;
; insert reset & interrupt vectors!
;

start:
mov TMOD#21h
mov TH0,#0d8h
mov TL0,#0f0h
mov TCON,#50h
setb TR0
mov IE,#82h
loop:
sjmp loop

timer0_isr:
cpl P1.0
clr TR0
mov TH0,#0d8h
mov TL0,#0f0h
setb TR0
reti

This code should generate a square wave (or reasonable approximation thereof). Your job: get it working, figure out WHY it works and modify for taste! Bonus points if you calculate the worst case error! Extra Bonus points if you modify it to generate three outputs at 120degrees phase shift between each (3 phase) and sweep the output frequency from 50 -100Hz at 1Hz/second. Super bonus is to use PWM to generate an approximate sine wave.