help with guitar 'gate' effect

i'm using a 16f873A PIC with 2 8-bit DIP switches connected to ports B and C (inputs). One of the pins on port A is connected to the gate of a FET, which forms part of a variable gain circuit through which the guitar signal passes. my plan is that the user sets each of the 16 switches to 1 or 0 to form a musical pattern as in a 16 step sequencer. the jfet cuts the signal level in and out.

I'm stuck with how to implement a tempo control? ie should I use an external clock and use a main loop that has fixed duration, or keep the system clock fixed, read in a pot using an ADC pin and do something fancy with the code?

damn, actually i can see a way to achieve what i want - with a tempo clock clocking a 4 bit counter controlling a 16 to 1 MUX. moral of the story: keep it is simple, don't assume the answer to everthing is a pic!