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f_LFXT1,LF oscillator crystal frequency range

Daichi Tanaka
Prodigy 80 points
Other Parts Discussed in Thread: MSP430F47197

    Application Note: SLAS626-March 2011 pg 42; Crystal Oscillator, LFXT1, low-frequency mode mentions that the f_LFTXT1,LF (test condition XTS = 0) the suggested crystal to use is a 32768 Hz.

Question1 :  What is the range that this IC allows for operation?  Example +/- 100Hz or +/-50ppm, etc.....

    We plan to use a +/- 20ppm 32768Hz crystal and calculated its possible temperature variance, but there is nowhere in the document that shows the Min/Max range that I can guarantee for this micro-controller MSP430F47197 to operate properly.   is there a range or suggested range allowed?

Question 2: if there is a shift in crystal oscillation, when it uses for MCLK, ACLK, etc. does it have internal PLL circuit or such to correct itself to get more accuracy?

Reason I am asking is is oscillation crystal is off, the frequency calculation timing becomes shifted also (effects a lot of internal sampling calculation).  I would like to determine if I need a even better ppm part to use.

Regards,

Daichi Tanaka 

  • 0
    Expert 1480 points

    Hello,

    @Question 1:

    The accuracy of your XTAL also determines the accuracy of your system. I do not think that there is any suggested or allowed range for 32768 XTAL. If there is a 42122Hz +/- 2 kHz XTAL available, it will work too.

    @ Question 2:

    The FLL+ module of your chip has an FLL. This control loop allows one to set the clock to a multiple of the XTAL clock (max. frequency is limited by DCO). It also produces a clock with a jitter, as the frequency of the DCO has to be controled to a multible of XTAL clock. But you need at least one trusted accurate  clock source (XTAL).

    Regards

    Marco

  • 0
    TI__Mastermind 22165 points

    Daichi Tanaka said:
    Reason I am asking is is oscillation crystal is off, the frequency calculation timing becomes shifted also (effects a lot of internal sampling calculation).  I would like to determine if I need a even better ppm part to use.

    In addition to the good information which Marco already gave, please check the two Application Notes below which might be helpful for you:

    1) MSP430 32-kHz Crystal Oscillators (Rev. B)

    2) MSP430 LFXT1 Oscillator Accuracy

    BR,

    Mo.

  • 0
    Guru 227245 points

    Daichi Tanaka said:
    What is the range that this IC allows for operation?  Example +/- 100Hz or +/-50ppm, etc.....

    Mso tdatasheets only give the typical frequency of 32768Hz (which also is the main use).
    Some list XT1 for maximum 50kHz. The minimum, however, is limited by teh oscilaltor fault detection, which requires a minimum frequency to 'detect ' the crystal oscillation. IIRC, the minimum frequency is 10kHz, below the crystal may or may not be detected as failing. (and below 10Hz, it will be detected as failing in any case).

    Daichi Tanaka said:
    if there is a shift in crystal oscillation, when it uses for MCLK, ACLK, etc. does it have internal PLL circuit or such to correct itself to get more accuracy?

    No. If you use a crystal, the crystal frequency is the reference.
    There is an FLL (4x/5x/6x family only) that may adjust the DCO based on teh crystal frequency, but there too the crystal is the reference.

    Daichi Tanaka said:
    is oscillation crystal is off, the frequency calculation timing becomes shifted also

    Yes, indeed. So if precision is important, use a crystal with tighter tolerance.

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