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Capacitor for crystal

Discussion in '8bit Microcontrollers' started by ClueLess, Aug 9, 2008.

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  1. ClueLess

    ClueLess Guest

    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?
  2. Rich Webb

    Rich Webb Guest

    First I'd read the datasheets and/or application notes for the
    microcontroller and for the crystal under consideration.
  3. 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
  4. ClueLess

    ClueLess Guest

    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
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