🛠 Programmer aux registres : interruption Timer/Counter1

Objectives

Move beyond the Arduino API to drive the ATmega32u4 "bare metal": configure Timer/Counter1 in CTC mode (Clear Timer on Compare Match) with a prescaler of 1024, trigger a hardware interrupt at 1 Hz, and toggle the LED on pin 13 (PB7) directly via the PINB register. You will understand the complete path: 16 MHz clock, prescaler, OCR1A, interrupt vector, and why an ISR must remain minimal.

Hardware

• 1 Arduino Leonardo (the sketch runs on top of the Arduino core, which remains necessary for initialization and the Caterina bootloader)
• No external components: the built-in LED is sufficient

Complete Code

// Clignotement 0,5 Hz par interruption Timer/Counter1 en mode CTC
// 16 MHz / 1024 (prescaler) / 15625 = 1 Hz d'increment compare

volatile uint8_t clignotements = 0;

void setup {
  DDRB |= (1 << DDB7);          // PB7 (Arduino D13) en sortie
  noInterrupts;
  TCCR1A = 0;
  TCCR1B = (1 << WGM12) | (1 << CS12) | (1 << CS10); // mode CTC, prescaler 1024
  TCNT1  = 0;
  OCR1A  = 15624;               // 16e6 / 1024 / (15624+1) = 1 Hz
  TIMSK1 |= (1 << OCIE1A);      // autorise l'interruption Compare Match A
  interrupts;
}

ISR(TIMER1_COMPA_vect) {
  PINB = (1 << PINB7);          // ecriture de 1 dans PINB = bascule la broche
  clignotements++;
}

void loop {
  // Le travail se fait dans l'ISR ; la boucle principale reste libre.
  // Exemple : Serial toutes les 2 bascules (0,5 Hz) si besoin.
}

Expected Verification

The LED blinks at 0.5 Hz (one toggle per second, meaning an off/on cycle every 2 seconds). Verify the accuracy by comparing 60 blinks against a watch: the error comes solely from the tolerance of the 16 MHz crystal. Suspend the loop with delay: the blinking continues, proving that the timer and an interrupt are driving the pin.

Common Errors

• Forgetting noInterrupts/interrupts around configuration: modifying TCCR1B/OCR1A while the timer is running can produce a spurious overflow.

• Not declaring the shared variable (clignotements) as volatile: without this keyword, the optimizer may remove its read in loop.
• ISR too long: an ISR that prints or delays breaks other interrupts, including the Leonardo's USB (the USB service is itself interrupt-driven) — the connection may freeze.
• Wrong toggle register: PINB = (1 << PINB7) toggles; PORTB ^= ... also works but performs a read-modify-write, which is less atomic in an interrupt context.
• Incorrect OCR1A precalculation: with the 1024 prescaler, 1 Hz corresponds to OCR1A = 15624; at 2 Hz, 7811; the classic mistake is forgetting the +1 for counting from zero.