soft_i2c.c

/*
 * soft_i2c
 *      This is a basic software implementation of I2C protocol 
 *      using WiringPi library GPIO.
 *
 *      https://github.com/electronicayciencia/wPi_soft_i2c
 *
 *      Reinoso G.
 */

 
#include <wiringPi.h>
#include <stdint.h>
#include <stdio.h>

#include "soft_i2c.h"


/* Pull: drives the line to level LOW */
void _i2c_pull(int pin) {
	pinMode(pin, OUTPUT);
	digitalWrite(pin, LOW);
	delayMicroseconds((1e6/I2C_FREQ)/2);
}

/* Release: releases the line and return line status */
int _i2c_release(int pin) {
	pinMode(pin, INPUT);
	delayMicroseconds((1e6/I2C_FREQ)/2);
	return digitalRead(pin);
}

/* In case of clock stretching or busy bus we must wait */
/* But not wait forever though */
void _i2c_release_wait(int pin) {
	int n = 0;

	pinMode(pin, INPUT);
	delayMicroseconds((1e6/I2C_FREQ)/2);
	while (!digitalRead(pin)) {
		if (++n >= 50)	{
			if (WARN) fprintf(stderr, "Warning: I2C Bus busy or defective. Pin %d is LOW for 5s.\n", pin);
			return;
		}
		delay(100);
		pinMode(pin, INPUT);
	}
	delayMicroseconds((1e6/I2C_FREQ)/2);
}

/* Initializes software emulated i2c */
i2c_t i2c_init(int scl, int sda) {
	i2c_t port;

	port.scl = scl;
	port.sda = sda;
	
	pinMode(scl, INPUT);
	pinMode(sda, INPUT);
	pullUpDnControl(scl, PUD_UP);
	pullUpDnControl(sda, PUD_UP);

	i2c_reset(port);

	return port;
}

/* Start: pull SDA while SCL is up*/
/* Best practice is to ensure the bus is not busy before start */
void i2c_start(i2c_t port) {
	if (!_i2c_release(port.sda)) 
		i2c_reset(port);
    _i2c_release_wait(port.scl);

	_i2c_pull(port.sda);
	_i2c_pull(port.scl);
}

/* Stop: release SDA while SCL is up */
void i2c_stop(i2c_t port) {
	_i2c_release_wait(port.scl);
	if (!_i2c_release(port.sda))
		i2c_reset(port);
}

/* Reset bus sequence */
void i2c_reset(i2c_t port) {
	int i;
	int n = 0;

	_i2c_release(port.sda);

	do {
		for (i = 0; i < 10; i++) {
			_i2c_pull(port.scl);
			_i2c_release(port.scl);
		}
		if (++n >= 100) {
			if (WARN) fprintf(stderr, "Warning: I2C Bus busy or defective. SDA doesn't go UP after reset.\n");
			return;
		}
		delay(10);
	} while (!digitalRead(port.sda));

	_i2c_pull(port.scl);
	_i2c_pull(port.sda);

	i2c_stop(port);
}

/* Sends 0 or 1: 
 * Clock down, send bit, clock up, wait, clock down again 
 * In clock stretching, slave holds the clock line down in order
 * to force master to wait before send more data */
void i2c_send_bit(i2c_t port, int bit) {
	if (bit)
		_i2c_release(port.sda);
	else
		_i2c_pull(port.sda);
		
	_i2c_release_wait(port.scl);
	_i2c_pull(port.scl);

	_i2c_pull(port.sda);
}

/* Reads a bit from sda */
int i2c_read_bit(i2c_t port) {
	int s;

	_i2c_release(port.sda);
	_i2c_release_wait(port.scl);
	s = digitalRead(port.sda);
	_i2c_pull(port.scl);
	_i2c_pull(port.sda);
	
	return s;
}

/* Sends 8 bit in a row, MSB first and reads ACK.
 * Returns I2C_ACK if device ack'ed */
int i2c_send_byte(i2c_t port, uint8_t byte) {
	int i;

	for (i = 0; i < 8; i++) {
		i2c_send_bit(port, byte & 0x80);
		byte = byte << 1;
	}

	return i2c_read_bit(port);
}

/* Reads a byte, MSB first */
uint8_t i2c_read_byte(i2c_t port) {
	int byte = 0x00;
	int i;

	for (i=0; i<8; i++) 
		byte = (byte << 1) | i2c_read_bit(port);

	return byte;
}