22

2570L–AVR–08/07

ATmega325/3250/645/6450

the selected address If EEMWE is zero, setting EEWE will have no effect. When EEMWE has

been written to one by software, hardware clears the bit to zero after four clock cycles. See the

description of the EEWE bit for an EEPROM write procedure.

• Bit 1 – EEWE: EEPROM Write Enable

The EEPROM Write Enable Signal EEWE is the write strobe to the EEPROM. When address

and data are correctly set up, the EEWE bit must be written to one to write the value into the

EEPROM. The EEMWE bit must be written to one before a logical one is written to EEWE, oth-

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 SPMCSR 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 250 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 8-1 lists the typical pro-

gramming time for EEPROM access from the CPU.

Table 8-1.

EEPROM Programming Time

Symbol

Number of Calibrated

RC Oscillator Cycles

Typical Programming Time

EEPROM write (from CPU)

27,072

3.4 ms