Caution:
Caution: An interrupt between step 5 and step 6 will make the write cycle fail, since the EEPROM master write
enable will time-out. If an interrupt routine accessing the EEPROM is interrupting another EEPROM access,
the EEAR or EEDR register will be modified, causing the interrupted EEPROM access to fail. It is
recommended to have the global interrupt flag cleared during all the steps to avoid these problems.
When the write access time has elapsed, the EEWE bit is cleared by hardware. The user software can poll this
bit and wait for a zero before writing the next byte. When EEWE has been set, the CPU is halted for two cycles
before the next instruction is executed.
• Bit 0 – EERE: EEPROM Read Enable
The EEPROM read enable signal EERE is the read strobe to the EEPROM. When the correct address is set up in the EEAR
register, the EERE bit must be written to a logic one to trigger the EEPROM read. The EEPROM read access takes one
instruction, and the requested data is available immediately. When the EEPROM is read, the CPU is halted for four cycles
before the next instruction is executed.
The user should poll the EEWE bit before starting the read operation. If a write operation is in progress, it is neither possible
to read the EEPROM, nor to change the EEAR register.
The calibrated oscillator is used to time the EEPROM accesses. Table 4-2 lists the typical programming time for EEPROM
access from the CPU.
Table 4-2. EEPROM Programming Time.
Symbol
Number of Calibrated RC Oscillator Cycles
Typ Programming Time
EEPROM write (from
CPU)
26368
3.3 ms
The following code examples show one assembly and one C function for writing to the EEPROM. The examples assume
that interrupts are controlled (e.g. by disabling interrupts globally) so that no interrupts will occur during execution of these
functions. The examples also assume that no flash boot loader is present in the software. If such code is present, the
EEPROM write function must also wait for any ongoing SPM command to finish.
Assembly Code Example
EEPROM_write:
; Wait for completion of previous write
sbic
rjmp
EECR,EEWE
EEPROM_write
; Set up address (r18:r17) in address register
out
out
EEARH, r18
EEARL, r17
; Write data (r16) to data register
out EEDR,r16
; Write logical one to EEMWE
sbi EECR,EEMWE
; Start eeprom write by setting EEWE
sbi
ret
EECR,EEWE
C Code Example
void EEPROM_write (unsigned int uiAddress, unsigned char ucData)
{
/* Wait for completion of previous write */
while(EECR & (1<<EEWE))
;
/* Set up address and data registers */
EEAR = uiAddress;
EEDR = ucData;
/* Write logical one to EEMWE */
EECR |= (1<<EEMWE);
/* Start eeprom write by setting EEWE */
EECR |= (1<<EEWE);
}
22
ATmega16/32/64/M1/C1 [DATASHEET]
7647O–AVR–01/15
MICROCHIP [ MICROCHIP ]
