// File: ds3231.cpp
// Read and display index register 2 from a DS3231 RTC on the I2C bus
// Author: Jim Merkle
#include <stdio.h>           // printf()
#include <sys/types.h>       // open()
#include <sys/stat.h>        // open()
#include <fcntl.h>           // open()
#include <sys/ioctl.h>       // ioctl()
#include <errno.h>           // errno
#include <string.h>          // strerror()
#include <unistd.h>          // close()
#include <linux/i2c-dev.h>   // struct i2c_msg
#include <linux/i2c.h>       // struct i2c_rdwr_ioctl_data

int main(int argc, char * argv[])
{
	struct i2c_msg ds3231msg[2];	// declare our two i2c_msg array
	unsigned char ucIndex = 2;
	unsigned char ucData = 0;
	// Load up transmit msg
	ds3231msg[0].addr = 0x68;
	ds3231msg[0].flags = 0;
	ds3231msg[0].len = sizeof(ucIndex);
	ds3231msg[0].buf = &ucIndex;
	// Load up receive msg
	ds3231msg[1].addr = 0x68;
	ds3231msg[1].flags = I2C_M_RD;
	ds3231msg[1].len = sizeof(ucData);
	ds3231msg[1].buf = &ucData;
	// Load up i2c_rdwr_ioctl_data
	struct i2c_rdwr_ioctl_data i2c_data; // declare our i2c_rdwr_ioctl_data structure
	i2c_data.msgs = ds3231msg;
	i2c_data.nmsgs = 2;

	// Open file descriptor to I2C bus
	int fd = open("/dev/i2c-1",O_RDWR);
	if(fd<0) {
		printf("Failed to open i2c-1.");
		return -1;
	}
	// With our file descriptor, perform I2C message transfers
	int result = ioctl(fd,I2C_RDWR,&i2c_data);
	if(result < 0)
	{
		printf("ioctl error: %s\n", strerror(errno));
		return -1;
	}
	// We have read a data byte from index 2...  Display it
	printf("Index 2: 0x%02X\n",ucData);
	close(fd);

	return 0;
}