Low-power and Sleep Modes

Most of the time, do nothing

Battery projects last longest when the MCU sleeps between short bursts of work. Sleep modes switch off parts of the chip: on the ATmega328P, idle stops the CPU but keeps peripherals and timers running, while power-down stops almost everything and wakes on an external interrupt, pin change or the watchdog timer. ESP32 boards offer light sleep and deep sleep, where most of the chip is off and wake-up behaves like a reset (use RTC memory or flash for state). Board-level parts also matter: a power LED, USB-serial chip or linear regulator can draw far more than the sleeping MCU, so a bare chip or a low-power board helps. Turn off unused peripherals (ADC, Wi-Fi) and measure current with a meter rather than trusting estimates. API details differ by core version; check the docs.

Power-down until a button press (ATmega328P)

avr-libc sleep functions; wake-up via external interrupt on pin 2 (Uno R3).

#include <avr/sleep.h>

const uint8_t WAKE_PIN = 2;

void wakeUp() { /* nothing: waking is the point */ }

void setup() {
  pinMode(WAKE_PIN, INPUT_PULLUP);
  pinMode(LED_BUILTIN, OUTPUT);
}

void loop() {
  digitalWrite(LED_BUILTIN, HIGH);   // "work" briefly
  delay(200);
  digitalWrite(LED_BUILTIN, LOW);

  set_sleep_mode(SLEEP_MODE_PWR_DOWN);
  noInterrupts();
  sleep_enable();
  attachInterrupt(digitalPinToInterrupt(WAKE_PIN), wakeUp, LOW);
  interrupts();
  sleep_cpu();                       // sleeps here until the interrupt
  sleep_disable();
  detachInterrupt(digitalPinToInterrupt(WAKE_PIN));
}

ESP32 deep sleep with a timer wake-up

ESP32 Arduino core; function names from the ESP-IDF sleep API, check the docs for your core version.

RTC_DATA_ATTR int bootCount = 0;   // survives deep sleep (RTC memory)

void setup() {
  Serial.begin(115200);
  bootCount++;
  Serial.printf("Boot number %d\n", bootCount);

  // ... take a reading, send it ...

  esp_sleep_enable_timer_wakeup(60ULL * 1000000ULL);  // 60 s in microseconds
  esp_deep_sleep_start();          // never returns; wake-up restarts setup()
}

void loop() {}

In power-down, millis() stops

On the ATmega328P, Timer0 stops in power-down sleep, so millis() does not advance while sleeping. Use the watchdog or an external RTC if you need to keep track of time.

त्वरित जाँच: What happens when an ESP32 wakes from deep sleep?

• It continues from the line after esp_deep_sleep_start()
• It restarts and runs setup() again, keeping only data stored in RTC memory or flash
• All variables keep their values automatically
• It waits for a USB connection