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How much is the deviation of the internal clock of MSP430FG438?

Moris Cho
Prodigy 120 points

Hi.

I need information that the deviation of the internal clock of MPS430FG438.

But, I can't find this informaton in datasheet and userguide.

Does anyone know this information?

The below code is my clock setting code for 7.99344 MHz

SCFI0 |= FN_4;                                   // x2 DCO freq, 8MHz nominal DCO

CFQCTL = 121;                                  // (121+1) x 32768 x 2 = 7.995392 MHz

FLL_CTL0 |= DCOPLUS + XCAP18PF;           // DCO+ set so freq= xtal x D x N+1

  • 0
    Guru 244290 points

    The FLL tries to adjust the DCO up or down so that there are 122 ticks of the (divided) DCO clock between two ticks of the reference clock.

    The two clocks are not exactly synchronized, so it is possible that you get 121 or 123 ticks, and the resulting adjustments.

    Assuming that the external crystal is perfectly accurate, you should get between 121 x 2 x 32768 kHz and 123 x 2 x 32768 kHz, i.e., between 7.929856 MHz and 8.060928 MHz. (Add to that the deviation of the crystal.)

    Please note that the modulator switches between two frequencies, so the values above are just averages; the actual clock will have lots of jitter.

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