> Hello Xyvind,

Not to be picky, but my first name does not start with an 'X'...

It's a Norwegian special character that is probably displayed as an
'X' on your screen, but it really looks like an 'O' with a slash
across.

It does sound a bit exotic, though... Xyvind...

> Again, you leave the room for error. Even if you read TMR0 and then
> check for TMR0 overflow, this doesn't solve the problem. TMR0 can read
> 255 and overflow before you check T0IF. So you get the error again.

Hmm.
I don't understand this.

If TMR0 overflows sometime in the beginning of the ISR (during
context save, for instance) this will be detected by my check.

I will read a low TMR0 value, but the high byte will be increased
since I check the INTCON,T0IF flag a tad later.

Immediately after I read TMR0 I also clear it, and this will prevent
a new overflow for some time.

Can you please explain in detail how this can go wrong?

>  Nikolai
>
> On Tuesday, February 08, 2000 Xyvind Kaurstad wrote:
> [snip]
> > The current ISR looks like this:
>
> >  org 0x04
>
> >  movwf W_TEMP
> >  swapf STATUS,0
> >  bcf STATUS,RP0
> >  movwf STAT_TEMP                ;Saves W and STATUS registers
>
> >  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
> >  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
>
> > nex_chk_1
>
> >  btfss INTCON,T0IF              ;Timer interrupt?
> >  goto end_handler               ;Nope, skip to end of handler
>
> >  incf TIMER,1                   ;Increase timer high byte by 1
> >  bcf INTCON,T0IF                ;Clears the T0IF interrupt
>
> > end_handler
>
> >  swapf STAT_TEMP,0
> >  movwf STATUS
> >  swapf W_TEMP,1
> >  swapf W_TEMP,0                 ;Restores W and STATUS registers
> >  retfie
>
>
> > Any good hints on jitter improvement?
>
> > The external interrupt is in the frequency range of 0 to 350 Hz, and
> > the TMR0 module has a prescaler of 1:2. The processor is running at
> > 10MHz.
>
> > -Xyvind
>
> > _______________________________________________
> > Xyvind 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)


_______________________________________________
¯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)