// Forum: https://forum.arduino.cc/t/time-almost-displaying-correctly-on-led-watch-matrix/1104816/2
// This Wokwi project: https://wokwi.com/projects/359845167514629121

#define NUM_COL 6   /// columns are anodes
#define NUM_ROW 6   /// rows are cathodes
#define COL_ON LOW  /// the following four lines of code control switching the multiplexed LEDs on and off
#define COL_OFF HIGH
#define ROW_ON HIGH
#define ROW_OFF LOW

// pins for cathode (-) connections on ATmega328p, columns in LED matrix
const int colLED[NUM_COL] = { 2, 3, 4, 5, 6, 7 };      
// pins for anode (+) connections on ATmega328p, rows in LED matrix
const int rowLED[NUM_ROW] = { 0, 1, A0, A1, A2, A3 };  

int hour         = 5;      // 0...12 or 0...23
int minute       = 35;     // 0...59
int dayoftheweek = 2;      // 1(sunday)...7

void setup() {

  for (int i = 0; i < NUM_COL; i++)  /// set all column pins to OUTPUT and OFF
  {
    pinMode(colLED[i], OUTPUT);        /// set column LEDs to output
    digitalWrite(colLED[i], COL_OFF);  /// turn all columns off
  }
  for (int j = 0; j < NUM_ROW; j++)  /// set all row pins to OUTPUT and OFF
  {
    pinMode(rowLED[j], OUTPUT);        /// set row LEDs to output
    digitalWrite(rowLED[j], ROW_OFF);  /// turn all rows off
  }
}

void loop() {
  displayTime();
}

// displayTime is the led matrix multiplexing function.
void displayTime() {
  // ----------------------------------------------------
  // hour
  // ----------------------------------------------------
  int h = hour;        // 0...11
  if (h == 0) 
    h = 12;            // 1...12

  int hourRow = (h - 1) / 2;
  int hourColumn = (h - 1) % 2;

  ledOn( hourRow, hourColumn);
  delayMicroseconds(150);
  ledOff( hourRow, hourColumn);

  // ----------------------------------------------------
  // minute
  // ----------------------------------------------------
  int m = minute;      // 0...59
  int minuteRow;
  if( m < 5)           // zero minutes is located at the bottom
    minuteRow = 5;
  else
    minuteRow = (m - 5) / 10;  // start at 5 at the top
  // A change of column for each 5 minutes: (m / 5)
  // But the 10,20,30 are the right column: + 1
  // There are two columns                : %2
  // The start column is                  : 2
  int minuteColumn = 2 + ((m / 5) + 1) % 2;

  ledOn( minuteRow, minuteColumn);
  delayMicroseconds(150);
  ledOff( minuteRow, minuteColumn);

  // minute remainder 1...4
  int r = m % 5;
  if( r > 0)          // leds off for seconds 0 and 5
  {
    r--;              // row 0 for remainder 1
    ledOn( r, 4);
    delayMicroseconds(100);
    ledOff( r, 4);
  }

  // ----------------------------------------------------
  // day of the week
  // ----------------------------------------------------
  int w = dayoftheweek - 1; // 1(sunday)...7 -> 0...6
  if( w > 0)                // no led for sunday
  {
    w--;                    // monday is row 0
    ledOn(w, 5);
    delayMicroseconds(150);
    ledOff(w, 5);
  }
}

// turn on specific LED
void ledOn(int row, int col) {
  digitalWrite(rowLED[row], HIGH);  // row pin to +3V
  digitalWrite(colLED[col], LOW);   // column pin to 0V
}

// turn off specific LED
void ledOff(int row, int col) {
  digitalWrite(rowLED[row], LOW);   // row pin to 0V
  digitalWrite(colLED[col], HIGH);  // column pin to +3V
}