For optiboot report

watchdog-test.ino

// ATmega328P watchdog test sketch
#include <avr/wdt.h>
int inByte = 0;
int led = 13;

void setup() {
  wdt_disable();
  Serial.begin(115200);
  Serial.print("ATmega328P watchdog test sketch\r\n");
  pinMode(led, OUTPUT);
  digitalWrite(led, LOW);
}

void loop() {
  Serial.print( "Press key to enable watchdog:\r\n"
                "1: WDTO_15MS\r\n"
                "2: WDTO_30MS\r\n"
                "3: WDTO_60MS\r\n"
                "4: WDTO_120MS\r\n"
                "5: WDTO_250MS\r\n"
                "6: WDTO_500MS\r\n"
                "7: WDTO_1S\r\n"
                "8: WDTO_2S\r\n"
                "9: WDTO_4S\r\n"
                "0: WDTO_8S\r\n"
                "any other key: jump back to 0x00\r\n\r\n");
  while (Serial.available() <= 0) {
  }
  inByte = Serial.read();
  switch (inByte) {
    case '1':
      wdt_enable(WDTO_15MS);
      break;
    case '2':
      wdt_enable(WDTO_30MS);
      break;
    case '3':
      wdt_enable(WDTO_60MS);
      break;
    case '4':
      wdt_enable(WDTO_120MS);
      break;
    case '5':
      wdt_enable(WDTO_250MS);
      break;
    case '6':
      wdt_enable(WDTO_500MS);
      break;
    case '7':
      wdt_enable(WDTO_1S);
      break;
    case '8':
      wdt_enable(WDTO_2S);
      break;
    case '9':
      wdt_enable(WDTO_4S);
      break;
    case '0':
      wdt_enable(WDTO_8S);
      break;
    default:
      Serial.print("Jump to 0x00\r\n\n");
      Serial.flush();
      asm volatile("jmp 0");
      break;
  }
  digitalWrite(led, HIGH);
  while (1) {};
}

Optiboot/optiboot#369