"David E. Olson" wrote:
>
> For those that replied to my 16-bit arithmetic. It all works now. Went the
> route of shifting and ORing.
>
> Next issue:
>
> I'm working on a speedometer where I'm using CCP1 and firing an interrupt on
> every falling edge so I can count the pulse. Then I have TMR1 rollover with
> a 1:8 prescale. In that interrupt, I'm storing my pulse count, clearing the
> pulse counter and moving on.
>
> This all works great but, every so often, TMR1 does not rollover properly.
> Sometimes it skips 1 or 2 cycles so my pulse count suddenly doubles or
> triples. Rather not indicate a sudden acceleration when there isn't one.
>
> Any way to stop this? I can do some threshold logic to make sure it doesn't
> count when I'm, let's say, 50% on either side of the previous value but, it
> seems like a hack.
>
> I'd like to stick with the pulse counting model since I need to also get an
> odometer out of this. The pulse count route seems to be an easier way to get
> the odometer and the speed at the same time.
>
> Think I should be using period measurement instead? At least I'd toss out
> the TMR interrupt and just have to deal with CCP...
>
> TIA
>
> -DO

I did a similar thing like this.
Capture on falling edge at 1:8 TMR1 prescale.

; Interrupt routine may capture many speed values,
; but the capture registers are only read every 500mS
; for a display update.

irqvec    movwf W_Hold        ; store w reg - ram page 0, 1 or whatever
        swapf status,w
        bcf status,rp0      ; ram page 0
        movwf S_Hold        ; store status in RP 0

        clrf status

        clrf tmr1L      ; reset tmr1
        clrf tmr1H

        bcf pir1,ccp1if ; (2) clear capture interrupt
        bsf flag1,Dspeed        ; (2) flag data available

        swapf S_Hold,w      ; restore Status
        movwf status
        swapf W_Hold        ; W reg restored from ram page
        swapf W_Hold,w
        retfie

When main loop is ready to update the displays, it checks if DSpeed is
set, and if so calls SpeedCal.

; It was determined during a calibration routine
; that at 64KPH, the capture value is 30Dh
;
; multiplied by 64, the value is C340h, which is the calibration
constant.
; Any speed value will do, but 64 is easy to multiply with.
;
; EXAMPLE
;
; While running, a value of 26Dh is captured.
;
; C340 / 26D = 50h -> 80KPH
;
; KPH result is in nratorL after division routine
; Division is standard Microchip 16 bit / 16 bit
;
SpeedCal  clrf intcon   ; disable interrupts
        clrf intcon

        movf ccpr1L,w   ; get capture data
        movwf denomL
        movf ccpr1H,w
        movwf denomH
        movf BaseL,w    ; calib H
        movwf nratorL
        movf BaseH,w    ; calib L
        movwf nratorH
        call divizn

; do what ever with speed data
; re-enable interrupts

        bcf pir1,ccp1if ; (2) clear capture interrupt
        movlw b'11000000'       ; re-enable interrupt
        movwf intcon    ; gie and peie = 1
        return

--
Best regards

Tony

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