NRF51 Flash write during brown-out

Question

The v3 reference manual of the nrf51 mentions: 
 
 "[The powerfail comparator] also provides hardware protection of data stored in program memory by preventing write instructions from being executed." (page 45) 
 
 The meaning of this is not entirely clear to me. Is this protection only active below 1.7V, or earlier? I'm trying to use the low-power comparator to generate an interrupt at 2.7V, then write something to flash and shut down. If I make sure that the power drop is slow enough so that there is enough time for my operations between the 2.7V and 1.7V, can I be sure that my data arrives in flash uncompromised?

Susheel Nuguru · Accepted Answer

This threshold is user set and you have to enable Power Fail comparator function explicitly . Please look into Power_Register ->POFCON

Regarding you write operation, all writes are blocked immediately after EVENTS_POFWARN is triggered. I verified this on 100028 board

#include <stdbool.h>
#include <stdint.h>
#include "nrf_delay.h"
#include "nrf_gpio.h"
#include "boards.h"

#define POF_PIN      (21)

void soc_hal_flash_word_unprotected_write(uint32_t * p_address, uint32_t value)
{
  NRF_NVMC->CONFIG = (NVMC_CONFIG_WEN_Wen << NVMC_CONFIG_WEN_Pos);
  while (NRF_NVMC->READY == NVMC_READY_READY_Busy)
  {
    // Do nothing.
  }
  *p_address = value;

  // Wait flash write to finish
  while (NRF_NVMC->READY == NVMC_READY_READY_Busy)
  {
      // Do nothing.
  }

  // Turn off flash write enable and wait until the NVMC is ready.
  NRF_NVMC->CONFIG = (NVMC_CONFIG_WEN_Ren << NVMC_CONFIG_WEN_Pos);
  while (NRF_NVMC->READY == NVMC_READY_READY_Busy)
  {
    // Do nothing.
  }
}

uint32_t *ptr = (uint32_t *)0X3C000;
void POWER_CLOCK_IRQHandler()
{
    if(NRF_POWER->EVENTS_POFWARN == 1)
    {
        soc_hal_flash_word_unprotected_write(ptr, 0XDEADBEEF);
        NRF_POWER->EVENTS_POFWARN = 0;
        nrf_gpio_pin_toggle(POF_PIN);
        
    }
}

/**
 * @brief Function for application main entry.
 */
int main(void)
{
    // Configure LED-pins as outputs.
    LEDS_CONFIGURE(1<<POF_PIN);
    LEDS_CONFIGURE(1<<22);
    nrf_gpio_pin_clear(POF_PIN);
    nrf_gpio_pin_clear(22);
    NRF_POWER->INTENSET = POWER_INTENSET_POFWARN_Msk;
    NRF_POWER->POFCON = ((POWER_POFCON_THRESHOLD_V23 << POWER_POFCON_THRESHOLD_Pos) | (POWER_POFCON_POF_Enabled << POWER_POFCON_POF_Pos));
    NVIC_EnableIRQ(POWER_CLOCK_IRQn);
    while(1)
    {
    if(*ptr == 0XDEADBEEF)
    {
        nrf_gpio_pin_set(22);
    }
    };
    
}

NRF51 Flash write during brown-out

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