On Thu, 27 Feb 2003, Wouter van Ooijen wrote:

> coding challenge:
>
> - 14-bit core PIC
> - forget about paging and banking issues
> - smallest possible code
> - assembler
>
> TMR1 is a free-running 16-bit timer that can be read as two file
> registers TMR1H and TMR1L. Note that there is no buffering in the
> reading, so for 00FF you might read FF as low byte and then 01 as high
> byte (just after the rollover).

How about this for reading the clock:

    movf   TMR1L,W       ; grab TMR1
    movwf  TMR1L_buff
    movf   TMR1H,W
    movwf  TMR1H_buff

  ; Check for rollover. If the low byte increments from ff to 00 then
  ; there's a rollover and we need to advance the high byte by one count
  ; However, the low byte may be any value between 0xfd and 0xff at
  ; the point we read it above. If it was then, well we have a rollover.
  ; One way to capture this condition is by monitoring the upper bit
  ; of TMR1L. If it is high when we read it above, and low when we read
  ; it next (just below), then a rollover has occurred. In fact, we
  ; have about 255 cycles to capture this rollover condition -
  ; anywhere from the first read being 0x80 to 0xff and the second
  ; read being 00 to 0x7f.  If interrupts are enabled, then we need
  ; to ensure no isr takes longer than 255 cycles.
  ;
    comf   TMR1L,W       ;
    andlw  0x80
    skpz
     incf  TMR1H_buff,F



>
> I have two file register bytes tH and tL that represent a TMR1 value
> that lies in the future. I want to wait until this 'moment' has arrived
> (or passed). Note that due to the above reading problem, the possibility
> of interrupts and one more issue (think for yourself) it is no use to
> loop and wait for a 'perfect match'. Looking for (tH,tL) >=
> (TMR1H,TMR1L) won't do either, because the tH,tL 'moment' might be
> beyond a roll-over from FFFF to 0000.
>
> Note that there are actually two challenges: first find a suitable
> algorithm, then encode it in assembler.
>
> The application of such code is to create a timing loop with zero
> cumulative error.


 Now check to see if you're close to the target value.

   if( abs(target - current) < threshold)
     you're close.

If threshold is 255, then this untested code might do the trick:

    movf   T1L,W
    subwf  TMR1L_Buff,F
    movf   T1H,W
    skpc
     incfsz T1H,W
      subwf TMR1H_buff,F

    skpnz
     goto   close

    skpc
     incfsz TMR1H_buff,W
      goto  not_close


This can be optimized....

Scott

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