ATMEL ATmega48, ATmega88, ATmega168 User Manual

BDTIC www.bdtic.com/ATMEL

Features

• High Performance, Low Power AVR

• Advanced RISC Architecture

– 131 Powerful Instructions – Most Single Clock Cycle Execution – 32 x 8 General Purpose Working Registers – Fully Static Operation – Up to 16 MIPS Throughput at 16 MHz – On-chip 2-cycle Multiplier

• Non-volatile Program and Data Memories

– 4/8/16K Bytes of In-System Self-Programmable Flash (ATmega48/88/168)

• Endurance: 75,000 Write/Erase Cycles

– Optional Boot Code Section with Independent Lock Bits

• In-System Programming by On-chip Boot Program

• True Read-While-Write Operation

– 256/512/512 Bytes EEPROM (ATmega48/88/168)

• Endurance: 100,000 Write/Erase Cycles – 512/1K/1K Byte Internal SRAM (ATmega48/88/168) – Programming Lock for Software Security

• 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 – Programmable Serial USART – Master/Slave SPI Serial Interface – Byte-oriented 2-wire Serial Interface – 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 – Five Sleep Modes: Idle, ADC Noise Reduction, Power-save, Power-down, and

Standby

• I/O and Packages

– 23 Programmable I/O Lines – Green/ROHS 32-lead TQFP and 32-pad QFN

• Operating Voltage:

– 2.7 - 5.5V for ATmega48/88/168

• Temperature Range: °C to 125°C

–-40

• Speed Grade:

– ATmega48/88/168: 0 - 8 MHz @ 2.7 - 5.5V, 0 - 16 MHz @ 4.5 - 5.5V

• Low Power Consumption

– Active Mode:

• 4 MHz, 3.0V: 1.8mA

– Power-down Mode:

• 5µA at 3.0V

8-Bit Microcontroller

8-bit Microcontroller with 8K Bytes In-System Programmable Flash

ATmega48 ATmega88 ATmega168

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1. Pin Configurations

Figure 1-1. Pinout ATmega48/88/168

(PCINT19/OC2B/INT1) PD3

(PCINT20/XCK/T0) PD4

GND

VCC

GND

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

TQFP Top View

PD2 (INT0/PCINT18)

PD1 (TXD/PCINT17)

PD0 (RXD/PCINT16)

1 2 3 4 5 6 7 8

(PCINT23/AIN1) PD7

(PCINT21/OC0B/T1) PD5

(PCINT22/OC0A/AIN0) PD6

PC6 (RESET/PCINT14)

PC5 (ADC5/SCL/PCINT13)

PC4 (ADC4/SDA/PCINT12)

PC3 (ADC3/PCINT11)

PC2 (ADC2/PCINT10)

PC1 (ADC1/PCINT9)

PC0 (ADC0/PCINT8)

ADC7

GND

AREF

ADC6

AVCC

PB5 (SCK/PCINT5)

(PCINT4/MISO) PB4

(PCINT1/OC1A) PB1

(PCINT2/SS/OC1B) PB2

(PCINT0/CLKO/ICP1) PB0

(PCINT3/OC2A/MOSI) PB3

(PCINT19/OC2B/INT1) PD3

(PCINT20/XCK/T0) PD4

GND

VCC

GND

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

NOTE: Bottom pad should be soldered to ground.

MLF Top View

PD2 (INT0/PCINT18)

PD1 (TXD/PCINT17)

PD0 (RXD/PCINT16)

1 2 3 4 5 6 7 8

(PCINT23/AIN1) PD7

(PCINT21/OC0B/T1) PD5

(PCINT22/OC0A/AIN0) PD6

PC6 (RESET/PCINT14)

PC5 (ADC5/SCL/PCINT13)

PC4 (ADC4/SDA/PCINT12)

PC3 (ADC3/PCINT11)

PC2 (ADC2/PCINT10)

PC1 (ADC1/PCINT9)

PC0 (ADC0/PCINT8)

ADC7

GND

AREF

ADC6

AVCC

PB5 (SCK/PCINT5)

(PCINT4/MISO) PB4

(PCINT1/OC1A) PB1

(PCINT2/SS/OC1B) PB2

(PCINT0/CLKO/ICP1) PB0

(PCINT3/OC2A/MOSI) PB3

1.1 Disclaimer

2. Overview

Typical values contained in this datasheet are based on simulations and characterization of other AVR microcontrollers manufactured on the same process technology. Min and Max values will be available after the device is characterized.

