Large actual baud rate variation when using Baud921600 setting (actual rate: 941176), even with HFCLK derived from using external clock source 32MHz +/-10ppm accuracy

RE: Large actual baud rate variation when using Baud921600 setting (actual rate: 941176), even with HFCLK derived from using external clock source 32MHz +/-10ppm accuracy

Håkon Alseth — Tue, 07 Sep 2021 10:10:01 GMT

Hi,

[quote user="James Lim"]

However this would mean that the HFCLK will always be on.

Do we have to manually trigger the HFCLK start stop for UART TXRX operation?

[/quote]

That is correct. The HFXO must be active when UART RX is active. If you have a added ready signal, this must then not only start the RXD receiver, but also start up the HFXO (and account for the time for settling: https://infocenter.nordicsemi.com/topic/ps_nrf5340/chapters/oscillators/doc/oscillators.html?cp=3_0_0_3_11_4_0#unique_817147230)

Kind regards,

Håkon

RE: Large actual baud rate variation when using Baud921600 setting (actual rate: 941176), even with HFCLK derived from using external clock source 32MHz +/-10ppm accuracy

James Lim — Tue, 07 Sep 2021 09:29:31 GMT

Thanks Håkon.

By manually triggering HFCLK using TASKS_HFCLKSTART, we are able to get the 941k frequency.

However this would mean that the HFCLK will always be on.

Do we have to manually trigger the HFCLK start stop for UART TXRX operation?

RE: Large actual baud rate variation when using Baud921600 setting (actual rate: 941176), even with HFCLK derived from using external clock source 32MHz +/-10ppm accuracy

Håkon Alseth — Thu, 02 Sep 2021 11:10:35 GMT

Hi,

It should be quite stable once the HFCLK source is set to HFXO and its state is running. The actual baudrate is a bit off, around 20kHz, but it shall follow the tolerance of the HFCLK source.

Based on your description, it sounds like you are running on HFINT (internal RC oscillator). Could you ensure that the HFXO is running by checking this register?

https://infocenter.nordicsemi.com/topic/ps_nrf5340/chapters/clock/doc/clock.html?cp=3_0_0_3_10_2_33#register.HFCLKSTAT

Kind regards,

Håkon