ATMEGA649 datasheet(21/375 Pages) ATMEL

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ATmega329/3290/649/6490

2552H–AVR–11/06

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, otherwise 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):

Wait until EEWE becomes zero.

Wait until SPMEN in SPMCSR becomes zero.

Write new EEPROM address to EEAR (optional).

Write new EEPROM data to EEDR (optional).

Write a logical one to the EEMWE bit while writing a zero to EEWE in EECR.

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 268 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 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

The calibrated Oscillator is used to time the EEPROM accesses. Table 2 lists the typical

programming time for EEPROM access from the CPU.

Table 2. EEPROM Programming Time

Symbol

Number of Calibrated

RC Oscillator Cycles

Typical Programming Time

EEPROM write (from CPU)

27,072

3.4 ms

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