🛠 Mode basse consommation : power-down et watchdog

Objectives

Put the ATmega32U4 to sleep in power-down mode (microampere-level consumption on the MCU) and wake it up every second using the watchdog, using direct AVR C programming. Understand what this mode implies on a board: USB is disconnected during sleep and the Serial Monitor sees nothing.

Hardware

• 1 Arduino Micro + micro-USB cable (or power supply on Vin)
• The onboard LED (pin 13) as a wake-up indicator
• Optional: an ammeter in series to measure current during sleep

Complete code

// Sommeil power-down + reveil par watchdog (~1 s), en AVR C direct.
// Attention : en power-down, l'USB CDC est mort ; pas de Serial pendant le sommeil.
#include <avr/sleep.h>
#include <avr/wdt.h>

volatile bool reveil = false;

ISR(WDT_vect) {          // le watchdog reveille : travail minimal
  reveil = true;
}

void dormir1s {
  MCUSR &= ~(1 << WDRF);           // effacer la raison de reset
  wdt_reset;
  WDTCSR |= (1 << WDCE) | (1 << WDE);  // autoriser la modification
  WDTCSR = (1 << WDP0) | (1 << WDP1) | (1 << WDIE);  // ~1 s, mode interruption
  set_sleep_mode(SLEEP_MODE_PWR_DOWN);               // mode le plus profond
  sleep_enable;
  sei;
  sleep_cpu;                     // dort jusqu'au watchdog
  sleep_disable;
}

void setup {
  pinMode(LED_BUILTIN, OUTPUT);
  wdt_disable;
}

void loop {
  digitalWrite(LED_BUILTIN, HIGH);  // temoin bref du reveil
  delay(30);
  digitalWrite(LED_BUILTIN, LOW);
  dormir1s;                       // ~1 s de sommeil profond
}

Expected verification

LED 13 blinks briefly every second. By inserting an ammeter between the power supply and the board, a significant drop in current is observed during sleep compared to wake-up. >

Common errors

• Expecting serial messages during sleep: the 32U4 is asleep, USB is disconnected; the Monitor displays a port error.
• Forgetting to re-enable sleep_disable or disarm the watchdog elsewhere: the wake-up behavior becomes inconsistent.
• Configuring the watchdog in reset mode instead of interrupt mode (WDIE bit): the board restarts instead of waking up.
• Relying on USB wake-up: in power-down, an external interrupt (pins 0, 1, 2, 3, 7) is better for wake-up via button.
• Leaving pins as floating inputs: they consume power; set them as outputs in a stable state or with pull-ups before sleeping.