...doesn't this work?


Dear all

Having served the first part of my pic apprenticeship (making leds flash,
counting on 7 segment displays, etc), I've moved on to meatier things.

My first stab was aimed at producing a measurement of a pulse length
(yes, alright dammit, it's a R/C thing!). Despite extensive de-bugging,
and appearing to work on MPSIM, the wretched thing keeps saying '00'. So,
back to the drawing board, how about an interrupt driven program that just
mirrors the input to an output pin? Here's my version (I'm sure it could be
done in 4 1/2 bytes of program code, but it mirrors the way my other prog's
supposed to work):

        list    p=16f84

#include "c:\progra~1\mplab\p16f84.inc"
left            equ     0x0f
right           equ     0x10
_w              equ     0x11
_status         equ     0x12
        ; You can assume WDT Off, PWRTE On, XT, 4Mhz Resonator
        org 0

        goto main

        org 4

        ; interrupt routine
        movwf _w                        ; save w
        movf status,w           ; and status
        movwf _status
        bcf intcon,intf         ;
        bsf status,rp0
        btfss option_reg,intedg ; are we here for a rising or
        goto pulse_off          ; falling edge

pulse_on
        bcf option_reg,intedg   ; next time, a falling edge
        bcf status,rp0
        bsf porta,2                     ; turn on a pin to match rb:0
        goto int_exit

pulse_off
        bsf option_reg,intedg   ; next time, a rising edge
        bcf status,rp0
        bcf porta,2                     ; turn off to match rb:0

int_exit
        movf _status,w          ; the usual gubbins
        movwf status
        swapf _w
        swapf _w,w                      ;
        retfie

main
        clrf tmr0
        bsf status,rp0
        clrw
        movwf trisa             ; make porta outputs (they are anyway?)
        movlw 1
        movwf trisb             ; make rb:0 an input
        movlw b'11000000'       ; wait for rising edge on rb:0
        movwf option_reg
        bcf status,rp0
        movlw b'10010000'       ; GIE and inte set
        movwf intcon
        clrw
        movwf left              ; stuff left over from my digit
        movwf right             ; displaying routines
        clrf porta
        clrf portb              ; does clrfing portb cause a problem?

loop    goto loop               ; hang about
        end


Again, it works on the simulator, toggling RB:0, but in 'real
life', all pins except RB:0 sit dormant. Can anyone suggest
why?

Regards

John M