const uint32_t ONE_SEC = 1000;
const int16_t LED_PIN = 12;

uint32_t prevTime = 0;

void setup() {
  Serial.begin(115200);
  pinMode(LED_PIN, OUTPUT);
}

void loop() {
  static bool ledState = false;

  if (millis() - prevTime >= ONE_SEC) {
    ledState = !ledState;
    digitalWrite(LED_PIN, ledState);
    Serial.println(ledState ? "On" : "Off");
    prevTime = millis();
  }
}