Atmel Electronic Components Datasheet



ATMEGA1280

8-BIT Microcontroller


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Atmel ATmega640/V-1280/V-1281/V-2560/V-2561/V
8-bit Atmel Microcontroller with 16/32/64KB In-System Programmable Flash
Features
High Performance, Low Power Atmel® AVR® 8-Bit Microcontroller
Advanced RISC Architecture
– 135 Powerful Instructions – Most Single Clock Cycle Execution
– 32 × 8 General Purpose Working Registers
– Fully Static Operation
– Up to 16 MIPS Throughput at 16MHz
– On-Chip 2-cycle Multiplier
High Endurance Non-volatile Memory Segments
– 64K/128K/256KBytes of In-System Self-Programmable Flash
– 4Kbytes EEPROM
– 8Kbytes Internal SRAM
– Write/Erase Cycles:10,000 Flash/100,000 EEPROM
– Data retention: 20 years at 85C/ 100 years at 25C
– 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
• Endurance: Up to 64Kbytes Optional External Memory Space
Atmel® QTouch® library support
– Capacitive touch buttons, sliders and wheels
– QTouch and QMatrix acquisition
– Up to 64 sense channels
JTAG (IEEE® std. 1149.1 compliant) Interface
– Boundary-scan Capabilities According to the JTAG Standard
– Extensive On-chip Debug Support
– Programming of Flash, EEPROM, Fuses, and Lock Bits through the JTAG Interface
Peripheral Features
– Two 8-bit Timer/Counters with Separate Prescaler and Compare Mode
– Four 16-bit Timer/Counter with Separate Prescaler, Compare- and Capture Mode
– Real Time Counter with Separate Oscillator
– Four 8-bit PWM Channels
– Six/Twelve PWM Channels with Programmable Resolution from 2 to 16 Bits
(ATmega1281/2561, ATmega640/1280/2560)
– Output Compare Modulator
– 8/16-channel, 10-bit ADC (ATmega1281/2561, ATmega640/1280/2560)
– Two/Four Programmable Serial USART (ATmega1281/2561, ATmega640/1280/2560)
– 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
– Six Sleep Modes: Idle, ADC Noise Reduction, Power-save, Power-down, Standby,
and Extended Standby
I/O and Packages
– 54/86 Programmable I/O Lines (ATmega1281/2561, ATmega640/1280/2560)
– 64-pad QFN/MLF, 64-lead TQFP (ATmega1281/2561)
– 100-lead TQFP, 100-ball CBGA (ATmega640/1280/2560)
– RoHS/Fully Green
Temperature Range:
– -40C to 85C Industrial
Ultra-Low Power Consumption
– Active Mode: 1MHz, 1.8V: 500µA
– Power-down Mode: 0.1µA at 1.8V
Speed Grade:
– ATmega640V/ATmega1280V/ATmega1281V:
• 0 - 4MHz @ 1.8V - 5.5V, 0 - 8MHz @ 2.7V - 5.5V
– ATmega2560V/ATmega2561V:
• 0 - 2MHz @ 1.8V - 5.5V, 0 - 8MHz @ 2.7V - 5.5V
– ATmega640/ATmega1280/ATmega1281:
• 0 - 8MHz @ 2.7V - 5.5V, 0 - 16MHz @ 4.5V - 5.5V
– ATmega2560/ATmega2561:
• 0 - 16MHz @ 4.5V - 5.5V
DATASHEET
2549Q–AVR–02/2014


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1. Pin Configurations
Figure 1-1. TQFP-pinout ATmega640/1280/2560
100 99 98 97 96 95 94 93 92 91 90 89 88 87 86 85 84 83 82 81 80 79 78 77 76
(OC0B) PG5 1
(RXD0/PCINT8) PE0 2
(TXD0) PE1 3
(XCK0/AIN0) PE2 4
(OC3A/AIN1) PE3 5
(OC3B/INT4) PE4 6
(OC3C/INT5) PE5 7
(T3/INT6) PE6 8
(CLKO/ICP3/INT7) PE7 9
VCC 10
GND 11
(RXD2) PH0 12
(TXD2) PH1 13
(XCK2) PH2 14
(OC4A) PH3 15
(OC4B) PH4 16
(OC4C) PH5 17
(OC2B) PH6 18
(SS/PCINT0) PB0 19
(SCK/PCINT1) PB1 20
(MOSI/PCINT2) PB2 21
(MISO/PCINT3) PB3 22
(OC2A/PCINT4) PB4 23
(OC1A/PCINT5) PB5 24
(OC1B/PCINT6) PB6 25
INDEX CORNER
75 PA3 (AD3)
74 PA4 (AD4)
73 PA5 (AD5)
72 PA6 (AD6)
71 PA7 (AD7)
70 PG2 (ALE)
69 PJ6 (PCINT15)
68 PJ5 (PCINT14)
67 PJ4 (PCINT13)
66 PJ3 (PCINT12)
65 PJ2 (XCK3/PCINT11)
64 PJ1 (TXD3/PCINT10)
63 PJ0 (RXD3/PCINT9)
62 GND
61 VCC
60 PC7 (A15)
59 PC6 (A14)
58 PC5 (A13)
57 PC4 (A12)
56 PC3 (A11)
55 PC2 (A10)
54 PC1 (A9)
53 PC0 (A8)
52 PG1 (RD)
51 PG0 (WR)
26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50
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Figure 1-2. CBGA-pinout ATmega640/1280/2560
Top view
1 2 3 4 5 6 7 8 9 10
A
B
C
D
E
F
G
H
J
K
Bottom view
10 9 8 7 6 5 4 3 2 1
A
B
C
D
E
F
G
H
J
K
Table 1-1. CBGA-pinout ATmega640/1280/2560
12345678
A
GND
AREF
PF0
PF2
PF5
PK0
PK3
PK6
B AVCC PG5 PF1 PF3 PF6 PK1 PK4 PK7
C PE2 PE0 PE1 PF4 PF7 PK2 PK5 PJ7
D PE3 PE4 PE5 PE6 PH2 PA4 PA5 PA6
E PE7 PH0 PH1 PH3 PH5 PJ6 PJ5 PJ4
F VCC PH4 PH6 PB0 PL4 PD1 PJ1 PJ0
G GND
PB1
PB2
PB5
PL2
PD0
PD5
PC5
H PB3 PB4 RESET PL1 PL3 PL7 PD4 PC4
J PH7 PG3 PB6 PL0 XTAL2 PL6 PD3 PC1
K PB7
PG4
VCC
GND
XTAL1
PL5
PD2
PD6
Note: The functions for each pin is the same as for the 100 pin packages shown in Figure 1-1 on page 2.
