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()).

[Stig24]

I enabled the HFXO but didn’t improve anything.

Made a test (on/off signal to module every two seconds) and observed if the correct signal reached my module. Also observed my 13 bytes trough a usb to ttl module, and the bytes is always correct.

What	Correct	Wrong
With flush, with usb-2-ttl connected, without HFXO	16	34
Without flush, with usb-2-ttl connected, without HFXO	12	38
Without flush, with usb-2-ttl connected, with HFXO	13	37
With flush, with usb-2-ttl connected, with HFXO	11	39
With flush, without usb-2-ttl connected, without HFXO	10	40
Eqvivalent python program module connected to ttl (no nRF52)	50	0

Cannot understand why with the nRF52 it is struggling that much and the usb-2-ttl show the correct bytes from nRF52.

Snip of code 

[Einar Thorsrud]

Hi,

With this code you will be using the HFXO (though it would be cleaner to start it with the clock driver as you are anyway using that), so that should be OK. The difference in baud rate between the two devices here is a bit more than 2%, so with this short frame I would expect a lower error rate, but apparently that is not the case.

The fact that the nRF cannot do exactly 921600 baud is a WH limitation, so that is just how it is. Does the other device support other baud rates? If so, can you pick one where there is a better match (assuming you can also live with a lower baudrate).

[Stig24]

What I find strange is when sending data to my module (which is a LiDAR) and at the same time reading this data with the usb-2-ttl the bytes is perfect, yet the LiDAR is not responding. I would assume the usb-2-ttl module shows a bit more flexibility in terms of baud rate.

I have tried contact the manufacturer of the LiDAR without luck, but I will try to reach out to them again.

Just to clarify it is not possible to “tune” the baud rate to get it more accurate?

 

Thanks for the help!