RE: nRF52805 — nRESET (P0.21) cannot be reconfigured as GPIO using UICR->PSELRESET[] (?)

Vidar Berg — Wed, 02 Jul 2025 16:19:37 GMT

Thanks for posting the update. I didn't refresh the page before posting so I didn't know that you had found the problem.

RE: nRF52805 — nRESET (P0.21) cannot be reconfigured as GPIO using UICR->PSELRESET[] (?)

Vidar Berg — Wed, 02 Jul 2025 16:17:05 GMT

Hello,

The UICR registers are in FLASH which means you must erase the UICR section before you can update existing register values. It's also important that you don't have the CONFIG_GPIO_AS_PINRESET symbol set in your bootloader or application as this will cause the UICR.PSELRESET registers to be set again by the startup code here:

Best regards,

Vidar

RE: nRF52805 — nRESET (P0.21) cannot be reconfigured as GPIO using UICR->PSELRESET[] (?)

Bill Russell — Wed, 02 Jul 2025 16:12:34 GMT

Just following up to say: I got it working

Erase the chip using SES or nrfjprog --eraseall
In your SES project:
- Uncomment the call to try_disable_reset_pin_once() (see code below)
Run the program from debugger (once)
Stop execution, comment the function call again
Run again from debugger

Code Example

#include "nrf_gpio.h"
#include <stdio.h>

#define TEST_PIN 21  // P0.21 = nRESET on nRF52805 with an LED attached

// Run this once to try writing UICR
static void try_disable_reset_pin_once(void)
{
    NRF_NVMC->CONFIG = NVMC_CONFIG_WEN_Wen;
    while (NRF_NVMC->READY == NVMC_READY_READY_Busy);

    NRF_UICR->PSELRESET[0] = (TEST_PIN << 0) | (1UL << 31);
    NRF_UICR->PSELRESET[1] = (TEST_PIN << 0) | (1UL << 31);

    NRF_NVMC->CONFIG = NVMC_CONFIG_WEN_Ren;
    while (NRF_NVMC->READY == NVMC_READY_READY_Busy);

    NVIC_SystemReset();
}

int main(void)
{
    // === Uncomment this line ONCE to write to UICR ===
    // try_disable_reset_pin_once();

    // === Read back the UICR values ===
    printf("UICR->PSELRESET[0] = 0x%08lX\n", NRF_UICR->PSELRESET[0]);
    printf("UICR->PSELRESET[1] = 0x%08lX\n", NRF_UICR->PSELRESET[1]);

    // Try to configure P0.21 as output and light LED
    nrf_gpio_cfg_output(TEST_PIN);
    nrf_gpio_pin_clear(TEST_PIN);  // Active-low LED ON

    // Optional second LED (red)
    nrf_gpio_cfg_output(4);
    nrf_gpio_pin_clear(4);  // Active-low LED ON

    while (1)
    {
        __WFE();
    }
}

0x80000015 = disconnect bit + GPIO 21
After reset, UICR->PSELRESET[x] retained values: 0x80000015
P0.21 now functions as normal output — confirmed with LED pull-down