Capacitor for crystal

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For very slow crystals it is necessary to add a series resistance in the crystal
circuit. How do you calculate its value with respect to the two capacitors that
connect the crystal to the ground?

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Thanks for your time and attention

ClueLess

Re: Capacitor for crystal
On Sat, 09 Aug 2008 19:32:53 +0530, the renowned ClueLess

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The resistor is there to limit drive power.. so it depends on crystal
characteristics (ESR, max drive power) and other things (drive
voltage, load caps). It's not a straightforward calculation,
particularly if you want to minimize power consumption and/or drive
power. Overdriving the crystal can lead to excessive drift or failure.
Typically 32kHz-ish crystals have *maximum* drive powers in the 1uW to
100nW range.


Best regards,
Spehro Pefhany
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Re: Capacitor for crystal
On Sat, 09 Aug 2008 10:59:14 -0400, Spehro Pefhany

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crystal
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Thanks for your response.

Actually the micro is running slower, say about 5%, than what it should (a
thirty second operation takes place in 31.5 seconds) so I was wondering whether
the RC values have any bearing.

Perhaps I should try to change the C value by inserting a trimmer instead of the
fixed cap.

Thanks again for your time and help

--

Thanks for your time and attention

ClueLess

Re: Capacitor for crystal

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it sounds like the intended operation is running slower than expected rather
than the crystal itself. if you can't measure the crystal frequency assume
it is close enough and adjust your timing withing the routine via software.
off the top of my head, adding C to the circuit will probably slow it
further... j



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