🛠 Mode basse consommation : sleep profond et watchdog

Objectives

Turn the Esplora into an autonomous thermometer-watchdog: wake up every second via the hardware watchdog, read the temperature sensor using the library, trigger a buzzer alert if the threshold is exceeded, then return to SLEEP_MODE_PWR_DOWN (the deepest sleep mode of the ATmega32u4). You will learn how to turn off the ADC and disable interrupts during sleep — and especially the limitations specific to this board: it is powered ONLY by USB (no documented jack), so the regulator and USB transceiver consume power continuously.

Hardware

• Arduino Esplora + micro-USB cable (or a USB battery such as a power bank for a portable setup).
• Esplora library; avr/sleep.h, avr/wdt.h (core).
• No external components.

Complete code

/* Veilleur de temperature en sommeil profond.
   Reveil 1 s (watchdog) -> lecture capteur -> bip si > 30 degres -> re-dormir.
   Attention : sur Esplora, seule l'alimentation USB est documentee ; le
   courant de la carte en veille reste domine par le regulateur/USB, pas par
   l'ATmega32u4 seul. */
#include <Esplora.h>
#include <avr/sleep.h>
#include <avr/wdt.h>

volatile bool reveille = false;

ISR(WDT_vect) {
  reveille = true;   // simple drapeau : le traitement se fait dans la loop
}

void dormir {
  Esplora.writeRGB(0, 0, 0);      // tout eteint avant de dormir
  Esplora.noTone;
  ADCSRA &= ~(1 << ADEN);         // ADC off (conseille avant sleep)
  set_sleep_mode(SLEEP_MODE_PWR_DOWN);
  sleep_enable;
  MCUCR |= (1 << BODSE) | (1 << BODS);  // eteind BOD (2 cycles)
  MCUCR = (1 << BODS);
  sei;
  sleep_cpu;                     // ici on dort jusqu'au watchdog
  // --- reprise d'execution ---
  cli;
  sleep_disable;
  ADCSRA |= (1 << ADEN);
}

void setup {
  // Watchdog en mode interruption seule : 1 s
  wdt_reset;
  WDTCSR = (1 << WDCE) | (1 << WDE);
  WDTCSR = (1 << WDIE) | (1 << WDP2);   // 1 s, interruption sans reset
  Serial.begin(9600);
}

void loop {
  dormir;
  if (!reveille) return;
  reveille = false;

  int t = Esplora.readTemperature(DEGREES_C);
  Serial.print("T="); Serial.print(t); Serial.println("C (reveil watchdog)");
  if (t > 30) {
    Esplora.writeRGB(255, 0, 0);
    Esplora.tone(880, 300);
    delay(400);
  }
}

Expected verification

The serial monitor displays a line approximately every second. Squeeze the board: above 30 °C, a beep and a red flash follow the wake-up. Between wake-ups, the CPU is in power-down (the L LED blinks if your sketch does so — here it does not). >

Common errors

1. Forgetting to clear the sleep flag: without sleep_disable upon waking, any future sleep_cpu will put the board to sleep forever; always structure your code as enable → sleep → disable.
2. Watchdog in reset mode: without the WDTCSR sequence above (WDIE without WDE), a timeout will restart the board instead of waking it up — the Caterina bootloader may then be re-triggered.
3. Processing inside the ISR: reading the multiplexer (settling time ~50 µs) inside an interrupt blocks everything; only the flag should be set within the ISR.
4. Expecting battery autonomy: the Esplora has neither a jack nor documented external power guidelines; for portable use, provide a USB battery cable, and know that sleep does not cut off the upstream regulator's consumption.
5. The serial monitor "breaks" sleep: on some platforms, keeping the CDC open prevents deep sleep; test on battery, with the cable unplugged, to measure the actual behavior.