9
GND
PA0
PA1
PA7
PJ3
PC7
PC6
PC3
PC0
PD7
10
VCC
PA2
PA3
PG2
PJ2
GND
VCC
PC2
PG1
PG0
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Figure 1-3. Pinout ATmega1281/2561
(OC0B) PG5
(RXD0/PCINT8/PDI) PE0
(TXD0/PDO) PE1
(XCK0/AIN0) PE2
(OC3A/AIN1) PE3
(OC3B/INT4) PE4
(OC3C/INT5) PE5
(T3/INT6) PE6
(ICP3/CLKO/INT7) PE7
(SS/PCINT0) PB0
(SCK/ PCINT1) PB1
(MOSI/ PCINT2) PB2
(MISO/ PCINT3) PB3
(OC2A/ PCINT4) PB4
(OC1A/PCINT5) PB5
(OC1B/PCINT6) PB6
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
INDEX CORNER
48 PA3 (AD3)
47 PA4 (AD4)
46 PA5 (AD5)
45 PA6 (AD6)
44 PA7 (AD7)
43 PG2 (ALE)
42 PC7 (A15)
41 PC6 (A14)
40 PC5 (A13)
39 PC4 (A12)
38 PC3 (A11)
37 PC2 (A10)
36 PC1 (A9)
35 PC0 (A8)
34 PG1 (RD)
33 PG0 (WR)
Note:
The large center pad underneath the QFN/MLF package is made of metal and internally connected to GND. It should
be soldered or glued to the board to ensure good mechanical stability. If the center pad is left unconnected, the pack-
age might loosen from the board.
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2. Overview
The ATmega640/1280/1281/2560/2561 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
ATmega640/1280/1281/2560/2561 achieves throughputs approaching 1 MIPS per MHz allowing the system
designer to optimize power consumption versus processing speed.
2.1 Block Diagram
Figure 2-1. Block Diagram
VCC
PF7..0
PK7..0
PJ7..0
PE7..0
RESET
GND
XTAL1
XTAL2
PA7..0
PG5..0
Power
Supervision
POR/ BOD &
RESET
Watchdog
Timer
Watchdog
Oscillator
Oscillator
Circuits /
Clock
Generation
PORTA (8)
PORT G (6)
PORT F (8)
PORT K (8)
PORT J (8)
PORT E(8)
JTAG
EEPROM
XRAM
A/D
Converter
Internal
Bandgap reference
CPU
Analog
Comparator
16 bit T/C3
16 bit T/C5
16 bit T/C4
FLASH
SRAM
16 bit T/C1
USART 0
USART 3
USART 1
PC7..0
PORT C (8)
TWI
SPI
8 bit T/C0
8 bit T/C2
USART 2
NOTE:
Shaded parts only available
in the 100-pin version.
Complete functionality for
the ADC,T/C4, and T/C5 only
available in the 100-pin version.
PORTD (8)
PORT B (8)
PORTH (8)
PORTL (8)
PD7..0
PB7..0
PH7..0
PL7..0
The Atmel® AVR® core combines a rich instruction set with 32 general purpose working registers. All the 32 regis-
ters 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.
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The ATmega640/1280/1281/2560/2561 provides the following features: 64K/128K/256K bytes of In-System Pro-
grammable Flash with Read-While-Write capabilities, 4Kbytes EEPROM, 8Kbytes SRAM, 54/86 general purpose
I/O lines, 32 general purpose working registers, Real Time Counter (RTC), six flexible Timer/Counters with com-
pare modes and PWM, four USARTs, a byte oriented 2-wire Serial Interface, a 16-channel, 10-bit ADC with
optional differential input stage with programmable gain, programmable Watchdog Timer with Internal Oscillator,
an SPI serial port, IEEE® std. 1149.1 compliant JTAG test interface, also used for accessing the On-chip Debug
system and programming and six software selectable power saving modes. The Idle mode stops the CPU while
allowing the SRAM, Timer/Counters, 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 mod-
ules 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. In Extended Standby mode, both the main Oscillator and the
Asynchronous Timer continue to run.
Atmel offers the QTouch® library for embedding capacitive touch buttons, sliders and wheels functionality into AVR
microcontrollers. The patented charge-transfer signal acquisition offersrobust sensing and includes fully
debounced reporting of touch keys and includes Adjacent Key Suppression® (AKS®) technology for unambiguous
detection of key events. The easy-to-use QTouch Suite toolchain allows you to explore, develop and debug your
own touch applications.
