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Why MSP430 RST Capacitor must be remove for Spy-Bi-Wire

CHEN YOUNG WU
Intellectual 540 points
Other Parts Discussed in Thread: MSP430G2232

Dear friends:

   On MSP430G2232 RST Pin, I use a 47K resistor connect to VCC and a 10pF Capacitor connect to GND for Power on reset.

   When I use Spy-Bi-Wire Interface to connect the device, It failed !

   When I removed out Capacitor, It works !

   Who can tell me why ?

Chen Young

  • 0
    Intellectual 595 points

    In this article, see the firts figure. Note B says the upper limit is 2.2nF for the capacitor to use. 

    http://processors.wiki.ti.com/index.php/JTAG_(MSP430)

    If your spy-bi-wire interface has this capacitance already, maybe you are making it to cross the limit by adding a parallel capacitance. 

  • 0 in reply to Gunay Turan
    Intellectual 540 points

    Dear Gunay :

     Thank you for your prompy answer.

     Yes, I checked that I put a 10nF capacitor on it not 10 pF.

     I put a 1.5nF capacitor and test normal.

    Thank you again !

    Chen Young

  • 0
    Guru 227245 points

    CHEN YOUNG WU said:
       Who can tell me why ?

    I can. But I guess you also want to know why and not only who could tell you why?

    SBW uses one signal (RST) for clocking data, and one wire for I/O. Sicne I/O is in both directions, clock signal and direction for both, FET and MSP, need to be an exact match. A capacitance on the reset line will cause a time-shift of the clock signal between FET and MSP, so beyond a certain point, both will send or receive at the same time and communication is broken.

    However, you can place a resistor between the capacitor and the RST pin. This significantly reduces the influence of the capacitor on the high-frequency SBW communication while still keeping its purpose to delay a reset on power-up. Then you can raise the capacitance far above 2.2nF.

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