At 09:31 AM 1/7/2004 -0500, you wrote:
>At 03:09 PM 1/7/2004 +0100, you wrote:
>>Hi all,
>>
>>I am now designing a electronic metronome for myself, PIC-based of course.
>>As an extra, since I'm using crystal precision anyway, I thought of
>>including a 440 Hz (an A note) reference tone to tune flutes etc.
>>But I can't find any standard (cheap) crystal that divides nicely in 440 Hz.
>>Do you know a simple solution, that does not introduce audible jitter
>>(that would effectively ruin the use as a tuner)?
>>Are self-adjusting timer loops good enough?
>>
>>Well, thanks in advance again,
>
>How about just using a capture-compare module and whatever crystal you like?

P.S. with a 4MHz crystal, you'd use 1e6/880 = 1136 for the adder for the
timer. That would be a nominal 440.14 Hz with zero jitter. Using a 20MHz
crystal, you would use 5682 for the adder, which would yield a nominal
439.986Hz, which is within 0.0031%.

Using a 16MHz crystal and timing of 4545/4546 (still no jitter) will
be as close to perfect as you could want (10ppm), while using very little
processor time. You'd use an interrupt to update the compare register-
the only requirement is that it be serviced within 1.1msec after the
compare hit.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

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