Hi everyone,

I'm in need of some help with a program I'm trying to put together.
I'm still a newbie here so this is just something I'm putting
together as a learning exercise and hopefully I'm just missing
something obvious.

Please note that the following is a cut down version of something
much larger that I'm putting together. I've cut it down to the
minimal version that I can that exhibits the behaviour I'm seeing.

I have a circuit with one LED (connected to PORTB,2) and a push
button switch (push-to-make) on PORTB,0. I have an interrupt
service routine which wakes up when the timer expires. The ISR
checks a register which I've named SEQ and if it is set to 1 it
turns the LED on, if it is set to 2 it turns the LED off.
Meanwhile, in the main program it loops around waiting for the pin
that the push button is connected to going low. At that point it
changes SEQ from 1 to 2 and then goes into and idle loop.

So, the program should start and light the LED by default. When I
press the button for the first time the LED should go out and stay
out until the system is reset. However, the LED just comes on and
stays on.

Here is the code:

----------
        list    P=16F84A
        include "p16f84a.inc"


LEDR1   equ     10h
SEQ     equ     17h

        org     0
        goto    Start

Init
        clrf    PORTA
        clrf    PORTB

        goto    InitCont
        goto    ISR             ; MUST BE INSTRUCTION 0x04
InitCont
        bsf     STATUS,5        ; Select bank 1

        movlw   b'0000'
        movwf   TRISA

        movlw   b'00000001'     ; Push button is connected to PORTB,0
        movwf   TRISB           ; LED is connected to PORTB,2

        movlw   b'00000111'     ; 1:256 pre-scaler on TMR0
        movwf   OPTION_REG

        bcf     STATUS,5        ; Select bank 0

        movlw   b'10100000'     ; Enable timer interrupts
        movwf   INTCON

        clrf    LEDR1
        movlw   d'1'
        movwf   SEQ

        retlw   0



ISR
        clrwdt
        bcf     INTCON,3        ; Reset timer interrupt

        movlw   d'1'            ; Subtract 1 from sequence number to
        subwf   SEQ,w           ; make it zero based
        addwf   PCL,f
        goto    Sequence1
        goto    Sequence2

Sequence1
        movlw   b'01111110'     ; Set LED pins high
        movwf   LEDR1
        goto    RefreshLEDs

Sequence2
        movlw   b'00000000'     ; Set LED pins low
        movwf   LEDR1
        goto    RefreshLEDs

RefreshLEDs
        movfw   LEDR1
        movwf   PORTB

        retfie



Start
        call    Init
Main
        clrwdt

        btfsc   PORTB,0
        goto    Main

        movlw   d'2'
        movwf   SEQ

Done
        clrwdt
        goto Done
END
-----------

I'm pretty happy that the circuit is ok as I have a simpler program
(not using interrupts, timers etc) which just turns the LED on
whilst the push button is held down and that works fine in the same
circuit. I can post that program or more details of the circuit if
it helps.

Thanks, and sorry for the longish post,
Matt

--
Matt N. Marsh
Email: matt@mattmarsh.net          Yahoo: marshmn
  Web: http://www.mattmarsh.net/  Jabber: mattmarsh@jabber.org
                                     MSN: matt@mattmarsh.net
                                     ICQ: 250467363
                                     AIM: MattMarshUK

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