4-Digit LED Clock

// A clock for the 4-digit display in the Elegoo UNO R3 Super Starter Kit.//// Wiring: display pin N goes to Arduino pin N+1 (display pin 1 -> Arduino 2,// ... display pin 12 -> Arduino 13). The four "digit" pins of the display// (6, 8, 9 and 12) go through a 220 ohm resistor on the way.//// Setting the time://   - From the computer: send a line like "T 14:30:00" over USB.//     Send "?" to ask the clock what time it thinks it is.//   - With buttons (optional): a button between A0 and GND adds an hour,//     a button between A1 and GND adds a minute. Hold a button to go faster.//// The dot after the second digit blinks every second. In 12-hour mode the// dot in the bottom-right corner means PM.// ---------- Settings you can change ----------const bool USE_24_HOUR = false;   // true = 14:30, false = 2:30const bool COMMON_ANODE = false;  // false for a 5461AS display, true for a 5461BSconst unsigned long MICROS_PER_SECOND = 1000000;  // raise if the clock runs fast, lower if slow// ---------- Pins ----------// Segment pins in the order A, B, C, D, E, F, G, dot.const byte SEGMENT_PINS[8] = {12, 8, 5, 3, 2, 11, 6, 4};// Digit pins, left to right.const byte DIGIT_PINS[4] = {13, 10, 9, 7};const byte SEGMENT_ON = COMMON_ANODE ? LOW : HIGH;const byte SEGMENT_OFF = COMMON_ANODE ? HIGH : LOW;const byte DIGIT_ON = COMMON_ANODE ? HIGH : LOW;// Which segments make each number. Bit 0 is segment A, bit 6 is segment G.const byte NUMBERS[10] = {  0b00111111,  // 0  0b00000110,  // 1  0b01011011,  // 2  0b01001111,  // 3  0b01100110,  // 4  0b01101101,  // 5  0b01111101,  // 6  0b00000111,  // 7  0b01111111,  // 8  0b01101111,  // 9};const byte DOT = 0b10000000;struct Button {  byte pin;  bool held;  unsigned long lastDownMillis;  unsigned long nextRepeatMillis;};Button hourButton = {A0};Button minuteButton = {A1};long secondsToday = 0;       // time of day, in seconds since midnightbool timeIsSet = false;      // until it is, the display flashesunsigned long lastTickMicros;byte frame[4];               // the segments each digit is showing right nowvoid setup() {  for (byte s = 0; s < 8; s++) {    pinMode(SEGMENT_PINS[s], OUTPUT);    digitalWrite(SEGMENT_PINS[s], SEGMENT_OFF);  }  for (byte d = 0; d < 4; d++) {    pinMode(DIGIT_PINS[d], INPUT);  // INPUT = disconnected, so the digit stays dark  }  pinMode(hourButton.pin, INPUT_PULLUP);  pinMode(minuteButton.pin, INPUT_PULLUP);  Serial.begin(9600);  Serial.println("Clock ready. Send the time like this: T 14:30:00");  lastTickMicros = micros();}void loop() {  keepTime();  readComputer();  if (clicked(hourButton)) addHour();  if (clicked(minuteButton)) addMinute();  drawTime();  refreshDisplay();}// Counts the seconds. Works off the Arduino's own crystal, so it can drift// a little over days.void keepTime() {  while (micros() - lastTickMicros >= MICROS_PER_SECOND) {    lastTickMicros += MICROS_PER_SECOND;    secondsToday = (secondsToday + 1) % 86400L;  }}void setTime(long seconds) {  secondsToday = seconds % 86400L;  lastTickMicros = micros();  timeIsSet = true;}void addHour() {  secondsToday = (secondsToday + 3600) % 86400L;  timeIsSet = true;}// Adds a minute (without changing the hour) and restarts the seconds at :00.void addMinute() {  long hours = secondsToday / 3600;  long minutes = (secondsToday / 60 % 60 + 1) % 60;  setTime(hours * 3600 + minutes * 60);}// Reads lines sent from the computer, one letter at a time, so the display// keeps running while a message is arriving.void readComputer() {  static char line[20];  static byte length = 0;  while (Serial.available()) {    char c = Serial.read();    if (c == '\n' || c == '\r') {      line[length] = '\0';      handleCommand(line);      length = 0;    } else if (length < sizeof(line) - 1) {      line[length++] = c;    }  }}void handleCommand(const char *line) {  int h, m, s;  if (sscanf(line, "T %d:%d:%d", &h, &m, &s) == 3 &&      h >= 0 && h < 24 && m >= 0 && m < 60 && s >= 0 && s < 60) {    setTime(h * 3600L + m * 60L + s);    Serial.print("Time set to ");    printTime();  } else if (line[0] == '?') {    printTime();  }}void printTime() {  char text[9];  sprintf(text, "%02d:%02d:%02d", (int)(secondsToday / 3600),          (int)(secondsToday / 60 % 60), (int)(secondsToday % 60));  Serial.println(text);}// Returns true once when a button is pressed, then again every so often// while it is held down.bool clicked(Button &b) {  unsigned long now = millis();  if (digitalRead(b.pin) == LOW) {    b.lastDownMillis = now;    if (!b.held) {      b.held = true;      b.nextRepeatMillis = now + 600;      return true;    }    if ((long)(now - b.nextRepeatMillis) >= 0) {      b.nextRepeatMillis = now + 150;      return true;    }  } else if (b.held && now - b.lastDownMillis > 50) {    b.held = false;  // only counts as let go once it has stayed up a moment  }  return false;}// Works out what each of the four digits should show.void drawTime() {  int hours = secondsToday / 3600;  int minutes = secondsToday / 60 % 60;  bool pm = hours >= 12;  if (!USE_24_HOUR) {    hours = hours % 12;    if (hours == 0) hours = 12;  }  bool firstHalfOfSecond = micros() - lastTickMicros < MICROS_PER_SECOND / 2;  frame[0] = (hours >= 10 || USE_24_HOUR) ? NUMBERS[hours / 10] : 0;  // no "0" in front of 1-9  frame[1] = NUMBERS[hours % 10];  frame[2] = NUMBERS[minutes / 10];  frame[3] = NUMBERS[minutes % 10];  if (firstHalfOfSecond) frame[1] |= DOT;  if (pm && !USE_24_HOUR) frame[3] |= DOT;  // Until the time has been set, flash on and off like a microwave clock.  if (!timeIsSet && !firstHalfOfSecond) {    for (byte d = 0; d < 4; d++) frame[d] = 0;  }}// Lighting every LED at once would pull too much power through the Arduino's// pins, so this lights one segment on at most two digits at a time and steps// through all 16 combinations about 100 times a second. That's too fast for// your eye to notice, so the numbers look steady.void refreshDisplay() {  static byte slot = 0;  static unsigned long lastSlotMicros = 0;  if (micros() - lastSlotMicros < 600) return;  lastSlotMicros = micros();  for (byte d = 0; d < 4; d++) pinMode(DIGIT_PINS[d], INPUT);  for (byte s = 0; s < 8; s++) digitalWrite(SEGMENT_PINS[s], SEGMENT_OFF);  byte segment = slot / 2;  byte firstDigit = (slot % 2) * 2;  for (byte d = firstDigit; d < firstDigit + 2; d++) {    if (frame[d] & (1 << segment)) {      pinMode(DIGIT_PINS[d], OUTPUT);      digitalWrite(DIGIT_PINS[d], DIGIT_ON);    }  }  digitalWrite(SEGMENT_PINS[segment], SEGMENT_ON);  slot = (slot + 1) % 16;}