🛠 Persister une calibration dans l'EEPROM de l'ATmega (512 octets ou 1 Ko)

Objectives

One of the true joys of the AVR compared to the Due (which has 0 bytes of EEPROM): the Duemilanove includes 512 bytes of EEPROM (ATmega168) or 1 KB (ATmega328), preserved without power. You will store a calibration value in a structure, write it only when it changes (EEPROM has limited write cycles), and verify that it survives a complete power cut.

Hardware

• 1 Arduino Duemilanove (ATmega328 recommended: 1 KB of EEPROM)
• 1 push button (pin 2 to GND) to request saving
• 1 potentiometer on A0 to simulate the measurement to be calibrated
• Mini-USB cable or standalone power supply

Complete code

// Calibration persistee en EEPROM (bibliothque EEPROM fournie avec le core)
#include <EEPROM.h>

struct Calibration {          // 4 octets
  uint16_t zero;              // offset de calibration
  uint16_t magic;             // marqueur de validite
};
const uint16_t MAGIC = 0xABCD;
const byte BOUTON = 2;

Calibration cal;

void setup {
  pinMode(BOUTON, INPUT_PULLUP);
  Serial.begin(9600);
  EEPROM.get(0, cal);                       // lecture adresse 0
  if (cal.magic != MAGIC) {                 // premiere vie de la carte ?
    cal.zero = analogRead(A0);
    cal.magic = MAGIC;
    EEPROM.put(0, cal);                     // ecriture initiale
    Serial.println("Calibration initialisee");
  } else {
    Serial.print("Calibration rechargee : zero=");
    Serial.println(cal.zero);
  }
}

void loop {
  if (digitalRead(BOUTON) == LOW) {         // bouton enfonce
    delay(50);                              // anti-rebond grossier
    if (digitalRead(BOUTON) == LOW) {
      cal.zero = analogRead(A0);
      EEPROM.put(0, cal);                   // écrit seulement sur demande
      Serial.print("zero enregistre : ");
      Serial.println(cal.zero);
      while (digitalRead(BOUTON) == LOW) { } // attend le relachement
    }
  }
}

Expected verification

First execution: "Calibration initialized". Turn off completely (unplug USB AND power supply), plug back in: the board displays "Calibration reloaded: zero=…" with the previous value, not a reset. Pressing the button with the potentiometer in a different position changes the persisted value. On a 168, the same structure occupies 4 of the 512 available bytes.

Common errors

• Writing to EEPROM in every loop: the AVR spec announces ~100,000 cycles per byte; an EEPROM.update (which only writes if the value changes) or an event-based write is mandatory for continuous operation.
• Writing beyond the actual size: 512 bytes on ATmega168, 1024 on ATmega328 — an EEPROM.put at address 1020 of a 4-byte structure works, at 1024 it silently overflows depending on the case.
• Believing that EEPROM.get on a new chip returns 0: a blank EEPROM is 0xFF; hence the magic marker to detect the first run.
• Changing the struct definition without changing the address or the magic number: the old content is reinterpreted arbitrarily — version your structures.
• Mixing up boards: a sketch compiled for 328 uploaded to 168 crashes before even talking to the EEPROM (see the Blink tutorial and processor selection).
• Comparing floats read from EEPROM with ==: prefer integers (here uint16_t) or a tolerance.