Detecting a Missing Pulse Using a PIC16F873A

GA...The timer can even be just a simple dumb delay loop which counts down, every time the pulse is seen it resets the counter to say 255, if the counter gets down to zero all hell breaks loose it sets the alarm.
Del

GA Simple and concise. No need for extraneous Bullsh*#.

You might find some useful stuff here too:-

Google search for "watchdog timer PIC"

Look for a retrigerable Monostable circuit on the web, a simple circuit using a 555 is more than enough.

I've already designed an analogue circuit using a 556 that then delivers a logic input to the PIC if a pusle is missed but as I have space left I decided to give it a go in code to simplify the circuit, I am a bit of a novice with pic assembly but have something that seems to do the job:

#include <P16F873.inc>

ORG 0x000 ; processor reset vector
clrf PCLATH ; ensure page bits are cleared



COUNT1 equ 20h
COUNT2 equ 21h
COUNT5 equ 24h

main

bsf STATUS,5
movlw 0x03
movwf TRISB
bcf STATUS,5 ;set up portb i/o's

movlw 0x00
movwf PORTB ;clear portb

wait btfss PORTB,0 ;wait for start
goto wait

start movlw 0x03
movwf COUNT3

loop btfsc PORTB,1
goto chkrmton ;go here if portb,1 goes low
call delay3
decfsz COUNT3,1 ;check portb,1 3 counts fail is reaches 0
goto loop


fail bsf PORTB,3
call delay
bcf PORTB,3
call delay
goto fail

chkrmton btfsc PORTB,0 ;if portb,0 is low, pass
goto start
pass bsf PORTB,2
goto pass

delay ;approx .45s @4MHz
movlw 0x06
movwf COUNT5
start3 decfsz COUNT1,1
goto start3
decfsz COUNT2,1
goto start3
decfsz COUNT5,1
goto start3
RETURN

end

This basically looks at portb 0 & 1, portb,0 starts the monitoring on portb,1 if a pulse is missed the delay times out and checks portb,0 and if still hi it jumps to a loop to flash an LED on portb,3. If portb,0 is low it lights an LED to signify no pulses missed.

Any edits to my very basic code graciously accepted

I've found your missing pulse...it was lost out in the snow.
I've brought it in, given it a nice hot drink of cocoa and sat it down in front of the fire.
I shall keep it as a family pet.
Del

I'm glad it's gone to a good home.