nRF52840DK uart - 921600 not working

[Stig24]

I’m using the nRF52840DK to communicate with a module over uart. My problem is that the module has a baud rate of 921600 and as far as my understanding the nRF52840 is using a baud rate of 941176 when set to 921600.  This leads to my commands (13 bytes) only works around 10-20% of the times, and the data received is not as expected (startbytes etc.).

I have tried using nrf_serial_flush as well but then nothing works. Also tried setting the baud rate manually without succeeding.

Any tips?

[Einar Thorsrud]

Hi,

The UART peripheral only supports some baud rates, but if you are close enough and the transfers are short enough it should be OK. It could be that there are other factors here as well. First of all, make sure you start the HFXO on the nRF. Without it you are using the HFINT, and  the frequency could be quite off (worst case up to ±8 % on the nRF52840, as you can see form the electrical specifications).

[Stig24]

Will try!

When trying to enable HFXO

NRF_CLOCK->EVENTS_HFCLKSTARTED = 0;
NRF_CLOCK->TASKS_HFCLKSTART    = 1;
while (NRF_CLOCK->EVENTS_HFCLKSTARTED == 0);

I get these fault messages and not sure why they occurs: 

[Einar Thorsrud]

Hi,

Based on the compiler errors it looks like you put these 3 lines outside of a function. Try moving them into a function (for instance in the beginning of main()).