Which is the frequency?

[Bruce]

Try this. At 20MHz you should have a 500kHz signal on the pin used.

DEFINE OSC 20
TRISB.0 = 0

ASM
loop ; @ 20MHz 1us high + 1uS low
    bsf portb,0   ; 200nS
    goto $+1      ; 600nS
    goto $+1      ; 1000nS high
    bcf portb,0   ; 200nS
    goto $+1      ; 600nS
    goto loop     ; 1000nS low
ENDASM

[Nitro]

just bear in mind that the pic will divide whatever the clock frequency is by 4
ie: if you have a 20mhz crystal, it will perform funcions @5mhz or one operation per 1/4 millionths of a second.