The device is manufactured using the Atmel high-density nonvolatile 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 opera-
tion. By combining an 8-bit RISC CPU with In-System Self-Programmable Flash on a monolithic chip, the Atmel
ATmega640/1280/1281/2560/2561 is a powerful microcontroller that provides a highly flexible and cost effective
solution to many embedded control applications.
The ATmega640/1280/1281/2560/2561 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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2.2 Comparison Between ATmega1281/2561 and ATmega640/1280/2560
Each device in the ATmega640/1280/1281/2560/2561 family differs only in memory size and number of pins. Table
2-1 summarizes the different configurations for the six devices.
Table 2-1. Configuration Summary
Device
ATmega640
ATmega1280
ATmega1281
ATmega2560
ATmega2561
Flash
64KB
128KB
128KB
256KB
256KB
EEPROM
4KB
4KB
4KB
4KB
4KB
RAM
8KB
8KB
8KB
8KB
8KB
General
Purpose I/O pins
86
86
54
86
54
16 bits resolution
PWM channels
12
12
6
12
6
Serial
USARTs
4
4
2
4
2
ADC
Channels
16
16
8
16
8
2.3 Pin Descriptions
2.3.1 VCC
Digital supply voltage.
2.3.2 GND
Ground.
2.3.3 Port A (PA7..PA0)
2.3.4
Port A is an 8-bit bi-directional I/O port with internal pull-up resistors (selected for each bit). The Port A output buf-
fers have symmetrical drive characteristics with both high sink and source capability. As inputs, Port A pins that are
externally pulled low will source current if the pull-up resistors are activated. The Port A pins are tri-stated when a
reset condition becomes active, even if the clock is not running.
Port A also serves the functions of various special features of the ATmega640/1280/1281/2560/2561 as listed on
page 75.
Port B (PB7..PB0)
2.3.5
Port B is an 8-bit bi-directional I/O port with internal pull-up resistors (selected for each bit). The Port B output buf-
fers 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.
Port B has better driving capabilities than the other ports.
Port B also serves the functions of various special features of the ATmega640/1280/1281/2560/2561 as listed on
page 76.
Port C (PC7..PC0)
Port C is an 8-bit bi-directional I/O port with internal pull-up resistors (selected for each bit). The Port C output buf-
fers 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.
Port C also serves the functions of special features of the ATmega640/1280/1281/2560/2561 as listed on page 79.
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2.3.6 Port D (PD7..PD0)
2.3.7
Port D is an 8-bit bi-directional I/O port with internal pull-up resistors (selected for each bit). The Port D output buf-
fers 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.
Port D also serves the functions of various special features of the ATmega640/1280/1281/2560/2561 as listed on
page 80.
Port E (PE7..PE0)
2.3.8
Port E is an 8-bit bi-directional I/O port with internal pull-up resistors (selected for each bit). The Port E output buf-
fers have symmetrical drive characteristics with both high sink and source capability. As inputs, Port E pins that are
externally pulled low will source current if the pull-up resistors are activated. The Port E pins are tri-stated when a
reset condition becomes active, even if the clock is not running.
Port E also serves the functions of various special features of the ATmega640/1280/1281/2560/2561 as listed on
page 82.
Port F (PF7..PF0)
2.3.9
Port F serves as analog inputs to the A/D Converter.
Port F also serves as an 8-bit bi-directional I/O port, if the A/D Converter is not used. Port pins can provide internal
pull-up resistors (selected for each bit). The Port F output buffers have symmetrical drive characteristics with both
high sink and source capability. As inputs, Port F pins that are externally pulled low will source current if the pull-up
resistors are activated. The Port F pins are tri-stated when a reset condition becomes active, even if the clock is not
running. If the JTAG interface is enabled, the pull-up resistors on pins PF7(TDI), PF5(TMS), and PF4(TCK) will be
activated even if a reset occurs.
Port F also serves the functions of the JTAG interface.
Port G (PG5..PG0)
Port G is a 6-bit I/O port with internal pull-up resistors (selected for each bit). The Port G output buffers have sym-
metrical drive characteristics with both high sink and source capability. As inputs, Port G pins that are externally
pulled low will source current if the pull-up resistors are activated. The Port G pins are tri-stated when a reset con-
dition becomes active, even if the clock is not running.
Port G also serves the functions of various special features of the ATmega640/1280/1281/2560/2561 as listed on
page 86.
2.3.10 Port H (PH7..PH0)
Port H is a 8-bit bi-directional I/O port with internal pull-up resistors (selected for each bit). The Port H output buf-
fers have symmetrical drive characteristics with both high sink and source capability. As inputs, Port H pins that are
externally pulled low will source current if the pull-up resistors are activated. The Port H pins are tri-stated when a
reset condition becomes active, even if the clock is not running.
Port H also serves the functions of various special features of the ATmega640/1280/2560 as listed on page 88.
2.3.11 Port J (PJ7..PJ0)
Port J is a 8-bit bi-directional I/O port with internal pull-up resistors (selected for each bit). The Port J output buffers
have symmetrical drive characteristics with both high sink and source capability. As inputs, Port J pins that are
externally pulled low will source current if the pull-up resistors are activated. The Port J pins are tri-stated when a
reset condition becomes active, even if the clock is not running. Port J also serves the functions of various special
features of the ATmega640/1280/2560 as listed on page 90.
