Features
ATmega328P
8-bit AVR Microcontroller with 32K Bytes In-System
Programmable Flash
DATASHEET
● High performance, low power AVR
®
8-bit microcontroller
● Advanced RISC architecture
● 131 powerful instructions – most single clock cycle execution
● 32 8 general purpose working registers
● Fully static operation
● Up to 16MIPS throughput at 16MHz
● On-chip 2-cycle multiplier
● High endurance non-volatile memory segments
● 32K bytes of in-system self-programmable flash program memory
● 1Kbytes EEPROM
● 2Kbytes internal SRAM
● Write/erase cycles: 10,000 flash/100,000 EEPROM
● 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
● 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
● Programmable serial USART
● Master/slave SPI serial interface
● Byte-oriented 2-wire serial interface (Phillips I
● Programmable watchdog timer with separate on-chip oscillator
● On-chip analog comparator
● Interrupt and wake-up on pin change
2
C compatible)
● 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
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I/O and packages
●
● 23 programmable I/O lines
● 32-lead TQFP, and 32-pad QFN/MLF
● Operating voltage:
● 2.7V to 5.5V for ATmega328P
● Temperature range:
● Automotive temperature range: –40°C to +125°C
● Speed grade:
● 0 to 8MHz at 2.7 to 5.5V (automotive temperature range: –40°C to +125°C)
● 0 to 16MHz at 4.5 to 5.5V (automotive temperature range: –40°C to +125°C)
● Low power consumption
● Active mode: 1.5mA at 3V - 4MHz
● Power-down mode: 1µA at 3V
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1. Pin Configurations
(PCINT19/OC2B/INT1) PD3
(PCINT20/XCK/T0) PD4
(PCINT6/XTAL1/TOSC1) PB6
(PCINT7/XTAL2/TOSC2) PB7
GND
VCC
GND
VCC 6
7
8
5
4
3
2
1
32 31 30 29 28 27 26 25
9 10111213141516
19
18
17
20
21
22
23
24 PC1 (ADC1/PCINT9)
(PCINT21/OC0B/T1) PD5
(PCINT0/CLKO/ICP1) PB0
(PCINT23/AIN1) PD7
(PCINT1/OC1A) PB1
(PCINT2/SS/OC1B) PB2
(PCINT3/OC2A/MOSI) PB3
(PCINT4/MISO) PB4
(PCINT22/OC0A/AIN0) PD6
PC0 (ADC0/PCINT8)
AVC C
PB5 (SCK/PCINT5)
ADC7
GND
AREF
ADC6
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)
(PCINT19/OC2B/INT1) PD3
(PCINT20/XCK/T0) PD4
(PCINT6/XTAL1/TOSC1) PB6
(PCINT7/XTAL2/TOSC2) PB7
GND
VCC
GND
VCC
NOTE: Bottom pad should be soldered to ground.
6
7
8
5
4
3
2
1
32 31 30 29
32 MLF Top View
28 27 26 25
9 10111213141516
19
18
17
20
21
22
23
24 PC1 (ADC1/PCINT9)
(PCINT21/OC0B/T1) PD5
(PCINT0/CLKO/ICP1) PB0
(PCINT23/AIN1) PD7
(PCINT1/OC1A) PB1
(PCINT2/SS/OC1B) PB2
(PCINT3/OC2A/MOSI) PB3
(PCINT4/MISO) PB4
PC0 (ADC0/PCINT8)
AVC C
PB5 (SCK/PCINT5)
ADC7
GND
AREF
ADC6
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)
TQFP Top View
Figure 1-1. Pinout
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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.
The various special features of port B are elaborated in Section 13.3.1 “Alternate Functions of Port B” on page 65 and
Section 8. “System Clock and Clock Options” on page 24.
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 input pin. 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 28-4 on page 261. Shorter pulses are not guaranteed to generate a
reset.
The various special features of port C are elaborated in Section 13.3.2 “Alternate Functions of Port C” on page 68.
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 Section 13.3.3 “Alternate Functions of Port D” on page 70.
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
supply voltage, V
CC
through a low-pass filter. Note that PC6..4 use digital
.
CC
1.1.8 AREF
AREF is the analog reference pin for the A/D converter.
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1.1.9 ADC7:6 (TQFP and QFN/MLF Package Onl y)
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.
