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Capacitors for LaunchPad external crystal

Andrew Palm
Intellectual 370 points

The LaunchPad comes with an external crystal.  The positions for matching capacitors on the LaunchPad board appear to be unpopulated, although the component list shows 12.5pf values for these.  So is the capacitance provided by the gaps actually 12.5pf and no internal cap setting is needed, or should the internal cap feature be set?  If the latter, to what value?

Cheers, Andy

PS.  I've looked for the answer by a search in this forum and the LaunchPad User manual.  I could not find an answer, so unless I missed it, I suggest this could go on the LaunchPad FAQ list.

  • 0
    TI__Expert 7845 points

    It is correct that the load caps for the crystal are not populated and you can actually just set them to the appropriate value in software using the internal cap feature.

  • 0 in reply to Harman
    Intellectual 370 points

    OK, thanks for clearing that up.  Do you happen to know what is the correct software capacitance setting to use with the crystal supplied with the LaunchPad? 

    Andy

  • 0 in reply to Andrew Palm
    Prodigy 70 points

    This depends on your board design.

     

    If ther eis virtually no parasitic capacitance on the lines between MSP and Crystal, the 12.5pF are good for most crystals. (check the datasheed what the crystal needs).
    But then, PCB design is usually less than optimal, so sometimes 10pF are the better choice.
    And maybe you want to trim the crystal frequency by applying an adjustable capacitor. Then maybe 6pF is the value to chose.
    And of you need to place your own capacitors because teh crystal needs something totally different, then 1pF is the value of choice, which is actually not a capacitor selection but rather an 'no capacitor' selection as 1pF is the parasitic capacitance of the MSP pins.

     

    Jens posted that in your previous Launchpad crystal thread.

    Do "incorrect" cap values lead to inaccurate crystal timing?

     

  • 0 in reply to Victor Youk52513
    Guru 227245 points

    Victor Youk said:
    Do "incorrect" cap values lead to inaccurate crystal timing?


    Yes. Small values will dampen oscillation (while slighly speeding it up), so oscillation will finally cease or not even start. Large values will cause an oscillation slowdown and increased power consumption. There is a relatively small (some pF) range in the middle where it makes not much of a difference.

    basically, charge flows from one capacitor through teh quartz (which is a capacitor itself) to the other side and back. At the resonance frequency of the quartz, this system has the lowest impedance and highest amplitude, so if all three match, it will self-adjust to this resonance frequency. If one or both sides are too large, this will cause more charge flowing through the crystal, resulting in increased mechanical stress (not noticeably life-time relevant if it isn't too much) and therefor a slowdown in oscillation frequency. If the capacitors are too small, the crystal will not swing at full scale and slightly speed up, while at a certain point the attached electronic won't be able to detect a stable oscillation.

    That's a very simplified description.

    The resulting speedup/slowdown will, if intentionally done by an adjustable capacitor, allow an increase of accuracy through careful calibration.

    Anyway, you should go for high-quality, low ESR, low tolerance capacitors when applying them to a quartz. The few cents you save on cheaper high-tolerance and high-ESR capacitors won't compensate for all the trouble you can get with them.

  • 0 in reply to Jens-Michael Gross
    Expert 1790 points

    Hello Victor,

    Please have an eye on the different definition of load capacitace. A 12.5 pF load for a crystal is on his pads. The load capacitance of external capacitor, parasitic, package is reference to ground. For example adding on each pin of crystal a (high-quality) 15 pF capacitor to ground is like a series circuit and loads the crystal by 1/(1/15 +1/15) = 7.5 pF.

    You can see this in MSP430x22x2 datasheet (SLA504D) on page 38 crystal oscilator, LFXT1, note 1. : parasitic and package capacitance of 2 pF on each pin result in a 1 pF effective load.

    Regards

    Guenther

  • 0
    Intellectual 370 points

    All the information on previous posts was informative and interesting.  However, none of them seemed to answer the basic newbie question, "If I use the clock crystal supplied with the LaunchPad, what should the XCAPx setting be in software?"  To try and find out, I emailed the the Microcrystal company and they supplied a datasheet describing their recommendations for the MSP430 line.  (A link is at the bottom of this post.)

    There are different recommendations depending on the MSP430 family, but for the MSP430x2xx line (which I ASSUME is good for the current Value Line MSP430G2xx chips supplied with the LaunchPad) their recommendation is XCAPx = 2 (bit setting 01).  This is listed in the Family Guide as approximately 10pf.  With the stray pin and board capacitance the total capacitance for the clock crystal is about 12.5pf. 

    Since the LaunchPad seems to be aimed at newbies, students, and hobbyists, I think this would make a good FAQ entry.  These folks will need to know a specific setting for the specific MSP430 chips and specific clock crystal supplied with the board.  Confusion is likely because there are unpopulated cap positions on the board that are listed as 12.5pf in the parts list in the LaunchPad User Guide and there is a 12.5pf setting for XCAPx (=3) which is NOT recommended by the crystal manufacturer.

    Unless I've missed something, I think this is the answer I was originally looking for.

    Best regards to all, Andy.

    2772.TI_MSP430_Crystal Recommendations.pdf

  • 0 in reply to Andrew Palm
    Expert 1790 points

    Hello Andy,

    The cristal used in the Launch pad requires 12.5 pF (see http://processors.wiki.ti.com/index.php/MSP430_LaunchPad_%28MSP-EXP430G2%29?DCMP=launchpad&HQS=Other+OT+launchpadwiki at chapter "LaunchPad Kit Contents").

    It is a Swiss quality crystal from MICRO CRYSTAL AG: MS3V-T1R 32.768kHz CL: 12.5pF +/-20ppm (datasheet at http://www.microcrystal.com/CMSPages/GetFile.aspx?nodeguid=21218980-0886-41a2-87fc-6b04e14f0226)

    I use a similar one in my design and experienced that it is tolerant on drive speed and wrong load setting.

    Regards

    Guenther

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