Jon,

Something like this?
Untried, hope that it helps

        Org     0x00
        Goto    Init

        Org     0x04
        DECFSZ  LED_COUNT
        GOTO    INTERRUPT_END
                        Turn_off_led
                        BCF     INTCON, T0IE    ; Disable Timer0 interrupt
INTERRUPT_END
                        BCF     INTCON, T0IF    ;clear the overflow bit
Retfie          ;end interrupt vector


Init
;Set up Timer0
        MOVLW   b'10000111'
        MOVWF   OPTION_REG
                        ; Prescaler 1:256
                        ; With 4MHz X-tal this will give
                        ; a interrupt 65,536 ms after TMR0
                        ; is enabled

Main
        ; Do your stuff

Turn_on_led
MOVLW   .7      ; The length for sync led
                ; 7 x 65ms = 455 ms
MOVWF   LED_COUNT
CLRF    TMR0    ; Clear Timer0 reg
BSF     INTCON, T0IE ; Enable TMR0 interrupt

GOTO    Main

HŠlsningar/ Regards,
Bjarne Nebelong
Manager Electrical Design
BINDOMATIC¨
mailto:bjarne.nebelong@bindomatic.se
+46-(0)8 709 58 46


To the PIC folks--

   I need a little bit of help.  I'm realitvely new to PICs, even newer to
their interrupts.  i'm writing a bit of code that needs to flash a "sync"
LED on a PIC16C74A using MASM (no basic/c stuff).  The problem is the sync
is too fast and barely visible.  I can't sacrifice the processing speed
(i.e., don't want to insert a NOP delay or the like), so i want to have
timer0 count-up and trigger an interrupt that cuts the LED off.

  Essentially when the code gets a sync -- it'll turn on the LED and start
the timer.  If another sync arrives before the timer expires, the sync will
simply reset the timer.  When the timer expires, it just turns off the LED.
(the LED is a pin on portD).  Simple enough, but it aint workin'.

  The question: can someone run me through (pseudo or code snip-its) what
needs to be done to setup a timer0 interrupt?  Thanks.

Here's my little test setup now (code from memory, not cut/paste)

org 0x00         ;reset vector
goto main

org 0x04         ;interrupt vector
turn off led
BCF INTCON, T0IF  ;clear the overflow bit
retfie            ;end interrupt vector

;sample code that should generate interrupt

main:

turn on LED

BCF   INTCON, T0IF  ;clear overflow bit
CLRF  TIMER0        ;reset timer0
BSF   INTCON, GIE   ;enable inerrupts
BSF   INTCON, T0IE  ;enable timer0 interrupt

;do stuff that just loops quitely, waiting for UART input on ;RX line via
polling

;end of sample code

  The LED should be getting turned of after the timer0 rolls from 0xFF to
0x00?  Right?  The LED never turns off.  Thanks for any help.

--jon