It depends quite a bit on 3 main factors...

1) What part derivative are you using? IE need to know what the clock to bus cycle divisor is.

2) What frequency oscillator is your procesor running at?

3) How do you want to generate the timing? By software or by timer?

If you answer these questions we can be of more help.


If you had standard 8051 with 12MHz oscillator (Bus cycles of 1uSec) and wanted to generate the highest possible frequency by way of software on P1.0 you would write code as:

LOOP:
  SETB P1.0 ;(2 bus cycles)
  CLR P1.0 ;(2 bus cycles)
  SJMP LOOP ;(2 bus cycles)
------------------------------------
---------------Total: (6 bus cycles) - Freq = 166.66 KHz


To make it a square wave it would be slower but the code:

LOOP_A:
  SETB P1.0 ;(2 bus cycles)
  SJMP LOOP_B ;(2 bus cycles)
LOOP_B:
  CLR P1.0 ;(2 bus cycles)
  SJMP LOOP_A ;(2 bus cycles)
------------------------------------
---------------Total: (8 bus cycles) - Freq = 125 KHz


Michael Karas