1.2 Disclaimer
Typical values contained in this datasheet are based on simulations and characterization of actual ATmega328P AVR®
microcontrollers manufactured on the typical process technology. automotive min and max values are based on
characterization of actual ATmega328P AVR microcontrollers manufactured on the whole process excursion (corner run).
1.3 Automotive Quality Grade
The ATmega328P have been developed and manufactured according to the most stringent requirements of the international
standard ISO-TS-16949. This data sheet contains limit values extracted from the results of extensive characterization
(temperature and voltage). The quality and reliability of the ATmega328P have been verified during regular product
qualification as per AEC-Q100 grade 1. As indicated in the ordering information paragraph, the products are available in only
one temperature.
Table 1-1. Temperature Grade Identification for Automotive Products
T emperature Temperature Identifier Comments
–40°C; +125°C Z Full automotive temperature range
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2. Overview
The Atmel® ATmega328P 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 ATmega328P achieves throughputs approaching 1MIPS per MHz
allowing the system designer to optimize power consumption versus processing speed.
2.1 Block Diagram
Figure 2-1. Block Diagram
GND VCC
DATA BUS
Watchdog
Timer
Watchdog
Oscillator
Oscillator
Circuits/
Clock
Generation
EEPROM
8-bit T/C 2
Power
Supervision
POR/ BOD
and
RESET
Flash
AVR
Analog
Comp.
debugWIRE
CPU
Program
Logic
SRAM
A/D Conv.16-bit T/C 18-bit T/C 0
Internal
Bandgap
AVC C
AREF
GND
2
6
USART 0 SPI TWI
PORT D (8) PORT B (8) PORT C (7)
RESET
XTAL[1..2]
PD[0..7] PB[0..7] PC[0..6] ADC[6..7]
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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 Atmel
®
ATmega328P provides the following features: 32K bytes of in-system programmable flash with read-while-write
capabilities, 1K bytes EEPROM, 2K 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 byteoriented 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.
The device is manufactured using Atmel 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 ATmega328P is a powerful microcontroller that provides
a highly flexible and cost effective solution to many embedded control applications.
The ATmega328P 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.
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3. Resources
A comprehensive set of development tools, application notes and datasheets are available for download on
http://www.atmel.com/avr.
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”.
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6. AVR CPU Core
Status and
Control
Interrupt
Unit
32 x 8
General
Purpose
Registers
ALU
Data Bus 8-bit
Data
SRAM
SPI
Unit
Instruction
Register
Instruction
Decoder
Watchdog
Timer
Analog
Comparator
EEPROM
I/O Lines
I/O Module n
Control Lines
Direct Addressing
Indirect Addressing
I/O Module 2
I/O Module 1
Program
Counter
Flash
Program
Memory
6.1 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 6-1. 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.
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The fast-access register file contains 32 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
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 ATmega328P has extended I/O space from 0x60 - 0xFF in SRAM where only the ST/STS/STD and
LD/LDS/LDD instructions can be used.
®
instructions have a single 16-bit word format. Every program memory address contains a 16- or
6.2 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 bit-functions. Some
implementations of the architecture also provide a powerful multiplier supporting both signed/unsigned multiplication and
fractional format. See Section “” on page 281 for a detailed description.
6.3 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.
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6.3.1 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 Value00000000
• 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 Section
“” on page 281 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 Section “”
on page 281 for detailed information.
• Bit 3 – V: Two’s Complement Overflow Flag
The two’s complement overflow flag V supports two’s complement arithmetics. See Section “” on page 281 for detailed
information.
• Bit 2 – N: Negative Flag
The negative flag N indicates a negative result in an arithmetic or logic operation. See Section “” on page 281 for detailed
information.
• Bit 1 – Z: Zero Flag
The zero flag Z indicates a zero result in an arithmetic or logic operation. See Section “” on page 281 for detailed
information.
• Bit 0 – C: Carry Flag
The carry flag C indicates a carry in an arithmetic or logic operation. See Section “” on page 281 for detailed information.
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6.4 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
● One 16-bit output operand and one 16-bit result input
Figure 6-2 shows the structure of the 32 general purpose working registers in the CPU.
Figure 6-2. AVR CPU General Purpose Working Registers
General R14 0x0E
Purpose R15 0x0F
Working R16 0x10
Registers R17 0x11
7 0 Addr.
