ResetDetector.c

#include <EEPROM.h>

#define FLAGSET     0x55
#define FLAGCLEAR   0xAA
#define FLAGADDRESS 00
#define FLAGTIMEOUT 3000

uint32_t flag = FLAGCLEAR;

void setup() {
  // put your setup code here, to run once:
  Serial.begin(115200);  Serial.println();
  pinMode(LED_BUILTIN, OUTPUT);
  digitalWrite(LED_BUILTIN, HIGH);

  //Activate the EEPROM before attempting to use it
  EEPROM.begin(sizeof(flag));

  //Check to see if the flag is still set from the previous boot
  if(checkFlag()) {
    //Clear the flag now it has been detected
    writeFlag(FLAGCLEAR);
    
    //Do the firmware reset here
    Serial.println("Flag Detected");
    digitalWrite(LED_BUILTIN, LOW);
  } 
  else {
    //Set the flag for detection with the next boot
    writeFlag(FLAGSET);
  }  

  // After setup, while away the end of the reset period
  while (millis() < FLAGTIMEOUT);
  writeFlag(FLAGCLEAR);

  Serial.println("Flag Checking Stopped");
}

void loop() {
  // put your main code here, to run repeatedly:
}

bool checkFlag() {
  flag = EEPROM.read(FLAGADDRESS);
  return flag == FLAGSET;
}

void writeFlag(uint32_t value) {
  flag = value;
  EEPROM.write(FLAGADDRESS, flag);
  EEPROM.commit();
}