20
8271D–AVR–05/11
ATmega48A/PA/88A/PA/168A/PA/328/P
8.3.1 Data Memory Access Times
This section describes the general access timing concepts for internal memory access. The
internal data SRAM access is performed in two clk
CPU
cycles as described in Figure 8-4.
Figure 8-4. On-chip Data SRAM Access Cycles
8.4 EEPROM Data Memory
The ATmega48A/PA/88A/PA/168A/PA/328/P contains 256/512/512/1Kbytes 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 Reg-
isters, the EEPROM Data Register, and the EEPROM Control Register.
”Memory Programming” on page 297 contains a detailed description on EEPROM Programming
in SPI or Parallel Programming mode.
8.4.1 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 8-2. 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.
clk
WR
RD
Data
Data
Address
Address valid
T1 T2 T3
Compute Address
Read
Write
CPU
Memory Access Instruction
Next Instruction
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