On 31 Aug 2004 at 8:20, Olin Lathrop wrote:

> Specifying an analog quantity as "exactly" is useless, and immediately
> tells us that the person doing the speccing doesn't know what he's
> doing.

    Or maybe it just tells us that we're holding that person to an
    unreasonably (or, at least, unusually) high standard of precision in
    his usage of the language. 

> Since 5mS can't be achieved "exactly", we can only ignore it and take
> our own guess about how close is close enough. 

    Right.  When someone asks for "exactly five milliseconds", but
    doesn't mention anything about the tolerance of his oscillator or
    about theoretical physics, MY guess -- and, I expect, yours -- is
    that he means "exactly five milliseconds, assuming that my
    oscillator is running exactly at its nominal frequency".

    It's like someone asking, "How can I get my PIC to just sit and wait
    forever for a pin to go high?"  I don't think he wants me to answer
    that by explaining how the Second Law of Thermodynamics makes
    "forever" impossible; he'd probably prefer that I just draw up a
    BTFSS/GOTO loop and remind him to disable the watchdog timer. 

> Given that, what's the best way to teach someone to time 5mS on a PIC?
> This is 1000s of instruction cycles even at a modest clock rate, so I
> think a busy loop shouldn't be the first recommendation.

    How likely is it that he needs to do something else while waiting
    those five milliseconds?  He didn't SAY that he did...

    If he's just waiting, a busy loop is by far the easiest and most
    appropriate solution. 

> >         DELAY5K:
> >             CALL    DELAY1K
> >             CALL    DELAY1K
> >             CALL    DELAY1K
> >             CALL    DELAY1K
> >         DELAY1K:
> >             MOVLW   249
> >         LOOP:
> >             ADDLW   -1
> >             SKPZ
> >             BRA     LOOP
> >             RETURN
> 
> If you've decided you want a busy loop for whatever reason, this sort
> of structure is reasonable.

    Thanks.

> However, getting the details just right can be very tricky.  Counting
> cycles isn't as easy as it may seem, especially for a beginner.

    True.  Good thing the PICLIST is here to help those beginners.

    -Andy

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