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xCAP selection

Dustin Lane
Intellectual 610 points
Other Parts Discussed in Thread: MSP430F2274

I am planning on using the Epson MC-146 32.768kHz crystal with a msp430F2274.  The load capacitance of the crystal is 9 pF.  I am unclear in the documentation what value I should set for xCAP.  I can choose between 1, 6, 10, and 12.5 pF.  Should this value match the load capacitance of the crystal or does it represent the individual values for each capacitor, CL1 and CL2?  What value should I choose?

Thanks.

  • 0
    Prodigy 30 points

    I'm fairly sure you should match it to the load capacitor value, but I'm new at this, lol.  I'm pretty sure I read somewhere in the datasheet of the msp430x2xx userguide that you match it to cload.  Just know i'm only like 80% sure lol.

  • 0 in reply to Brian Pugh
    TI__Genius 12770 points

    Dustin,

    Brian is correct, the load capacitance seen by the crystal oscillator should match the load capacitance stated in the crystal's datasheet.

    Typically external crystals have a range for load capacitance, with 9pF most likely being the nominal choice for load capacitance in the case.

    This range would be something like 7pF, 9pF and 12pF.

    The MSP430F2274 supports 1pF, 5.5pF, 8.5pF and 11.5pF, with 9pF being the nominal load capacitance as you mentioned this would lead to an the BCSCTL3 register equal to XCAP_2  for 8.5pF (i.e. " BCSCTL3 |= XCAP_2; " ).

    The values for the load capcitance for the MSP430F2274 are noted on page 38 of the MSP430F2274 device datasheet.

    Best regards,

    Austin Miller

  • 0 in reply to Austin Miller
    Expert 2080 points

    Some crystals manufacturer recommends a XCAP1 setting for a 9pF crystal, XCAP2 for the 12.5pF version on the MSP430 serie 2xxx.

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