The ATmega48/88/168 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 ATmega48/88/168 achieves throughputs approaching 1 MIPS per MHz allowing the system designer to optimize power consumption versus processing speed.

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2.1 Block Diagram

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

USART 0

8bit T/C 2

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

Internal

Bandgap

Analog

Comp.

SPI TWI

SRAMFlash

EEPROM

Watchdog

Oscillator

Watchdog

Timer

Oscillator

Circuits /

Clock

Generation

Power

Supervision

POR / BOD &

RESET

VCC

GND

PROGRAM

LOGIC

debugWIRE

GND

AREF

AVCC

DATA B U S

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

RESET

XTAL[1..2]

CPU

ATmega48/88/168 Automotive

Figure 2-1. Block Diagram

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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.

The ATmega48/88/168 provides the following features: 4K/8K/16K bytes of In-System Programmable Flash with Read-While-Write capabilities, 256/512/512 bytes EEPROM, 512/1K/1K bytes 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 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.

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 non-volatile 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 ATmega48/88/168 is a powerful microcontroller that provides a highly flexible and cost effective solution to many embedded control applications.

The ATmega48/88/168 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 Automotive Quality Grade

The ATmega48-15AZ, ATmega88-15AZ and ATmega168-15AZ have been developed and manufactured according to the most stringent requirements of the international standard ISO-TS-16949 grade 1. This data sheet contains limit values extracted from the results of extensive characterization (Temperature and Voltage). The quality and reliability of the ATmega48-15AZ, ATmega88-15AZ and ATmega168-15AZ have been verified during regular product qualification as per AEC-Q100.

As indicated in the ordering information paragraph (see “Ordering Information” on page 325), the products are available in three different temperature grades, but with equivalent quality and reliability objectives. Different temperature identifiers have been defined as listed in Table 2-1.

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ATmega48/88/168 Automotive

Table 2-1. Temperature Grade Identification for Automotive Products

Temperature

-40 ; +85 T

-40 ; +105 T1 Reduced Automotive Temperature Range

-40 ; +125 Z Full AutomotiveTemperature Range

2.3 Comparison Between ATmega48, ATmega88, and ATmega168

The ATmega48, ATmega88 and ATmega168 differ only in memory sizes, boot loader support, and interrupt vector sizes. Table 2-2 summarizes the different memory and interrupt vector sizes for the three devices.

Table 2-2. Memory Size Summary

Device Flash EEPROM RAM Interrupt Vector Size

ATmega48 4K Bytes 256 Bytes 512 Bytes 1 instruction word/vector

Identifier Comments

Similar to Industrial Temperature Grade but with Automotive Quality

ATmega88 8K Bytes 512 Bytes 1K Bytes 1 instruction word/vector

ATmega168 16K Bytes 512 Bytes 1K Bytes 2 instruction words/vector

ATmega88 and ATmega168 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 ATmega48, there is no Read-While-Write support and no separate Boot Loader Section. The SPM instruction can execute from the entire Flash.

2.4 Pin Descriptions

2.4.1 VCC

Digital supply voltage.

2.4.2 GND

Ground.

2.4.3 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.

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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.

2.4.4 Port C (PC5..0)

2.4.5 PC6/RESET

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.

The various special features of Port B are elaborated in “Alternate Functions of Port B” on page

70 and “System Clock and Clock Options” on page 24.

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.

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 8-1 on page 43. 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

74.

2.4.6 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

77.

2.4.7 AV

AVCC is the supply voltage pin for the A/D Converter, PC3..0, and ADC7..6. It should be externally connected to V to V

through a low-pass filter. Note that PC6..4 use digital supply voltage, VCC.

