8.4 EEPROM data memory
The ATmega164A/164PA/324A/324PA/644A/644PA/1284/1284P contains 512/1K/2K/4Kbytes 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. The access between the EEPROM and the
CPU is described in the following, 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 page 291, page
301 and page 306 respectively.
EEPROM Read/Write Access
The EEPROM Access Registers are accessible in the I/O space. See ”Register Description” on page 24 for
The write access time for the EEPROM is given in Table 8-2 on page 26. A self-timing function, however, lets
the user software detect when the next byte can be written. If the user code contains instructions that write the
EEPROM, some precautions must be taken. In heavily filtered power supplies, VCC 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 Section “8.4.2” on page 22 for details on how to avoid problems in
In order to prevent unintentional EEPROM writes, a specific write procedure must be followed. Refer to the
description of the EEPROM Control Register for details on this.
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.
Preventing EEPROM corruption
During periods of low VCC, the EEPROM data can be corrupted because the supply voltage is too low for the
CPU and the EEPROM to operate properly. These issues are the same as for board level systems using
EEPROM, and the same design solutions should be applied.
An EEPROM data corruption can be caused by two situations when the voltage is too low. First, a regular write
sequence to the EEPROM requires a minimum voltage to operate correctly. Secondly, the CPU itself can
execute instructions incorrectly, if the supply voltage is too low.
EEPROM data corruption can easily be avoided by following this design recommendation:
Keep the AVR RESET active (low) during periods of insufficient power supply voltage. This can be done by
enabling the internal Brown-out Detector (BOD). If the detection level of the internal BOD does not match the
needed detection level, an external low VCC reset Protection circuit can be used. If a reset occurs while a write
operation is in progress, the write operation will be completed provided that the power supply voltage is