Rainbow-electronics ATmega8515L Manual do Utilizador descarregar pdf (Página 21)

ATmega8515(L)

2512A–AVR–04/02

Thenext codeexamplesshow assembly andCfunctionsforreading the EEPROM.The

examples assume that interrupts are controlledsothat no interruptswill occurduring

execution of these functions.

Preventing EEPROM

Corruption

During periods oflow V

, the EEPROM data can be corruptedbecausethe supply volt-

age is too lowfor the CPU and the EEPROM to operate properly.Theseissues arethe

same asforboard levelsystems using EEPROM, and the same design solutionsshould

beapplied.

An EEPROM data corruption can be causedbytwo situationswhen thevoltage is too

low. First, a regularwrite sequencetothe EEPROM requires aminimumvoltage to

operate correctly. Secondly, the CPU itself can execute instructions incorrectly, if the

supply voltage is too low.

EEPROM data c orruption can easily beavoidedbyfollowing th isdesign

recommendation:

Keep the AVR RESETactive (low) during periods of insufficient powersupply volt-

age. Thiscan be done by enabling theinternalBrown-out Detector(BOD). If the

detection level of theinternalBODdoes not match theneededdetection level, an

externallow V

Reset Protection circuit can beused. If aReset occurs whilea

write operation is in progress, the write operation will be completedprovided that the

powersupply voltage issufficient.

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

CCode 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;

}

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