Interfacing with I2C Devices - Revision history

Yoder: Removed ECE497 category

2012-10-08T13:50:04Z

Removed ECE497 category

2011-07-27T14:42:47Z

‎Tested Devices

Garbear: /* Opening the Bus */ added

 tags for clarity

2011-06-13T00:44:21Z

‎Opening the Bus: added <pre> tags for clarity

2011-06-13T00:41:16Z

‎Cross Compilation: fixed code formatting (missing template {{#source}}?)

2011-06-13T00:40:50Z

‎Completed Code: fixed code formatting (missing template {{#source}}?)

2011-06-13T00:40:17Z

‎Writing to the ADC: fixed code formatting (missing template {{#source}}?)

2011-06-13T00:39:23Z

‎Reading from the ADC: fixed code formatting

2011-06-13T00:32:49Z

‎Beagleboard I2C2 Enable: fixed link

2011-05-09T19:40:21Z

‎Initiating communication with the AD7991

2011-05-09T19:40:03Z

‎Initiating communication with the AD7991

@@ Line 1: / Line 1: @@
 [[Category:Robotics]]
 [[Category:BeagleBoard]]
 This page is meant to provide some basic information about how to interface with [http://en.wikipedia.org/wiki/I%C2%B2C I²C] devices through the /dev/i2c interface.  The I²C bus is commonly used to connect relatively low-speed sensors and other peripherals to equipment varying in complexity from a simple microcontroller to a full-on motherboard.  I²C is extremely popular due to its ease-of-use and ability to control multiple peripherals while utilizing only two pins on the host controller.  Although I²C has a variety of modes, this page will deal purely with communication between a Linux-based master and a slave peripheral for the time being.

@@ Line 198: / Line 198: @@
 * [http://www.analog.com/en/analog-to-digital-converters/ad-converters/ad7991/products/product.html AD7991 Quad Input ADC]
-** Utilized with [http://www.acroname.com/robotics/parts/SharpGP2D12-15.pdf Sharp GP2D12 IR Range Sensors]
+** Utilized with [http://www.acroname.com/robotics/parts/SharpGP2D12-15.pdf Sharp GP2D12 IR Range Sensors].  (That link is broken.  [http://www.acroname.com/robotics/parts/c_Sensors.html Here] is a list of sensors from that site.
 * [http://www.robot-electronics.co.uk/htm/srf08tech.shtml SRF08 Ultrasonic Range Sensors]
 * [http://www.analog.com/static/imported-files/data_sheets/ADXL345.pdf ADXL345 3-axis Accelerometer]

@@ Line 36: / Line 36: @@
 In order to communicate with an I²C peripheral with this simple structure, you must first open the bus for reading and writing like you would any file.  A call to [http://en.wikipedia.org/wiki/Open_(system_call) open] must be used rather than fopen so that writes to the bus are not buffered.  Open returns a new file descriptor (a non-negative integer) which can then be used to configure the bus.  A typical reason for failure at this stage is a lack of permissions to access /dev/i2c-2.  Adding the user to a group which has permissions to access the file will alleviate this problem, as will adjusting the file permissions to enable user access.  Adding a [http://xgoat.com/wp/2008/01/29/i2c-device-udev-rule/ udev rule to set the I²C device group] is the most permanent solution.
- #source: c|
+<pre>
- int file;
+int file;
- char *filename = "/dev/i2c-2";
+char *filename = "/dev/i2c-2";
- if ((file = open(filename, O_RDWR)) < 0) {
+if ((file = open(filename, O_RDWR)) < 0) {
      /* ERROR HANDLING: you can check errno to see what went wrong */
      perror("Failed to open the i2c bus");
      exit(1);
-−
+}
 === Initiating communication with the AD7991 ===

@@ Line 186: / Line 186: @@
 For example, using the CodeSourcery ARM toolchain:
-{{#source: bash|
+<pre>
 $ gcc CROSS-COMPILE=arm-none-linux-gnueabi ARCH=arm i2c_interface.c -o i2c_binary
-}}
+</pre>
 The resulting binary can then be moved to the embedded device and executed.

@@ Line 112: / Line 112: @@
 === Completed Code ===
-{{#source: c|
+<pre>
 #include <glib.h>
 #include <glib/gprintf.h>
@@ Line 178: / Line 178: @@
+     }
+}
-}}
+</pre>
 === Cross Compilation ===

@@ Line 92: / Line 92: @@
 Some devices require an internal address to be sent prior to the data to specify the register on the external device to access.  The AD7991 contains only one configuration register, and as such, does not require a internal register selection address to be sent to the device.  For devices with more than one configuration register, the address of the register should be written first, followed by the data to be placed there.  See the datasheet specific to the part for more details.
-{{#source: c|
+<pre>
 //unsigned char reg = 0x10; // Device register to access
 //buf[0] = reg;
@@ Line 104: / Line 104: @@
      printf("\n\n");
+}
-}}
+</pre>
 === Detecting Errors ===

@@ Line 64: / Line 64: @@
 The code in the else block below calculates the voltage present at the ADC pin assuming a 5 volt reference/supply voltage.  The AD7991 samples a 12 bit value, which is read back as two eight bit values.  Rather than waste the empty bits, two of them are also used to signal the channel data being sent.  See page 21 of the datasheet for more details.
-{{#source: c|
+<pre>
 char buf[10] = {0};
 float data;
@@ Line 84: / Line 84: @@
+     }
+}
-}}
+</pre>
 === Writing to the ADC ===

@@ Line 15: / Line 15: @@
 See [[BeagleBoard#Linux_kernel]] and [[BeagleBoardLinuxKernel]] for details on how to recompile your Linux kernel.  To enable i2c2 specifically during that process, enable the setting during the "make menuconfig" step.
-It is important to note that the method of enabling i2c2 varies depending on your kernel and applied patches.  Recent kernel versions have changed how [[BeagleBoardPinMux#Recent_Kernels]] | PIN_MUX settings]] are set.  For me, running the openembedded stable/2009 kernel (2.6.29-r46) was enough.  By default i2c2 was properly configured and enabled.  A fast way to check is to see if /dev/i2c-2 exists (But this does not check proper mux settings).
+It is important to note that the method of enabling i2c2 varies depending on your kernel and applied patches.  Recent kernel versions have changed how [[BeagleBoardPinMux#Recent_Kernels | PIN_MUX settings]] are set.  For me, running the openembedded stable/2009 kernel (2.6.29-r46) was enough.  By default i2c2 was properly configured and enabled.  A fast way to check is to see if /dev/i2c-2 exists (But this does not check proper mux settings).
 I2C2 is pinned out as pins 23 (SDA) and 24 (SCL) on the expansion header.

@@ Line 52: / Line 52: @@
   int addr = 0b00101001;          // The I2C address of the ADC
   if (ioctl(file, I2C_SLAVE, addr) < 0) {
       printf("Failed to acquire bus access and/or talk to slave.\n");