R0 0x00
R1 0x01
R2 0x02
…
R13 0x0D
…
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 6-2, 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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6.4.1 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 6-3.
Figure 6-3. The X-, Y-, and Z-registers
15 XH XL 0
X-register 7 0 7 0
R27 (0x1B) R26 (0x1A)
15 YH YL 0
Y-register 7 0 7 0
R29 (0x1D) R28 (0x1C)
15 ZH ZL 0
Z-register 7 0 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).
6.5 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 Figure 7-2 on page 18.
See Table 6-1 for stack pointer details.
Table 6-1. Stack Pointer instructions
Instruction Stack pointer Description
PUSH Decremented by 1 Data is pushed onto the stack
CALL
ICALL
Decremented by 2 Return address is pushed onto the stack with a subroutine call or interrupt
RCALL
POP Incremented by 1 Data is popped from the stack
RET
RETI
Incremented by 2
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
Return address is popped from the stack with return from subroutine or return
from interrupt
®
architecture is so small that only
SPL is needed. In this case, the SPH register will not be present.
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6.5.1 SPH and SPL – Stack Pointer High and Stack Point er Low Register
clk
CPU
1st Instruction Fetch
1st Instruction Execute
2nd Instruction Fetch
T1 T2 T3 T4
2nd Instruction Execute
3rd Instruction Fetch
3rd Instruction Execute
4th Instruction Fetch
clk
CPU
T1
Register Operands Fetch
Result Write Back
ALU Operation Execute
Total Execution Time
T2 T3 T4
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/WR/WR/WR/WR/WR/WR/WR/W
Initial Value RAMEND RAMEND RAMEND RAMEND RAMEND RAMEND RAMEND RAMEND
RAMEND RAMEND RAMEND RAMEND RAMEND RAMEND RAMEND RAMEND
6.6 Instruction Execution Timing
This section describes the general access timing concepts for instruction execution. The AVR® CPU is driven by the CPU
clock clk
Figure 6-4 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 1MIPS per MHz with the corresponding
unique results for functions per cost, functions per clocks, and functions per power-unit.
Figure 6-4. The Parallel Instruction Fetches and Instruction Executions
, directly generated from the selected clock source for the chip. No internal clock division is used.
CPU
Figure 6-5 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.
Figure 6-5. Single Cycle ALU Operation
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6.7 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 27. “Memory Programming” on page 241 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 Section 11. “Interrupts” on page 49. 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 Section 11. “Interrupts” on page 49 for more information. The reset vector can
also be moved to the start of the boot flash section by programming the BOOTRST fuse, see Section 26. “Boot Loader
Support – Read-While-Write Self-Programming” on page 229.
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.
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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.
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) */
6.7.1 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.
16
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7. AVR Memories
0x0000
0x3FFF
Boot Flash Section
Program Memory
Application Flash Section
7.1 Overview
This section describes the different memories in the ATmega328P. The AVR® architecture has two main memory spaces,
the data memory and the program memory space. In addition, the ATmega328P features an EEPROM memory for data
storage. All three memory spaces are linear and regular.
7.2 In-System Reprogrammable Flash Program Memory
The ATmega328P contains 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 16K 16. For software security, the flash program memory space
is divided into two sections, boot loader section and application program section in ATmega328P. See SELFPRGEN
description in Section 25.3.1 “SPMCSR – Store Program Memory Control and Status Register” on page 228 and Section
26.9.1 “SPMCSR – Store Program Memory Control and Status Register” on page 239 for more details.
The flash memory has an endurance of at least 10,000 write/erase cycles. The ATmega328P program counter (PC) is 14
bits wide, thus addressing the 16K program memory locations. The operation of boot program section and associated boot
lock bits for software protection are described in detail in Section 25. “Self-Programming the Flash, ATmega328P” on page
223 and Section 26. “Boot Loader Support – Read-While-Write Self-Programming” on page 229. Section 27. “Memory
Programming” on page 241 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 Section 6.6 “Instruction Execution Timing” on page 14.
Figure 7-1. Program Memory Map ATmega328P
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7.3 SRAM Data Memory
32 Registers
Data Memory
0x0000 - 0x001F
0x0020 - 0x005F
0x0060 - 0x00FF
0x0100
0x08FF
64 I/O Registers
160 Ext I/O Registers
Internal SRAM
(1048 x 8)
A
Figure 7-2 shows how the ATmega328P SRAM memory is organized.
