thanks a lot for valuable inputs.but i still have two doubts.
1. in normal operation no write cycle is executed.with power-on it is just read configuration. write routine is executed only in menu mode and that too when the menu is 'store'.so if a power fail occures, the chances of having these combinations are very less.
2. when the board was touched-up with a soldering iron, (with all chips in place) it got corrupted.(this happened twice).
does this not indicate that the chip got corrupted by itself due to spikes or power outrages.
dsu