (OCR0A and OCR0B) are 8-bit registers. Interrupt
request (abbreviated to Int.Req. in the figure) signals are all visible in the Timer Interrupt Flag Register (TIFR0). All
interrupts are individually masked with the Timer Interrupt Mask Register (TIMSK0). TIFR0 and TIMSK0 are not
shown in the figure.

The Timer/Counter can be clocked internally, via the prescaler, or by an external clock source on the T0 pin. The
Clock Select logic block controls which clock source and edge the Timer/Counter uses to increment (or decrement)
its value. The Timer/Counter is inactive when no clock source is selected. The output from the Clock Select logic is
referred to as the timer clock (clk

The double buffered Output Compare Registers (OCR0A and OCR0B) are compared with the Timer/Counter value
at all times. The result of the compare can be used by the Waveform Generator to generate a PWM or variable fre­quency output on the Output Compare pins (OC0A and OC0B). See ”Using the Output Compare Unit” on page 122
for details. The compare match event will also set the Compare Flag (OCF0A or OCF0B) which can be used to
generate an Output Compare interrupt request.

count sequence. The TOP value can be assigned to be the fixed value 0xFF
(MAX) or the value stored in the OCR0A Register. The assignment is depen­dent on the mode of operation.

).

T0

15.3 Timer/Counter Clock Sources

The Timer/Counter can be clocked by an internal or an external clock source. The clock source is selected by the
Clock Select logic which is controlled by the Clock Select (CS02:0) bits located in the Timer/Counter Control Reg­ister (TCCR0B). For details on clock sources and prescaler, see ”Timer/Counter0 and Timer/Counter1 Prescalers”

on page 139.

15.4 Counter Unit

The main part of the 8-bit Timer/Counter is the programmable bi-directional counter unit. Figure 15-2 shows a
block diagram of the counter and its surroundings.

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Figure 15-2. Counter Unit Block Diagram

DATA BUS

count

TCNTn Control Logic

clear

direction

bottom

Signal description (internal signals):

count Increment or decrement TCNT0 by 1.

direction Select between increment and decrement.

clear Clear TCNT0 (set all bits to zero).

top

TOVn

(Int.Req.)

clk

Tn

Clock Select

Edge

Detector

( From Prescaler )

Tn

clk

Tn

top Signalize that TCNT0 has reached maximum value.

bottom Signalize that TCNT0 has reached minimum value (zero).

Depending of the mode of operation used, the counter is cleared, incremented, or decremented at each timer clock
(clk

). clkT0 can be generated from an external or internal clock source, selected by the Clock Select bits

T0

(CS02:0). When no clock source is selected (CS02:0 = 0) the timer is stopped. However, the TCNT0 value can be
accessed by the CPU, regardless of whether clk
counter clear or count operations.

The counting sequence is determined by the setting of the WGM01 and WGM00 bits located in the Timer/Counter
Control Register (TCCR0A) and the WGM02 bit located in the Timer/Counter Control Register B (TCCR0B). There
are close connections between how the counter behaves (counts) and how waveforms are generated on the Out­put Compare outputs OC0A and OC0B. For more details about advanced counting sequences and waveform
generation, see ”Modes of Operation” on page 99.

The Timer/Counter Overflow Flag (TOV0) is set according to the mode of operation selected by the WGM02:0 bits.
TOV0 can be used for generating a CPU interrupt.

15.5 Output Compare Unit

The 8-bit comparator continuously compares TCNT0 with the Output Compare Registers (OCR0A and OCR0B).
Whenever TCNT0 equals OCR0A or OCR0B, the comparator signals a match. A match will set the Output Com­pare Flag (OCF0A or OCF0B) at the next timer clock cycle. If the corresponding interrupt is enabled, the Output
Compare Flag generates an Output Compare interrupt. The Output Compare Flag is automatically cleared when
the interrupt is executed. Alternatively, the flag can be cleared by software by writing a logical one to its I/O bit loca­tion. The Waveform Generator uses the match signal to generate an output according to operating mode set by the
WGM02:0 bits and Compare Output mode (COM0x1:0) bits. The max and bottom signals are used by the Wave­form Generator for handling the special cases of the extreme values in some modes of operation (”Modes of

Operation” on page 99).

Timer/Counter clock, referred to as clkT0 in the following.

is present or not. A CPU write overrides (has priority over) all

T0

Figure 15-3 shows a block diagram of the Output Compare unit.

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Figure 15-3. Output Compare Unit, Block Diagram

OCFnx (Int.Req.)

= (8-bit Comparator )

OCRnx

OCnx

DATA B U S

TCNTn

WGMn1:0

Waveform Generator

top

FOCn

COMnx1:0

bottom

The OCR0x Registers are double buffered when using any of the Pulse Width Modulation (PWM) modes. For the
normal and Clear Timer on Compare (CTC) modes of operation, the double buffering is disabled. The double buff­ering synchronizes the update of the OCR0x Compare Registers to either top or bottom of the counting sequence.
The synchronization prevents the occurrence of odd-length, non-symmetrical PWM pulses, thereby making the
output glitch-free.

