ATmega169PV-8MCH datasheet(22/393 Pages) ATMEL

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8018N–AVR–08/09

ATmega169P

7.3

EEPROM Data Memory

The ATmega169P contains 512 bytes of data EEPROM memory. It is organized as a separate

data space, in which single bytes can be read and written. The EEPROM has an endurance of at

least 100,000 write/erase cycles. This section describes the access between the EEPROM and

the CPU, specifying the EEPROM Address Registers, the EEPROM Data Register, and the

EEPROM Control Register.

For a detailed description of SPI, JTAG and Parallel data downloading to the EEPROM, see

”Serial Downloading” on page 309, ”Programming via the JTAG Interface” on page 314, and

”Parallel Programming Parameters, Pin Mapping, and Commands” on page 298 respectively.

7.3.1

EEPROM Read/Write Access

The EEPROM Access Registers are accessible in the I/O space.

The write access time for the EEPROM is given in Table 7-1 on page 23. A self-timing function,

however, lets the user software detect when the next byte can be written. If the user code con-

tains instructions that write the EEPROM, some precautions must be taken. In heavily filtered

power supplies, V

CC is likely to rise or fall slowly on power-up/down. This causes the device for

some period of time to run at a voltage lower than specified as minimum for the clock frequency

used. See ”Preventing EEPROM Corruption” on page 26 for details on how to avoid problems in

these situations.

In order to prevent unintentional EEPROM writes, a specific write procedure must be followed.

When the EEPROM is read, the CPU is halted for four clock cycles before the next instruction is

executed. When the EEPROM is written, the CPU is halted for two clock cycles before the next

instruction is executed.

The following procedure should be followed when writing the EEPROM (the order of steps 3 and

4 is not essential). See ”EEPROM Register Description” on page 27 for supplementary descrip-

tion for each register bit:

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 279 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.

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