const int P[] = {11, 10, 9, 8};
const int M[] = {7, 6, 5, 4};
const char k4x4 [4][4] = {
  {'1', '2', '3', 'A'},
  {'4', '5', '6', 'B'},
  {'7', '8', '9', 'C'},
  {'*', '0', '#', 'D'}
};


void setup() {
  for (int i = 0; i < 4; i++)
  {
    pinMode(P[i], OUTPUT);
    pinMode(M[i], INPUT_PULLUP);
    digitalWrite(P[i], HIGH);
  }
  Serial.begin(9600);
  Serial.println("begin");
}

void loop() {
  char a = GetKey4x4();
  if(a != 0)
  {
    Serial.print(a);
  }
}

char GetKey4x4()
{
  static unsigned long timer;
  static char olda;
  char a = 0;

  if((timer + 50) > millis()) return 0;

  for (byte p = 0; p < 4; p++)
  {
    digitalWrite(P[p], LOW);
    for(byte m = 0; m < 4; m++)
    {
      if(!digitalRead(M[m]))
      {
        a = k4x4[p][m];
      }
    }
    digitalWrite(P[p], HIGH);
  }

  timer = millis();
  if(a == olda) return 0;
  olda = a;
  return a;
}