ATMEGA640 datasheet(34/40 Pages) ATMEL

34 / 40 page

2549NS–AVR–05/11

ATmega640/1280/1281/2560/2561

Non-Read-While-Write area of flash not functional

The Non-Read-While-Write area of the flash is not working as expected. The problem is

related to the speed of the part when reading the flash of this area.

Problem Fix/Workaround

- Only use the first 248K of the flash.

- If boot functionality is needed, run the code in the Non-Read-While-Write area at maximum

1/4th of the maximum frequency of the device at any given voltage. This is done by writing

the CLKPR register before entering the boot section of the code.

Part does not work under 2.4 volts

The part does not execute code correctly below 2.4 volts.

Problem Fix/Workaround

Do not use the part at voltages below 2.4 volts.

Incorrect ADC reading in differential mode

The ADC has high noise in differential mode. It can give up to 7 LSB error.

Problem Fix/Workaround

Use only the 7 MSB of the result when using the ADC in differential mode.

Internal ADC reference has too low value

The internal ADC reference has a value lower than specified.

Problem Fix/Workaround

- Use AVCC or external reference.

- The actual value of the reference can be measured by applying a known voltage to the

ADC when using the internal reference. The result when doing later conversions can then be

calibrated.

IN/OUT instructions may be executed twice when Stack is in external RAM

If either an IN or an OUT instruction is executed directly before an interrupt occurs and the

stack pointer is located in external ram, the instruction will be executed twice. In some cases

this will cause a problem, for example:

- If reading SREG it will appear that the I-flag is cleared.

- If writing to the PIN registers, the port will toggle twice.

- If reading registers with interrupt flags, the flags will appear to be cleared.

Problem Fix/Workaround

There are two application workarounds, where selecting one of them, will be omitting the

issue:

- Replace IN and OUT with LD/LDS/LDD and ST/STS/STD instructions.

- Use internal RAM for stack pointer.

EEPROM read from application code does not work in Lock Bit Mode 3

When the Memory Lock Bits LB2 and LB1 are programmed to mode 3, EEPROM read does

not work from the application code.

ATMEGA640 Datasheet(PDF) 34 Page - ATMEL Corporation