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

[Mahendra Tailor]

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

[Håkon Alseth]

Hi,

 

In this scenario, I'd recommend starting the external HFCLK to ensure a stable tolerance of your UART.

Best regards,

Håkon

[Mahendra Tailor]

Thank you for getting back to me.

(1) I thought the HCLK is always ON? Otherwise the uart rx would never receive anything at anytime.
(2) Did you mean the HFXO?
(3) Please explain how I can detect that I am getting into that operation zone?
(4) What is the extra current consumption I could expect to see with HFXO?

[Mahendra Tailor]

Also I will be most grateful if you could explain what to do with regards to uart operation when the tolerance is +/- 8% as to how to detect that so that I can use the HFXO

[Håkon Alseth]

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