The data books give some pretty anemic figures for 65c22 I/O currents-- you know, the old 3mA low and 400uA high, give or take.

Practical experience told me these VIAs could do far more; so back when we were on the Delphi forum, I experimented with the Rockwell 65c22's I had and posted the results: 90-110mA shorted to ground and pulling up, and 16-20mA shorted to Vcc and pulling down. PB was slightly stronger than the other I/O lines. For practical loads, the valid TTL levels could be achieved with a pull-up resistor of 100 ohms to Vcc or a pull-down resistor of 220 ohms to ground. Obviously it was not symmetrical, and the designers had the TTL-type loads in mind where the low logic level is closer to ground than the high logic level is to Vcc, and the low passes more current.

For a project I'm planning, I wanted to see what the WDC VIA will do-- not that I need 100mA per bit, but I need pull-ups and pull-down resistors on various lines that are a little stronger than what the data sheet says it will support. Hoping (and suspecting) again that the data sheet is all washed up, I repeated the experiment now that I have some WDC parts. This one was more symmetrical, doing 52mA pulling up from a short to ground or pulling down from a short to Vcc. The heaviest load I could put on it and still get a valid TTL low (.8V max) was 180 ohms to Vcc. (That's about 24mA.) As for pulling up with a resistor to ground however, even with 100 ohms I still got over 3.3V (33mA).

Of course the manufacturer does not specifically guarantee these high current capabilities, but I think it's quite safe to assume they will remain in this ballpark. Hopefully someone will be pleased to find this means the 65c22 is more versatile than they thought. There would be no reason to short the outputs in practical use, and I'm sure internal heating would dramatically shorten the life of the part if you regularly run outputs at 24mA or more; but if you need a 50% duty cycle at 6mA when the book says it's not possible, or have a heavyish capacitive load and need a fast rise time, now you know it can handle it.

Garth