Re: C= IC's

From: Nicolas Welte (
Date: 2000-12-01 09:34:56

David Ross wrote:
> On Thu, 30 Nov 2000 wrote:
> > The 8520 is also in one of my 1571's.  My other 1571 uses a 6526 in the
> > same socket.
> What's the difference between the 8520 and the 6526?  Isn't it something
> with the TOD clock?

Correct. The 6526 (and 8521) have a BCD counter that is fed by the TOD
input through a configurable divider with a factor of 5 or 6, making it
count 10ths of seconds on 50 and 60Hz power lines. The 8520 on the other
hand does not have the divider and the configure bit for it, and has a
straight binary counter instead. It simply counts events on the TOD
input directly. Still funny that the technical documents on the 8520
still call it a TOD input (which means Time Of Day).

There are several different CIA implementations that can be
distinguished either by part number or software behaviour:
6526 (pre 1987)
8521 (1986 only?)
6526 (past 1987)

The two different 6526 have a different interrupt timing that differs by
one clock cycle on some events, but I don't know the exact details. The
8521 should have been a replacement for the first 6526, it even has the
same IRQ timing, but somehow even Commodore put only one of them into
each C64/C128. Some issue must have prevented its use in the other
socket. And the 8520 has that different TOD behaviour.

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