, even if the ADC is not used. If the ADC is used, it should be connected

2.4.8 AREF

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

2.4.9 ADC7..6 (TQFP and QFN Package Only)

In the TQFP and QFN 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.

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3. About Code Examples

Flash

Program

Memory

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

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.

4. AVR CPU Core

4.1 Introduction

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.

4.2 Architectural Overview

Figure 4-1. Block Diagram of the AVR Architecture

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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 operands 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 ATmega48/88/168 has Extended I/O space from 0x60 - 0xFF in SRAM where only the ST/STS/STD and LD/LDS/LDD instructions can be used.

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4.3 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 bit-functions. 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.

4.4 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.

The AVR Status Register – SREG – is defined as:

Bit 76543210

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W Initial Value00000000

ATmega48/88/168 Automotive

ITHSVNZCSREG

• 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 arithmetics. See the “Instruction Set Description” for detailed information.

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• 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.

4.5 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

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

Figure 4-2 shows the structure of the 32 general purpose working registers in the CPU.

Figure 4-2. AVR CPU General Purpose Working Registers

70Addr.

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 4-2, 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.

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4.5.1 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 4-3.

Figure 4-3. 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 7070

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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4.6 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. The Stack Pointer Register always points to the top of the Stack. Note that the Stack is implemented as growing from higher memory locations to lower memory locations. This implies that a Stack PUSH command decreases the Stack Pointer.

The Stack Pointer points to the data SRAM Stack area where the Subroutine and Interrupt Stacks are located. This Stack space in the data SRAM must be defined by the program before any subroutine calls are executed or interrupts are enabled. The Stack Pointer must be set to point above 0x0100, preferably RAMEND. The Stack Pointer is decremented by one when data is pushed onto the Stack with the PUSH instruction, and it is decremented by two when the return address is pushed onto the Stack with subroutine call or interrupt. The Stack Pointer is incremented by one when data is popped from the Stack with the POP instruction, and it is incremented by two when data is popped from the Stack with return from subroutine RET or return from interrupt RETI.

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.

Bit 151413121110 9 8

SP15 SP14 SP13 SP12 SP11 SP10 SP9 SP8 SPH

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

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4.7 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

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 chip. No internal clock division is used.

Figure 4-4 shows the parallel instruction fetches and instruction executions enabled by the Har-

vard 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 4-4. The Parallel Instruction Fetches and Instruction Executions

Figure 4-5 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

CPU

Figure 4-5. Single Cycle ALU Operation

4.8 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 278 for details.

The lowest addresses in the program memory space are by default defined as the Reset and

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Interrupt Vectors. The complete list of vectors is shown in “Interrupts” on page 52. The list also determines the priority levels of the different interrupts. The lower the address the higher is the

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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 52 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, ATmega88

and ATmega168” on page 263.

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) */

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

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) */

4.8.1 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.

5. AVR ATmega48/88/168 Memories

This section describes the different memories in the ATmega48/88/168. The AVR architecture has two main memory spaces, the Data Memory and the Program Memory space. In addition, the ATmega48/88/168 features an EEPROM Memory for data storage. All three memory spaces are linear and regular.

5.1 In-System Reprogrammable Flash Program Memory

The ATmega48/88/168 contains 4/8/16K bytes 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 2/4/8K x 16. For software security, the Flash Program memory space is divided into two sections, Boot Loader Section and Application Program Section in ATmega88 and ATmega168. ATmega48 does not have separate Boot Loader and Application Program sections, and the SPM instruction can be executed from the entire Flash. See SELFPRGEN description in section

“Store Program Memory Control and Status Register – SPMCSR” on page 258 and page 268for

more details.

The Flash memory has an endurance of at least 75,000 write/erase cycles. The ATmega48/88/168 Program Counter (PC) is 11/12/13 bits wide, thus addressing the 2/4/8K 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, ATmega48” on

page 256 and “Boot Loader Support – Read-While-Write Self-Programming, ATmega88 and ATmega168” on page 263. “Memory Programming” on page 278 contains a detailed description

on Flash Programming in SPI- or Parallel Programming mode.

