/* This demo code demonstrates the bidirectional possibilities
    of the PCF8574 with the Wire.h -library.  R.Schneider
 */
#include <Wire.h>    // Required for I2C communication
#define PCF8574adr0 0x20
volatile byte PCFmerker = 0 ; 

void setup() {
  Serial.begin(9600); 
  Wire.begin( PCF8574adr0 );
  pinMode( 2, INPUT_PULLUP ) ; 
  attachInterrupt( digitalPinToInterrupt( 2 ), isrpcf, CHANGE ); 
}

void isrpcf()
{   // Wenn LOW-Pegel, dann merken!
  if( digitalRead(2) == LOW )  PCFmerker = 1; 
  if( digitalRead(2) == HIGH ) PCFmerker = 2; 
}

byte mirror( byte input )
{
  static String col[4] = {"blau ", "gr\365n ", "gelb ", "rot " }; 
  byte output = 0xFF, i = 0;
  input = (~input) & 0x0F ;  
  if( input & 0x01 ) { output &= 0x7F; i=0; }
  if( input & 0x02 ) { output &= 0xBF; i=1; }
  if( input & 0x04 ) { output &= 0xDF; i=2; }
  if( input & 0x08 ) { output &= 0xEF; i=3; }
  if( input > 0 ) 
  { Serial.print( String(input) + " " + col[i] ); 
    Wire.beginTransmission(PCF8574adr0);
    Wire.write( output );
    Wire.endTransmission();
    //delay(100); 
  }
  return( output); 
}

void loop() {
  static byte input =0, output=0, isrZaehler = 0; 
  
  if( PCFmerker == 1 ) 
  {
    Wire.requestFrom( PCF8574adr0 , 1);
    if (Wire.available()) {
      input = Wire.read();
    }
    Wire.endTransmission();
    output = mirror( input );
    PCFmerker = 0 ; 
    isrZaehler++; 
    if( isrZaehler % 10 == 0 ) Serial.println(); 
  }
  if( PCFmerker > 1 ) 
  {
    Wire.beginTransmission(PCF8574adr0);
    Wire.write( 0xFF ); // 4 LEDs löschen
    Wire.endTransmission();
    PCFmerker = 0; 
  }
  // M8 etwas anderes Sinnvolles! 
}