Crystal capacitance

Discussion in 'General Electronics Chat' started by Flappie, Jan 9, 2014.

  1. Flappie

    Thread Starter New Member

    Jan 7, 2014

    I've been searching this forum and google'd a lot about it, but I can't find a clear answer to my question. Or let's say I don't know what's best for me.

    I'm using an ATmega644A-PU and I want to connect an external crystal of 20MHz to it. It will be connected like this:
    The crystal has a HC49 socket. I haven't bought the crystal yet, but I can choose between a crystal with a load capacitance (CL) of 50pF or one with a CL of 20pF. I can also choose one with a CL of 18pF (costs 1 cent more, so that won't be an issue :p).

    I also haven't bought the capacitors C1 and C2 yet.

    My question: which crystal is my best choice (what CL value)? What value do I choose for C1 & C2?

    The datasheet of my MCU tells me the recommended range for C1 & C2 is 12 - 22pF.

    The datasheets of these crystals tell me the shunt capacitance (C0) is 7pF maximum (I don't know whether this is relevant or not).

    Since I haven't bought any of these components yet, I'm trying to determine which components to buy in order to get the best setup.

    Thank you very much in advance!!
  2. Galen

    New Member

    Jan 9, 2014
    I used to have crystals made for Mil Std equipment. Generally 20pf was the load value. It should be noted that if you get an order of 100 crystals, some may not be as tunable as others. Start-up is also an issue for certain circuit designs. The low frequency you are using shouldn't be a problem. Old timers would unsolder the case and lower the frequency with pencil lead or raise the frequency by pencil eraser.
  3. Flappie

    Thread Starter New Member

    Jan 7, 2014
    Thanks for your reply.

    I heard from others that 18pF for C1 & C2 is a 'general taken value.'
    Would it be a good setup to take 18pF for C1 and a crystal with a load capacitance of 20pF?
  4. MrChips


    Oct 2, 2009
    18pF is typical. You are not going to experience any problems with 12-22pF.
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  5. bertus


    Apr 5, 2008

    The attached PDF's will tell you somethings about the loading capacitors.

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  6. crutschow


    Mar 14, 2008
    Remember that your wiring and the microprocessor input will add several pF of stray capacitance which should be subtracted from the value for the added capacitors.
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  7. alfacliff

    Well-Known Member

    Dec 13, 2013
    unless your trying for extreme accuracy on the oscilator, dont worry about it too much the load capacitance will affect the crystal frequency a little bit. if too big, it may not oscilate.
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