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2.3.12 Port K (PK7..PK0)
Port K serves as analog inputs to the A/D Converter.
Port K is a 8-bit bi-directional I/O port with internal pull-up resistors (selected for each bit). The Port K output buffers
have symmetrical drive characteristics with both high sink and source capability. As inputs, Port K pins that are
externally pulled low will source current if the pull-up resistors are activated. The Port K pins are tri-stated when a
reset condition becomes active, even if the clock is not running.
Port K also serves the functions of various special features of the ATmega640/1280/2560 as listed on page 92.
2.3.13 Port L (PL7..PL0)
Port L is a 8-bit bi-directional I/O port with internal pull-up resistors (selected for each bit). The Port L output buffers
have symmetrical drive characteristics with both high sink and source capability. As inputs, Port L pins that are
externally pulled low will source current if the pull-up resistors are activated. The Port L pins are tri-stated when a
reset condition becomes active, even if the clock is not running.
Port L also serves the functions of various special features of the ATmega640/1280/2560 as listed on page 94.
2.3.14 RESET
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 “System and Reset Characteristics” on page 360. Shorter
pulses are not guaranteed to generate a reset.
2.3.15 XTAL1
Input to the inverting Oscillator amplifier and input to the internal clock operating circuit.
2.3.16 XTAL2
Output from the inverting Oscillator amplifier.
2.3.17 AVCC
AVCC is the supply voltage pin for Port F and the A/D Converter. It should be externally connected to VCC, even if
the ADC is not used. If the ADC is used, it should be connected to VCC through a low-pass filter.
2.3.18 AREF
This is the analog reference pin for the A/D Converter.
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3. Resources
A comprehensive set of development tools and application notes, and datasheets are available for download on
http://www.atmel.com/avr.
4. About Code Examples
This documentation contains simple code examples that briefly show how to use various parts of the device. Be
aware that not all C compiler vendors include bit definitions in the header files and interrupt handling in C is com-
piler dependent. Confirm with the C compiler documentation for more details.
These code examples assume that the part specific header file is included before compilation. 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".
5. 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.
6. Capacitive touch sensing
The Atmel® QTouch® Library provides a simple to use solution to realize touch sensitive interfaces on most Atmel
AVR® microcontrollers. The QTouch Library includes support for the QTouch and QMatrix acquisition methods.
Touch sensing can be added to any application by linking the appropriate Atmel QTouch Library for the AVR Micro-
controller. This is done by using a simple set of APIs to define the touch channels and sensors, and then calling the
touch sensing API’s to retrieve the channel information and determine the touch sensor states.
The QTouch Library is FREE and downloadable from the Atmel website at the following location:
www.atmel.com/qtouchlibrary. For implementation details and other information, refer to the Atmel QTouch Library
User Guide - also available for download from the Atmel website.
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7. AVR CPU Core
7.1 Introduction
This section discusses the AVR core architecture in general. The main function of the CPU core is to ensure cor-
rect program execution. The CPU must therefore be able to access memories, perform calculations, control
peripherals, and handle interrupts.
7.2 Architectural Overview
Figure 7-1. Block Diagram of the AVR Architecture
Data Bus 8-bit
Flash
Program
Memory
Instruction
Register
Instruction
Decoder
Control Lines
Program
Counter
Status
and Control
32 x 8
General
Purpose
Registers
ALU
Interrupt
Unit
SPI
Unit
Watchdog
Timer
Analog
Comparator
Data
SRAM
EEPROM
I/O Module1
I/O Module 2
I/O Module n
I/O Lines
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 × 8-bit general purpose working registers with a single clock cycle
access time. This allows single-cycle Arithmetic Logic Unit (ALU) operation. In a typical ALU operation, two oper-
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ands are output from the Register File, the operation is executed, and the result is stored back in the Register File
– in one clock cycle.
Six of the 32 registers can be used as three 16-bit indirect address register pointers for Data Space addressing –
enabling efficient address calculations. One of the these address pointers can also be used as an address pointer
for look up tables in Flash program memory. These added function registers are the 16-bit X-, Y-, and Z-register,
described later in this section.
The ALU supports arithmetic and logic operations between registers or between a constant and a register. Single
register operations can also be executed in the ALU. After an arithmetic operation, the Status Register is updated
to reflect information about the result of the operation.
Program flow is provided by conditional and unconditional jump and call instructions, able to directly address the
whole address space. Most AVR instructions have a single 16-bit word format. Every program memory address
contains a 16-bit 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 sub-
routines 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 Reg-
ister File, 0x20 - 0x5F. In addition, the ATmega640/1280/1281/2560/2561 has Extended I/O space from 0x60 -
0x1FF in SRAM where only the ST/STS/STD and LD/LDS/LDD instructions can be used.
7.3 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 Summary” on page 404 for a detailed
description.
7.4 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 Summary” on page 404. 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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7.4.1 SREG – AVR Status Register
The AVR Status Register – SREG – is defined as:
Bit
0x3F (0x5F)
Read/Write
Initial Value
7
I
R/W
0
6
T
R/W
0
5
H
R/W
0
4
S
R/W
0
3
V
R/W
0
2
N
R/W
0
1
Z
R/W
0
0
C
R/W
0
SREG
• 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 inter-
rupts 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 Summary” on
page 404.
• 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 oper-
ated 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 Summary” on page 404 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 Summary” on page 404 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 Sum-
mary” on page 404 for detailed information.
