ATMEGA640 datasheet(24/449 Pages) ATMEL

24 / 449 page

ATmega640/1280/1281/2560/2561

2549K–AVR–01/07

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.

Note:

1. See “About Code Examples” on page 9.

Preventing EEPROM

Corruption

During periods of low V

CC, the EEPROM data can be corrupted because the supply volt-

age 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

CC 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.

Assembly Code Example(1)

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

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;

}

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