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]

Is it possible to measure the external clock frequency?

[Stig24]

I see - by the way - what does nRF52840DK stands for? I assume RF - Radio frequency, DK - development kit, and maybe n = nordic. Haven found any good information on this.

[Einar Thorsrud]

Nobody knows :D Well, almost, at least. There is sort of a system:

• All RF devices made by nordic the last decades has a name starting with nRF (think "Nordic RF").
• The series (or generation if you like) is defined by the first to digits (51, 52, 53 for short range devices).
• And the rest of the numbers are typically so that a higher number is more high-end (think more peripherals and more memory) than the lower number devices.

[Stig24]

Just to clarify, why can the crystal produce 921600 or more close? 32MHz / 35 =914285 why can't this baud rate be produced? And by the way, my esp8266 uses a 26MHz crystal -> 928571 

[Einar Thorsrud]

I cannot disclose details of how tis is implemented internally in the IC.

[Stig24]

So I have been adjusting the Prescaler and cc to achieve 941176 in baud rate, but I only manage to achieve 1Mhz and 888Khz. How can I adjust it to achieve 941176?

[Einar Thorsrud]

Which prescaler and CC? The only register you can write to to modify the baudrate of the UART or UARTE peripheral is the BAUDRATE register, and you should only use values that are described in the register description in the product specification.

To achieve 941176, write 0x0F000000 (251658240) to the BAUDRATE register as stated in the product specification (typically by setting the sdk_config.h macro for the driver accordingly. Note that if using the UART peripheral instead of UARTE, the value is different (0x0EBED000 = 247386112), which you can also see in the product specification.

[Stig24]

I tried to reply to the message where you gave me this code: 

#include <stdint.h>
#include "boards.h"

int main(void)
{
    nrf_gpio_range_cfg_output(LED_1, LED_2); 
    nrf_gpio_pin_clear(LED_1);
    
    NRF_CLOCK->TASKS_HFCLKSTART = 1;
    while (NRF_CLOCK->EVENTS_HFCLKSTARTED == 0)
    {
    }
    NRF_CLOCK->EVENTS_HFCLKSTARTED = 0;
    
    NRF_GPIOTE->CONFIG[0] = GPIOTE_CONFIG_MODE_Task << GPIOTE_CONFIG_MODE_Pos |
                            GPIOTE_CONFIG_POLARITY_Toggle << GPIOTE_CONFIG_POLARITY_Pos |
                            LED_2 << GPIOTE_CONFIG_PSEL_Pos | 
                            GPIOTE_CONFIG_OUTINIT_Low << GPIOTE_CONFIG_OUTINIT_Pos;
                            
    NRF_TIMER1->PRESCALER = 0;
    NRF_TIMER1->CC[0] = 1;
    NRF_TIMER1->SHORTS = TIMER_SHORTS_COMPARE0_CLEAR_Enabled << TIMER_SHORTS_COMPARE0_CLEAR_Pos;
    NRF_TIMER1->TASKS_START = 1;
    
    NRF_PPI->CH[0].EEP = (uint32_t) &NRF_TIMER1->EVENTS_COMPARE[0];
    NRF_PPI->CH[0].TEP = (uint32_t) &NRF_GPIOTE->TASKS_OUT[0];
    
    NRF_PPI->CHENSET = PPI_CHENSET_CH0_Enabled << PPI_CHENSET_CH0_Pos;
    
    while (true)
    {
        
    }
}

[Einar Thorsrud]

Aha, I did not notice that. You will not be able to get the exact same frequency. The point was that you can use this to measure the accuracy. The accuracy over time will be the same regardless of the frequency of the toggeling pin (so regardless of the prescaler and CC), as the clock source / oscillator is the same. So this was an answer to how you can measure the accuracy of the HFXO. In short, output a known frequency and use a frequency counter to check the deviation from the intended frequency and the measured frequency. Assuming a perfect frequency counter, this will tell you the accuracy of your HFXO.

[Stig24]

I see, but there is not a way to actually measure the exact 941176? 

[Einar Thorsrud]

Output it via GPIOTE using a timer like I showed? You can't (the math don't add up). That is was not the point with that code.