• Bit 2 – N: Negative Flag
The Negative Flag N indicates a negative result in an arithmetic or logic operation. See the “Instruction Set Sum-
mary” on page 404 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 Summary” on
page 404 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 Summary” on page
404 for detailed information.
7.5 General Purpose Register File
The Register File is optimized for the AVR Enhanced RISC instruction set. In order to achieve the required perfor-
mance 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
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• Two 8-bit output operands and one 16-bit result input
• One 16-bit output operand and one 16-bit result input
Figure 7-2 shows the structure of the 32 general purpose working registers in the CPU.
Figure 7-2. AVR CPU General Purpose Working Registers
General
Purpose
Working
Registers
70
R0
R1
R2
R13
R14
R15
R16
R17
R26
R27
R28
R29
R30
R31
Addr.
0x00
0x01
0x02
0x0D
0x0E
0x0F
0x10
0x11
0x1A
0x1B
0x1C
0x1D
0x1E
0x1F
X-register Low Byte
X-register High Byte
Y-register Low Byte
Y-register High Byte
Z-register Low Byte
Z-register High Byte
7.5.1
Most of the instructions operating on the Register File have direct access to all registers, and most of them are sin-
gle cycle instructions.
As shown in Figure 7-2, each register is also assigned a data memory address, mapping them directly into the first
32 locations of the user Data Space. Although not being physically implemented as SRAM locations, this memory
organization provides great flexibility in access of the registers, as the X-, Y- and Z-pointer registers can be set to
index any register in the file.
The X-register, Y-register, and Z-register
The registers R26..R31 have some added functions to their general purpose usage. These registers are 16-bit
address pointers for indirect addressing of the data space. The three indirect address registers X, Y, and Z are
defined as described in Figure 7-3.
Figure 7-3. The X-, Y-, and Z-registers
X-register
15
7
R27 (0x1B)
XH
07
R26 (0x1A)
XL
0
0
Y-register
15
7
R29 (0x1D)
YH
07
R28 (0x1C)
YL
0
0
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 incre-
ment, and automatic decrement (see the “Instruction Set Summary” on page 404 for details).
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7.6 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 inter-
rupts are enabled. The Stack Pointer must be set to point above 0x0200. The initial value of the stack pointer is the
last address of the internal SRAM. The Stack Pointer is decremented by one when data is pushed onto the Stack
with the PUSH instruction, and it is decremented by two for ATmega640/1280/1281 and three for
ATmega2560/2561 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 for ATmega640/1280/1281 and three for ATmega2560/2561 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
15 14 13 12 11 10
9
8
0x3E (0x5E)
SP15
SP14
SP13
SP12
SP11
SP10
SP9
SP8
SPH
0x3D (0x5D)
SP7
SP6
SP5
SP4
SP3
SP2
SP1
SP0
SPL
76543210
Read/Write
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
Initial Value
0
0
1
0
0
0
0
1
11111111
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7.6.1 RAMPZ – Extended Z-pointer Register for ELPM/SPM
Bit
0x3B (0x5B)
Read/Write
Initial Value
7
RAMPZ7
R/W
0
6
RAMPZ6
R/W
0
5
RAMPZ5
R/W
0
4
RAMPZ4
R/W
0
3
RAMPZ3
R/W
0
2
RAMPZ2
R/W
0
1
RAMPZ1
R/W
0
0
RAMPZ0
R/W
0
RAMPZ
For ELPM/SPM instructions, the Z-pointer is a concatenation of RAMPZ, ZH, and ZL, as shown in Figure 7-4. Note
that LPM is not affected by the RAMPZ setting.
Figure 7-4. The Z-pointer used by ELPM and SPM
Bit
(Individually)
7
0
RAMPZ
Bit (Z-pointer)
23
16
7
ZH
15
0
8
70
ZL
70
7.6.2
The actual number of bits is implementation dependent. Unused bits in an implementation will always read as zero.
For compatibility with future devices, be sure to write these bits to zero.
EIND – Extended Indirect Register
Bit
0x3C (0x5C)
Read/Write
Initial Value
7
EIND7
R/W
0
6
EIND6
R/W
0
5
EIND5
R/W
0
4
EIND4
R/W
0
3
EIND3
R/W
0
2
EIND2
R/W
0
1
EIND1
R/W
0
0
EIND0
R/W
0
EIND
For EICALL/EIJMP instructions, the Indirect-pointer to the subroutine/routine is a concatenation of EIND, ZH, and
ZL, as shown in Figure 7-5. Note that ICALL and IJMP are not affected by the EIND setting.
Figure 7-5. The Indirect-pointer used by EICALL and EIJMP
Bit
(Individually)
Bit (Indirect-
pointer)
7
EIND
23
0
16
7
ZH
15
0
8
7
ZL
7
0
0
The actual number of bits is implementation dependent. Unused bits in an implementation will always read as zero.
For compatibility with future devices, be sure to write these bits to zero.
7.7 Instruction Execution Timing
This section describes the general access timing concepts for instruction execution. The AVR CPU is driven by the
CPU clock clkCPU, directly generated from the selected clock source for the chip. No internal clock division is used.
Figure 7-6 on page 17 shows the parallel instruction fetches and instruction executions enabled by the Harvard
architecture and the fast-access Register File concept. This is the basic pipelining concept to obtain up to 1 MIPS
per MHz with the corresponding unique results for functions per cost, functions per clocks, and functions per
power-unit.
