The EEPROM (Electrically Erasable Programmable Read-Only Memory) library in Arduino allows you to store data persistently even after power loss. EEPROM is useful for saving settings, calibration data, or logs.

1. Including the EEPROM Library
First, include the EEPROM library in your sketch:

cpp
#include <EEPROM.h>

2. Basic EEPROM Functions
A. EEPROM.write(address, value)
Writes a byte (0–255) to a specific EEPROM address.

Example:

cpp
EEPROM.write(0, 123); // Write value 123 to address 0

B. EEPROM.read(address)
Reads a byte (0–255) from an EEPROM address.

Example:

cpp
byte storedValue = EEPROM.read(0); // Read from address 0
Serial.println(storedValue); // Prints "123"

C. EEPROM.update(address, value)
Only writes if the value differs from what’s already stored (reduces wear).

Example:

cpp
EEPROM.update(0, 200); // Updates only if different

D. EEPROM.put(address, data)
Stores any data type (e.g., int, float, struct).

Example:

cpp
float sensorCalibration = 3.14;
EEPROM.put(10, sensorCalibration); // Store at address 10

E. EEPROM.get(address, data)
Retrieves any data type stored with EEPROM.put().

Example:

cpp
float readValue;
EEPROM.get(10, readValue); // Read float from address 10
Serial.println(readValue); // Prints "3.14"

3. Storing Strings (Char Arrays)
Since EEPROM works with bytes, strings must be written character by character.

Writing a String

cpp
void writeString(int addr, const String &data) {
  for (int i = 0; i < data.length(); i++) {
    EEPROM.write(addr + i, data[i]);
  }
  EEPROM.write(addr + data.length(), '\0'); // Null terminator
}

Reading a String

cpp
String readString(int addr) {
  char data[50]; // Max expected length
  int len = 0;
  do {
    data[len] = EEPROM.read(addr + len);
    len++;
  } while (data[len - 1] != '\0' && len < 50);
  return String(data);
}

Example Usage

cpp
writeString(20, "Hello EEPROM!"); // Store at address 20
String message = readString(20);  // Retrieve
Serial.println(message);          // Prints "Hello EEPROM!"

4. EEPROM Size & Limitations

⚠️ Wear Leveling Warning:

• EEPROM has a limited write cycle life (~100,000 writes per cell).
• Use EEPROM.update() instead of EEPROM.write() when possible.

5. Clearing EEPROM
To reset all EEPROM values to 0:

cpp
void clearEEPROM() {
  for (int i = 0; i < EEPROM.length(); i++) {
    EEPROM.write(i, 0);
  }
}

6. Full Example: Storing Sensor Data

cpp
#include <EEPROM.h>

struct SensorData {
  float temperature;
  int humidity;
  char timestamp[20];
};

void setup() {
  Serial.begin(9600);
  EEPROM.begin(); // Required for ESP boards

  // Write data
  SensorData data = {25.5, 60, "2024-01-01 12:00"};
  EEPROM.put(0, data); // Store at address 0

  // Read data
  SensorData retrievedData;
  EEPROM.get(0, retrievedData);

  Serial.print("Temp: "); Serial.println(retrievedData.temperature);
  Serial.print("Humidity: "); Serial.println(retrievedData.humidity);
  Serial.print("Time: "); Serial.println(retrievedData.timestamp);
}

void loop() {}

Key Takeaways

• Use EEPROM.write()/read() for bytes.
• Use EEPROM.put()/get() for complex data (structs, floats, etc.).
• Prefer EEPROM.update() to reduce wear.
• ESP8266/ESP32 require EEPROM.begin(size) before use.
• Clear EEPROM if corrupted.