1-Wire

• 1-Wire bus description (wikipedia.org)
• Design guide

Example program:

#include <LiquidCrystal.h>
#include <OneWire.h>

OneWire ow(12);
LiquidCrystal lcd(3, 2, 1, 4, 5, 6, 7);
byte addr[8];
String keyStatus="";

void setup(void) {
  lcd.begin(8, 2);
  lcd.print( "iButton-" );
  lcd.setCursor(0, 1);
  lcd.print( "--reader" );
}

void loop(void) {
  getKeyCode();

  if ( keyStatus == "ok" ) {
    int i;

    lcd.setCursor(0, 0);
    for( i = 7;  i > 3; i-- )
      printHex( addr[i] );

    lcd.setCursor(0, 1);
    for( i = 3; i >= 0; i-- )
      printHex( addr[i] );
  }
  else if ( keyStatus != "nothing" ) { 
    lcd.clear();
    lcd.print(keyStatus);
  }

  delay(50);
}

void getKeyCode(){
  if ( !ow.search(addr) ) {
    ow.reset_search();
    keyStatus = "nothing";
    return;
  }

  if ( OneWire::crc8(addr, 7) != addr[7] ) {
    keyStatus = "CRC err";
    return;
  }

  if ( addr[0] != 0x01 ) {
    keyStatus = "DS1990A";
    return;
  }

  keyStatus = "ok";
  ow.reset();
}

void printHex(byte x)
{
  lcd.print( x / 16, HEX );
  lcd.print( x % 16, HEX );
}