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I'm very familiar with the '22, having used it a lot for a wide range of things. (See the "Tip of the Day" topic in this forum, which has lots of tips about using the '22.) However I've done very little with the '20-- or actually the '21, which is the 6520's CMOS equivalent. This '21 was on a commercially-made STD-bus SBC which also had a '22. Since the '22 is so much better, I'm sure the only reason they had both on the same board was to run legacy software that required the '20. As far as I can tell:
The only advantages the '20 might have over the '22 are the additional chip-select line, and a separate IRQ output for each port. In most cases, these are negligible advantages. Otherwise, the '22 can do everything the 20 can, plus a lot more. The '22 has the counter/timers and synchronous serial port, all with various operating modes. The '20 only has the two parallel ports-- nothing else.
The 6522's pin-out is slightly different from that of the '20, so it cannot be considered a drop-in replacement. In addition, software written for the '20 will have to be modified to do the same job on the '22. Certain operations are just a little more awkward on the the '20. For example, before reading or writing with RS0 low, bit 2 of the appropriate control register must first have been written in order to tell it if you want to access the data direction registers or the ports themselves. Separate addresses in the '22 simplify this.
Garth
_________________ http://WilsonMinesCo.com/ lots of 6502 resources
The "second front page" is http://wilsonminesco.com/links.html .
What's an additional VIA among friends, anyhow?
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