21
8024A–AVR–04/08
ATmega8HVA/16HVA
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 EEPE bit is cleared by hardware. The user soft-
ware can poll this bit and wait for a zero before writing the next byte. When EEPE has been set,
the CPU is halted for two cycles before the next instruction is executed.
Caution:
A BOD reset during EEPROM write will invalidate the result of the ongoing operation.
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 EEPE 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 and the programming time will
therefore depend on the calibrated oscillator frequency. Table 8-2 lists the typical programming
time for EEPROM access from the CPU.
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 glo-
bally) 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.
Table 8-2.
EEPROM Programming Time
Symbol
Number of Calibrated RC
Oscillator Cycles
Typ Programming Time,
fOSC = 4.0 MHz
EEPROM write
(from CPU)
13 600
3.4 ms
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