Right load capacitor for crystal

Discussion in 'Embedded Systems and Microcontrollers' started by Vindhyachal Takniki, Jan 24, 2015.

  1. Vindhyachal Takniki

    Thread Starter Member

    Nov 3, 2014
    348
    6
    1. I have to connect two crystal , one for RTC & other for main clock, i have selected below crystals:
    http://www.digikey.com/product-search/en?x=0&y=0&lang=en&site=us&keywords=370-1191-ND
    http://www.digikey.com/product-search/en?vendor=0&keywords=SA315H16.00MF15XA-ND

    2. Datasheet of both caps says 12.5pf load capacitance.

    3. Does that mean I have to selected both CL1 & CL2 as 12.5pF each?

    Edit: If yes , 12.5pf caps are not available on digikey , can I use 10pF instead with same cyrstal



    4. Datasheet of MCU for 16Mhz says:
    For CL1 and CL2, it is recommended to use high-quality external ceramic capacitors in the 5 pF to 25 pF range (typ.), designed for high-frequency applications, and selected to match the requirements of the crystal or resonator (see Figure 30). CL1 and CL2 are usually the same size. The crystal manufacturer typically specifies a load capacitance which is the series combination of CL1 and CL2. PCB and MCU pin capacitance must be included (10 pF can be used as a rough estimate of the combined pin and board capacitance) when sizing CL1 and CL2.

    5. Datasheet of MCu for 32.768Khz says:
    For CL1 and CL2 it is recommended to use high-quality external ceramic capacitors in the 5 pF to 15 pF range selected to match the requirements of the crystal or resonator (see Figure 31). CL1 and CL2, are usually the same size. The crystal manufacturer typically specifies a load capacitance which is the series combination of CL1 and CL2.
    Load capacitance CL has the following formula: CL = CL1 x CL2 / (CL1 + CL2) + Cstray where Cstray is the pin capacitance and board or trace PCB-related capacitance. Typically, it is between 2 pF and 7 pF.
     
  2. JohnInTX

    Moderator

    Jun 26, 2012
    2,341
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    I usually start with 10-22pf then tune as described in AN849 and AN826 (from Microchip but applicable to most others). AN849 in particular is an excellent resource.

    Good luck.
     
  3. Vindhyachal Takniki

    Thread Starter Member

    Nov 3, 2014
    348
    6
    @JohnInTX , even if datasheet says 12.5pf, I have used 10pf & its working.
    Thanks
     
  4. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    7,386
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    I've found the exact value of cap not to be too critical. Of course, I've also never measured the exact frequency the thing was running at either.
     
  5. BobTPH

    Active Member

    Jun 5, 2013
    782
    114
    The capacitance given is the series capacitance of the 2 capacitors + stray capacitance. So typically you would use about 2 x C - x, where x is a few pF. In this case I would use 22pF.
    Bob
     
  6. ScottWang

    Moderator

    Aug 23, 2012
    4,853
    767
    Did you measure the frequency?
     
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