ESP32 DHT22 microSD CSV Logger

#include <Arduino.h>
#include <WiFi.h>
#include <SPI.h>
#include <SD.h>
#include <DHT.h>
#include <time.h>
#include <sys/time.h>
#include <esp_sntp.h>
#include <math.h>
#include <cstring>

const char* SSID = "YOUR_WIFI_SSID";
const char* PASSWORD = "YOUR_WIFI_PASSWORD";
constexpr uint8_t DHT_PIN = 4, SD_CS = 25;
constexpr uint32_t SAMPLE_MS = 10000, WIFI_WINDOW_MS = 15000, WIFI_RETRY_MS = 30000;
const char* FILE_NAME = "/climate.csv";
const char* HEADER = "uptime_ms,utc,time_status,temp_c,humidity_pct,sensor_status\n";
DHT dht(DHT_PIN, DHT22);
volatile bool clockSet = false;
bool cardReady = false, wifiTrying = false, ntpStarted = false;
uint32_t wifiStarted = 0, lastSample = 0;

void timeReceived(struct timeval*) { clockSet = true; }
void startWiFi() {
  wifiStarted = millis(); wifiTrying = true;
  WiFi.begin(SSID, PASSWORD);
  Serial.println("Starting bounded Wi-Fi attempt");
}
void networkTask() {
  uint32_t now = millis();
  if (WiFi.status() == WL_CONNECTED) {
    wifiTrying = false;
    if (!ntpStarted) {
      sntp_set_time_sync_notification_cb(timeReceived);
      configTime(0, 0, "pool.ntp.org", "time.nist.gov"); // UTC, asynchronous SNTP.
      ntpStarted = true;
    }
  } else {
    if (wifiTrying && now - wifiStarted >= WIFI_WINDOW_MS) {
      WiFi.disconnect(false); wifiTrying = false;
      Serial.println("Wi-Fi unavailable; sampling continues without waiting");
    }
    if (!wifiTrying && now - wifiStarted >= WIFI_RETRY_MS) startWiFi();
  }
}
bool initializeCard() {
  if (!SPI.begin(18, 19, 23, SD_CS)) return false;
  if (!SD.begin(SD_CS, SPI, 4000000) || SD.cardType() == CARD_NONE) return false;
  File file = SD.open(FILE_NAME, FILE_APPEND);
  if (!file) return false;
  bool ok = true;
  if (file.size() == 0) ok = file.print(HEADER) == strlen(HEADER);
  file.close();
  return ok;
}
void recordSample() {
  float h = dht.readHumidity(), t = dht.readTemperature();
  bool sensorOK = isfinite(h) && isfinite(t);
  char utc[25] = "";
  time_t stamp = time(nullptr);
  bool hasTime = clockSet && stamp >= 1704067200; // Reject an obviously unset clock.
  if (hasTime) {
    struct tm tmUTC;
    gmtime_r(&stamp, &tmUTC);
    strftime(utc, sizeof(utc), "%Y-%m-%dT%H:%M:%SZ", &tmUTC);
  }
  String row = String(millis()) + "," + utc + "," + (hasTime ? "SET_THIS_BOOT" : "UNSET") + ",";
  if (sensorOK) row += String(t, 1) + "," + String(h, 1) + ",ok\n";
  else row += ",,sensor_error\n";
  Serial.print(row);
  if (!cardReady) { Serial.println("NOT SAVED: card unavailable; no replay buffer"); return; }
  File file = SD.open(FILE_NAME, FILE_APPEND);
  if (!file) { cardReady = false; Serial.println("Open failed; fix card and reset"); return; }
  size_t written = file.print(row);
  file.close();
  if (written != row.length()) {
    cardReady = false;
    Serial.println("Short write; stop logging, inspect file and reset after repair");
  } else Serial.println("Row accepted by filesystem; not a power-loss durability guarantee");
}
void setup() {
  Serial.begin(115200);
  dht.begin();
  cardReady = initializeCard();
  Serial.println(cardReady ? "CSV card ready" : "SD init failed; Serial sampling still runs");
  WiFi.mode(WIFI_STA); WiFi.setAutoReconnect(false);
  startWiFi();
  lastSample = millis(); // First DHT22 sample after 10 seconds.
}
void loop() {
  networkTask();
  uint32_t now = millis();
  if (now - lastSample >= SAMPLE_MS) { lastSample = now; recordSample(); }
  delay(2);
}