The ATmega328P 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 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 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 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 2048 bytes of internal data
SRAM in the ATmega328P are all accessible through all these addressing modes. The register file is described in
Section 6.4 “General Purpose Register File” on page 12.
Figure 7-2. Data Memory Map
7.3.1 Data Memory Access Times
18
This section describes the general access timing concepts for internal memory access. The internal data SRAM access is
performed in two clk
cycles as described in Figure 7-3.
CPU
Figure 7-3. On-chip Data SRAM Access Cycles
clk
CPU
ddress
Data
WR
Data
RD
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T1
Memory Access Instruction
T2 T3
Address validCompute Address
Write
Read
Next Instruction
7.4 EEPROM Data Memory
The Atmel® ATmega328P contains 1Kbyte 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.
Section 27. “Memory Programming” on page 241 contains a detailed description on EEPROM programming in SPI or
parallel programming mode.
7.4.1 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 7-2 on page 22. 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 Section 7.4.2 “Preventing EEPROM Corruption” on page 19 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.
7.4.2 Preventing EEPROM Corruption
is likely to rise or fall slowly on power-up/down. This
CC
During periods of low V
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 will be completed provided that the power supply voltage is sufficient.
7.5 I/O Memory
The I/O space definition of the ATmega328P is shown in Section “” on page 275.
All ATmega328P 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
ATmega328P 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 AVR
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.
the EEPROM data can be corrupted because the supply voltage is too low for the CPU and the
CC,
reset protection circuit can be used. If a reset occurs while a write operation is in progress, the write
CC
®
, the CBI and SBI
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7.5.1 General Purpose I/O Registers
The Atmel® ATmega328P 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.
7.6 Register Description
7.6.1 EEARH and EEARL – The EEPROM Address Register
Bit 151413121110 9 8
0x22 (0x42) –––––––EEAR8EEARH
0x21 (0x41) EEAR7 EEAR6 EEAR5 EEAR4 EEAR3 EEAR2 EEAR1 EEAR0 EEARL
76543210
Read/Write R R R R R R R R/W
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 X
XXXXXXXX
• Bits 15..9 – Res: Reserved Bits
These bits are reserved bits in the Atmel ATmega328P 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 256/512/512/1K bytes
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.
EEAR8 is an unused bit in ATmega328P and must always be written to zero.
7.6.2 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
• 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.
7.6.3 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 – Res: Reserved Bits
These bits are reserved bits in the Atmel ATmega328P 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 7-1. 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 7-1. EEPROM Mode Bits
EEPM1 EEPM0 Programming Time Operation
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
• 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 SELFPRGEN 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
Section 26. “Boot Loader Support – Read-While-Write Self-Programming” on page 229 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.
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The calibrated oscillator is used to time the EEPROM accesses. Table 7-2 lists the typical programming time for EEPROM
access from the CPU.
Table 7-2. EEPROM Programming Time
Symbol Number of Calibrated RC Oscillator Cycles Typical 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.
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;
}
7.6.4 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
7.6.5 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
7.6.6 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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8. System Clock and Clock Options
8.1 Clock Systems and their Distribution
Figure 8-1 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 Section 9. “Power Management and Sleep Modes” on page 34. The clock systems are detailed
below.
Figure 8-1. Clock Distribution
Asynchronous
Timer/Counter
Timer/Counter
Oscillator
General I/O
Modules
clk
I/O
clk
ASY
External Clock
ADC
clk
ADC
AVR Clock
Control Unit
System Clock
Prescaler
Source clock Watchdog clock
Clock
Multiplexer
Oscillator
Crystal
CPU Core
clk
CPU
clk
FLASH
Reset Logic Watchdog Timer
RAM
Watchdog
Oscillator
Low-frequency
Crystal Oscillator
Flash and
EEPROM
Calibrated RC
Oscillator
8.1.1 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.
8.1.2 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 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
8.1.3 Flash Clock – clk
The flash clock controls operation of the flash interface. The flash clock is usually active simultaneously with the CPU clock.
24
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CPU
FLASH
is halted, TWI address recognition in all sleep modes.
I/O
8.1.4 Asynchronous Ti mer 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 real-time counter even when
the device is in sleep mode.
ASY
8.1.5 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.
8.2 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
Table 8-1. 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.
8.2.1 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.
