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

[mattmacleod]

Hello, 

I have an nrf5340-dk board and I am trying to play around with the different baud rates through changing the overlay file (as shown): 

I am using teraterm to view the output data and I am not able to view the data with any of the available baud rates between 250Kbps and 1Mbps (115200bps, 250Kbps and 1Mbps all work).

I have tried the following  SDK versions (1.4.2,1.6.1,1.7.0 )  in addition to making a simple APP only containing a while loop printing "hello world" and I was not able to view a readable output on teraterm (shown below): 

#include <zephyr.h>

#include <sys/printk.h>

void main(void)

{

    while(true){

        printk("A\n");

    }

}

I have probed the Rx and Tx pins (P0.20, P0.22) on the nrf with an oscilloscope and I can see the appropriate signals (similar to 250kbps and 1Mbps) at 460800 bps, however the output is being received but is gibberish at 460800bps. 

I would appreciate any suggestions for debugging this. 

Thanks, 

[Håkon Alseth]

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

[mattmacleod]

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

[Håkon Alseth]

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