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CC1354P10: 32kHz Load Cap Values in the EVM appear wrong on the Schematic and BOM

Blake Carpenter
Expert 7045 points
Part Number: CC1354P10


Hi Wireless Support, 

My customer noted the lack of identical load caps in the 32.768kHz crystal on the EVM.  https://www.ti.com/tool/LP-EM-CC1354P10

Note they show 12pF and 15pF which is unusual as they are normally identical values unless tuned empirically with a scope and a pure frequency (a lot of work).  OR is it just a typo?

If we use the following calculator (I checked  the underlying math) https://ecsxtal.com/crystal-load-capacitance-calculator/

I see that our chosen part on the EVM - TZ3581F TAI-SAW  https://www.taisaw.com/assets/PDF/TZ3581A%20_Rev.1.pdf gives us a 7pF CL 

Assuming ~2pF on the stray board capacitance gives us a 10pF value and ~5pF gives us 4pF.

Did we assume ~1pF on stray capacitance?  OR did we actually measure the values by putting out a pure frequency and tuning the caps as implied here https://www.ti.com/lit/an/swra372c/swra372c.pdf ?

If the schematic is correct and we intended to make these two values different we should probably make a note on the schematic or we will answer this question a dozen times.

Also let me know if my calcs were in error above.

Thanks!   

Blake

  • +1
    TI__Mastermind 18695 points

    Hi Blake,

    These values (12 pF and 15 pF) are intentional and were fine-tuned in the lab to account for the parasitic capacitances of the PCB layout.

    So, if the customer is copying the reference design we would recommend using the 12/15 pF combination to reduce the risk of needing to fine-tune it on their board.

    A more detailed App Note covering the crystal oscillator theory and selection is SWRA495 (Crystal Oscillator and Crystal Selection for the CC26xx and CC13xx Family of Wireless MCUs): https://www.ti.com/lit/swra495 - I would recommend referring to this for our CC13xx devices.

    Regards,

    Zack

  • 0 in reply to ZC
    TI__Expert 7045 points

    Thanks Zack!  Great answer - if we keep getting this question - we might make a note on the schematic.  I notice there are several other notes on that page already.

    Thanks!

    Blake