Pic16F88 and a 20mhz Xtal need advice.

Discussion in 'General Electronics Chat' started by Geoffr67, Jan 2, 2012.

  1. Geoffr67

    Thread Starter Member

    Dec 5, 2011
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    Ok im setting up a circuit and while ive used the ceramic osc in 20mhz id like to use a 20mhz crystal this time to provide more accurate timing of my PWM software i wrote. This will be the first time i am useing a 20Mhz Xtal, can anyone with pic experience give me some recommendations as to what crystal to use and a ballpark of Capacitor values to go along with it to establish oscillation?
    Thank you!
     
  2. joeyd999

    AAC Fanatic!

    Jun 6, 2011
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    You need a parallel resonant crystal and a 15-18pf ceramic cap on each lead to ground. It really is that simple.
     
  3. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    If you have a frequency counter, use on 18pf cap on one side, and a trimmer cap on the other until you are at exactly 20Mhz. Make a simple program that toggles an output each time through the main loop and you should get a sold 5Mhz square wave from a PIC if you want to use a scope. If it is off, tweak the trimmer cap a bit until it is on.

    You may want to do this where the device will actually be located, and avoid as much thermal change as possible. If there will be a wide temperature working band, look at an oven controlled crystal instead, for more $$.
     
  4. SgtWookie

    Expert

    Jul 17, 2007
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  5. Geoffr67

    Thread Starter Member

    Dec 5, 2011
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    im on mouser looking for one that fits the bill, but many of the datasheets do not definitively specify if its series or parallel cut crystal.do you have an example of a suitable part?
    Thank you!
     
  6. Geoffr67

    Thread Starter Member

    Dec 5, 2011
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  7. Markd77

    Senior Member

    Sep 7, 2009
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    That datasheet doesn't give much to go on.
    I think that if you look at the mouser page, and if you see a capacitance value (eg. 20pF) in the item description, then it should be a parallel cut crystal.
    If you accidentally get a series one it will most likely still work, but the frequency will be slightly off.

    http://qooljaq.com/CrystalOscillator.htm
     
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