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Figure 7-6. The Parallel Instruction Fetches and Instruction Executions
T1 T2 T3
clkCPU
1st Instruction Fetch
1st Instruction Execute
2nd Instruction Fetch
2nd Instruction Execute
3rd Instruction Fetch
3rd Instruction Execute
4th Instruction Fetch
T4
Figure 7-7 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 7-7. Single Cycle ALU Operation
T1
T2 T3
T4
clkCPU
Total Execution Time
Register Operands Fetch
ALU Operation Execute
Result Write Back
7.8 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 325 for details.
The lowest addresses in the program memory space are by default defined as the Reset and Interrupt Vectors.
The complete list of vectors is shown in “Interrupts” on page 101. 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 sec-
tion by setting the IVSEL bit in the MCU Control Register (MCUCR). Refer to “Interrupts” on page 101 for more
information. The Reset Vector can also be moved to the start of the Boot Flash section by programming the
BOOTRST Fuse, see “Memory Programming” on page 325.
When an interrupt occurs, the Global Interrupt Enable I-bit is cleared and all interrupts are disabled. The user soft-
ware 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 writ-
ing 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
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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 nec-
essarily 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 */
__disable_interrupt();
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 pend-
ing 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) */
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7.8.1 Interrupt Response Time
The interrupt execution response for all the enabled AVR interrupts is five clock cycles minimum. After five clock
cycles the program vector address for the actual interrupt handling routine is executed. During these five 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 five 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 five clock cycles. During these five clock cycles, the Program
Counter (three bytes) is popped back from the Stack, the Stack Pointer is incremented by three, and the I-bit in
SREG is set.
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8. AVR Memories
This section describes the different memories in the ATmega640/1280/1281/2560/2561. The AVR architecture has
two main memory spaces, the Data Memory and the Program Memory space. In addition, the
ATmega640/1280/1281/2560/2561 features an EEPROM Memory for data storage. All three memory spaces are
linear and regular.
8.1 In-System Reprogrammable Flash Program Memory
The ATmega640/1280/1281/2560/2561 contains 64K/128K/256K bytes On-chip In-System Reprogrammable Flash
memory for program storage, see Figure 8-1. Since all AVR instructions are 16 bit or 32 bit wide, the Flash is orga-
nized as 32K/64K/128K × 16. For software security, the Flash Program memory space is divided into two sections,
Boot Program section and Application Program section.
The Flash memory has an endurance of at least 10,000 write/erase cycles. The ATmega640/1280/1281/2560/2561
Program Counter (PC) is 15/16/17 bits wide, thus addressing the 32K/64K/128K program memory locations. The
operation of Boot Program section and associated Boot Lock bits for software protection are described in detail in
“Boot Loader Support – Read-While-Write Self-Programming” on page 310. “Memory Programming” on page 325
contains a detailed description on Flash data serial downloading using the SPI pins or the JTAG interface.
Constant tables can be allocated within the entire program memory address space (see the LPM – Load Program
Memory instruction description and ELPM - Extended Load Program Memory instruction description).
Timing diagrams for instruction fetch and execution are presented in “Instruction Execution Timing” on page 16.
Figure 8-1. Program Flash Memory Map
Address (HEX)
0
Application Flash Section
0x7FFF/0xFFFF/0x1FFFF
Boot Flash Section
8.2 SRAM Data Memory
Figure 8-2 on page 22 shows how the ATmega640/1280/1281/2560/2561 SRAM Memory is organized.
The ATmega640/1280/1281/2560/2561 is a complex microcontroller with more peripheral units than can be sup-
ported within the 64 location reserved in the Opcode for the IN and OUT instructions. For the Extended I/O space
from $060 - $1FF in SRAM, only the ST/STS/STD and LD/LDS/LDD instructions can be used.
The first 4,608/8,704 Data Memory locations address both the Register File, the I/O Memory, Extended I/O Mem-
ory, and the internal data SRAM. The first 32 locations address the Register file, the next 64 location the standard
I/O Memory, then 416 locations of Extended I/O memory and the next 8,192 locations address the internal data
SRAM.
An optional external data SRAM can be used with the ATmega640/1280/1281/2560/2561. This SRAM will occupy
an area in the remaining address locations in the 64K address space. This area starts at the address following the
internal SRAM. The Register file, I/O, Extended I/O and Internal SRAM occupies the lowest 4,608/8,704 bytes, so
when using 64Kbytes (65,536 bytes) of External Memory, 60,478/56,832 Bytes of External Memory are available.
See “External Memory Interface” on page 27 for details on how to take advantage of the external memory map.
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When the addresses accessing the SRAM memory space exceeds the internal data memory locations, the exter-
nal data SRAM is accessed using the same instructions as for the internal data memory access. When the internal
data memories are accessed, the read and write strobe pins (PG0 and PG1) are inactive during the whole access
cycle. External SRAM operation is enabled by setting the SRE bit in the XMCRA Register.
Accessing external SRAM takes one additional clock cycle per byte compared to access of the internal SRAM.
This means that the commands LD, ST, LDS, STS, LDD, STD, PUSH, and POP take one additional clock cycle. If
the Stack is placed in external SRAM, interrupts, subroutine calls and returns take three clock cycles extra
because the three-byte program counter is pushed and popped, and external memory access does not take
advantage of the internal pipe-line memory access. When external SRAM interface is used with wait-state, one-
byte external access takes two, three, or four additional clock cycles for one, two, and three wait-states respec-
tively. Interrupts, subroutine calls and returns will need five, seven, or nine clock cycles more than specified in the
instruction set manual for one, two, and three wait-states.
