> -----Original Message-----
> From: Graham North [SMTP:graham_from_malton@YAHOO.CO.UK]
> Sent: Thursday, July 03, 2003 9:38 AM
> To:   PICLIST@MITVMA.MIT.EDU
> Subject:      [PIC]: 18FXX20 Timer 3 as external counter?
>
> Ops, forgot the tag last time.
>
> Hi everyone,
>
> I'm trying to use Timer 3 on the 18F8720 to count the number of pulses. I
> have a function generator connected to RC0 outputing a square wave, but am
> getting nothing.
>
> Timer 3 is set up as below:
>
> ;***********************************************************************
> ; Timer 3 --  Used to count the count.
> ;***********************************************************************
>  movlw B'00000110'   ; Timer 3 Off, No Prescale, Not Sychronised
>  movwf T3CON     ; Ext input
>  bcf IPR2,TMR3IP   ; Timer 3 low priority interrupt
>  bsf PIE2,TMR3IE   ; Timer 3 Interrupt enable
>  clrf TMR3H
>  clrf TMR3L
>  bsf T3CON,TMR3ON  ; Start Timer 3 (Count)
>
>
> I'll try and describe what I'm trying to do here. I have a circuit
> outputting pulses, I want to count how many I get in 25ms. The max freq
> the pulses will be is 100kHz, so the 16 bit counter should never overflow.
> The interupt is there just incase something screwy happens and it does
> overflow.
>
> So every 25ms I read TMR3L and TMR3H, then clear them, wait 25ms and read
> them again, forever! I currently have a function generator attached to RC0
> putting in a square wave to simulate the circuitry. Every time I read
> TMR3L and TMR3H they are zero.
>
> Have I done something stupid? Or missed something obvious? I've been
> trying to debug this all afternoon with no luck.
>
>
The first thing that springs to mind is that you have cleared bit 0 of T3CON
which means that timer 3 is stopped:

 movlw B'00000110'   ; Timer 3 Off, No Prescale, Not Sychronised

Should be

 movlw B'00000111'

Regards

Mike


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