🛠 Data logger sur EEPROM : persistance sans usure

Objectives

Make the most of the UNO's smallest storage space: the 1 KB of EEPROM on the ATmega328P, which survives power cuts (unlike the 2 KB of SRAM). You will design a data format, write only when the value changes (the EEPROM has a finite write endurance, typically around 100,000 cycles per cell), and verify recovery after a restart.

Hardware

• 1 Arduino UNO R3 + USB-B cable (or a 7–12 V jack power supply to test autonomy)
• 1 LDR + 10 kΩ resistor in a voltage divider on A1
• 1 push button on pin 2 to GND (to trigger saving)
• The ATmega328P is in a socket: in case of failure, you can replace the chip for a few euros, and the board remains usable.

Complete Code

// Logger de maximum de luminosité, persistant en EEPROM (1 Ko).
#include <EEPROM.h>

const uint8_t LDR = A1;
const uint8_t BTN = 2;

// Adresse 0 : un en-tête « magique » pour détecter une EEPROM jamais initialisée.
const uint16_t MAGIC = 0xA5A5;
uint16_t maxVu = 0;
uint16_t dernierEnregistre = 0;

void setup {
  Serial.begin(9600);
  pinMode(BTN, INPUT_PULLUP);

  uint16_t entete = 0;
  EEPROM.get(0, entete);
  if (entete != MAGIC) {
    // Première initialisation : pas de données valides.
    EEPROM.put(0, MAGIC);
    EEPROM.put(2, (uint16_t)0);
    maxVu = 0;
    Serial.println(F("EEPROM initialisee"));
  } else {
    EEPROM.get(2, maxVu);
    Serial.print(F("Restaure max=")); Serial.println(maxVu);
  }
}

void loop {
  int v = analogRead(LDR);

  if (v > maxVu) {
    maxVu = v;
    EEPROM.put(2, maxVu);        // écrivons SEULEMENT quand ça change
    Serial.print(F("nouveau max=")); Serial.println(maxVu);
  }

  // Bouton : dump du contenu à la demande.
  if (digitalRead(BTN) == LOW) {
    Serial.print(F("etat EEPROM[2..3]="));
    Serial.println(EEPROM.read(2) | (EEPROM.read(3) << 8));
    delay(300);                  // anti-rebond grossier pour ce bouton de debug
  }

  Serial.print(F("ldr=")); Serial.println(v);
  delay(100);
}

Expected Verification

Note the maximum value, unplug the USB (or plug in the jack), then reconnect: the Serial Monitor displays "Restore max=..." with the preserved value — SRAM is lost, EEPROM is not. Also check wear: with a threshold that causes constant writing, you waste write cycles unnecessarily; the code only calls put when there is an actual change.

Common Errors

• Calling put/get in loop every iteration: at 10 writes/s, you approach the 100,000 cycles/cell limit in just a few days. Write on event or use a deadband.
• Forgetting the magic header: on the first boot, a blank EEPROM contains 0xFF everywhere (65535), so your "max" value will be permanently incorrect.
• Writing a float (4 bytes) without thinking: EEPROM.put handles it, but using EEPROM.write byte by byte with incorrect ordering results in inconsistent values.
• Beware of clones: some low-cost clones have fuses configured differently; if the EEPROM seems dead, test the library with a minimal sketch before doubting your code.
• Confusing EEPROM (1 KB, persistent) with flash (32 KB, including 0.5 KB for the bootloader): for large volumes, you need an SPI SD card, not the EEPROM.