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MSP-TS430PZ100A: XCAPxPF value setting

Daichi Tanaka
Prodigy 80 points
Other Parts Discussed in Thread: MSP430F47197, MSP-TS430PZ100A

TI Member and supporters:

    For the MSP430F47197 Microcontroller with the MSP-TS430PZ100A development board,  our design requires the crystal oscillator to be very accurate for the A/D conversion.  With this development board comes with a crystal of MS1V-T1K (32.768kHz) and a  suggestion for 12pF capacitor on C1 and C2 for the crystal capacitance load to use however no suggested setting for the internal cap (Softare Lingo: XCAPxPF).

   I was reviewing the documents of the "Micro Crystal Switzerland"  website and they provide suggested recommendation for the internal micro-controller setting and the capacitance load for the MSP430 series.  However, the information we would like were not available for the TI Micro-controller: MSP430F47197 that we planned to use. 

~LINK~
<http://www.microcrystal.com/Support/Support.aspx>

MSP430F47197 is the MSP430x4xx series and in the website for capacitance document suggestion for the crystal setting Quotes:
    "Recommended Setting:   Cx = 18pF (OSCCAPx = 3)   Corresponding crystal's CL: 9.0pF"

However, the TI document for the MSP430x4xx does not have such "OSCCAPx" setting for software (only for the older x4xx series model does).
The only setting for the new IC in regards to the Cx for the MSPF43047197 (TI website: slau056j.pdf pg 304 FLL_CTL0, FLL+ Control Register 0) is the following:

<<
XCAPxPF   Bits 5−4
Oscillator capacitor selection. These bits select the effective capacitance
seen by the LFXT1 crystal or resonator. Should be set to 00 if the
high-frequency mode is selected for LFXT1 with XTS_FLL = 1.
00 ~1 pF
01 ~6 pF
10 ~8 pF
11 ~10 pF
>>

   Can someone provide us with the recommended setting for the best corresponding crystal value software setting for this development board?   


Thanks

  • 0
    Guru 227245 points

    Teh available on-chip capacitances differ between MSPs. I don' tknwo why. Maybe because Ti has detected a shift in the market of common crystals. However, you always have the option to attach external capacitors and use the lowes possible CP setting (plain pin capacitance).

    But in your case, 12pF isn't far from the available 10pF setting.. You may add the remaining 2pF by some increased parasitic line capacitance on the board traces :)
    Note that the 10pF XCAP is the effective load, not the individual pin capacitance (which is twice the value).

  • 0 in reply to Jens-Michael Gross
    Prodigy 80 points

    Under SLAA322B-April 2009 "MSP430 32kHz Crystal Oscillators" Section 2.1 Effective Load Capacitance

       Suggested in the document recommends a capacitor load twice of internal load capacitance of the crystal.  As an example based on there recommendation,  if we use a 12.5pF load capacitance crystal we would need a approximately 25pF capacitance load for the MSP430 Microcontroller per pin (CL1 / CL2 in the document).   With that assumption, calculating using the example capacitance load per pin based in the document, we would require a  22pF (+2pF parasitic).  However that still means we are missing 1pF somewhere...  which I am assuming it is covered by the XCAPxPF=0 (1pF) but it had no explanation about the software setting or maybe they are assuming tolerance of the capacitance and it has not considering software setting.   (Personal comment... what frustrated even more is there frequency vs load capacitance graph and they show a effective load capacitance suggestion for 0ppm target at 7pF.... confused me even more but that is another story...)

       Going back to my original question, the Development board, when they suggest using a external 12pF capacitance load for the MSP430F47197 which uses a crystal that has a 12.5pF load capacitance, I would just like to know what "XCAPxPF" setting they recommend using for frequency target of 32768Hz.  I will figure out the detailed calculations later since it is always vague.

    D.T. 

  • 0 in reply to Daichi Tanaka
    Guru 227245 points

    The capacitor settings for the internal load capacitance of the MSP mean the effective load capacitance seen on the crystal. For the 1x family, this is fixed 12pF, for all others, this is selectable. Which values are available depends on the specific MSP (see datasheet). The users guide lists the settings available on all MSPs of this family when the users guide was first written, but new MSPs with different options have been added since. This is causing a bit confusion.

    However, these settings already contain the parasitic pin capacitance (but of course not the PCB trace parasitic capacitance).

    So if the MSP setting is XCAP12,5pF, then 2*25pF including pin capacitance are exposed. If the MSP has different settings, then the capacitances are different.

    The suggestions you got are probably directed at a different model of the MSP430 family.

    In your case, the 47197, 1pF, 5.5, 8.5 and 11 pF are available. If 11pF isn't what you want, you'll have to add external caps anyway, and whether this is 1,5pF or 12pF...
    You can try with any combination and output ACLK, driven by the crystal, to a port pin and check it. Then you know how much your PCB adds to the load. :)

    About the 7pF in the diagram, well, watch crystals usually have a nonlinear temperature coefficient. The highest frequency is at 25°C, and above and below they swing slower. A lighter capacitive load may compensate that for a different temperature. Just an idea.

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