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Load Capacitance on F28335 controlCARD

Gautam Iyer
Guru 192875 points

Hi Tiers,

Here's what I observed, controlCard has a crystal oscillator with Digikey part number: 644-1043-1-ND ie NX5032GA-30. http://media.digikey.com/pdf/Data%20Sheets/NDK%20PDFs/NX5032GA.pdf

If you check its datasheet, the load capacitance mentioned is 8pF but that on F28335 controlCard its 33pF (the same is mentioned in its BOM and Schematic). Generally "Wrong load capacitance might result in an inaccurate clock" with many more side-effects.

Actually I've tested my product with 33pF and works fine but I'm unsure of later effects. I was about to release the same for production when I noticed this. Can anyone please shed some light (as in why 33pF was used while developing the controlCard)?

Regards,

Gautam

  • 0
    TI__Guru 57255 points

    Gautam,

    You are correct in your assessment.  Using 33pF for these capacitors is a bit on the high side.  This was an oversight in the design and has been noted as something that needs to be fixed in the schematic.

    Instead of 33pF, a value around 12-16pF is probably more applicable.

    Incorrect load capacitance values often lead to slow startups, inaccurate clocks, or -- in the worst of situations -- clock failure.  (in general we recommend that customers work with the crystal vendor to characterize the crystal's operation with the MCU)

    Hopefully, this answers your question.


    Thank you,
    Brett

  • 0 in reply to Brett Larimore
    Guru 192875 points

    Thanks Brett for the detailed description.

    Regards,

    Gautam