ATmega8(L)
erwise no EEPROM write takes place. The following procedure should be followed when writing
the EEPROM (the order of steps 3 and 4 is not essential):
1. Wait until EEWE becomes zero.
2. Wait until SPMEN in SPMCR becomes zero.
3. Write new EEPROM address to EEAR (optional).
4. Write new EEPROM data to EEDR (optional).
5. Write a logical one to the EEMWE bit while writing a zero to EEWE in EECR.
6. Within four clock cycles after setting EEMWE, write a logical one to EEWE.
The EEPROM can not be programmed during a CPU write to the Flash memory. The software
must check that the Flash programming is completed before initiating a new EEPROM write.
Step 2 is only relevant if the software contains a boot loader allowing the CPU to program the
Flash. If the Flash is never being updated by the CPU, step 2 can be omitted. See “Boot Loader
Support – Read-While-Write Self-Programming” on page 209 for details about boot
programming.
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 soft-
ware 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 1 lists the typical pro-
gramming time for EEPROM access from the CPU.
Table 1. EEPROM Programming Time
Number of Calibrated RC
Symbol
Oscillator Cycles(1)
Typ Programming Time
EEPROM Write (from CPU)
8448
8.5 ms
Note:
1. Uses 1 MHz clock, independent of CKSEL Fuse settings.
21
2486T–AVR–05/08
ATMEL [ ATMEL ]
