diff --git a/slave/slave/lib/i2c.c b/slave/slave/lib/i2c.c
--- a/slave/slave/lib/i2c.c
+++ b/slave/slave/lib/i2c.c
@@ -4,3 +4,201 @@
  * Created: 11/7/2012 7:18:23 PM
  *  Author: kripperger
  */ 
+
+#include <inttypes.h>
+#include <compat/twi.h>
+#include "../config.h"
+#include "i2c.h"
+
+
+/* I2C clock in Hz */
+#define SCL_CLOCK  100000L
+
+
+
+/*************************************************************************
+ Initialization of the I2C bus interface. Need to be called only once
+*************************************************************************/
+void i2c_init(void)
+{
+  /* initialize TWI clock: 100 kHz clock, TWPS = 0 => prescaler = 1 */
+  
+  TWSR = 0;                         /* no prescaler */
+  TWBR = ((F_CPU/SCL_CLOCK)-16)/2;  /* must be > 10 for stable operation */
+
+}/* i2c_init */
+
+
+
+/*************************************************************************	
+  Issues a start condition and sends address and transfer direction.
+  return 0 = device accessible, 1= failed to access device
+*************************************************************************/
+unsigned char i2c_start(unsigned char address)
+{
+    uint8_t   twst;
+
+	// send START condition
+	TWCR = (1<<TWINT) | (1<<TWSTA) | (1<<TWEN);
+
+	// wait until transmission completed
+	while(!(TWCR & (1<<TWINT)));
+
+	// check value of TWI Status Register. Mask prescaler bits.
+	twst = TW_STATUS & 0xF8;
+	if ( (twst != TW_START) && (twst != TW_REP_START)) return 1;
+
+	// send device address
+	TWDR = address;
+	TWCR = (1<<TWINT) | (1<<TWEN);
+
+	// wail until transmission completed and ACK/NACK has been received
+	while(!(TWCR & (1<<TWINT)));
+
+	// check value of TWI Status Register. Mask prescaler bits.
+	twst = TW_STATUS & 0xF8;
+	if ( (twst != TW_MT_SLA_ACK) && (twst != TW_MR_SLA_ACK) ) return 1;
+
+	return 0;
+
+}/* i2c_start */
+
+
+
+/*************************************************************************
+ Issues a start condition and sends address and transfer direction.
+ If device is busy, use ack polling to wait until device is ready
+ 
+ Input:   address and transfer direction of I2C device
+*************************************************************************/
+void i2c_start_wait(unsigned char address)
+{
+    uint8_t   twst;
+
+
+    while ( 1 )
+    {
+	    // send START condition
+	    TWCR = (1<<TWINT) | (1<<TWSTA) | (1<<TWEN);
+    
+    	// wait until transmission completed
+    	while(!(TWCR & (1<<TWINT)));
+    
+    	// check value of TWI Status Register. Mask prescaler bits.
+    	twst = TW_STATUS & 0xF8;
+    	if ( (twst != TW_START) && (twst != TW_REP_START)) continue;
+    
+    	// send device address
+    	TWDR = address;
+    	TWCR = (1<<TWINT) | (1<<TWEN);
+    
+    	// wail until transmission completed
+    	while(!(TWCR & (1<<TWINT)));
+    
+    	// check value of TWI Status Register. Mask prescaler bits.
+    	twst = TW_STATUS & 0xF8;
+    	if ( (twst == TW_MT_SLA_NACK )||(twst ==TW_MR_DATA_NACK) ) 
+    	{    	    
+    	    /* device busy, send stop condition to terminate write operation */
+	        TWCR = (1<<TWINT) | (1<<TWEN) | (1<<TWSTO);
+	        
+	        // wait until stop condition is executed and bus released
+	        while(TWCR & (1<<TWSTO));
+	        
+    	    continue;
+    	}
+    	//if( twst != TW_MT_SLA_ACK) return 1;
+    	break;
+     }
+
+}/* i2c_start_wait */
+
+
+
+/*************************************************************************
+ Issues a repeated start condition and sends address and transfer direction 
+
+ Input:   address and transfer direction of I2C device
+ 
+ Return:  0 device accessible
+          1 failed to access device
+*************************************************************************/
+unsigned char i2c_rep_start(unsigned char address)
+{
+    return i2c_start( address );
+
+}/* i2c_rep_start */
+
+
+
+/*************************************************************************
+ Terminates the data transfer and releases the I2C bus
+*************************************************************************/
+void i2c_stop(void)
+{
+    /* send stop condition */
+	TWCR = (1<<TWINT) | (1<<TWEN) | (1<<TWSTO);
+	
+	// wait until stop condition is executed and bus released
+	while(TWCR & (1<<TWSTO));
+
+}/* i2c_stop */
+
+
+
+/*************************************************************************
+  Send one byte to I2C device
+  
+  Input:    byte to be transfered
+  Return:   0 write successful 
+            1 write failed
+*************************************************************************/
+unsigned char i2c_write( unsigned char data )
+{	
+    uint8_t   twst;
+    
+	// send data to the previously addressed device
+	TWDR = data;
+	TWCR = (1<<TWINT) | (1<<TWEN);
+
+	// wait until transmission completed
+	while(!(TWCR & (1<<TWINT)));
+
+	// check value of TWI Status Register. Mask prescaler bits
+	twst = TW_STATUS & 0xF8;
+	if( twst != TW_MT_DATA_ACK) return 1;
+	return 0;
+
+}/* i2c_write */
+
+
+
+/*************************************************************************
+ Read one byte from the I2C device, request more data from device 
+ 
+ Return:  byte read from I2C device
+*************************************************************************/
+unsigned char i2c_readAck(void)
+{
+	TWCR = (1<<TWINT) | (1<<TWEN) | (1<<TWEA);
+	while(!(TWCR & (1<<TWINT)));    
+
+    return TWDR;
+
+}/* i2c_readAck */
+
+
+
+/*************************************************************************
+ Read one byte from the I2C device, read is followed by a stop condition 
+ 
+ Return:  byte read from I2C device
+*************************************************************************/
+unsigned char i2c_readNak(void)
+{
+	TWCR = (1<<TWINT) | (1<<TWEN);
+	while(!(TWCR & (1<<TWINT)));
+	
+    return TWDR;
+
+}/* i2c_readNak */
diff --git a/slave/slave/lib/i2c.h b/slave/slave/lib/i2c.h
--- a/slave/slave/lib/i2c.h
+++ b/slave/slave/lib/i2c.h
@@ -10,6 +10,55 @@
 #define I2C_H_
 
