Re: Playing samples on both new and old SIDs

From: Nicolas Welte (
Date: 2000-07-27 17:38:02

Richard Atkinson wrote:
> Whoops, it was in Knowledge Base and there's a full explanation from
> Nicolas about it being due to faulty CIAs not the 8500 or anything else.
> Incidentally, Nicolas, is this the 8521 you're referring to? I have a 64C
> in which one of the CIAs is an 8521 and the other is a 6526A! So perhaps
> Commodore were aware of the bug in the 8521 and used up remaining supplies
> somewhere where it wouldn't matter.

There must be far more versions of the CIA than the type numbers tell us.
The chips with the different timing that cause the problems are normal
MOS/CSG 6526 (1,2,3 MHz) with a date code of '87 and later. They were
introduced with the C64E and the 128DCR and were not replaced till the end.
I bet those are HMOS-II versions, even if they're still labeled 6526.

In 1986, Commodore built some C64 and 128 machines with the strange chip
combination you described above. One is a 8521, the other is a normal 6526
(pre 1987). There must be a bug in the 8521 for sure, but this chip has at
least the same IRQ timing for the timers as the old 6526 chips! I don't
remember which of the two CIA slots has the 8521, do you? Judging from the
type number, the 8521 was Commodore's first attempt to make a 6526
compatible HMOS-II CIA. The 8520 was introduced much earlier with the
Amiga, but we all know it has the TOD clock replaced with a 24bit binary
event counter.

And then there is the good old NMOS 6526. I found that especially some
versions from 1984 that are marked 6526R4 die with a stupid failure: they
draw too much current from one or more internal data lines and mess up
reading from the colour RAM. I just observed this in the second machine.
Too bad, the chips are otherwise still okay, but one had to put them behind
a wall of bus drivers to keep the rest of the system running. 

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