Mike,

I know it sounds "anal" but a list of all explicit requirements and any derived (implicit) ones, would be really helpful just to ensure that they're all met?

You also should apprise us of what the components you're going to use will be. For example, while we know you have an 80C51BH MCU, what are the memory devices? This will help with the timing.

From what I've gathered, you've decided that you want to write data to your program store from the PC parallel port. Is that correct? I've also gathered that you want to control the operation of the circuit from the PC parallel port. Is that correct?

Now ... as for the oscillator ... I've routinely made my own crude crystal oscillator by soldering SMT (0805-packaged) 1k0 resistors between pins 13 and 12 and pins 11 and 10 of a 74AC04. By soldering a 150 pf capacitor between pins 12 and 11, I now have an oscillator that will use a crystal connected to pins 10 and 13. In order to make it compatible with "standard" TTL oscillators, I connect pins 10 and 9 and take my output from pin 8. This seems to work quite happily in the range from 4 MHz to 25 MHz, though your results may differ. I've also used 7404, LS04, and HC will probably work as well.

If you want to change the oscillator source, you'll have to construct a circuit of some sort that swaps the clock sources without producing glitches or pulses shorter than the minimum (~41 ns, in this case) that the MCU will tolerate. This is not an easy trick, BTW.

Once you have a list, you can design data paths and switching logic to meet them.

RE