twi_functions.c

/*
 * Copyright (c) 2014, sat3ll
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, 
 * with or without modification, are permitted provided that the following conditions are met:
 *
 * 1. Redistributions of source code must retain the above copyright notice, 
 *    this list of conditions and the following disclaimer.
 * 
 * 2. Redistributions in binary form must reproduce the above copyright notice, 
 * this list of conditions and the following disclaimer in the documentation and/or 
 * other materials provided with the distribution.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY EXPRESS 
 * OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED 
 * WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO 
 * EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, 
 * INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT 
 * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, 
 * OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, 
 * WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) 
 * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */

#include <avr/io.h>
#include <util/twi.h>
#include "include/twi_functions.h"


/* Set TWI Speed */
void init_twi(void)
{
	TWSR = 0; /* No prescalar */
	TWBR = 192UL; /* 40kHz with 16MHz crystal */
}

/* Sent TWI start condition */
static void twi_start(void)
{
	TWCR = (1<<TWINT)|(1<<TWSTA)|(1<<TWEN);
	while(!(TWCR & (1<<TWINT)));
}

/* Send TWI restart condition */
static void twi_restart(void)
{
	TWCR = (1<<TWINT)|(1<<TWSTA)|(1<<TWEN);
	while(!(TWCR & (1<<TWINT)));
}

/* Read a register and do not send ACK bit */
static uint8_t readNACK(void)
{
	TWCR = (1<<TWINT)|(1<<TWEN);
	while(!(TWCR & (1<<TWINT)));
	while(TW_STATUS != TW_MR_DATA_NACK);
	return TWDR;
}

/* Send stop condition */
static void twi_stop(void)
{
	TWCR = (1<<TWINT)|(1<<TWSTO)|(1<<TWEN);
}

/* Read a register from a device */
uint8_t readReg(uint8_t address, uint8_t register_location)
{
	uint8_t result = 0;
	
	twi_start();
	TWDR = (address<<1) + TW_WRITE;
	TWCR = (1<<TWINT)|(1<<TWEN);
	while(!(TWCR & (1<<TWINT)));
	if (TW_STATUS != TW_MT_SLA_ACK)
		return 0;
		
	TWDR = register_location;
	TWCR = (1<<TWINT)|(1<<TWEN);
	while(!(TWCR & (1<<TWINT)));
	if (TW_STATUS != TW_MT_DATA_ACK)
		return 0;
		
	twi_restart();
	TWDR = (address<<1) + TW_READ;
	TWCR = (1<<TWINT)|(1<<TWEN);
	while(!(TWCR & (1<<TWINT)));
	if (TW_STATUS != TW_MR_SLA_ACK)
		return 0;
	
	result = readNACK();
	twi_stop();
	return result;	
}