Hi Michael,

There's always this kind of a problem when you try to use someone else's program for your needs, especially when it is lacking in comments. I'm not going to do your homework for you, but here's what I would do if I were you:

First I would get the datasheets for both of the key chips (89C51 and 8253). You need to fully understand how they work.

Second I'd go back to where I found the code to find out more about it. For example, was it an example of good working code, or did it have a problem that you've now inherited?

Since this is a two chip problem, running a simulator alone won't help, unless you write your own and include both chips. I've written a simple 8051 simulator as an exercise in understanding the hardware better; I'd probably modify a copy to implement the second chip. You'll probably need to prototype the hardware, and spend some time with a scope looking to see how it works.

Once working, then I would try modifying the code so it better suits my needs.

If you don't understand the hardware and the existing code first, success will be much more difficult.

Good luck!

Dennis