ATmega128 datasheet(25/386 Pages) ATMEL

25 / 386 page

2467S–AVR–07/09

ATmega128

The next code examples show assembly and C functions for reading the EEPROM. The exam-

ples assume that interrupts are controlled so that no interrupts will occur during execution of

these functions.

EEPROM Write During

Power-down Sleep

Mode

When entering Power-down sleep mode while an EEPROM write operation is active, the

EEPROM write operation will continue, and will complete before the write access time has

passed. However, when the write operation is completed, the Oscillator continues running, and

as a consequence, the device does not enter Power-down entirely. It is therefore recommended

to verify that the EEPROM write operation is completed before entering Power-down.

Preventing EEPROM

Corruption

During periods of low V

CC, 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. Sec-

ondly, 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 a reset occurs while a write oper-

Assembly Code Example

EEPROM_read:

; Wait for completion of previous write

sbic

EECR,EEWE

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

unsigned char

EEPROM_read(unsigned int uiAddress)

{

/* Wait for completion of previous write */

while(EECR & (1<<EEWE))

;

/* Set up address register */

EEAR = uiAddress;

/* Start eeprom read by writing EERE */

EECR |= (1<<EERE);

/* Return data from data register */

return EEDR;

}

ATmega128 Datasheet(PDF) 25 Page - ATMEL Corporation

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