> > Immediately after I read TMR0 I also clear it, and this will prevent
> > a new overflow for some time.
>
> Sorry, I missed this instruction. There is a slim chance that TMR0
> overflows in the same instruction when you read it. As I understand
> from datasheets, read is made in the beginning of instruction cycle,
> and write - in the end. So actually, there is still some room for
> error. I don't know if this can matter. Best way is compare with this
> (INTF part of your routine):

I actually discovered this myself yesterday.
If I read 0xFF from TMR0 the INTF flag is already set, and my check
of the INTF-flag will actually generate a big jitter.

I solved it by checking if my TIMER_STR (copy of TMR0) is 0xFF.
If it is, I just skip the INTF-test.


Like this:

movf TMR0,0                     ;Read TMR0
clrf TMR0
movwf TIMER_STR         ;Store it in TIMER_STR

comf TIMER_STR,0        ;Complementing it, store in W
skpnz                           ;If it was 0xFF the Z flag will be set
goto $+3                        ;and the goto will be performed

btfss INTCON,T0IF       ;These two instructions will be skipped
incf TIMER_STR+1,1      ;only if TIMER_STR == 0xFF


This solved my problem!
Thanks for all help.


> >> >  btfss INTCON,INTF              ;RPM-interrupt?
> >> >  goto nex_chk_1                 ;Nope, skip to end of handler
> >>
> >> >  bsf STAT,0                     ;Signals that an RPM pulse-count
> >> >  is ready
> >>
> >> >  movf TMR0,0
>       btfsc INTCON,T0IF
>        movlw 255
> >> >  clrf TMR0                      ;Clears TMR0
> >> >  movwf TIMER_STR                ;Stores cur. TMR0 value in
> >> >  TIMER_STR low byte ;btfss INTCON,T0IF              ;Timer
> >> >  interrupt pending? ;goto $+3
> >> >  ;Nope, skips next two ;instructions
> >>    ;incf TIMER,1                   ;Increases TIMER
> >> >  manually
> >>    bcf INTCON,T0IF                ;Clears the timer interrupt
> >>
> >> >  movf TIMER,0
> >> >  movwf TIMER_STR+1              ;Stores TIMER in TIMER_STR high
> >> >  byte clrf TIMER
> >>
> >> >  bcf INTCON,INTF                ;Clears the RPM-interrupt
>
> Bye,
>  Nikolai


_______________________________________________
¯yvind Kaurstad, oyvind@safetel.no

Safetel AS
P.B. 405, N-4067 Stavanger, Norway
Tel: +47 51 81 78 80 (Switchboard)
     +47 51 81 78 82 (Office)
     +47 51 81 78 93 (Fax)