I'be been asked to implement a square wave with a frequency of 100KHz. However, I am not allowed to use timers. I know how to implement this with timers but am unsure how to do it without.

I was thinking of using a counter to count from a certain number down to 0 and then every time this loop finishes, complement the output. Does this sound right? If so, how can I figure out the number that needs to be counted down from (x in the code below). Using the fact that the 8051 has a 12Mhz crystal I thought I can do this: (Can I?)

Start: mov A, #x
Loop: dec A
jnz loop
cpl p1.5
jmp Start