The 82C55 chip does not have a CLK pin so it does not have a direct specification of a maximum clock rate that is simple to calculate. Instead the maximum frequency of the part is related to how fast you can perform the read and write cycles to the part. You need to check the data sheet for the actual part you are using and then compare the data sheet timing specifications for the read and write cycles to the timing you are generating from your microcontroller. An oscilloscope is very handy for this process.

In your application, compatibility at the higher crystal frequency depends upon how you have the 82C55 connected to the microcontroller. (I.E if you are using the external bus inteface versus hanging the part on port pins and bit banging the control and data lines to the part). It also depends a lot on what "speed grade" of the 82C55 that you are using. When these chips were generally available they came in flavors of "standard speed", "-5", and "-8".

If you are making a production design you should consider carefully the use of the 82C55. This chip is getting increasingly harder to purchase as more and more manufactureres drop it from their production line.

Michael Karas