unsigned long newMillis = 0, oldMillis = 0; // 1. event time keepers
unsigned long interval = 500;               // 2. LED blink rate in milliseconds
bool state;                                 // 3. the state of the LED
byte buttonPin = 2;                         // 4, button pin

void setup() {
  Serial.begin(115200);             // 1. configure Serial Monitor baud
  pinMode(LED_BUILTIN, OUTPUT);     // 2. configure LED_BUILTIN/DIO pin 13 as OUTPUT
  pinMode(buttonPin, INPUT_PULLUP); // 3. configure button pin as INPUT with internal pullup resistor
}

void loop() {
  blink();                                // 1. call the "blink" function
  pause();                                // 2. call the "pause button" function
}

void blink() {
  newMillis = millis();                   // 1. start an event
  if (newMillis - oldMillis > interval) { // 2. compare difference in event times to interval
    oldMillis = newMillis;                // 3. store the new event start time
    state = !state;                       // 4. flip the current state for the LED
    digitalWrite(LED_BUILTIN, state);     // 5. set the LED to the current state
    if (state) Serial.print("1");         // 6. show current state if HIGH in Serial Monitor
    else Serial.print("0");               // 7. show current state if LOW in Serial Monitor
  }
}

void pause() {
  while (!digitalRead(buttonPin)) {}      // 1. hold here while button is being pressed
}