Datasheet

20
2570N–AVR–05/11
ATmega325/3250/645/6450
Figure 8-3. On-chip Data SRAM Access Cycles
8.3 EEPROM Data Memory
The Atmel ATmega325/3250/645/6450 contains 1/2K 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 Regis-
ters, 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 280, page 284, and page 268 respectively.
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 8-1. A self-timing function, however,
lets the user software detect when the next byte can be written. If the user code contains instruc-
tions 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 21. 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.
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 clock continues running, and as a
clk
WR
RD
Data
Data
Address
Address valid
T1 T2 T3
Compute Address
Read
Write
CPU
Memory Access Instruction
Next Instruction