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

Question

Hi all,

I'm working with the nRF52805 (QFN16) and was curious if the nRESET pin (P0.21) can be disabled via the UICR->PSELRESET[n] mechanism, as is possible on other nRF52 chips like the 52832.

After reviewing the nRF52805 PS v1.4 and Nordic’s online documentation (including this UICR page), I saw PSELRESET[0] and PSELRESET[1] described, including the CONNECT bit (bit 31) to disable the reset function.

Used a debugger (J-Link + SES) to write the following:

NRF_UICR->PSELRESET[0] = (21 << 0) | (1UL << 31);  // P0.21, disconnect
NRF_UICR->PSELRESET[1] = (21 << 0) | (1UL << 31);

Reset the MCU (NVIC_SystemReset()).
After reboot, printed the UICR values:
```
UICR->PSELRESET[0] = 0x00000015
UICR->PSELRESET[1] = 0x00000015
```
(Expected 0x80000015 if the disconnect bit was honored.)
Tried configuring P0.21 as a GPIO and driving it low to light an LED.

The values were written without the CONNECT bit sticking (bit 31 stayed cleared).
The pin still behaves as a hardware reset input.
Any attempt to drive it low is still reading high — P0.21 is not usable as GPIO.
Meanwhile, other GPIOs (e.g. P0.04) behave normally.

TL;DR:

Although PSELRESET[n] is listed in nRF52805 documentation and accessible via firmware, the disconnect bit is not honored, and P0.21 always functions as nRESET.

Is this expected behavior, and should we consider the PSELRESET feature non-functional on the nRF52805 even in Rev 1.4?
If so, could the docs be clarified to explicitly note that reset remapping is not supported on this part, despite the register existing?

Thanks, Bill!

Bill Russell · Answer

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

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

TL;DR:

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