🛠 Sortir des sentiers : registres AVR et interruption externe

Objectives

Program the ATmega32U4 at the register level: configure an external interrupt on a falling edge (registers EICRA and EIMSK), write a short Interrupt Service Routine (ISR), and understand the pattern of using a volatile flag rather than performing long tasks within the ISR. On the Micro, external interrupts are available on pins 0, 1, 2, 3, and 7.

Hardware

• 1 Arduino Micro + micro-USB cable
• 1 push button between pin 3 and GND (pin 3 carries external interrupt INT0; internal pull-up enabled by the code)
• The onboard LED (pin 13) serves as an indicator

Complete code

// Interruption externe sur front descendant, a la main (sans attachInterrupt).
// Broche 3 du Micro = INT0. On configure les registres puis on ecrit l'ISR.
volatile bool evenement = false;   // drapeau partage ISR -> boucle

void setup {
  DDRB  &= ~(1 << DDB0);          // sens entree de la broche 13 cote port B
  EIMSK &= ~(1 << INT0);          // INT0 desactive pendant la config
  EICRA = (1 << ISC01) | (0 << ISC00);  // INT0 sur front descendant
  EIMSK |= (1 << INT0);           // autoriser INT0
  // Broche 3 : entree + pull-up (le noyau Arduino gere le pull via PORT)
  pinMode(3, INPUT_PULLUP);
  pinMode(LED_BUILTIN, OUTPUT);
  sei;                          // autorisation globale des interruptions
  Serial.begin(9600);
}

ISR(INT0_vect) {                  // executee sur chaque front descendant
  evenement = true;               // travail minimal : lever un drapeau
}

void loop {
  if (evenement) {
    evenement = false;
    digitalWrite(LED_BUILTIN, !digitalRead(LED_BUILTIN));
    Serial.println("INT0 : front descendant detecte");
  }
}

Expected verification

Each press (falling edge) toggles LED 13 and prints a line; the monitor remains responsive because only a flag toggle occurs in the ISR. By bringing your finger close to the long-wired button, you may see spurious triggers: this is real, hence the pull-up and debouncing on the loop side if necessary.

Common errors

• Calling delay or Serial.println in the ISR: the ISR blocks system time (millis) and corrupts serial output; only raise a flag.
• Forgetting volatile on the shared flag: the compiler optimizes the loop read and the event seems lost.
• Forgetting sei: EIMSK is armed but interrupts are globally masked, so nothing happens.
• Thinking that pins 0/1 are good interrupt inputs here: they are used for Serial1 (RX/TX); prefer 2, 3, or 7.
• Be aware of known quirks of the Caterina bootloader: it executes SPM operations with interrupts enabled and may leave an interrupt armed upon exit (AVR core issues #122 and #218); properly reinitialize EIMSK in setup.