Blink

uint32_t blinkTime;

void setup() {
  Serial.begin(9600);
  pinMode(LED_BUILTIN, OUTPUT);
}
void loop() {

  if(millis() - blinkTime > 1000){
    blinkTime = millis();
    digitalWrite(LED_BUILTIN, ! digitalRead(LED_BUILTIN));
    Serial.println(digitalRead(LED_BUILTIN));
  }

}