🛠 Sommeil profond et watchdog pour économiser la pile

Objectives

Extend the battery life of the 110 mAh battery on the SimpleSnap: put the ATmega328P into power-down mode, wake it up with the watchdog, and disable the ADC before sleeping. We also address the 8 MHz vs 16 MHz pitfall: a sketch configured for 16 MHz runs everything twice as fast (incorrect delay, shifted UART).

Hardware

• 1 LilyPad SimpleSnap (ATmega328P @ 8 MHz; F_CPU must be set to 8000000, board "LilyPad Arduino")
• FTDI adapter for uploading and measuring
• Multimeter set to ammeter mode between the battery and the board (optional, to measure sleep current)
• >

Complete Code

// Sommeil power-down + réveil watchdog (~4 s) — LilyPad ATmega328P @ 8 MHz
#include <avr/sleep.h>
#include <avr/wdt.h>

const uint8_t LED = 13;

ISR(WDT_vect) { /* simple réveil : le travail se fait dans loop */ }

void setup {
  // F_CPU = 8000000 (carte « LilyPad Arduino ») : jamais 16 MHz ici !
  pinMode(LED, OUTPUT);
  ADCSRA &= ~_BV(ADEN);          // ADC coupé : économie immédiate
  set_sleep_mode(SLEEP_MODE_PWR_DOWN); // mode le plus profond
  wdt_reset;
  MCUSR  &= ~_BV(WDRF);
  WDTCSR  = _BV(WDCE) | _BV(WDE);      // suite d'accès aux fuseaux watchdog
  WDTCSR  = _BV(WDIE) | _BV(WDP3) | _BV(WDP0); // 2^15 cycles / 8 MHz ≈ 4,1 s
}

void loop {
  digitalWrite(LED, HIGH);       // « battement de cœur » bref
  delay(20);
  digitalWrite(LED, LOW);

  set_sleep_enable(true);
  sleep_cpu;                   // dort jusqu'à l'expiration du watchdog
}

Expected Verification

The LED flashes briefly, then the board sleeps for about 4 seconds; the pattern repeats indefinitely. The ammeter inserted between the battery and the board drops significantly below the awake current (LED on + CPU at 8 MHz). If the rhythm seems twice as fast, F_CPU is set to 16 MHz: re-select the LilyPad board and re-upload.

Common Errors

• Board or fuses set to 16 MHz: delay and watchdog are twice as short, UART is shifted — the #1 pitfall of the LilyPad line (8 MHz, versus 16 MHz on Uno/Leonardo).
• Forgetting to disable the ADC before sleep: idle consumption is multiplied.
• Trying to use delay during sleep: timers are frozen in power-down mode; only the watchdog (or an external interrupt) wakes the board.
• Believing the entire board is asleep: the regulator/charger MCP73831 also consumes power; the bare MCU figure from the datasheet is an underestimate.
• Leaving a wdt_enable from a previous sketch: the board restarts in a loop; call wdt_disable in setup if needed.