erik wrote:

> The maximum frequency would be about 71kHz.  That means that I must
> evaluate the configuration switches, check a timer/ counter, and turn
> on an output, plus execute the rest of the program in about 72
> instructions. (that's if there are no jumps or gotos)

  That sounds like a 20MHz PIC.

> It's becoming pretty clear that I just won't be able to do it.

  This, Erik, is called "programming".  It's actually quite easy once
you get the hang of it. ;-) Your mistake is to assume you must do all
these things on *every* pass of the timing loop.

  Once you drop this assumption, you think in terms of getting the
timing loop done in a certain number of instructions with a few left
over.  You now use those instructions do do *part* of the non-critical
routines each time you do the timing job.

  You may either call the frequency generation code as a subroutine
regularly through the other code, or use a state-machine dispatcher
which returns to the timing code.  The PIC is very much "purpose-built"
to run isosynchronous code.

  I presume the timing code will use a phase accumulator type synth?
--
  Cheers,
        Paul B.