Hi Durgai,

I suggest an serial EEPROM e.g. 24C02 to store during power off.
But the write cycles are limited to 1,000,000.
I have tested a single byte and it fails after 22,000,000.
But further write cycles still possible on all other bytes.
Maybe these limits are high enough for your application.

The other way, count inside the RAM and write into EEPROM only before power failed.
There are several ways to generate an early power fail signal. You can feed the unregulated voltage before the 7805 into a comparator. You can use the comparator inside the AT89C2051 or a supervisor circuit. E.g. the MAX813L with power on reset, power fail warning and watchdog.


Peter