ISR(TWI_vect) {
   // Whatever will happen here
}

static void i2cEventReceived(uint8_t data) {
	// Do whatever we want with the received data
}
static void i2cEventBusError() {
	// What do we do in case of bus error?
}
static uint8_t i2cEventTransmit() {
	// Generate next byte that will be sent to the master
}


static void i2cSlaveInit(uint8_t address) {
	cli();

    /*
        Respond to general calls and calls towards us
    */
	TWAR = (address << 1) | 0x01;

    /*
        Set TWIE (TWI Interrupt enable), TWEN (TWI Enable),
        TWEA (TWI Enable Acknowledgement), TWINT (Clear
        TWINT flag by writing a 1)
    */
	TWCR = 0xC5;

	sei();
	return;
}

static void i2cSlaveShutdown() {
	cli();

	TWCR = 0;
	TWAR = 0;

	sei();
	return;
}

ISR(TWI_vect) {
	switch(TW_STATUS) {
        /*
            Note: TW_STATUS is an macro that masks
            status bits from TWSR)
        */
		case TW_SR_SLA_ACK:
		case TW_SR_DATA_ACK:
			/*
				We have received data. This is now contained
                in the TWI data register (TWDR)
			*/
			i2cEventReceived(TWDR);
			break;
		case TW_ST_SLA_ACK:
		case TW_ST_DATA_ACK:
			/*
				Either slave selected (SLA_ACK) and data
                requested or data transmitted, ACK received
				and next data requested
			*/
			TWDR = i2cEventTransmit();
			break;
		case TW_BUS_ERROR:
			i2cEventBusError();
			break;
		default:
			break;
	}
    /*
        Set TWIE (TWI Interrupt enable), TWEN (TWI
        Enable), TWEA (TWI Enable Acknowledgement),
        TWINT (Clear TWINT flag by writing a 1)
    */
	TWCR = 0xC5;
}

#include <avr/wdt.h>
#include <avr/interrupt.h>
#include <util/twi.h>
#include <stdint.h>
#define TWI_ADDRESS 0x14

static uint8_t lastByte = 0x00;

static void i2cEventReceived(uint8_t data) {
	// Do whatever we want with the received data
	lastByte = data;
}
static void i2cEventBusError() {
	// Ignore
	return;
}
static uint8_t i2cEventTransmit() {
	lastByte = lastByte + 1;
	return lastByte;
}


static void i2cSlaveInit(uint8_t address) {
	cli();

    /*
        Respond to general calls and calls towards us
    */
	TWAR = (address << 1) | 0x01;
    /*
        Set TWIE (TWI Interrupt enable), TWEN (TWI
        Enable), TWEA (TWI Enable Acknowledgement),
        TWINT (Clear TWINT flag by writing a 1)
    */
	TWCR = 0xC5;

	sei();
	return;
}

static void i2cSlaveShutdown() {
	cli();

	TWCR = 0;
	TWAR = 0;

	sei();
	return;
}

ISR(TWI_vect) {
	switch(TW_STATUS) {
        /*
            Note: TW_STATUS is an macro that masks
            status bits from TWSR)
        */
		case TW_SR_SLA_ACK:
		case TW_SR_DATA_ACK:
			/*
				We have received data. This is now contained
                in the TWI data register (TWDR)
			*/
			i2cEventReceived(TWDR);
			break;
		case TW_ST_SLA_ACK:
		case TW_ST_DATA_ACK:
			/*
				Either slave selected (SLA_ACK) and data
                requested or data transmitted, ACK received
				and next data requested
			*/
			TWDR = i2cEventTransmit();
			break;
		case TW_BUS_ERROR:
			i2cEventBusError();
			break;
		default:
			break;
	}
    /*
        Set TWIE (TWI Interrupt enable), TWEN (TWI Enable),
        TWEA (TWI Enable Acknowledgement), TWINT (Clear
        TWINT flag by writing a 1)
    */
	TWCR = 0xC5;
}


void setup() {
	/*
		Disable watchdog, initialize serial
	*/
	wdt_disable();