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ATmega48/88/168 Automotive

0x0000

0x7FF

Program Memory

Application Flash Section

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 Tim-

ing” on page 12.

Figure 5-1. Program Memory Map, ATmega48

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Figure 5-2. Program Memory Map, ATmega88 and ATmega168

Program Memory

0x0000

Application Flash Section

Boot Flash Section

0x0FFF/0x1FFF

5.2 SRAM Data Memory

Figure 5-3 shows how the ATmega48/88/168 SRAM Memory is organized.

The ATmega48/88/168 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 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 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 Z-register.

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 bytes of internal data SRAM in the ATmega48/88/168 are all accessible through all these addressing modes. The Register File is described in “General Purpose Regis-

ter File” on page 10.

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Figure 5-3. Data Memory Map

clk

Data

Address

Address valid

T1 T2 T3

Compute Address

Read

Write

CPU

Memory Access Instruction

Next Instruction

Data Memory

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5.2.1 Data Memory Access Times

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

Figure 5-4. On-chip Data SRAM Access Cycles

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

Internal SRAM

(512/1024/1024 x 8)

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

0x0100

0x02FF/0x04FF/0x04F

cycles as described in Figure 5-4.

CPU

5.3 EEPROM Data Memory

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The ATmega48/88/168 contains 256/512/512 bytes 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 278 contains a detailed description on EEPROM Programming

in SPI or Parallel Programming mode.

5.3.1 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 5-2. 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

is likely to rise or fall slowly on power-up/down. This 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 22 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.

5.3.2 The EEPROM Address Register – EEARH and EEARL

Bit 151413121110 9 8

–––––––EEAR8EEARH

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..9 – Res: Reserved Bits

These bits are reserved bits in the ATmega48/88/168 and will always read as zero.

• Bits 8..0 – EEAR8..0: EEPROM Address

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

EEAR8 is an unused bit in ATmega48 and must always be written to zero.

5.3.3 The EEPROM Data Register – EEDR

Bit 76543210

MSB LSB EEDR

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W Initial Value00000000

• Bits 7..0 – EEDR7.0: 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.

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5.3.4 The EEPROM Control Register – EECR

Bit 76543210

– – 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 – Res: Reserved Bits

These bits are reserved bits in the ATmega48/88/168 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 5-1. 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.

Table 5-1. EEPROM Mode Bits

EEPM1 EEPM0

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

0 1 1.8 ms Erase Only

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Programming

Time Operation

1 0 1.8 ms Write Only

1 1 – Reserved for future use

• 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.

• 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 SELFPRGEN in SPMCSR becomes zero.

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

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

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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, ATmega88 and ATmega168” on page 263 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.

• 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 5-2 lists the typical programming time for EEPROM access from the CPU.

Table 5-2. EEPROM Programming Time

Symbol Number of Calibrated RC Oscillator Cycles Typ Programming Time

EEPROM write (from CPU)

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.

26,368 3.3 ms

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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;

}

5.3.5 Preventing EEPROM Corruption

During periods of low V too low for the CPU 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 be used. If a reset occurs while a write operation is in progress, the write operation will be completed provided that the power supply voltage is sufficient.

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the EEPROM data can be corrupted because the supply voltage is

CC,

reset Protection circuit can

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5.4 I/O Memory

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The I/O space definition of the ATmega48/88/168 is shown in “Register Summary” on page 318.

All ATmega48/88/168 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 ATmega48/88/168 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.

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.

5.4.1 General Purpose I/O Registers

The ATmega48/88/168 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.

