Button: Debounce & Interrupt

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#include <Arduino.h>

const int BTN = 0,
  LED = 2;
volatile bool pressed = false;
void IRAM_ATTR onPress() {
  pressed = true;
}
void setup() {
  pinMode(LED, OUTPUT);
  pinMode(BTN, INPUT_PULLUP);
  attachInterrupt(digitalPinToInterrupt(BTN), onPress, FALLING);
}
void loop() {
  static unsigned long last = 0;
  if (pressed && millis() - last > 50) {
    last = millis();
    pressed = false;
    digitalWrite(LED, !digitalRead(LED));
  }
}