EMAC I²C Programming

TODO: {{#todo:Review; (12.4.13-15:35->MD+);(12.26.13-14:05->MW+)|Michael Welling|oe 4,oe 5,mw,Review,md,mg}}

Background Information

I²C is a low-speed (400 kHz max.) two wire serial interface used to connect to a variety of sensor and I/O devices. The interface uses two bi-directional open-drain I/Os, SDA and SCL, to communicate to devices based on an address encoded within the data transmission. SDA is a data signal which sends and receives serially transmitted data. SCL is a clock signal which is used to determine when to latch data from the SDA line.

For more information about the protocol see the following page: http://en.wikipedia.org/wiki/I²C

This page specifically covers the usage of the Linux I²C i2c-dev driver.

For more information about the Linux I²C subsystem: https://i2c.wiki.kernel.org/index.php/Main_Page

Accessing the i2c-dev Interface

When the i2c-dev driver is properly loaded it exposes a device interface for each available I²C interface. Each interface will have an associated character device node at /dev/i2c-X, where X is replaced by a number representing the interface. This device node allows the user to access the I²C interface directly from a C program.

To open a I²C device node from a C program, use the standard open command as follows:

int fd;
    ⋮
fd = open("/dev/i2c-0", O_RDWR);

Once the device is open, the I2C_SLAVE ioctl command is used to set the address of the slave I²C device:

int ret;
int addr = 0x0e;
    ⋮
ret = ioctl(fd, I2C_SLAVE, addr);

After the device address is set, the device can be accessed with the standard read and write commands:

int data = 0xaa;
    ⋮
ret = write(fd, &data, 1);
    ⋮
ret = read(fd, &data, 1);

Below is a functional code listing that puts all of the commands together, showing the required include files and error checking. For a more complete example refer to the I²C demo in the EMAC SDK.

Simple C Code Example

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/ioctl.h>
#include <sys/types.h>
#include <sys/stat.h>
#include "i2c-dev.h"

int main(int argc, char * argv[])
{
    int fd;
    int ret;
    int addr = 0x0e;
    int data = 0xaa;

    fd = open("/dev/i2c-0", O_RDWR);
    
    if(fd < 0) {
        fprintf(stderr, "Cannot open i2c device\n");
        exit(-1);
    }

    ret = ioctl(fd, I2C_SLAVE, addr);

    if(ret < 0) {
        fprintf(stderr, "Cannot set slave address\n");
        exit(-1);
    }

    ret = write(fd, &data, 1);

    if(ret != 1)
        fprintf(stderr, "Cannot write i2c device\n");

    ret = read(fd, &data, 1);

    if(ret != 1)
        fprintf(stderr, "Cannot read i2c device\n");
    else
        printf("Byte read from i2c was %x\n", data);

    exit(0);
}

The i2c-dev.h header is taken from the lm-sensor project and defines the i2c character device interface. This file is included in the i2c test demo included in the EMAC SDK.