/* Callback demo — hardware interrupt (Wokwi, Arduino Uno)
 *
 * A button on pin 2 fires a CALLBACK (an ISR) on every press. Notice: loop() never
 * reads the button. The hardware "calls back" onPress() the instant the pin falls —
 * that's the interrupt/callback pattern, vs. polling the pin on a schedule.
 *
 * Good practice shown: the ISR stays TINY (just flags the event); the real work
 * happens back in loop(). Shared vars touched by the ISR are 'volatile'.
 *
 * Board: Arduino Uno.  Pin 2 = INT0 (a dedicated interrupt pin).  No library needed.
 * Wiring: a pushbutton, one leg to D2, the diagonal leg to GND (uses internal pull-up).
 */
const int BTN = 2;              // INT0-capable pin on the Uno
const int LED = LED_BUILTIN;    // pin 13

volatile uint32_t pressCount = 0;   // written by the ISR -> must be volatile
volatile bool     pressed    = false;

void onPress() {               // <-- THE CALLBACK (interrupt service routine). Keep it tiny.
  pressCount++;
  pressed = true;
}

void setup() {
  Serial.begin(9600);
  pinMode(LED, OUTPUT);
  pinMode(BTN, INPUT_PULLUP);
  // Register the callback: call onPress() on a HIGH->LOW edge (button press).
  attachInterrupt(digitalPinToInterrupt(BTN), onPress, FALLING);
  Serial.println("Ready. Note: loop() is NOT polling the button.");
}

void loop() {
  Serial.println("...loop doing its own thing...");
  if (pressed) {               // handle the event the callback flagged
    pressed = false;
    digitalWrite(LED, !digitalRead(LED));
    Serial.print("  >> callback fired!  count = ");
    Serial.println(pressCount);
  }
  delay(500);                  // loop is slow on purpose — the callback still fires instantly
}