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HFINT has frequency tolerance of +/-1.5% typ and +/-8% max

Hi

The 840 datasheet (page 90) states that the HFINT tolerance is +/-1.5% typical and +/-8% max.

Under normal operation, the HFINT is running and so if the device (like in our module) the uart is open and running, it's baudrate is going to be derived from HFINT.

That +/-1.5% tolerance is fine for UART operation.

However how can the uart operate if we get a batch of chips that have the 8% tolerance?

We have hit the issue in testing because as we get the temperature down to -35 centigrade we start getting uart errors.

Will be most grateful for clarification on all this, especially the 8% max tolerance

Look forward to hearing from you

Regards

Mahendra

Parents Reply
  • 1 and 2) Yes, I meant the HFXO. Using a stable clock source is the only way you can get a stable baudrate over the whole temperature spectrum.

    3. There is no definite way to check this. You could check the temperature (TEMP peripheral) or if you get framing errors on the UART, then start the HFXO.

    4. Depends on the crystal chosen. If you do not turn it off before going to sleep, you'll get an added suspend current of the HFXO, provided that no other devices are requesting the HFCLK.

    Best regards,

    Håkon

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