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Choosing capacitators for crystal on AVR uC

Discussion in 'Electronic Basics' started by Willem, Feb 14, 2004.

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  1. Willem

    Willem Guest

    Hi all,

    I've made a little hello world at90s8515 uC board with only a couple of
    leds. The crystal specified a capacitance of 32pF so that's what I used but
    the crystal is not oscillating. When I desolder the crystal and put an
    external clock from the STK500 development board it's running fine. I found
    this basic rule about the capacitance you need:

    capacitance = 2 * (LC - trace_capacitance)

    Where:
    LC = capacitance specified by crystal manufacturer
    trace_capacitance = capacitance from traces and wires etc which is typical
    7-10pF

    Is this a good basic rule of thumb? And how much can it be of this
    capacitance? I read it influences the frequency of the crystal a bit but you
    can't push it too much. But I couldn't find how much you can push it and
    if/how this depends on the crystal used.

    Any information is appreciated.

    Regards,
    Willem
     

  2. This is what I found on the net a while ago, which is pretty much the same
    as your formula above.

    Crystal Capacitor Formula (Parallel Mode)
    -----------------------------------------
    Let: CL = Crystal's load capacitance
    CP = Circuit's parasitic capacitance
    CI = Chip's input capacitance
    C1 = Crystal oscillator capacitor 1
    C2 = Crystal oscillator capacitor 2

    Then: C1 = C2 = 2 * CL - (CP + CI)

    Example: For a crystal with 12pF load capacitance (CL)
    and assuming CP + CI = 5pF (typical), we have
    C1 = C2 = 2 * 12pF - 5pF = 19pF. We can then
    use two 18pF capacitors (closest standard
    value).
    ---

    Note that you need a parallel mode crystal and *two* capacitors, which you
    put between the crystal terminals and GND, like this:

    .. C1 ||
    .. .-----||-------+--------
    .. | || |
    .. | _-_ TO
    .. | XTAL |___| MCU
    .. | -
    .. | C2 || |
    .. +-----||-------+--------
    .. | ||
    .. |
    .. ===
    .. GND

    cheers,
    Costas
     
  3. CBarn24050

    CBarn24050 Guest

    Hi, ive allways used 20p and never had a problem on any micro or any crystal.
     
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