Nous allons faire communiquer 2 atmega48 de chez ATMEL via la communication twi (i2c). Dans un premier temps, nous allons détailler un cas simple : un atmega en mode master et l'autre en mode slave.

La communication est initiée par le maitre qui envoie d'un bit de START. Ensuite, il envoie l'adresse de l'esclave sur 7 bits. Le 8ème bit sert à indiquer la direction l'échange (écriture ou lecture).

/*
 * twislave.c
 *
 *  Created on: 26 févr. 2010
 *      Author: gdo
 */

/* ATMEGA48 @20MHz
 * LOW FUSE : F7
 * CKDIV8=1		no divided clock by 8
 * CKOUT=1		no clock output on PORTB
 * SUT1..0=11	slowly rising power
 * CKSEL3..0=0111	full swing crystal oscillator
 */

#include <avr/interrupt.h>
#include<util/twi.h>

void twiSlaveInitialise(unsigned char myAddress);

ISR(TWI_vect)
{
	switch (TWSR)
	{
	/* Slave Transmitter*/
	case (TW_ST_SLA_ACK): /* 0xA8 : SLA+R received, ACK returned*/
		break;
	case (TW_ST_ARB_LOST_SLA_ACK): /* 0xB0 : arbitration lost in SLA+RW, SLA+R received, ACK returned */
		break;
	case (TW_ST_DATA_ACK): /* 0xB8 : data transmitted, ACK received */
		break;
	case (TW_ST_DATA_NACK):/*0xC0 : data transmitted, NACK received */
		break;
	case (TW_ST_LAST_DATA): /* 0xC8 : last data byte transmitted, ACK received */
		break;

		/* Slave Receiver */
	case (TW_SR_SLA_ACK):/* 0x60 : SLA+W received, ACK returned */
		break;
	case (TW_SR_ARB_LOST_SLA_ACK):/*0x68 : arbitration lost in SLA+RW, SLA+W received, ACK returned */
		break;
	case (TW_SR_GCALL_ACK): /*0x70 : general call received, ACK returned */
		break;
	case (TW_SR_ARB_LOST_GCALL_ACK): /* 0x78 : arbitration lost in SLA+RW, general call received, ACK returned */
		break;
	case (TW_SR_DATA_ACK): /* 0x80 : data received, ACK returned */
		break;
	case (TW_SR_DATA_NACK): /* 0x88 : data received, NACK returned */
		break;
	case (TW_SR_GCALL_DATA_ACK): /* 0x90 : general call data received, ACK returned */
		break;
	case (TW_SR_GCALL_DATA_NACK):/* 0x98 : general call data received, NACK returned */
		break;
	case (TW_SR_STOP):/* 0xA0 : stop or repeated start condition received while selected */
		break;

	default:
		break;
	}

	TWCR |= 0b10000000; // TWINT flag bit is cleared
}

void twiSlaveInitialise(unsigned char myAddress)
{
	PRR = (0 << PRTWI); // the PRTWI bit in PRR must be written to zero to enable the TWI
	TWAR = myAddress + 1; // Set own TWI slave address. Accept TWI General Calls.
	/* TWINT = 0 :
	 * TWEA = 1 : enable ACK
	 * TWSTA = 0: start condition
	 * TWSTO = 0 : stop condition
	 * TWEN = 1 : enable the TWI
	 * TWIE = 1 : enable TWI interrupt */
	TWCR = (0 << TWINT) | (1 << TWEA) | (0 << TWSTA) | (0 << TWSTO) | (1
			<< TWEN) | (1 << TWIE);
}

int main(void)
{
	twiSlaveInitialise(0x20);
	SREG = (1 << SREG_I); /* The Global Interrupt Enable bit must be set for the interrupts to be enabled */

	while (1)
	{
	}
	return 1;
}

/*
 * twiMaster.c
 *
 *  Created on: 25 nov. 2009
 *      Author: gdo
 */

/* ATMEGA48 @20MHz
 * pull up resistor 1.5kohm
 * LOW FUSE : F7
 * CKDIV8=1		no divided clock by 8
 * CKOUT=1		no clock output on PORTB
 * SUT1..0=11	slowly rising power
 * CKSEL3..0=0111	full swing crystal oscillator
	*/

#include<util/twi.h>

//sets bitrate and prescaler
void twiMasterInitialise(unsigned char bitRate, unsigned char prescaler)
{
	PRR = (0 << PRTWI); // the PRTWI bit in PRR must be written to zero to enable the TWI
	TWSR = prescaler & 0x03;
	TWBR = bitRate;
}

void twiError(void)
{

}

void twiWait(unsigned char myStatus)
{
	while (!(TWCR & (1 << TWINT)))
		;
	if ((TWSR & 0xF8) != myStatus)
	{
		twiError();
	}
}

/* send START condition bit */
void twiStart(void)
{
	/* TWINT : TWI interrupt flag
	 * TWSTA : TWI START condition bit
	 * TWSTO : TWI STOP condition bit
	 * TWEN : TWI enable bit */
	TWCR = (1 << TWINT) | (1 << TWSTA) | (0 << TWSTO) | (1 << TWEN);
	twiWait(TW_START);
}

/* send slave address */
void twiAddress(unsigned char myAddress)
{
	TWDR = myAddress;
	TWCR = (1 << TWINT) | (0 << TWSTA) | (0 << TWSTO) | (1 << TWEN);
	twiWait(TW_MT_SLA_ACK); // 0x18, 0x20 or 0x38
}

/* send data */
void twiData(unsigned char myData)
{
	TWDR = myData;
	TWCR = (1 << TWINT) | (0 << TWSTA) | (0 << TWSTO) | (1 << TWEN);
	twiWait(TW_MT_DATA_ACK);
}

/* send STOP condition bit */
void twiStop(void)
{
	TWCR = (1 << TWINT) | (0 << TWSTA) | (1 << TWSTO) | (1 << TWEN);
}

/* write data to slaveAddress */
void twiWriteData(unsigned char slaveAddress, unsigned char data)
{
	twiStart();
	twiAddress(slaveAddress);
	twiData(data);
	twiStop();
}


int main(void)
{
	twiMasterInitialise(92, 0);	//set TWBR = 92 for 100kHz SCL @ 20MHz

	while (1)
	{
		twiWriteData(0x20, 0x05); //write 0x05 @0x20
	}
	return 1;
}