RFID time clock with an LCD and a DS1307 on an Arduino Uno

// RFID time clock: swipe a card to clock in, and the board logs who and when.
//
//   LCD RS -> pin 2   E -> A3   D4 -> pin 8   D5 -> pin 7   D6 -> A2   D7 -> pin 4
//   RC522 SDA (SS) -> pin 10   SCK -> 13   MOSI -> 11   MISO -> 12
//         RST -> A1   3.3V -> 3.3V   GND -> GND
//   DS1307 clock SDA -> A4, SCL -> A5
//   mode button -> A0, to GND (INPUT_PULLUP)     passive buzzer -> pin 3
//
// The LCD shows the date and the time. Press the button to switch to
// enrolling: each new card you hold to the reader takes the next name from
// the list below. Press it again to go back. From then on, a known card shows
// its name and sends a line to the serial monitor:
//
//   $card,name,date,time%
//
// The $ and % mark where each record starts and ends, so a program on the
// computer can pick them out of the stream and save them as a spreadsheet.
// Change the names before you upload it to your own board.

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

const int SS_PIN = 10;
const int RST_PIN = A1;
const int BUTTON = A0;
const int BUZZER = 3;
const int PEOPLE = 6;

const char *NAMES[PEOPLE] = {"Ada", "Ben", "Cleo", "Dev", "Esme", "Femi"};

MFRC522 reader(SS_PIN, RST_PIN);
RTC_DS1307 rtc;
LiquidCrystal lcd(2, A3, 8, 7, A2, 4);  // RS, E, D4, D5, D6, D7

unsigned long cards[PEOPLE];
int enrolled = 0;
bool enrolling = false;
bool buttonWas = false;
unsigned long buttonAt = 0;
unsigned long lastShown = 0;
unsigned long messageUntil = 0;

void print2(Print &out, int value) {
  if (value < 10) out.print('0');
  out.print(value);
}

void printDate(Print &out, const DateTime &t) {
  out.print(t.year());
  out.print('-');
  print2(out, t.month());
  out.print('-');
  print2(out, t.day());
}

void printTime(Print &out, const DateTime &t) {
  print2(out, t.hour());
  out.print(':');
  print2(out, t.minute());
  out.print(':');
  print2(out, t.second());
}

// The card's first four ID bytes as one number.
unsigned long cardId() {
  unsigned long id = 0;
  for (byte i = 0; i < 4 && i < reader.uid.size; i++) id = id << 8 | reader.uid.uidByte[i];
  return id;
}

int personOf(unsigned long id) {
  for (int i = 0; i < enrolled; i++) {
    if (cards[i] == id) return i;
  }
  return -1;
}

void message(const char *top, const char *bottom) {
  lcd.clear();
  lcd.print(top);
  lcd.setCursor(0, 1);
  lcd.print(bottom);
  messageUntil = millis() + 2000;
}

void showMode() {
  if (enrolling) {
    message("Enrolling", "Hold up a card");
    messageUntil = 0;
    Serial.println("enrolling: hold up a card");
  } else {
    Serial.println("clocking: hold up a card");
    lastShown = 0;
  }
}

void showClock() {
  DateTime now = rtc.now();
  lcd.clear();
  printDate(lcd, now);
  lcd.setCursor(0, 1);
  printTime(lcd, now);
}

void swiped(unsigned long id) {
  int who = personOf(id);
  if (enrolling) {
    if (who >= 0) {
      message(NAMES[who], "already enrolled");
    } else if (enrolled < PEOPLE) {
      cards[enrolled] = id;
      message(NAMES[enrolled], "enrolled");
      Serial.print("enrolled ");
      Serial.println(NAMES[enrolled]);
      enrolled++;
      tone(BUZZER, 880, 200);
    } else {
      message("The list is full", "");
    }
    return;
  }
  if (who < 0) {
    message("Unknown card", "Enroll it first");
    Serial.println("unknown card");
    tone(BUZZER, 110, 400);
    return;
  }
  DateTime now = rtc.now();
  message(NAMES[who], "clocked in");
  tone(BUZZER, 1047, 150);
  Serial.print('$');
  Serial.print(id, HEX);
  Serial.print(',');
  Serial.print(NAMES[who]);
  Serial.print(',');
  printDate(Serial, now);
  Serial.print(',');
  printTime(Serial, now);
  Serial.println('%');
}

void setup() {
  pinMode(BUTTON, INPUT_PULLUP);
  Serial.begin(9600);
  lcd.begin(16, 2);
  lcd.print("Starting up..");
  SPI.begin();
  reader.PCD_Init();
  if (!rtc.begin()) Serial.println("no clock found: check SDA and SCL");
  if (!rtc.isrunning()) rtc.adjust(DateTime(F(__DATE__), F(__TIME__)));
  Serial.println("Mokxi Uno: RFID time clock. The button on A0 switches to enrolling.");
  showMode();
}

void loop() {
  bool down = digitalRead(BUTTON) == LOW;
  if (down != buttonWas && millis() - buttonAt > 30) {
    buttonWas = down;
    buttonAt = millis();
    if (down) {
      enrolling = !enrolling;
      showMode();
    }
  }
  if (reader.PICC_IsNewCardPresent() && reader.PICC_ReadCardSerial()) {
    swiped(cardId());
    reader.PICC_HaltA();
  }
  if (!enrolling && millis() > messageUntil && millis() - lastShown >= 1000) {
    lastShown = millis();
    showClock();
  }
}