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CC2340R5: Sizing of LEXT capacitor?

Cameron Wutzke
Expert 3542 points
Part Number: CC2340R5

Tool/software:

Hey!

How should I size my 32.768KHz oscillator caps? Can't find a portion in the datasheet. Couldn't get a clear answer from this guy either: https://www.ti.com/lit/an/swra495k/swra495k.pdf 

Here's how we do it on the EVM: I notice one cap is 12pF and one is 15pF. Curious if there's a reason they're different and if I should do the same. For reference, I'm using this external crystal: (should fall in the LEXT datasheet spec) 

Thanks!~ 

  • 0
    TI__Genius 17955 points

    Hi,

    The total load capacitance is what matters, so the capacitors C81 and C91 were tuned to a more exact value of total capacitance (hence why they are not equal values).

    The design files are found here: LP-EM-CC2340R5 (SWRR381): https://www.ti.com/lit/zip/swrc381

    From the BOM, C81 and C91 are 0201 component size.

    Regards,

    Zack

  • 0 in reply to ZC
    TI__Expert 3542 points

    Thanks Zack!

    ...

    I am confused by the datasheet spec. I see the y1 on BOM is ~7pF:

    Wouldn't the 12pf / 15pf be too much? Thanks

  • 0 in reply to Cameron Wutzke
    TI__Genius 17955 points

    Hi,

    There are 2 factors to consider:

    • The load capacitors are actually in series when considering the total crystal oscillator circuit; for an explanation of this see Section 2.3 and Section 3.3 of SWRA495 (Crystal Oscillator and Crystal Selection for the CC26xx and CC13xx Family of Wireless MCUs): www.ti.com/lit/swra495 
    • The PCB parastics must also be taken into account.

    Regards,

    Zack