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MSP430G2 LaunchPad 32KHz Crystal Issue

Murat Aksoy
Prodigy 50 points
Other Parts Discussed in Thread: MSP430G2231, MSP430G2211

Hi,

I don't know how to describe the problem. I have 2x MSP430G2 v1.4 LaunchPads. One has MSP430G2231 and the other has MSP430G2211 running. I've written a simple programming polling TAIFG bit and toggling LEDs one on the left every second (G2211) the one on the right every half second (G2231). Capacitance is set to 12.5 pF. There are several problems. The crystals sometimes work and sometimes don't. If they do work, they sometimes stop. For some reason when I upload the program now and power up the LaunchPad via Flasher, the reset button doesn't work.

And the last one that makes me create a thread here is that when I connect both the LaunchPads to the same USB power adapter, and when the crystals are stuck if I touch the board's (with G2211) USB the other one starts working. The crystal stops resonating again after I take my finger off. If I put my finger on it again, it starts resonating from where it left resonating. Could it be my body's capacitance, my body being an antenna, the RF signals the university tower is emitting and the board is picking, GND issue with the USB power adapter or with universities system, or the combinations of all? This behavior is similar.

Some more notes: I've also tried 10 pF capacitance which seems to be fine for now on G2231 and not so fine on G2211.

P.S:The video was processing at the time I created this thread. 

Murat

  • 0
    Guru 92251 points

    The FAQ in the MSP-EXP430G2 LaunchPad Evaluation Kit User's Guide says

    7. I soldered the 32-kHz crystal to the board and the oscillation is not starting.

    The MSP430 driving capabilities for the low-frequency crystal is limited, because it is designed for low-power applications. To ensure proper operation, the load on these pins must be as small as possible, the matching capacitors (12.5 pF for 32.768 kHz) for the crystal must be soldered to the board, and the resistors R28 and R29 must be removed. Measuring the frequency of the oscillation with an oscilloscope typically disturbs the oscillation.

    Have R28 and R29 been removed?

  • 0 in reply to Chester Gillon
    Prodigy 50 points
    Thanks for the info. It's funny watching and reading so many how to solder crystal etc videos and articles, non of them desoldered R28 and R29 from the board. It seems to be doing good now. I've read somewhere there I should choose 10 pF with the capacitance of pins it adds up to 12.5 pF in parallel. I'll try to bench both settings with a more accurate clock to see which one is a better solution.
  • 0 in reply to Murat Aksoy
    Guru 227245 points
    Pin capacitance is 2pF on each pin. Which gives a load capacitance of 1pF. If the crystal requires 12.5pF load capacitance, two 23pF capacitors (minus trace stray capacitance) are needed (as the two capacitors are seen in series by the crystal so their capacitance halves)
    However, when using the XCAPx bits for capacitance, the offered values already include pin capacitance and the calculation, so you simply set the best-fitting value to the required load: XCAP_3. (provided you have no additional trace capacitance, which you had through the resistors)

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