Re: 6522 vs 6526 (was Re: 6502 to 8501: new details)

From: Nicolas Welte (
Date: 2002-02-26 10:32:41

Spiro Trikaliotis wrote:
> To Nikolas: I doubt that a 6529 is more common than a 6522, is it? OTOH,
> a 6821 might be usefull for this purpose, but I'm not sure if that is
> more common than the 6522.

You're right of course, the 6529 is not a common chip at all, I only used it
as an example how small a single port I/O chip should be. The 6821 would be a
good choice, but unfortunately it's DDR is on the same address as the data
register itself and you have to select between the two via a write to the
control register. To keep the timing compatible with the original CPU, you
had to clock the 6821 at twice the speed and generate the approprite write to
the control register in hardware. Then there's also the 8255, but I think
that one has the big disadvantage of not being able to configure the data
direction for each bit separately, only one nibble at a time, IIRC.


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