
18
ATmega64A [DATASHEET]
8160D–AVR–02/2013
Figure 8-3.
On-chip Data SRAM Access Cycles
8.3
EEPROM Data Memory
The ATmega64A contains 2 Kbytes 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 Regis-
ters, the EEPROM Data Register, and the EEPROM Control Register.
Parallel Programming mode.
8.3.1
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 8-5 on page 31. 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, 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 mini-
problems in these situations.
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.
8.3.2
EEPROM Write During Power-down Sleep Mode
When entering Power-down Sleep mode while an EEPROM write operation is active, the EEPROM write operation
will continue, and will complete before the Write Access time has passed. However, when the write operation is
completed, the oscillator continues running, and as a consequence, the device does not enter Power-down
entirely. It is therefore recommended to verify that the EEPROM write operation is completed before entering
Power-down.
clk
WR
RD
Data
Address
Address Valid
T1
T2
T3
Compute Address
Read
Wr
ite
CPU
Memory Access Instruction
Next Instruction