Capacitor for crystal

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

  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?
     
    ClueLess, Aug 9, 2008
    #1
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  2. ClueLess

    Rich Webb Guest

    First I'd read the datasheets and/or application notes for the
    microcontroller and for the crystal under consideration.
     
    Rich Webb, Aug 9, 2008
    #2
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  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
     
    Spehro Pefhany, Aug 9, 2008
    #3
  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
     
    ClueLess, Aug 10, 2008
    #4
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