How to use the UART with the registers?

Question

I would like to use the UART at the lowest possible level and Ihave created a very simple example which uses the UART bus with the registers. The example should write word "hello" once a second to the UART, which has been set to pins 11 and 12. However, it seems that the pins remain at low all the time. I am using SES, nRF5 (16.0.0) and nRF52-DK without any additions or modifications.

I am using the leds of the board to debug the issue. I can see that leds 1-3 are on, which should mean that the registers have been set and the UART has been enabled. Led 4 flashes about once a second which should mean that the word is written to the UART and the ready event has happened as expected. However, the logic analyzer proves that nothing is written and the pins remain low at all the times.

Do you have any idea what might be wrong? The register map of the UART is very limited and therefore I did not expect any remarkable problems. Apparently I am missing something.

#include <stdint.h>
#include <string.h>

#include "nrf_delay.h"
#include "nrf.h"
#include "nrf_gpio.h"


#define GPIO_PIN_17 17
#define GPIO_PIN_18 18
#define GPIO_PIN_19 19
#define GPIO_PIN_20 20

#define LED_ON  0
#define LED_OFF 1

int main (void)
{
    char test_string [] = "Hello"; 

    uint8_t n;

    nrf_gpio_range_cfg_output(17,20);   // Leds

    nrf_gpio_cfg_output(11);            // UART TX

    nrf_gpio_pin_write(GPIO_PIN_17, LED_OFF);
    nrf_gpio_pin_write(GPIO_PIN_18, LED_OFF);
    nrf_gpio_pin_write(GPIO_PIN_19, LED_OFF);
    nrf_gpio_pin_write(GPIO_PIN_20, LED_OFF);

    nrf_gpio_pin_write(GPIO_PIN_17, LED_ON);

    NRF_UART0->BAUDRATE = 0x00275000;   // 9,600 bps

    NRF_UART0->CONFIG   = 0x00000000;   // Hardware flow control disabled
                                        // Parity bit disabled

    NRF_UART0->PSELRTS  = 0xFFFFFFFF;   // Disconnected
    NRF_UART0->PSELCTS  = 0xFFFFFFFF;   // Disconnected
    
    NRF_UART0->PSELTXD  = 0x00000800;   // Pin 11
    NRF_UART0->PSELRXD  = 0x00001000;   // Pin 12
    
    NRF_UART0->PSELTXD  = 0xFFFFF7FF;   // Pin 11
    NRF_UART0->PSELRXD  = 0xFFFFEFFF;   // Pin 12

    NRF_UART0->ENABLE   = 0x00000004;   // Enable

    nrf_gpio_pin_write(GPIO_PIN_18, LED_ON);

    NRF_UART0->TASKS_STARTTX = 1;                       // Start TX

    nrf_gpio_pin_write(GPIO_PIN_19, LED_ON);
    
    while (1) {
        for (n = 0;n < strlen(test_string);n++) {
            NRF_UART0->TXD = test_string[n];            // Write to the TX register

            while (NRF_UART0->EVENTS_TXDRDY == 0) { }   // Wait until the byte has been sent

            NRF_UART0->EVENTS_TXDRDY = 0;
        }

        nrf_gpio_pin_write(GPIO_PIN_20, LED_ON);

        nrf_delay_ms(500);

        nrf_gpio_pin_write(GPIO_PIN_20, LED_OFF);

        nrf_delay_ms(500);
    }

    NRF_UART0->TASKS_STOPTX = 0;                        // Stop TX
}

hmolesworth · Accepted Answer

Pin registers are different from mask registers, so try just using the pin number and not a '1' shifted by pin number places: 
 // RW PSELTXD [0..31] Pin number configuration for UART TXD signal
// RW PSELRXD [0..31] Pin number configuration for UART RXD signal
// Disconnected 0xFFFFFFFF Disconnect

 NRF_UART0->PSELTXD = 11; // Pin 11 assuming pins numbered 0-31
 NRF_UART0->PSELRXD = 12; // Pin 12 
 Pins can be numbered depending on your definitions of course; 1-32 or 0-31, I use 0-31; I should add that you have to explicitly set Tx as an output pin and Rx an input pin; I don't see Rx in the code above.

How to use the UART with the registers?

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