Capacitor for crystal

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
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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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"it's the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
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Spehro Pefhany
crystal
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
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ClueLess
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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jim

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