🛠 Stocker des journaux dans la flash avec LittleFS

Objectives

Use the LittleFS file system mounted in the ESP32-S2 internal flash: format the partition, create a log file in append mode, record timestamped measurements, and read them back. This technique is useful because flash performs much better with sequential writes than random writes.

Hardware

• ESP32-S2 board and USB cable
• Computer with the "ESP32 LittleFS data upload" plugin (optional for preloading files)
• Sensor or potentiometer on GPIO3 (ADC1) to generate measurements

Complete code

// Journalisation dans LittleFS sur ESP32-S2
#include <LittleFS.h>

const char *CHEMIN_LOG = "/journal.csv";
const uint8_t CAPTEUR_PIN = 3;   // ADC1, reste disponible meme avec WiFi
uint32_t compteur = 0;

void setup {
  Serial.begin(115200);
  delay(500);
  if (!LittleFS.begin(true)) {   // true = formate si vierge
    Serial.println("Echec montage LittleFS");
    return;
  }
  Serial.println("LittleFS monte.");
  File f = LittleFS.open(CHEMIN_LOG, "a");
  if (!f) { Serial.println("Ouverture impossible"); return; }
  if (f.size == 0) f.println("millis;mesure");   // en-tete
  f.printf("%lu;demarrage\n", (unsigned long)millis);
  f.close;
}

void loop {
  compteur++;
  if (compteur % 10 == 0) {                       // une ligne toutes les 10 s
    uint16_t brut = analogRead(CAPTEUR_PIN);
    File f = LittleFS.open(CHEMIN_LOG, "a");
    if (f) {
      f.printf("%lu;%u\n", (unsigned long)millis, brut);
      f.close;
      Serial.println("Ligne ecrite.");
    }
  }
  delay(1000);
}

Expected verification

After the first boot (formatting), the console displays "LittleFS mounted." Every ten seconds, a line "millis;value" is added to the file. Restart the board using the reset button: the file is not erased, the millis timestamp restarts from zero, but previous measurements remain readable via a small reading sketch or the upload plugin.

Common errors

• Writing on every loop without buffering: flash wears out; group writes (every N seconds).
• Forgetting f.close: data remains in the buffer and is lost on the next reset.
• LittleFS.begin without parameters on a blank card: mounting fails; pass true to allow formatting.
• Default partition too small: if your logs grow, choose a partition with more space for the filesystem in the IDE.