®
clock generator, and routed to the appropriate modules.
(1)
8.2.2 Clock Startup Sequence
Any clock source needs a sufficient VCC to start oscillating and a minimum number of oscillating cycles before it can be
considered stable.
To ensure sufficient V
all other reset sources. Section 10. “System Control and Reset” on page 40 describes the start conditions for the internal
reset. The delay (t
CKSELx fuse bits. The selectable delays are shown in Table 8-2. The frequency of the watchdog oscillator is voltage
dependent as shown in Section 29. “Typical Characteristics” on page 268.
Table 8-2. Number of Watchdog Oscillator Cycles
Typ Time-out (VCC = 5.0V) Typ Time-out (VCC = 3.0V) Number of Cycles
Main purpose of the delay is to keep the AVR in reset until it is supplied with minimum V
actual voltage and it will be required to select a delay longer than the V
external brown-out detection circuit should be used. A BOD circuit will ensure sufficient V
the time-out delay can be disabled. Disabling the time-out delay without utilizing a brown-out detection circuit is not
recommended.
, the device issues an internal reset with a time-out delay (t
CC
) is timed from the watchdog oscillator and the number of cycles in the delay is set by the SUTx and
TOUT
0ms 0ms 0
4.1ms 4.3ms 512
65ms 69ms 8K (8,192)
) after the device reset is released by
TOUT
. The delay will not monitor the
rise time. If this is not possible, an internal or
CC
CC
before it releases the reset, and
CC
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The oscillator is required to oscillate for a minimum number of cycles before the clock is considered stable. An internal ripple
C2
XTAL2 (TOSC2)
XTAL1 (TOSC1)
GND
C1
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
start-up time is included.
8.3 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 8-2. 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 Section 8.4 “Full Swing Crystal Oscillator” on page 27.
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 8-3. For ceramic resonators, the capacitor values
given by the manufacturer should be used.
Figure 8-2. Crystal Oscillator Connections
is assumed to be at a sufficient level and only the
CC
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 8-3.
Table 8-3. Low Power Crystal Oscillator Operating Modes
Frequency Range
(MHz)
Recommended Range for
Capacitors C1 and C2 (pF)
0.4 to 0.9 – 100
(2)
CKSEL3..1
(1)
0.9 to 3.0 12 to 22 101
3.0 to 8.0 12 to 22 110
8.0 to 16.0 12 to 22 111
Notes: 1. .This option should not be used with crystals, only with ceramic resonators.
2. If 8MHz frequency exceeds the specification of the device (depends on V
), the CKDIV8 fuse can be
CC
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.
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The CKSEL0 fuse together with the SUT1..0 fuses select the start-up times as shown in Table 8-4.
Table 8-4. 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
Start-up Time from Power-
down and Power-save
258CK 14CK + 4.1ms
258CK 14CK + 65ms
1KCK 14CK
1KCK 14CK + 4.1ms
1KCK 14CK + 65ms
Additional Delay from
Reset (V
= 5.0V) CKSEL0 SUT1..0
CC
(1)
(1)
(2)
(2)
(2)
0 00
0 01
0 10
0 11
1 00
Crystal oscillator, BOD enabled 16KCK 14CK 1 01
Crystal oscillator, fast rising
power
Crystal oscillator, slowly rising
power
16KCK 14CK + 4.1ms 1 10
16KCK 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.
8.4 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 8-2 on page 26. 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 Section 8.3 “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 8-6 on page 28. For ceramic resonators, the
capacitor values given by the manufacturer should be used.
The operating mode is selected by the fuses CKSEL3..1 as shown in Table 8-5.
Table 8-5. Full Swing Crystal Oscillator operating modes
Frequency Range
0.4 - 16 12 - 22 011
Notes: 1. The frequency ranges are preliminary values. Actual values are TBD.
2. If 8MHz frequency exceeds the specification of the device (depends on V
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)
= 2.7 to 5.5V.
