AT90S2313 datasheet(39/92 Pages) ATMEL

39 / 92 page

AT90S2313

0839I–AVR–06/02

EEPROM Read/Write

Access

The EEPROM Access Registers are accessible in the I/O space.

The write access time is in the range of 2.5 - 4 ms, depending on the VCC voltages. A

self-timing function, however, lets the user software detect when the next byte can be

written. If the user code contains code that writes the EEPROM, some precaution 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. CPU operation under these condi-

tions may cause the Program Counter to perform unintentional jumps and eventually

execute the EEPROM write code. To secure EEPROM integrity, the user is advised to

use an external under-voltage reset circuit in this case.

In order to prevent unintentional EEPROM writes, a specific write procedure must be fol-

lowed. Refer to the description of the EEPROM Control Register for details on this.

When the EEPROM is written, the CPU is halted for two clock cycles before the next

instruction is executed. When the EEPROM is read, the CPU is halted for four clock

cycles before the next instruction is executed.

EEPROM Address Register –

EEAR

• Bit 7 – Res: Reserved Bit

This bit is a reserved bit in the AT90S2313 and will always read as zero.

• Bit 6..0 – EEAR6..0: EEPROM Address

The EEPROM Address Register (EEAR6..0) specifies the EEPROM address in the 128

bytes EEPROM space. The EEPROM data bytes are addressed linearly between 0 and

127.

EEPROM Data Register –

EEDR

• Bit 7..0 – EEDR7..0: EEPROM Data

For the EEPROM write operation, the EEDR Register contains the data to be written to

the EEPROM in the address given by the EEAR Register. For the EEPROM read oper-

ation, the EEDR contains the data read out from the EEPROM at the address given by

EEAR.

Bit

7654

3210

$1E ($3E)

–

EEAR6

EEAR5

EEAR4

EEAR3

EEAR2

EEAR1

EEAR0

EEAR

Read/Write

R/W

Initial value

0000

Bit

7654

3210

$1D ($3D)

MSB

LSB

EEDR

Read/Write

R/W

Initial value

0000

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