Practical case: Arduino UNO RFID Clock-In to SD Card

/*
 * Project: RFID Attendance SD Logger
 * Target: Arduino UNO R3 + MFRC522 + MicroSD + DS3231
 * Description: Reads RFID tag UIDs, fetches timestamp from RTC, 
 *              and logs the data to ATTEND.CSV on the SD card.
 */

#include <SPI.h>
#include <MFRC522.h>
#include <SD.h>
#include <Wire.h>
#include <RTClib.h>

// --- Pin Definitions ---
#define SD_CS_PIN    4
#define RFID_CS_PIN  10
#define RFID_RST_PIN 9

// --- Object Instantiation ---
MFRC522 mfrc522(RFID_CS_PIN, RFID_RST_PIN);
RTC_DS3231 rtc;

// --- Global Variables ---
const char* logFileName = "ATTEND.CSV";

void setup() {
  // Initialize serial communications
  Serial.begin(9600);
  while (!Serial) { delay(10); } // Wait for serial port to connect

  Serial.println(F("--- RFID Attendance Logger Initialization ---"));

  // 1. Initialize SPI Bus
  SPI.begin();

  // 2. Initialize DS3231 RTC (I2C)
  if (!rtc.begin()) {
    Serial.println(F("ERROR: Couldn't find RTC."));
    while (1) { delay(10); }
  }
  if (rtc.lostPower()) {
    Serial.println(F("WARNING: RTC lost power. Time may be inaccurate."));
  } else {
    Serial.println(F("RTC initialized successfully."));
  }

  // 3. Initialize MicroSD Card (SPI)
  // Disable RFID SPI temporarily to ensure clean SD init
  pinMode(RFID_CS_PIN, OUTPUT);
  digitalWrite(RFID_CS_PIN, HIGH); 

  Serial.print(F("Initializing SD card..."));
  if (!SD.begin(SD_CS_PIN)) {
    Serial.println(F("ERROR: SD card initialization failed!"));
    Serial.println(F("Check formatting (FAT32), wiring, and CS pin."));
    while (1) { delay(10); } // Halt if SD fails
  }
  Serial.println(F("SD card initialized."));

  // Write CSV Header if file doesn't exist
  if (!SD.exists(logFileName)) {
    File dataFile = SD.open(logFileName, FILE_WRITE);
    if (dataFile) {
      dataFile.println(F("Timestamp,UID"));
      dataFile.close();
      Serial.println(F("Created new ATTEND.CSV with headers."));
    } else {
      Serial.println(F("ERROR: Could not create file on SD card."));
    }
  }

  // 4. Initialize MFRC522 RFID (SPI)
  mfrc522.PCD_Init();
  // Optional: Increase antenna gain if tags are hard to read
  // mfrc522.PCD_SetAntennaGain(mfrc522.RxGain_max);
  Serial.println(F("MFRC522 initialized successfully."));

  Serial.println(F("--- System Ready. Waiting for RFID tags ---"));
}

void loop() {
  // Look for new RFID cards
  if (!mfrc522.PICC_IsNewCardPresent()) {
    return;
  }

  // Select one of the cards
  if (!mfrc522.PICC_ReadCardSerial()) {
    return;
  }

  // Get Current Time from RTC
  DateTime now = rtc.now();

  // Format Timestamp: YYYY-MM-DD HH:MM:SS
  char timeBuffer[20];
  snprintf(timeBuffer, sizeof(timeBuffer), "%04d-%02d-%02d %02d:%02d:%02d",
           now.year(), now.month(), now.day(),
           now.hour(), now.minute(), now.second());

  // Format UID to Hex String
  String uidString = "";
  for (byte i = 0; i < mfrc522.uid.size; i++) {
    if (mfrc522.uid.uidByte[i] < 0x10) {
      uidString += "0";
    }
    uidString += String(mfrc522.uid.uidByte[i], HEX);
  }
  uidString.toUpperCase();

  // Print to Serial Monitor
  Serial.print(F("Scanned: "));
  Serial.print(timeBuffer);
  Serial.print(F(" | UID: "));
  Serial.println(uidString);

  // Log to SD Card
  File dataFile = SD.open(logFileName, FILE_WRITE);
  if (dataFile) {
    dataFile.print(timeBuffer);
    dataFile.print(F(","));
    dataFile.println(uidString);
    dataFile.close();
    Serial.println(F(" -> Logged to SD card successfully."));
  } else {
    Serial.println(F(" -> ERROR: Failed to open ATTEND.CSV for writing."));
  }

  // Halt PICC to prevent reading the same card repeatedly in a single tap
  mfrc522.PICC_HaltA();
  // Stop encryption on PCD
  mfrc522.PCD_StopCrypto1();

  // Brief delay to prevent rapid-fire multiple reads
  delay(1000); 
}