Memory in Deep Sleep

RTC (Real-Time Clock) memory is a small portion of memory in the ESP32 that remains powered during deep sleep. It allows data retention while keeping power consumption low.

Key Features of RTC Memory

• Preserved in deep sleep, but lost in hibernation mode.
• Limited to 8KB of RTC slow memory (retained in deep sleep).
• Useful for storing variables like counters, sensor readings, or flags across sleep cycles.

This is the simplest method to store and retrieve data across deep sleep cycles.

#include <esp_sleep.h>

RTC_DATA_ATTR int bootCount = 0;  // Stored in RTC memory

void setup() {
    Serial.begin(115200);
    delay(1000);

    bootCount++;  // Increment counter across deep sleep cycles
    Serial.print("Boot count: ");
    Serial.println(bootCount);

    Serial.println("ESP32 entering deep sleep...");

    // Sleep for 10 seconds
    esp_sleep_enable_timer_wakeup(10 * 1000000);
    esp_deep_sleep_start();
}

void loop() {
    // This will not execute after wake-up
}

Explanation

• The variable bootCount is stored in RTC slow memory using RTC_DATA_ATTR.
• The value persists after deep sleep but resets on power loss or full reset.
• The ESP32 wakes up, prints the counter, and goes back to sleep.