#include <Wire.h> 
#define PCF8574_ADDRESS 0x20

void setup()
{
  init_keypad();
  Serial.begin(9600);
  delay(100);
}

void loop()
{
 	test_keypad(); 
}

void init_keypad()
{
  Wire.begin();
}

void test_keypad()
{
  char tav;
  Serial.println("waiting for key");
 
  tav = scanKey(true);
  Serial.println("key pressed");
  if (tav!=0)
  {
    //Serial.print("tav not 0");
    Serial.println(tav);    
  }
  delay(100);
}

byte prevKey = 0;

char scanKey(bool wait)
{
  byte returnValue;
  do 
  {
    byte num, num1;
    num = 0;
    num1 = writeReadPcf(0xf7);  //read first row 1111011B
    num1>>=4;
    switch (num1)
    {
      case 0x7: num = '1'; break;
      case 0xb: num = '2'; break;
      case 0xd: num = '3'; break;
      case 0xe: num = 'A'; break;
    }
    if (!num)
    {
      num1 = writeReadPcf(0xfb);  //read second row 11111011B
      num1>>=4;
      switch (num1)
      {
        case 0x7: num = '4'; break;
        case 0xb: num = '5'; break;
        case 0xd: num = '6'; break;
        case 0xe: num = 'B'; break;
      }
    }
    if (!num)
    {
      num1 = writeReadPcf(0xfd);  //read third row 11111101B
      num1>>=4;
      switch (num1)
      {
        case 0x7: num = '7'; break;
        case 0xb: num = '8'; break;
        case 0xd: num = '9'; break;
        case 0xe: num = 'C'; break;
      }
    }
    if (!num)
    {
      num1 = writeReadPcf(0xfe);  //read fourthrow 11111110B
      num1>>=4;
      switch (num1)
      {
        case 0x7: num = '*'; break;
        case 0xb: num = '0'; break;
        case 0xd: num = '#'; break;
        case 0xe: num = 'D'; break;
      }
    }
    //Serial.println(num);
    if (num == 0) 
    {
      returnValue = prevKey;
      prevKey = 0;
    }
    else
    {
      prevKey = num;
      returnValue = 0;
    }
    delay(100);
  }
  while (wait && returnValue==0);
  return returnValue;
}

byte writeReadPcf(byte key)
{
  byte temp;
  Wire.beginTransmission(PCF8574_ADDRESS);
  Wire.write(key);
  Wire.endTransmission();
  delay(10);
  Wire.requestFrom(PCF8574_ADDRESS,1);
  if(Wire.available())
  {
    temp = Wire.read();
  }
  //Serial.println(temp);
  Wire.endTransmission();
  delay(100);
  return temp;
}