The five different addressing modes for the data memory cover: Direct, Indirect with Displacement, Indirect, Indi-
rect 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-register
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, and the 4,196/8,192 bytes of internal data SRAM in the
ATmega640/1280/1281/2560/2561 are all accessible through all these addressing modes. The Register File is
described in “General Purpose Register File” on page 13.
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Figure 8-2.
Data Memory Map
Address (HEX)
0 - 1F
20 - 5F
60 - 1FF
200
21FF
2200
32 Registers
64 I/O Registers
416 External I/O Registers
Internal SRAM
(8192 × 8)
External SRAM
(0 - 64K × 8)
FFFF
8.2.1 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 clkCPU cycles as described in Figure 8-3.
Figure 8-3. On-chip Data SRAM Access Cycles
T1
T2
T3
clk
CPU
Address
Data
WR
Data
RD
Compute Address
Address valid
Memory Access Instruction
Next Instruction
8.3 EEPROM Data Memory
The ATmega640/1280/1281/2560/2561 contains 4Kbytes 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.
For a detailed description of SPI, JTAG and Parallel data downloading to the EEPROM, see “Serial Downloading”
on page 338, “Programming via the JTAG Interface” on page 342, and “Programming the EEPROM” on page 333
respectively.
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8.3.1 EEPROM Read/Write Access
The EEPROM Access Registers are accessible in the I/O space, see “Register Description” on page 34.
The write access time for the EEPROM is given in Table 8-1. 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 pre-
cautions must be taken. In heavily filtered power supplies, VCC 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 fre-
quency used. See “Preventing EEPROM Corruption” on page 25. for details on how to avoid problems in these
situations.
In order to prevent unintentional EEPROM writes, a specific write procedure must be followed. See the description
of the EEPROM Control Register for details on this; “Register Description” on page 34.
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.
The calibrated Oscillator is used to time the EEPROM accesses. Table 8-1 lists the typical programming time for
EEPROM access from the CPU.
Table 8-1. EEPROM Programming Time
Symbol
Number of Calibrated RC Oscillator Cycles
EEPROM write (from CPU)
26,368
Typ Programming Time
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 (for example 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 soft-
ware. If such code is present, the EEPROM write function must also wait for any ongoing SPM command to finish.
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Assembly Code Example(1)
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(1)
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);
}
Note: 1. See “About Code Examples” on page 10.
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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(1)
EEPROM_read:
; Wait for completion of previous write
sbic EECR,EEPE
rjcmp 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(1)
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;
}
Note: 1. See “About Code Examples” on page 10.
8.3.2 Preventing EEPROM Corruption
During periods of low VCC, the EEPROM data can be corrupted because the supply voltage is 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 VCC reset Protection circuit can 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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8.4 I/O Memory
8.4.1
The I/O space definition of the ATmega640/1280/1281/2560/2561 is shown in “Register Summary” on page 399.
All ATmega640/1280/1281/2560/2561 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 Summary” on page 404 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
ATmega640/1280/1281/2560/2561 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 -
0x1FF 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.
General Purpose I/O Registers
The ATmega640/1280/1281/2560/2561 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. Gen-
eral Purpose I/O Registers within the address range 0x00 - 0x1F are directly bit-accessible using the SBI, CBI,
SBIS, and SBIC instructions. See “Register Description” on page 34.
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9. External Memory Interface
With all the features the External Memory Interface provides, it is well suited to operate as an interface to memory
devices such as External SRAM and Flash, and peripherals such as LCD-display, A/D, and D/A. The main features
are:
Four different wait-state settings (including no wait-state)
Independent wait-state setting for different External Memory sectors (configurable sector size)
The number of bits dedicated to address high byte is selectable
Bus keepers on data lines to minimize current consumption (optional)
9.1 Overview
When the eXternal MEMory (XMEM) is enabled, address space outside the internal SRAM becomes available
using the dedicated External Memory pins (see Figure 1-3 on page 4, Table 13-3 on page 75, Table 13-9 on page
79, and Table 13-21 on page 86). The memory configuration is shown in Figure 9-1.
Figure 9-1.
External Memory with Sector Select
Memory Configuration A
0x0000
Internal memory
Lower sector
SRW01
SRW00
0x21FF
0x2200
External Memory
(0 - 60K x 8)
Upper sector
SRL[2..0]
SRW11
SRW10
0xFFFF
9.1.1 Using the External Memory Interface
The interface consists of:
• AD7:0: Multiplexed low-order address bus and data bus
A15:8: High-order address bus (configurable number of bits)
• ALE: Address latch enable
• RD: Read strobe
WR: Write strobe
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9.1.2
The control bits for the External Memory Interface are located in two registers, the External Memory Control Regis-
ter A – XMCRA, and the External Memory Control Register B – XMCRB.
When the XMEM interface is enabled, the XMEM interface will override the setting in the data direction registers
that corresponds to the ports dedicated to the XMEM interface. For details about the port override, see the alter-
nate functions in section “I/O-Ports” on page 67. The XMEM interface will auto-detect whether an access is internal
or external. If the access is external, the XMEM interface will output address, data, and the control signals on the
ports according to Figure 9-3 on page 29 (this figure shows the wave forms without wait-states). When ALE goes
from high-to-low, there is a valid address on AD7:0. ALE is low during a data transfer. When the XMEM interface is
enabled, also an internal access will cause activity on address, data and ALE ports, but the RD and WR strobes
will not toggle during internal access. When the External Memory Interface is disabled, the normal pin and data
direction settings are used. Note that when the XMEM interface is disabled, the address space above the internal
SRAM boundary is not mapped into the internal SRAM. Figure 9-2 illustrates how to connect an external SRAM to
the AVR using an octal latch (typically “74 × 573” or equivalent) which is transparent when G is high.
