This routine is pretty crude, but it's a starting point.
RA0 will change state approx every second with a 4MHz crystal.

list p=16f84,c=140    ; processor type
        ;
TimerH equ 0ch
TimerM equ 0dh
TimerL equ 0eh
PAData equ 0fh
Status equ 03h
PortA  equ 05h
rp0    equ 5h
        ;
        org 0h
        ;
        movlw 5h
        movwf TimerH
        clrf TimerM
        clrf TimerL
        clrf PAData
        bsf Status,rp0
        clrf PortA        ; all outputs
        bcf Status,rp0
        clrf PortA
        ;
Timer   decfsz TimerL     ; time wasting loop
        goto Timer
        decfsz TimerM
        goto Timer
        decfsz TimerH
        goto Timer
        ;
        movf PAData,w     ; send new data to PortA
        movwf PortA
        xorlw 1h          ; alternate bit 0
        movwf PAData
        movlw 5h
        movwf TimerH
        goto Timer
        ;
        ;
end

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