딥 슬립과 전력 관리

Why Power Management Matters

Running an ESP32 continuously at full Wi-Fi power draws ~240 mA. A 1000 mAh battery would last ~4 hours. With periodic deep sleep, the same battery can last weeks for a sensor that wakes, takes a reading, publishes over MQTT, then sleeps again.

Deep Sleep Basics

In deep sleep, only the RTC (Real-Time Clock) subsystem remains active. All other peripherals — CPU, Wi-Fi, Bluetooth, ADC — are powered off.

#define uS_TO_S 1000000ULL // microseconds → seconds

void setup() {
  Serial.begin(115200);
  Serial.println("Waking up...");

  // --- do your work here (read sensor, send MQTT, etc.) ---
  float temp = 25.4;
  Serial.printf("Temp: %.1f°C\n", temp);

  Serial.println("Going to sleep for 30 seconds...");
  delay(100); // let Serial flush

  esp_sleep_enable_timer_wakeup(30 * uS_TO_S);
  esp_deep_sleep_start(); // does not return
}

void loop() { /* never reached */ }

Every wake-up runs setup() from the beginning.

RTC Memory — Persisting Data

Normal variables reset on each wake. Store persistent data in RTC-retained SRAM:

RTC_DATA_ATTR int wakeCount = 0; // survives deep sleep

void setup() {
  wakeCount++;
  Serial.printf("Wake #%d\n", wakeCount);
  // ...
  esp_deep_sleep_start();
}

External GPIO Wake-up

Wake when a GPIO goes LOW (e.g., a button press or motion sensor trigger):

#define WAKE_PIN GPIO_NUM_33

esp_sleep_enable_ext0_wakeup(WAKE_PIN, 0); // 0 = wake on LOW

For multiple pins (any one triggers wake):

esp_sleep_enable_ext1_wakeup((1ULL << GPIO_NUM_33) | (1ULL << GPIO_NUM_34),
  ESP_EXT1_WAKEUP_ANY_LOW);

Touch Wake-up

The ESP32 has capacitive touch inputs on GPIO 2, 4, 12, 13, 14, 15, 27, 32, 33:

esp_sleep_enable_touchpad_wakeup();
touchSleepWakeUpEnable(T0, 40); // T0 = GPIO4, threshold 40

Duty Cycle Calculation

Active time: 2 seconds (connect Wi-Fi + publish + disconnect)
Sleep time:  58 seconds
Average current ≈ (2/60 × 200 mA) + (58/60 × 0.05 mA) ≈ 6.7 mA
Battery life (1000 mAh) ≈ 1000 / 6.7 ≈ 149 hours ≈ 6 days

Exercises

1. Build a sensor node: wake every 60 s, read DHT22, publish to MQTT, deep sleep
2. Add an RTC counter that increments each wake; publish it as wakeCount to MQTT
3. Measure average current with a USB power meter before and after adding sleep