nrf5340-dk: Unable to read serial output using printk as baud rates between 250kbps and 1Mbps

RE: nrf5340-dk: Unable to read serial output using printk as baud rates between 250kbps and 1Mbps

Håkon Alseth — Wed, 27 Oct 2021 11:09:37 GMT

Hi Matt,

The default clock source for the baudrate generator is the HFCLK, which can be sourced by a inaccurate RC oscillator or the external HFXO (which has a tolerance of nom. < 40 ppm):

https://infocenter.nordicsemi.com/topic/ps_nrf5340/chapters/clock/doc/clock.html

If the clock source is inaccurate, the outcome is that the baudrate is also inaccurate. By starting the external HFXO source, you will then switch to a more accurate source.

Wrt. to the "BAUDRATE" register, the target and actual baud are both shown. For your chosen one it states:

460800 baud (actual rate: 457143)

Meaning it is almost 4 kHz off its target, assuming a accurate clock source.

Kind regards,

Håkon

RE: nrf5340-dk: Unable to read serial output using printk as baud rates between 250kbps and 1Mbps

mattmacleod — Tue, 26 Oct 2021 16:16:13 GMT

Hi Håkon,

Sorry I dont understand what I am looking at in the lines you shared in that example. Where are the baud settings?

Thanks,

Matt

RE: nrf5340-dk: Unable to read serial output using printk as baud rates between 250kbps and 1Mbps

Håkon Alseth — Tue, 26 Oct 2021 11:28:39 GMT

Hi,

The actual rate, provided that you're sourcing from the HFXO, is almost 4kHz off the target baud rate:

https://infocenter.nordicsemi.com/topic/ps_nrf5340/uarte.html?cp=3_0_0_6_37_8_42#register.BAUDRATE

When running with the HFCLK internal RC oscillator, you will add a tolerance of several percent on top of the above mentioned frequency/baud rate. You can try to start the HFXO (see here for an example: https://github.com/nrfconnect/sdk-nrf/blob/v1.7.0/samples/bluetooth/direct_test_mode/src/dtm.c#L497-L527) and see if this improves the scenario, but in general; I would recommend avoiding 460k8 and 921k6 due to their actual rates.

Kind regards,

Håkon