The OCR0x Register access may seem complex, but this is not case. When the double buffering is enabled, the
CPU has access to the OCR0x Buffer Register, and if double buffering is disabled the CPU will access the OCR0x
directly.

15.5.1 Force Output Compare

In non-PWM waveform generation modes, the match output of the comparator can be forced by writing a one to
the Force Output Compare (FOC0x) bit. Forcing compare match will not set the OCF0x Flag or reload/clear the
timer, but the OC0x pin will be updated as if a real compare match had occurred (the COM0x1:0 bits settings
define whether the OC0x pin is set, cleared or toggled).

15.5.2 Compare Match Blocking by TCNT0 Write

All CPU write operations to the TCNT0 Register will block any compare match that occur in the next timer clock
cycle, even when the timer is stopped. This feature allows OCR0x to be initialized to the same value as TCNT0
without triggering an interrupt when the Timer/Counter clock is enabled.

15.5.3 Using the Output Compare Unit

Since writing TCNT0 in any mode of operation will block all compare matches for one timer clock cycle, there are
risks involved when changing TCNT0 when using the Output Compare Unit, independently of whether the
Timer/Counter is running or not. If the value written to TCNT0 equals the OCR0x value, the compare match will be
missed, resulting in incorrect waveform generation. Similarly, do not write the TCNT0 value equal to BOTTOM
when the counter is downcounting.

The setup of the OC0x should be performed before setting the Data Direction Register for the port pin to output.
The easiest way of setting the OC0x value is to use the Force Output Compare (FOC0x) strobe bits in Normal
mode. The OC0x Registers keep their values even when changing between Waveform Generation modes.

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Be aware that the COM0x1:0 bits are not double buffered together with the compare value. Changing the
COM0x1:0 bits will take effect immediately.

15.6 Compare Match Output Unit

The Compare Output mode (COM0x1:0) bits have two functions. The Waveform Generator uses the COM0x1:0
bits for defining the Output Compare (OC0x) state at the next compare match. Also, the COM0x1:0 bits control the
OC0x pin output source. Figure 15-4 shows a simplified schematic of the logic affected by the COM0x1:0 bit set­ting. The I/O Registers, I/O bits, and I/O pins in the figure are shown in bold. Only the parts of the general I/O port
control registers (DDR and PORT) that are affected by the COM0x1:0 bits are shown. When referring to the OC0x
state, the reference is for the internal OC0x Register, not the OC0x pin. If a system reset occur, the OC0x Register
is reset to “0”.

Figure 15-4. Compare Match Output Unit, Schematic

COMnx1

COMnx0
FOCn

clk

I/O

Waveform
Generator

DQ

1

OCnx

DQ

PORT

DATA BUS

DQ

DDR

0

OCnx

Pin

The general I/O port function is overridden by the Output Compare (OC0x) from the Waveform Generator if either
of the COM0x1:0 bits are set. However, the OC0x pin direction (input or output) is still controlled by the Data Direc­tion Register (DDR) for the port pin. The Data Direction Register bit for the OC0x pin (DDR_OC0x) must be set as
output before the OC0x value is visible on the pin. The port override function is independent of the Waveform Gen­eration mode.

The design of the Output Compare pin logic allows initialization of the OC0x state before the output is enabled.
Note that some COM0x1:0 bit settings are reserved for certain modes of operation. See ”Register Description” on

page 105.

15.6.1 Compare Output Mode and Waveform Generation

The Waveform Generator uses the COM0x1:0 bits differently in Normal, CTC, and PWM modes. For all modes,
setting the COM0x1:0 = 0 tells the Waveform Generator that no action on the OC0x Register is to be performed on
the next compare match. For compare output actions in the non-PWM modes refer to Table 15-2 on page 105. For
fast PWM mode, refer to Table 15-3 on page 105, and for phase correct PWM refer to Table 15-4 on page 106.

A change of the COM0x1:0 bits state will have effect at the first compare match after the bits are written. For non­PWM modes, the action can be forced to have immediate effect by using the FOC0x strobe bits.

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15.7 Modes of Operation

The mode of operation, i.e., the behavior of the Timer/Counter and the Output Compare pins, is defined by the
combination of the Waveform Generation mode (WGM02:0) and Compare Output mode (COM0x1:0) bits. The
Compare Output mode bits do not affect the counting sequence, while the Waveform Generation mode bits do.
The COM0x1:0 bits control whether the PWM output generated should be inverted or not (inverted or non-inverted
PWM). For non-PWM modes the COM0x1:0 bits control whether the output should be set, cleared, or toggled at a
compare match (See ”Compare Match Output Unit” on page 98).

For detailed timing information refer to ”Timer/Counter Timing Diagrams” on page 103.

