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C6748 RTC crystal capacitors

Dean Hofstetter
Expert 2585 points

The C6748 datasheet doesn't specify what capacitors to use with the 32.768kHz crystal.  What is the recommendation?

As a side note, the main DSP crystal oscillator says it wants 10-20pF of load capacitance, and the load capacitance is the SERIES combination of C1 and C2.  We are using 33pF caps for C1 and C2, which should result in a 16.5pF of load capacitance.  Is this correct?

Thanks, Dean

  • 0
    TI__Guru**** 177776 points

    Dean

    Capacitor requirement typically comes with the crystal. In general, 22pF would be a good choice for the crystal, the key thing is to probe the crystal output and make sure you are getting the 32.768kHz frequency out of the crystal.

    For capacitor in parallel, total capacitance is the sum of the individual capacitors' capacitances.

    Thanks

    David

  • 0 in reply to David (ASIC) Liu
    Expert 2585 points

    David

    Thanks for taking a stab at my question, but I'm still confused.  If the capacitor requirements "typically come from the crystal", then why does the C6748 datasheet specify 10-20pF of load capacitance for the main DSP crystal.  I'm still after the caps recommended for the RTC crystal, and I only bring up that point about main DSP crystal to counter your point.

    I've looked at a few EVM board built around the C6748.  All of them have 18pF for the two caps in the RTC circuit, which is close to your 22pF (and where you came up with that number, I have no idea).

    You also failed to answer my side note question from the original post.  I know that for capacitors in parallel you add them up.  However, the C6748 datasheet says SERIAL.  I'm just making sure the datasheet really means SERIAL, and that 33pF for the DSP crystal caps is fine.

    Thanks, Dean

  • 0 in reply to Dean Hofstetter
    TI__Guru**** 177776 points

    Dean

    The datasheet can only give a general range of the load capacitance. Actual load capacitance varies from crystal to crystal depending on the manufacturer, tolerance, etc. I did a search on the digikey site for a 32.768kHz crystal, and load capacitance varies from 6pF to 12.5pF, http://www.digikey.com/product-search/en/crystals-and-oscillators/crystals/852333?k=32.768kHz. Other crystals, as you found out, has 18pF load capacitance.

    On your side question, my take is that when it says serial combination, it means adding C1 and C2 capacitance together, not necessary mean they are in series with each other.

    Thanks
    David

  • 0 in reply to David (ASIC) Liu
    Expert 2585 points

    As far as the RTC caps, I'll go with the crystal recommendation, since there appears to be no specific C6748 requirement.  As I said, multiple EVM boards used 18pF, and our crystal datasheet says 18pF, so 18pF it is.  Thanks for answering that part of my question.

    As far as my side question, now you are starting to understand my confusion for the main DSP crystal.  I also originally read the datasheet to mean adding C1 and C2 together.  The datasheet says it wants 10-20pF of total capacitance, and if we assume parallel caps, that means C1 and C2 should be 5-10pF.  However, every EVM board I've looked at shows C1 and C2 being 33pF.  If you now compute the total capacitance of C1 and C2 in SERIES with a 33pF value, you get 16.5pF (which is right in the sweet spot).  This is also what started me down the whole RTC path.  Why would the DSP crystal circuit be so picky and require 10-20pF of load capacitance, and yet the RTC circuit has no requirement.

    What do you think?  33pF for C1 and C2 for the main DSP crystal?

    Thanks, Dean

  • 0 in reply to Dean Hofstetter
    TI__Guru**** 177776 points

    Dean

    If I read the whole sentence, it says "Typical load capacitance values are 10-20 pF, where the load capacitance is the series combination of C1 and C2." So I have to agree with you that 33pF is for C1 and C2, which would give each capacitor the capacitance of 16.5pF.

    For RTC, similiar requirement should be in place, I don't know why it is not specified in the datasheet.

    Thanks

    David