 
+#if (__GNUC__ * 100 + __GNUC_MINOR__) < 304
+#error "This library requires AVR-GCC 3.4 or later, update to newer AVR-GCC compiler !"
+#endif
+
+
+#include <avr/io.h>
+
+
+/** defines the data direction (reading from I2C device) in i2c_start(),i2c_rep_start() */
+#define I2C_READ    1
+
+/** defines the data direction (writing to I2C device) in i2c_start(),i2c_rep_start() */
+#define I2C_WRITE   0
+
+
+
+void i2c_init(void);	//initialize the I2C master interace. Need to be called only once
+
+void i2c_stop(void);	//Terminates the data transfer and releases the I2C bus
+
+unsigned char i2c_start(unsigned char addr);	//Issues a start condition and sends address and transfer direction 
+	//addr is the address and transfer direction of I2C device
+	//Returns   0   device accessible 
+	//Returns   1   failed to access device 
+
+unsigned char i2c_rep_start(unsigned char addr);	//Issues a repeated start condition and sends address and transfer direction
+	//addr is the address and transfer direction of I2C device
+	//Returns   0   device accessible
+	//Returns   1   failed to access device
+	
+void i2c_start_wait(unsigned char addr);	//Issues a start condition and sends address and transfer direction. If device is busy, use ack polling to wait until device ready 
+	//addr is the address and transfer direction of I2C device
+
+unsigned char i2c_write(unsigned char data);	//Send one byte to I2C device
+	//data is the byte to be transfered
+	//Returns   0   write successful
+	//Returns   1   write failed
+
+unsigned char i2c_readAck(void);	//read one byte from the I2C device, request more data from device
+	//Returns byte read from I2C device
+
+unsigned char i2c_readNak(void);	//read one byte from the I2C device, read is followed by a stop condition
+	//Returns byte read from I2C device
+
+unsigned char i2c_read(unsigned char ack);	//read one byte from the I2C device. Implemented as a macro, which calls either i2c_readAck or i2c_readNak
+#define i2c_read(ack)  (ack) ? i2c_readAck() : i2c_readNak(); 
+	//When ack is 1: send ack, request more data from device
+	//When ack is 0: send nak, read is followed by a stop condition
+	//Returns byte read from I2C device
 
 
 
diff --git a/slave/slave/lib/led.c b/slave/slave/lib/led.c
--- a/slave/slave/lib/led.c
+++ b/slave/slave/lib/led.c
@@ -6,6 +6,7 @@
  */
 
  #include <avr/io.h>
+ #include "led.h"
 
  void led_configure()
  {
diff --git a/slave/slave/slave.c b/slave/slave/slave.c
--- a/slave/slave/slave.c
+++ b/slave/slave/slave.c
@@ -13,8 +13,10 @@
 
 #include "config.h"
 
+#include <inttypes.h>
 #include <avr/io.h>
 #include <util/delay.h>
+#include <compat/twi.h>
 
 //#include "lib/serial.h"
 #include "lib/led.h"
@@ -39,6 +41,7 @@ int main(void)
 	uint8_t moduleID = io_getModuleId(); //Slave Module ID from rotary dip switch
 	
 	
+	
     while(1)
     {