Difference between revisions of "EMAC I²C Programming"

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{{todo|Write I2C Article; (12.4.13-15:35->MD+)|Michael Welling|project=oe 4,oe 5,mw,notstarted,md}}
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{{todo|SEOKWREV; (12.4.13-15:35->MD+);(12.26.13-14:05->MW+);(12.26.13-14:50->MG+);(03.04.14-16:25->BS-);(03.26.14-16:55->BS+);(04.14.14-14:55->BS+)|Michael Welling|project=oe 4,oe 5,mw,md,mg,bs,SEOKWREV}}
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{{#seo:
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|title=EMAC I²C Programming
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|titlemode=append
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|keywords=I²C Programming,i2c-dev Interface
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|description=Programming I²C device.
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}}
 
=== Background Information ===
 
=== Background Information ===
−
I²C is a low-speed two wire serial interface used to connect to a variety of sensor and I/O devices.
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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.
−
The interface uses bi-directional open-drain I/Os, SDA and SCL, to communicate to devices based on an address encoded in the data transmission. SDA is a data signal which sends and receives information using a protocol to address the appropriate device. 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:
 
For more information about the protocol see the following page:
−
http://en.wikipedia.org/wiki/I%C2%B2C
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http://en.wikipedia.org/wiki/I²C
 +
 
 +
This page specifically covers the usage of the Linux I²C i2c-dev driver.
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 +
For more information about the Linux I²C subsystem:
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https://i2c.wiki.kernel.org/index.php/Main_Page
 +
 
 +
=== Accessing the i2c-dev Interface ===
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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 <code>/dev/i2c-X</code>, 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.
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To open an I²C device node from a C program, use the standard <code>open</code> command as follows:
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<syntaxhighlight lang="c">
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int fd;
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    ⋮
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fd = open("/dev/i2c-0", O_RDWR);
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</syntaxhighlight>
 +
 
 +
 
 +
Once the device is open, the <code>I2C_SLAVE ioctl</code> command is used to set the address of the slave I²C device:
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<syntaxhighlight lang="c">
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#include "i2c-dev.h"
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    ⋮
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int ret;
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int addr = 0x0e;
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    ⋮
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ret = ioctl(fd, I2C_SLAVE, addr);
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</syntaxhighlight>
 +
 
 +
 
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After the device address is set, the device can be accessed with the standard <code>read</code> and <code>write</code> commands:
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<syntaxhighlight lang="c">
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int data = 0xaa;
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    ⋮
 +
ret = write(fd, &data, 1);
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    ⋮
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ret = read(fd, &data, 1);
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</syntaxhighlight>
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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 [[Example_i2c_test | I²C demo]] in the EMAC SDK.
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==== Simple C Code Example ====
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<syntaxhighlight lang="c">
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#include <stdio.h>
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#include <stdlib.h>
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#include <unistd.h>
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#include <sys/ioctl.h>
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#include <sys/types.h>
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#include <sys/stat.h>
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#include "i2c-dev.h"
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int main(int argc, char * argv[])
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{
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    int fd;
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    int ret;
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    int addr = 0x0e;
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    int data = 0xaa;
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    fd = open("/dev/i2c-0", O_RDWR);
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    if(fd < 0) {
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        fprintf(stderr, "Cannot open i2c device\n");
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        exit(-1);
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    }
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    ret = ioctl(fd, I2C_SLAVE, addr);
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    if(ret < 0) {
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        fprintf(stderr, "Cannot set slave address\n");
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        exit(-1);
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    }
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    ret = write(fd, &data, 1);
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    if(ret != 1)
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        fprintf(stderr, "Cannot write i2c device\n");
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    ret = read(fd, &data, 1);
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    if(ret != 1)
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        fprintf(stderr, "Cannot read i2c device\n");
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    else
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        printf("Byte read from i2c was %x\n", data);
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    exit(0);
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}
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</syntaxhighlight>
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{{imbox | type=notice | text=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.}}

Latest revision as of 14:55, 14 April 2014

TODO: {{#todo:SEOKWREV; (12.4.13-15:35->MD+);(12.26.13-14:05->MW+);(12.26.13-14:50->MG+);(03.04.14-16:25->BS-);(03.26.14-16:55->BS+);(04.14.14-14:55->BS+)|Michael Welling|oe 4,oe 5,mw,md,mg,bs,SEOKWREV}}

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 an 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: 

#include "i2c-dev.h"
    ⋮
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.
Retrieved from "https://wiki.emacinc.com/index.php?title=EMAC_I²C_Programming&oldid=3601"