© Raimond Spekking / CC BY-SA 4.0 (via Wikimedia Commons), Mitsubishi M5L2764K-5377 , CC BY-SA 4.0

(Not a picture of brand that suffered the problem, but could happen to any brand.)

Today I was working with some ancient 2764 8kB EPROMs. The symptom I was seeing was that the firmware only partially worked in the circuit. A bit later on, even fewer functions worked. Right away I suspected the EPROM of bit rot. Another EPROM of a different brand worked fine. There are various explanations for this but it all comes down to age and sensitivity of the on-chip amplifiers dropping. May also have to do with the programming algorithm. It might work better if I zap it more than once next time I program that chip. And to wait a while before verifying the EPROM against the file.

I might try to write a self-checksumming routine for firmware. Software like SRecord can automatically generate a checksum to be inserted in spare bytes. Question is what to do when the self-check fails. Blink a LED perhaps.

Addendum: Testing shows that appears to be more likely to happen with EPROMs that require 21V programming. I've read a post on EEVblog that the the programmer I have, the TL866A, isn't really capable of attaining that voltage even though it's advertised so. Newer TL866 models removed the 21V option. So the EPROM may be incompletely programmed and may "fade" with time. Fortunately I have 12.5V programming voltage EPROMs that I can use, and can retire the older chips.