🛠 Sommeil profond et réveil par watchdog

Objectives

Reduce the Nano Every's power consumption for a battery-powered sensor: put the core to sleep in SLEEP_MODE_PWR_DOWN and wake it up periodically using the ATmega4809 watchdog. On this family, the watchdog triggers a reset by default: we use this reset as an alarm, distinguishing a watchdog restart from a normal startup.

Hardware

• 1 × Arduino Nano Every
• External power supply (battery/LiPo via VIN) to measure the actual gain — the USB bridge module remains powered as long as USB is connected
• Optional: microammeter or power consumption measurement module

Complete code

// Sommeil profond, reveil ~1 s via watchdog (reset comme alarme)
#include <avr/sleep.h>
#include <avr/wdt.h>

void setup {
  pinMode(LED_BUILTIN, OUTPUT);
  wdt_reset;
  set_sleep_mode(SLEEP_MODE_PWR_DOWN);  // mode le plus profond
  sleep_disable;                       // armement a la volee
  wdt_enable(WDTO_1S);                   // delai 1 s
}

void loop {
  digitalToggle(LED_BUILTIN);            // signal d'activite brève
  delay(50);
  wdt_reset;                           // eviter le reset immediat
  sleep_enable;
  sleep_cpu;                           // dort jusqu'au reset WDT
  // A ce point : le watchdog a reinitialise la carte, setup tourne
  // a nouveau avant de revenir ici.
}

Expected verification

LED 13 blinks briefly every second. Between two blinks, the core is off: when powered by battery and with USB disconnected, a microammeter should show an average significantly lower than active mode (the exact gain depends on the mode and any peripherals left active).

Common errors

• Measuring the gain with USB connected: the serial bridge module and its regulator consume power constantly; without power on VIN and with the cable disconnected, the measurement is meaningless.
• Forgetting periodic wdt_reset: the watchdog resets the board during operation instead of serving as a clock.
• Confusing models: on the 328P, you exit sleep via a watchdog interrupt; on the 4809 family, the default behavior is a reset — structure your code as "short task → sleep".
• Expecting millis to continue: during deep sleep, the Arduino counter stops; after waking up, rely on an external RTC clock if you need accurate time.
• Peripherals left powered (blinking LED on a module, sensor in heating mode): they drain the budget faster than the core.

• In case of an unexplained reset, check the reset cause register before suspecting the code.