• Bit 1 – EEWE: EEPROM Write Enable
The EEPROM Write Enable Signal (EEWE) is the write strobe to the EEPROM. When
address and data are correctly set up, the EEWE bit must be set to write the value into
the EEPROM. When the write access time (typically 2.5 ms at VCC = 5V and 4 ms at
VCC = 2.7V) has elapsed, the EEWE bit is cleared (zero) by hardware. The user soft-
ware can poll this bit and wait for a zero before writing the next byte. When EEWE has
been set, the CPU is halted for two cycles before the next instruction is executed.
• Bit 0 – EERE: EEPROM Read Enable
The EEPROM Read Enable Signal (EERE) is the read strobe to the EEPROM. When
the correct address is set up in the EEAR register, the EERE bit must be set. When the
EERE bit is cleared (zero) by hardware, requested data is found in the EEDR register.
The EEPROM read access takes one instruction and there is no need to poll the EERE
bit. When EERE has been set, the CPU is halted for four cycles before the next instruc-
tion is executed.
Caution: If an interrupt routine accessing the EEPROM is interrupting another EEPROM
access, the EEAR or EEDR register will be modified, causing the interrupted EEPROM
access to fail. It is recommended to have the global interrupt flag cleared during
EEPROM write operation to avoid these problems.
Prevent EEPROM
Corruption
During periods of low VCC, the EEPROM data can be corrupted because the supply volt-
age is too low for the CPU and the EEPROM to operate properly. These issues are the
same as for board-level systems using the EEPROM, and the same design solutions
should be applied.
An EEPROM data corruption can be caused by two situations when the voltage is too
low. First, a regular write sequence to the EEPROM requires a minimum voltage to
operate correctly. Secondly, the CPU itself can execute instructions incorrectly, if the
supply voltage for executing instructions is too low.
EEPROM data corruption can easily be avoided by following these design recommen-
dations (one is sufficient):
1. Keep the AVR RESET active (low) during periods of insufficient power supply
voltage. This is best done by an external low VCC Reset Protection circuit, often
referred to as a Brown-out Detector (BOD). Please refer to application note AVR
180 for design considerations regarding power-on reset and low-voltage
detection.
2. Keep the AVR core in Power-down Sleep mode during periods of low VCC. This
will prevent the CPU from attempting to decode and execute instructions, effec-
tively protecting the EEPROM registers from unintentional writes.
3. Store constants in Flash memory if the ability to change memory contents from
software is not required. Flash memory cannot be updated by the CPU, and will
not be subject to corruption.
26
AT90S1200
0838H–AVR–03/02
ATMEL [ ATMEL ]
