22
ATmega8A [DATASHEET]
8159E–AVR–02/2013
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.
8.6.4
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.
8.6.5
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. Second, 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
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
in
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;
}