5.4.2 General Purpose I/O Register 2 – GPIOR2

Bit 76543210

MSB LSB GPIOR2

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W Initial Value00000000

5.4.3 General Purpose I/O Register 1 – GPIOR1

Bit 76543210

MSB LSB GPIOR1

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W Initial Value00000000

5.4.4 General Purpose I/O Register 0 – GPIOR0

Bit 76543210

MSB LSB GPIOR0

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W Initial Value00000000

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6. System Clock and Clock Options

General I/O

Modules

Asynchronous Timer/Counter

CPU Core RAM

clk

I/O

clk

ASY

AVR Clock

Control Unit

clk

CPU

Flash and EEPROM

clk

FLASH

Source clock

Watchdog Timer

Watchdog

Oscillator

Reset Logic

Clock

Multiplexer

Watchdog clock

Calibrated RC

Oscillator

Timer/Counter

Oscillator

Crystal

Oscillator

Low-frequency

Crystal Oscillator

External Clock

ADC

clk

ADC

System Clock

Prescaler

6.1 Clock Systems and their Distribution

Figure 6-1 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 Manage-

ment and Sleep Modes” on page 36. The clock systems are detailed below.

Figure 6-1. Clock Distribution

6.1.1 CPU Clock – clk

6.1.2 I/O Clock – clk

6.1.3 Flash Clock – clk

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CPU

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.

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 some external interrupts are detected by asynchronous logic, allowing such interrupts to be detected even if the I/O clock is halted. Also note that start condition detection in the USI module is carried out asynchronously when clk

FLASH

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

is halted, TWI address recognition in all sleep modes.

I/O

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6.1.4 Asynchronous Timer Clock – clk

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

6.1.5 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.

6.2 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 6-1. 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 128 kHz RC Oscillator 0011

ASY

(1)

Calibrated Internal RC Oscillator 0010

External Clock 0000

Reserved 0001

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

6.2.1 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.

6.2.2 Clock Startup Sequence

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

To ensure sufficient V the device reset is released by all other reset sources. “System Control and Reset” on page 41 describes the start conditions for the internal reset. The delay (t Oscillator and the number of cycles in the delay is set by the SUTx and CKSELx fuse bits. Th e selectable delays are shown in Table 6-2. The frequency of the Watchdog Oscillator is voltage dependent as shown in “Register Summary” on page 318.

to start oscillating and a minimum number of oscillating

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

) is timed from the Watchdog

TOUT

) after

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Table 6-2. Number of Watchdog Oscillator Cycles

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

0 ms 0 ms 0

4.1 ms 4.3 ms 4K (4,096)

65 ms 69 ms 8K (8,192)

Main purpose of the delay is to keep the AVR in reset until it is supplied with minimum VCC. The delay will not monitor the actual voltage and it will be required to select a delay longer than the V

rise time. If this is not possible, an internal or external Brown-Out Detection circuit should be

used. A BOD circuit will ensure sufficient V

before it releases the reset, and the time-out delay

can be disabled. Disabling the time-out delay without utilizing a Brown-Out 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

assumed to be at a sufficient level and only the start-up time is included.

6.3 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 6-2. 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 28.

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 6-3. For ceramic resonators, the capacitor values given by the manufacturer should be used.

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XTAL2

XTAL1

GND

Figure 6-2. 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 6-3

on page 27.

Table 6-3. Low Power Crystal Oscillator Operating Modes

Recommended Range for Capacitors C1

Frequency Range

0.4 - 0.9 100

(1)

(MHz) CKSEL3..1

(2)

(3)

and C2 (pF)

–

0.9 - 3.0 101 12 - 22

3.0 - 8.0 110 12 - 22

8.0 - 16.0 111 12 - 22

Notes: 1. The frequency ranges are preliminary values. Actual values are TBD.

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

3. If 8 MHz frequency exceeds the specification of the device (depends on V

), 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

6-4.

Table 6-4. 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

Start-up Time from

Power-down and

Power-save

258 CK 14CK + 4.1 ms

258 CK 14CK + 65 ms

1K CK 14CK

Additional Delay

from Reset

(VCC = 5.0V) CKSEL0 SUT1..0

(1)

(2)

000

001

010

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Ceramic resonator, fast rising power

Ceramic resonator, slowly rising power

1K CK 14CK + 4.1 ms

1K CK 14CK + 65 ms

(2)

011

100

Table 6-4. Start-up Times for the Low Power Crystal Oscillator Clock Selection (Continued)

Oscillator Source / Power Conditions

Crystal Oscillator, BOD enabled

Crystal Oscillator, fast rising power

Crystal Oscillator, slowly rising power

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.

