#define KEY_COL_1 A0
#define KEY_COL_2 A1
#define KEY_COL_3 A2
#define KEY_COL_4 A3
#define KEY_ROW_1 9
#define KEY_ROW_2 8
#define KEY_ROW_3 7
#define KEY_ROW_4 6

char keys[4][4] = {
  {'1', '2', '3', 'A'},
  {'4', '5', '6', 'B'},
  {'7', '8', '9', 'C'},
  {'*', '0', '#', 'D'}
};

char getKey() {
  for (byte row = 0; row < 4; row++) {
    pinMode(KEY_ROW_1, OUTPUT);
    pinMode(KEY_ROW_2, OUTPUT);
    pinMode(KEY_ROW_3, OUTPUT);
    pinMode(KEY_ROW_4, OUTPUT);
    digitalWrite(KEY_ROW_1, HIGH);
    digitalWrite(KEY_ROW_2, HIGH);
    digitalWrite(KEY_ROW_3, HIGH);
    digitalWrite(KEY_ROW_4, HIGH);
    
    switch (row) {
      case 0:
        digitalWrite(KEY_ROW_1, LOW);
        break;
      case 1:
        digitalWrite(KEY_ROW_2, LOW);
        break;
      case 2:
        digitalWrite(KEY_ROW_3, LOW);
        break;
      case 3:
        digitalWrite(KEY_ROW_4, LOW);
        break;
    }

    for (byte col = 0; col < 4; col++) {
      if (digitalRead(KEY_COL_1 + col) == LOW) {
        while (digitalRead(KEY_COL_1 + col) == LOW);
        return keys[row][col];
      }
    }
  }
  return '\0';
}

void setup() {
  Serial.begin(9600);
  pinMode(KEY_COL_1, INPUT_PULLUP);
  pinMode(KEY_COL_2, INPUT_PULLUP);
  pinMode(KEY_COL_3, INPUT_PULLUP);
  pinMode(KEY_COL_4, INPUT_PULLUP);
}

void loop() {
  char key = getKey();
  if (key != '\0') {
    Serial.println(key);
    delay(100);
  }
}