	/*
		Inputs are - after reset - configured as
		input (tristate) with pullups disabled (as
		they should be)
	*/

	/*
		Initialize TWI slave
	*/
	i2cSlaveInit(TWI_ADDRESS);
}

void loop() {
	// Do nothing here - this is entirely interrupt driven
}

#include <sys/cdefs.h>
#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
#include <sys/ioctl.h>
#include <errno.h>
#include <unistd.h>
#include <dev/iicbus/iic.h>

#define IIC_DEVICE "/dev/iic1"

/*
	This method provides a simple way
	of "scanning" an I2C bus by checking
	on which adresses devices exist. Note that
	since there is no real auto discovery on I2C
	this MAY trigger actions on some devices that
	are not desired - it's realized just as a write
	followed by a read.
*/
static void scanI2CBus(int fd) {
	unsigned int i;
	uint8_t buf[2] = { 0, 0 };
	struct iic_msg msg[2];
	struct iic_rdwr_data rdwr;

	msg[0].flags = !IIC_M_RD;
	msg[0].len   = sizeof(buf);
	msg[0].buf   = buf;

	msg[1].flags = IIC_M_RD;
	msg[1].len  = sizeof(buf);
	msg[1].buf  = buf;

	rdwr.nmsgs = 2;

	for(i = 1; i < 128; i++) {
		// Set address
		msg[0].slave = i;
		msg[1].slave = i;

		rdwr.msgs = msg;
		if(ioctl(fd, I2CRDWR, &rdwr) >= 0) {
			// Success - we have found a device
			printf("Device found: %02x\n", i);
		}
	}
}

static char i2cRead(int fd) {
        uint8_t buf[1];

        struct iic_msg msg[1];
        struct iic_rdwr_data rdwr;

        /*
            Assuming the code above for the
            AVR has been used with address 0x14
        */
        msg[0].slave = 0x14 << 1;
        msg[0].flags = IIC_M_RD;
        msg[0].len   = sizeof(buf);
        msg[0].buf   = buf;

        rdwr.msgs = msg;
        rdwr.nmsgs = 1;

        if(ioctl(fd, I2CRDWR, &rdwr) < 0)  {
                perror("I2CRDWR");
                return(0xFF);
        }

        return buf[0];
}

static void i2cSend(int fd, char value) {
        uint8_t buf[1];
        struct iic_msg msg;
        struct iic_rdwr_data rdwr;

        buf[0] = value;
        /*
            Assuming the code above for the AVR
            has been used with address 0x14
        */
        msg.slave = 0x14 << 1;
        msg.flags = 0;
        msg.len   = sizeof( buf );
        msg.buf   = buf;

        rdwr.msgs = &msg;
        rdwr.nmsgs = 1;

        if (ioctl(fd, I2CRDWR, &rdwr) < 0)  {
                perror("I2CRDWR");
        }
}


int main ( int argc, char **argv )  {
	int fd;

	if ((fd = open(IIC_DEVICE, O_RDWR)) < 0 )  {
		perror("open");
		return -1;

	}

	scanI2CBus(fd);

	printf("\n\nDoing some read and write tests\n\n");

	i2cSend(fd, 0x08);
	printf("First read: %u\n", (unsigned int)i2cRead(fd));
	printf("Second read: %u\n", (unsigned int)i2cRead(fd));
	printf("Third read: %u\n", (unsigned int)i2cRead(fd));


	close(fd);

	return 0;
}

clang -o i2cexample -Wall -ansi -std=c99 -pedantic ./i2cexample.c

user@FreePI_TSP:~/i2ctest # ./i2cexample
Device found: 28
Device found: 29


Doing some read and write tests

First read: 9
Second read: 10
Third read: 11