Whenever the NVRAM is put in the circuit, it is given the vcc voltage. Please clarify

I'm not a hardware guy, so this is probably better explained by someone else, but here goes anyway...

Whenever you apply power to or remove power from an integrated circuit, transients/voltage spikes are likely to occur. In most PCB designs, bypass caps are located near each device and are connected to VCC and GND to prevent voltage spikes on VCC due to that devices.

When you power a device up or down, power does not instantly appear. It takes a certain amount of time for the voltage to "come up to level".

So, it is possible that one of the address lines may come up to 5 volts BEFORE VCC does. Or, you board may have a lot of capacitance on the RD/WR line and when you power down, RD/WR may stay high longer than VCC does. Either of these 2 situations can cause problems with NVRAM devices.

I designed the software for several boards that used NVRAM. The problems we had were that the units were installed in areas where there were lots of power noise and lightning strikes. The NVRAMs were a constant source of problems since the freshness seal was invoked all the time. You can test this by dumping the RAM contents and you should see a repeating 7F in the first half and 80 in the second half (or something similar).

Jon