Arduino uno/mega2560 Code
//******************************************************//

/*
Arduino_4key_uart
This example code is in the public domain.
*/
// Define key pin
int K1 = 13;
int K2 = 12;
int K3 = 11;
int K4 = 10;

// variables will change:
int buttonState = 0; // variable for reading the pushbutton status

// the setup routine runs once when you press reset:
void setup() {

// initialize the keys pin as an input.
pinMode(K1, INPUT);
pinMode(K2, INPUT);
pinMode(K3, INPUT);
pinMode(K4, INPUT);

//Activate key pin internal pull-up resistors
digitalWrite(K1, HIGH);
digitalWrite(K2, HIGH);
digitalWrite(K3, HIGH);
digitalWrite(K4, HIGH);

// initialize serial communications at 9600 bps:
Serial.begin(9600);
}
// the loop routine runs over and over again forever:
void loop() {

buttonState = digitalRead(K1);
if(!buttonState)Serial.print("k1 \n" );
do
{
buttonState = digitalRead(K1);
}
while(!buttonState);//Wait button release

buttonState = digitalRead(K2);
if(!buttonState)Serial.print("k2 \n" );
do
{
buttonState = digitalRead(K2);
}
while(!buttonState);//Wait button release
buttonState = digitalRead(K3);
if(!buttonState)Serial.print("k3 \n" );
do
{
buttonState = digitalRead(K3);
}
while(!buttonState);//Wait button release

buttonState = digitalRead(K4);
if(!buttonState)Serial.print("k4 \n" );
do
{
buttonState = digitalRead(K4);
}
while(!buttonState);//Wait button release
delay(100);
}
//******************************************************//