Mr. Chu:

You will need to read the data sheet to figure out how the 8253 chip works.

If I were you I would put the 8235 chip in the junk box and start using a modern 8052 type microcontroller that has a PCA array on board. It is a much more flexible set of timers and your software will have direct on-chip access to the registers instead of you having to build an external bus interface to the 8253.

[At least I hope you were using 82C54 chips]. The old 8253 chips are complete junk. Long years ago I was designing with Intel parts and we were having no end of problems with the 8253 parts. We got all over Intel and finally the Intel FAE gave us a proprietatry internal document that was over 35 pages that was entitled - How the 8253 Really Works. Believe me...this is no joke.

Michael Karas