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MSP430F67451 extyernal high-frequency clock

Other Parts Discussed in Thread: MSP430F67451

Part number: MSP430F67451
Ref document: SLAS815C.PDF

We have applied an exteral 24 MHz clock source, with XT1BYPASS = 1, and it works, apparently normally.

We find no support for this idea in the above-referenced datasheet. Is our present practice a reasonable choice?

Our reason for use of a high-frequency external clock source is that we find lower timing jitter as a
result. We are applying the SD24B ADC in this system, and we observe better data quality with
our choice as opposed to use of the large multiplication factor with a 32.768 KHz crystal.

  • This is described in the documents ‘Family User’s Guide’ and ‘Device Data Sheet’, both documents can be found at the TI device internet page.

  • The User's Guide says:

    XT1 may be used with an external clock signal on the XIN pin in either LF or HF mode by setting XT1BYPASS. When used with an external signal, the external frequency must meet the data sheet parameters for the chosen mode.

    However, the MSP430F67451 data sheet never mentions HF mode for XT1, so I would have to conclude that this is not allowed.

  • LF/HF mode applies only to the crystal oscillator himself. When XT1BYPASS is set the Port-pin acts as a normal General Purpose I/O with Schmitt-Trigger Inputs and their parameters.

  • Hello Leo and Clemens. Your comments are appreciated. Clemens, your interpretation is the reason why, in design review, we decided to raise the question. Leo, your interpretation was also our reasoning in applying our external clock. We're about to build 15,000 of these... Our circuit does work. We used the external clock because we found better data quality in the SD24 by doing so, compared to use of an LF clock with multiplication. The datasheet does not explicitly say "you can apply an HF clock here", but we've found that it's valuable to do so. It was a concern among some of our workgroup that this isn't supported by TI, since it isn't promoted by TI in the datasheet as a clocking option, so, we've looked to this forum for some confirmation of our thinking.
  • Note: the DCO's jitter is caused by the modulation (it literally switches between two different frequencies). If you disable the FLL and set the MODx bits to zero, you should get a stable clock (which then isn't adjusted for future voltage or temperature changes).

  • All,

    Please refer to below post as well:
    e2e.ti.com/.../1532118

    This particular subset of parts comes from a larger family that may or may not have different features and/or modes. TI can only guarantee the operation, features, and specifications of a given device that are outlined within it's datasheet. Anything outside of that is not tested. It is true that in some use-cases an MSP430 MCU may operate correctly outside of the specified limits, but performance is not guaranteed and such operations are done at the risk of the user and are not recommended by TI.

    Regards,
    JH