15.7.1 Normal Mode

The simplest mode of operation is the Normal mode (WGM02:0 = 0). In this mode the counting direction is always
up (incrementing), and no counter clear is performed. The counter simply overruns when it passes its maximum 8­bit value (TOP = 0xFF) and then restarts from the bottom (0x00). In normal operation the Timer/Counter Overflow
Flag (TOV0) will be set in the same timer clock cycle as the TCNT0 becomes zero. The TOV0 Flag in this case
behaves like a ninth bit, except that it is only set, not cleared. However, combined with the timer overflow interrupt
that automatically clears the TOV0 Flag, the timer resolution can be increased by software. There are no special
cases to consider in the Normal mode, a new counter value can be written anytime.

The Output Compare unit can be used to generate interrupts at some given time. Using the Output Compare to
generate waveforms in Normal mode is not recommended, since this will occupy too much of the CPU time.

15.7.2 Clear Timer on Compare Match (CTC) Mode

In Clear Timer on Compare or CTC mode (WGM02:0 = 2), the OCR0A Register is used to manipulate the counter
resolution. In CTC mode the counter is cleared to zero when the counter value (TCNT0) matches the OCR0A. The
OCR0A defines the top value for the counter, hence also its resolution. This mode allows greater control of the
compare match output frequency. It also simplifies the operation of counting external events.

The timing diagram for the CTC mode is shown in Figure 15-5. The counter value (TCNT0) increases until a com-
pare match occurs between TCNT0 and OCR0A, and then counter (TCNT0) is cleared.

Figure 15-5. CTC Mode, Timing Diagram

OCnx Interrupt Flag Set

TCNTn

OCn
(Toggle)

Period

1 4

2 3

(COMnx1:0 = 1)

An interrupt can be generated each time the counter value reaches the TOP value by using the OCF0A Flag. If the
interrupt is enabled, the interrupt handler routine can be used for updating the TOP value. However, changing TOP
to a value close to BOTTOM when the counter is running with none or a low prescaler value must be done with
care since the CTC mode does not have the double buffering feature. If the new value written to OCR0A is lower
than the current value of TCNT0, the counter will miss the compare match. The counter will then have to count to
its maximum value (0xFF) and wrap around starting at 0x00 before the compare match can occur.

For generating a waveform output in CTC mode, the OC0A output can be set to toggle its logical level on each
compare match by setting the Compare Output mode bits to toggle mode (COM0A1:0 = 1). The OC0A value will

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not be visible on the port pin unless the data direction for the pin is set to output. The waveform generated will have

f

OCnx

f

clk_I/O

2 N 1 OCRnx+

--------------------------------------------------=

a maximum frequency of f
the following equation:

The N variable represents the prescale factor (1, 8, 64, 256, or 1024).

As for the Normal mode of operation, the TOV0 Flag is set in the same timer clock cycle that the counter counts
from MAX to 0x00.

15.7.3 Fast PWM Mode

The fast Pulse Width Modulation or fast PWM mode (WGM02:0 = 3 or 7) provides a high frequency PWM wave­form generation option. The fast PWM differs from the other PWM option by its single-slope operation. The counter
counts from BOTTOM to TOP then restarts from BOTTOM. TOP is defined as 0xFF when WGM2:0 = 3, and
OCR0A when WGM2:0 = 7. In non-inverting Compare Output mode, the Output Compare (OC0x) is cleared on the
compare match between TCNT0 and OCR0x, and set at BOTTOM. In inverting Compare Output mode, the output
is set on compare match and cleared at BOTTOM. Due to the single-slope operation, the operating frequency of
the fast PWM mode can be twice as high as the phase correct PWM mode that use dual-slope operation. This high
frequency makes the fast PWM mode well suited for power regulation, rectification, and DAC applications. High
frequency allows physically small sized external components (coils, capacitors), and therefore reduces total sys­tem cost.

In fast PWM mode, the counter is incremented until the counter value matches the TOP value. The counter is then
cleared at the following timer clock cycle. The timing diagram for the fast PWM mode is shown in Figure 15-6. The
TCNT0 value is in the timing diagram shown as a histogram for illustrating the single-slope operation. The diagram
includes non-inverted and inverted PWM outputs. The small horizontal line marks on the TCNT0 slopes represent
compare matches between OCR0x and TCNT0.

OC0

= f

/2 when OCR0A is set to zero (0x00). The waveform frequency is defined by

clk_I/O

Figure 15-6. Fast PWM Mode, Timing Diagram

OCRnx Interrupt Flag Set

OCRnx Update and
TOVn Interrupt Flag Set

TCNTn

OCnx

OCnx

Period

1

2 3

4 5 6 7

(COMnx1:0 = 2)

(COMnx1:0 = 3)

The Timer/Counter Overflow Flag (TOV0) is set each time the counter reaches TOP. If the interrupt is enabled, the
interrupt handler routine can be used for updating the compare value.

In fast PWM mode, the compare unit allows generation of PWM waveforms on the OC0x pins. Setting the
COM0x1:0 bits to two will produce a non-inverted PWM and an inverted PWM output can be generated by setting
the COM0x1:0 to three: Setting the COM0A1:0 bits to one allows the OC0A pin to toggle on Compare Matches if

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