Problem with understanding EPROMs and how they're written to

Question

I've got a problem understanding how an EPROM (UV-erasable) works.

This question may seem very dumb, but what would happen, if I just try to write contents to an EPROM, which has already been written to? Will it just not work? Or will the contents on the EPROM be corrupted? I don't really understand why I can't just overwrite the whole thing, without erasing it first through UV-Exposure.

Maybe somebody with a better understanding of this can explain it to me. Thanks in advance for your help.

Answer 1

EPROM cells can only be electrically written 1->0 and can only be erased with UV 0->1

So you can electrically overwrite as many times as you like, and get the correct result, provided you never want to change a 0 to a 1.

To change even one cell from 0-1 you have to erase the entire array (well, unless you're really accurate with a UV laser and/or masking the bare die, but that's more like hacking).

Answer 2

While it is possible to program unprogrammed bits in a device that has already been programmed, it is possible for programmed bits in the same row as a bit being programmed to experience a phenomenon documented by Atmel as programming erase.

This is caused by asperities on top of the poly on the non-floating gate. These projections can cause a high local electric field that can cause electrons on the floating gates of bits not being programmed to acquire sufficient energy to pass through the gate oxide. Atmel called this "jumping" the oxide barrier.