Re: 6526 failure modes?

From: Nicolas Welte (
Date: 2002-02-14 10:14:54

Sam Laur wrote:
> There are a few different possible scenarios. The worst is that
> the whole chip gets fried either due to latch-up or if the static
> charge is large enough to destroy other stuff besides the I/Os.

The strangest failure mode I ever saw (two times!) is a leakage current on
one or more of the CIA host interface pins. I had one CIA that somewhow
pulled the reset line so much, that the C64 resettet from time to time. Also,
it put a load onto the data lines in such a way that everything in the C64
still worked, except the color RAM couldn't be read by the CPU anymore. The
explanation is that the color RAM data is routed through a 4066 analog switch
to the CPU, and the signal becomes too weak to override the noise/load from
the faulty CIA. You can imagine that I had some trouble to find the faulty
chip in this C64!

The other CIA behaved almost the same (color RAM failure), but it didn't have
problems with the reset line. I found this fault rather quickly :-) It's
interesting that I'm pretty sure both CIAs were rather old 6526R4 models from
'83 or '84. Since then I'll try to stay away from using those as a spare part
or even using a C64 with them installed for interfacing anything.


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