6.4 Full Swing Crystal Oscillator

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 26. 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 6-6. For ceramic resonators, the capacitor values given by the manufacturer should be used.

Start-up Time from

Power-down and

Power-save

16K CK 14CK 1 01

16K CK 14CK + 4.1 ms 1 10

16K CK 14CK + 65 ms 1 11

= 2.7 - 5.5 volts.

Additional Delay

from Reset

(VCC = 5.0V) CKSEL0 SUT1..0

The operating mode is selected by the fuses CKSEL3..1 as shown in Table 6-5.

Table 6-5. Full Swing Crystal Oscillator operating modes

Frequency Range

Notes: 1. The frequency ranges are preliminary values. Actual values are TBD.

2. If 8 MHz 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.

(1)

(MHz) CKSEL3..1

0.4 - 20 011 12 - 22

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(2)

Recommended Range for Capacitors C1

and C2 (pF)

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XTAL2

XTAL1

GND

Figure 6-3. Crystal Oscillator Connections

Table 6-6. Start-up Times for the Full Swing 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 + 4.1 ms

258 CK 14CK + 65 ms

1K CK 14CK

1K CK 14CK + 4.1 ms

1K CK 14CK + 65 ms

16K CK 14CK 1 01

16K CK 14CK + 4.1 ms 1 10

16K CK 14CK + 65 ms 1 11

Additional Delay

from Reset

= 5.0V) CKSEL0 SUT1..0

(1)

(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.

7530H–AVR–02/09

6.5 Low Frequency Crystal Oscillator

The device can utilize a 32.768 kHz watch crystal as clock source by a dedicated Low Frequency Crystal Oscillator. The crystal should be connected as shown in Figure 6-2. When this Oscillator is selected, start-up times are determined by the SUT Fuses and CKSEL0 as shown in

Table 6-7.

Table 6-7. Start-up Times for the Low Frequency Crystal Oscillator Clock Selection

Power Conditions

BOD enabled 1K CK 14CK

Fast rising power 1K CK 14CK + 4.1 ms

Slowly rising power 1K CK 14CK + 65 ms

BOD enabled 32K CK 14CK 1 00

Fast rising power 32K CK 14CK + 4.1 ms 1 01

Slowly rising power 32K CK 14CK + 65 ms 1 10

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

application.

6.6 Calibrated Internal RC Oscillator

The calibrated internal RC Oscillator by default provides a 8.0 MHz clock. The frequency is nominal value at 3V and 25°C. 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 6-8. If selected, it will operate with no external components. During reset, hardware loads the calibration byte into the OSCCAL Register and thereby automatically calibrates the RC Oscillator. At 3V and 25°C, this calibration gives a frequency of 8 MHz ± 1%. The tolerance of the internal RC oscillator remains better than ±10% within the whole automotive temperature and voltage ranges (2.7V to 5.5V, -40°C to +125°C). The oscillator can be calibrated to any frequency in the range 7.3 - 8.1 MHz within ±1% accuracy, by changing the OSCCAL register. 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 “Calibration Byte” on

page 282.

Start-up Time from

Power-down and

Power-save

Reserved 0 11

Reserved 1 11

Additional Delay

from Reset

(VCC = 5.0V) CKSEL0 SUT1..0

(1)

000

001

010

Table 6-8. Internal Calibrated RC Oscillator Operating Modes

Frequency Range

7.3 - 8.1 0010

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

2. The frequency ranges are preliminary values. Actual values are TBD.

3. If 8 MHz frequency exceeds the specification of the device (depends on V Fuse can be programmed in order to divide the internal frequency by 8.

(2)

(MHz) CKSEL3..0

ATmega48/88/168 Automotive

(1)(3)

), the CKDIV8

7530H–AVR–02/09

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