Datasheet

Figure 9-3. On-chip Data SRAM Access Cycles
clk
WR
RD
Data
Data
Address
Address valid
T1 T2 T3
Compute Address
Read
Write
CPU
Memory Access Instruction
Next Instruction
9.4. EEPROM Data Memory
The ATmega644A contains 2K 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. 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.
See the related links for a detailed description on EEPROM Programming in SPI or Parallel Programming
mode.
Related Links
MEMPROG- Memory Programming on page 362
9.4.1. 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 9-2. 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 minimum for the clock frequency used. Please refer to Preventing EEPROM Corruption for
details on how to avoid 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.
9.4.2. Preventing EEPROM Corruption
During periods of low V
CC,
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.
Atmel ATmega644A [DATASHEET]
Atmel-42716C-ATmega644A_Datasheet_Complete-10/2016
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