CC
(2)
Recommended Range for
Capacitors C1 and C2 (pF) CKSEL3..1
), the CKDIV8 fuse can be
CC
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Figure 8-3. Crystal Oscillator Connections
C2
XTAL2 (TOSC2)
C1
XTAL1 (TOSC1)
GND
Table 8-6. 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
Start-up Time from Power-
down and Power-save
258CK 14CK + 4.1ms
258CK 14CK + 65ms
1KCK 14CK
1KCK 14CK + 4.1ms
1KCK 14CK + 65ms
Additional Delay from
Reset (VCC = 5.0V) CKSEL0 SUT1..0
(1)
(1)
(2)
(2)
(2)
0 00
0 01
0 10
0 11
1 00
Crystal oscillator, BOD enabled 16KCK 14CK 1 01
Crystal oscillator, fast rising
power
Crystal oscillator, slowly rising
power
16KCK 14CK + 4.1ms 1 10
16KCK 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.
28
ATmega328P [DATASHEET]
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8.5 Low Frequency Crystal Oscillator
C2CL 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. ATmega328P oscillator is optimized for very low power consumption, and thus when selecting crystals,
see Table 8-7 for maximum ESR recommendations on 6.5pF, 9.0pF and 12.5pF crystals
Table 8-7. Maximum ESR Recommendation for 32.768 kHz 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 of typical 6pF at each TOSC pin. The external
capacitance (C) needed at each TOSC pin can be calculated by using:
(1)
where CL is the load capacitance for a 32.768kHz crystal specified by the crystal vendor and C
is the total stray
S
capacitance for one TOSC pin.
Crystals specifying load capacitance (CL) higher than 6pF, require external capacitors applied as described in
Figure 8-2 on page 26.
The low-frequency crystal oscillator must be selected by setting the CKSEL fuses to “0110” or “0111”, as shown in Ta b le 8 - 9 .
Start-up times are determined by the SUT fuses as shown in Table 8-8.
Table 8-8. Start-up Times for the Low-frequency Crystal Oscillator Clock Selection
SUT1..0 Additional Delay from Reset (VCC = 5.0V) Recommended Usage
00 4CK Fast rising power or BOD enabled
01 4CK + 4.1ms Slowly rising power
10 4CK + 65ms Stable frequency at start-up
11 Reserved
Table 8-9. Start-up Times for the Low-frequency Crystal Oscillator Clock Selection
Start-up Time from
CKSEL3..0
(1)
0100
Power-down and Power-save
1KCK
Recommended Usage
0101 32KCK 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
8.6 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 28-1 on page 260 for more details. The device is shipped
with the CKDIV8 fuse programmed. See Section 8.11 “System Clock Prescaler” on page 32 for more details.
This clock may be selected as the system clock by programming the CKSEL fuses as shown in Table 8-10 on page 30. 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 28-1 on page 260.
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By changing the OSCCAL register from SW, see Section 8.12.1 “OSCCAL – Oscillator Calibration Register” on page 32, it is
possible to get a higher calibration accuracy than by using the factory calibration. The accuracy of this calibration is shown
as User calibration in Table 28-1 on page 260.
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 Section 27.4 “Calibration Byte” on page
244.
Table 8-10. Internal Calibrated RC Oscillator Operating Modes
Nominal Frequency (MHz) CKSEL3..0
8 0010
Notes: 1. The device is shipped with this option selected.
2. If 8MHz frequency exceeds the specification of the device (depends on V
programmed in order to divide the internal frequency by 8.
When this oscillator is selected, start-up times are determined by the SUT fuses as shown in Table 8-11.
Table 8-11. Start-up Times for the Internal calibrated RC Oscillator Clock Selection
Power Conditions
BOD enabled 6CK 14CK
Fast rising power 6CK 14CK + 4.1ms 01
Slowly rising power 6CK 14CK + 65ms
Notes: 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.
8.7 128 kHz 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 8-12.
.
Table 8-12. 128kHz Internal Oscillator Operating Modes
(1)(2)
Start-up Time from Power-down and
Power-save
Reserved 11
), the CKDIV8 fuse can be
CC
Additional Delay from Reset
(VCC = 5.0V) SUT1..0
(1)
(2)
00
10
30
Nominal Frequency CKSEL3..0
128kHz 0011
When this clock source is selected, start-up times are determined by the SUT fuses as shown in Table 8-13.
Table 8-13. Start-up Times for the 128 kHz Internal Oscillator
Start-up Time from Power-down and
Power Conditions
BOD enabled 6CK 14CK
Power-save Additional Delay from Reset SUT1..0
(1)
Fast rising power 6CK 14CK + 4ms 01
Slowly rising power 6CK 14CK + 64ms 10
Reserved 11
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.
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