🛠 Timer1 et registres AVR : le tempo sans delay

Objectives

Program the ATmega328P "bare metal": timer1 in CTC mode, registers TCCR1A/TCCR1B/TCNT1/OCR1A, prescaler calculation at 8 MHz, output toggling via GPIO registers. The LilyPad board is explicit — no embedded radio (Wi-Fi/Bluetooth only via external sewn-on modules) — so mastering registers and timers is what unlocks precise timing and power efficiency.

Hardware

• 1 LilyPad SimpleSnap (ATmega328P @ 8 MHz)
• 1 LED on pad 9 (Arduino pin 9 = PB1 = timer1 OC1A output on ATmega328P)
• >
• FTDI adapter for serial time scale (optional)

Complete code

// Timer1 en CTC : carré précis sur la pastille 9 (PB1), sans delay ni tone
// 8 MHz / prescaler 8 = 1 MHz au timer ; 1 MHz / (OCR1A+1 = 250) = 4 kHz d'ISR

volatile uint16_t battements = 0;

void setup {
  DDRB  |= _BV(PB1);          // pastille 9 = PB1 en sortie
  TCCR1A = 0;                 // mode normal côté comparaison de sortie
  TCCR1B = _BV(WGM12)         // CTC : TCNT1 repart à 0 sur comparaison avec OCR1A
         | _BV(CS11);         // prescaler /8 -> horloge timer à 1 MHz
  OCR1A  = 249;               // 1 MHz / 250 = une ISR toutes les 250 µs
  TIMSK1 = _BV(OCIE1A);       // autoriser l'ISR « comparaison A » du timer1
  sei;                      // activer les interruptions globales
}

ISR(TIMER1_COMPA_vect) {      // exécutée toutes les 250 µs
  if (++battements >= 2000) { // 2000 x 250 µs = 0,5 s
    battements = 0;
    PINB = _BV(PB1);          // écrire 1 dans PINB bascule la broche (XOR matériel)
  }
}

void loop {
  // 100 % libre : lecture de capteurs, calculs, ou sleep entre deux battements
}

Expected verification

The LED blinks at 1 Hz (0.5 s per state) with the precision of the 8 MHz crystal, without a single delay: loop remains available for other tasks or sleep. Changing OCR1A makes the frequency follow exactly the calculation 8 MHz / 8 / 250 / 2000 / 2. A logic analyzer on the pad sees a clean 1 Hz square wave.

Common errors

• Precomputing OCR1A for 16 MHz: everything blinks twice as fast — the LilyPad is at 8 MHz, redo the calculation.
• Forgetting sei or the OCIE1A bit of TIMSK1: the ISR never triggers, the LED stays frozen.
• Modifying TCCR1B without resetting TCNT1 to 0: an immediate interrupt arrives with a stale counter.
• Trying tone in parallel on the same pad: tone (Arduino core) and this code compete for timer1.
• Looking for an embedded radio: no LilyPad has integrated Wi-Fi/Bluetooth; radios are external sewn-on modules, otherwise stick to AVR peripherals (timer, ADC, UART, SPI, internal I²C).