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Capacitance for microcontroller timing crystal?

Discussion in 'Microcontrollers, Programming and IoT' started by eem2am, Dec 14, 2012.

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  1. eem2am

    eem2am

    422
    0
    Aug 3, 2009
    Hello, we are using an SAM3N00B microcontroller and have a 32.768KHz crystal connected to it, as well as a 4MHz crystal connected to it, (on the other crystal connection pins)

    Do you know which capacitance value goes with these crystals?...Also do you know if we need series resistors or not?

    32.768KHZ crystal datasheet: (ABS25-32.768KHz) = Farnell 1611824
    http://www.abracon.com/Resonators/abs25.pdf

    4MHz crystal datasheet: (ABLS-4.000MHz-B2-T) = Farnell 1652553
    http://www.abracon.com/Resonators/abls.pdf


    Here is the microcontroller datasheet: (SAM3N00B)
    http://www.atmel.com/Images/doc11011.pdf
     
  2. BobK

    BobK

    7,682
    1,688
    Jan 5, 2010
    Multiply the load capacitance by 2 (beause the two are in series) then subtract a little for stray capacitance.

    I..e for the first one, load capacitance 12.5 x 2 = 25pF use 22pF

    for the second, load capacitance 18 x 2 = 36pF use 33pF

    Bob
     
  3. eem2am

    eem2am

    422
    0
    Aug 3, 2009
    Thanks, ....i notice on page 714 (bottom) of the SAM3N datasheet (linked above) that they recomend subtracting the microcontrollers internal oscillator capacitance from the crystals load capacitance and then multiplying that figure by two.

    ....do you think this is not necessary?
     
  4. BobK

    BobK

    7,682
    1,688
    Jan 5, 2010
    My experience is with PICs, which don't seem to have this internal capacitance. So I would go with what your datasheet recommends.

    Bob
     
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