Address Latch Requirements
Due to the high-speed operation of the XRAM interface, the address latch must be selected with care for system
frequencies above 8MHz @ 4V and 4MHz @ 2.7V. When operating at conditions above these frequencies, the typ-
ical old style 74HC series latch becomes inadequate. The External Memory Interface is designed in compliance to
the 74AHC series latch. However, most latches can be used as long they comply with the main timing parameters.
The main parameters for the address latch are:
• D to Q propagation delay (tPD)
• Data setup time before G low (tSU)
• Data (address) hold time after G low (TH)
The External Memory Interface is designed to guaranty minimum address hold time after G is asserted low of th =
5ns. Refer to tLAXX_LD/tLLAXX_ST in “External Data Memory Timing” Tables 31-11 through Tables 31-18 on pages 367
- 370. The D-to-Q propagation delay (tPD) must be taken into consideration when calculating the access time
requirement of the external component. The data setup time before G low (tSU) must not exceed address valid to
ALE low (tAVLLC) minus PCB wiring delay (dependent on the capacitive load).
Figure 9-2. External SRAM Connected to the AVR
AVR
SRAM
D[7:0]
AD7:0
ALE
DQ
G
A[7:0]
A15:8
RD
WR
A[15:8]
RD
WR
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9.1.3 Pull-up and Bus-keeper
9.1.4
The pull-ups on the AD7:0 ports may be activated if the corresponding Port register is written to one. To reduce
power consumption in sleep mode, it is recommended to disable the pull-ups by writing the Port register to zero
before entering sleep.
The XMEM interface also provides a bus-keeper on the AD7:0 lines. The bus-keeper can be disabled and enabled
in software as described in “XMCRB – External Memory Control Register B” on page 38. When enabled, the bus-
keeper will keep the previous value on the AD7:0 bus while these lines are tri-stated by the XMEM interface.
Timing
External Memory devices have different timing requirements. To meet these requirements, the XMEM interface
provides four different wait-states as shown in Table 9-3 on page 37. It is important to consider the timing specifica-
tion of the External Memory device before selecting the wait-state. The most important parameters are the access
time for the external memory compared to the set-up requirement. The access time for the External Memory is
defined to be the time from receiving the chip select/address until the data of this address actually is driven on the
bus. The access time cannot exceed the time from the ALE pulse must be asserted low until data is stable during a
read sequence (see tLLRL+ tRLRH - tDVRH in Tables 31-11 through Tables 31-18 on pages 367 - 370). The different
wait-states are set up in software. As an additional feature, it is possible to divide the external memory space in two
sectors with individual wait-state settings. This makes it possible to connect two different memory devices with dif-
ferent timing requirements to the same XMEM interface. For XMEM interface timing details, refer to Table 31-11 on
page 367 to Table 31-18 on page 370 and Figure 31-9 on page 370 to Figure 31-12 on page 372 in the “External
Data Memory Timing” on page 367.
Note that the XMEM interface is asynchronous and that the waveforms in the following figures are related to the
internal system clock. The skew between the internal and external clock (XTAL1) is not guarantied (varies between
devices temperature, and supply voltage). Consequently, the XMEM interface is not suited for synchronous
operation.
Figure 9-3.
External Data Memory Cycles without Wait-state (SRWn1=0 and SRWn0=0)(1)
T1 T2 T3 T4
System Clock (CLKCPU)
ALE
A15:8 Prev. addr.
Address
DA7:0 Prev. data
Address XX
Data
WR
DA7:0 (XMBK = 0) Prev. data
Address
Data
DA7:0 (XMBK = 1) Prev. data
Address XXXXX
Data
XXXXXXXX
RD
Note: 1. SRWn1 = SRW11 (upper sector) or SRW01 (lower sector), SRWn0 = SRW10 (upper sector) or SRW00 (lower sec-
tor). The ALE pulse in period T4 is only present if the next instruction accesses the RAM (internal or external).
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Figure 9-4. External Data Memory Cycles with SRWn1 = 0 and SRWn0 = 1(1)
T1 T2 T3 T4
System Clock (CLKCPU)
ALE
A15:8 Prev. addr.
Address
DA7:0 Prev. data
Address XX
Data
WR
DA7:0 (XMBK = 0) Prev. data
Address
Data
DA7:0 (XMBK = 1) Prev. data
Address
Data
RD
T5
Note:
1. SRWn1 = SRW11 (upper sector) or SRW01 (lower sector), SRWn0 = SRW10 (upper sector) or SRW00 (lower sec-
tor).
The ALE pulse in period T5 is only present if the next instruction accesses the RAM (internal or external).
Figure 9-5. External Data Memory Cycles with SRWn1 = 1 and SRWn0 = 0(1)
T1
T2
T3 T4
T5
System Clock (CLKCPU)
T6
ALE
A15:8 Prev. addr.
Address
DA7:0 Prev. data
Address XX
Data
WR
DA7:0 (XMBK = 0) Prev. data
Address
Data
DA7:0 (XMBK = 1) Prev. data
Address
Data
RD
Note:
1. SRWn1 = SRW11 (upper sector) or SRW01 (lower sector), SRWn0 = SRW10 (upper sector) or SRW00 (lower sec-
tor).
The ALE pulse in period T6 is only present if the next instruction accesses the RAM (internal or external).
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