Hi Paul,

The short answer is you can, but we're approaching the limit of what a 'good ol' industry standard' 8051 can do - with or without timers!

Let's see, a 100kHz squarewave has a 10us period during which 5us-on/5us-off.

Now, using a 'standard 8051 - 12MHz' part timing, a jump instruction takes 2 usecs and an Acc increment takes 1 usec. So you may have to try something like this instead:

Start:
nop
cpl p1.5
nop
sjmp start:

Or alternatively, if you want to toggle the waveform on/off:

sqrwave:
jb waveflag,waveform_end
cpl p1.5
sjmp sqrwave

waveform_end:
ajmp main

This code would allow you to control the output waveform from an interrupt service routine.

Valentin