More about I/O

Inter Integrated Circuits (I²C)

I²C (Inter-Integrated Circuit)(alternately spelled I2C or IIC, most commonly pronounced I-squared-C) is a multimaster serial single-ended computer bus invented by the Philips semiconductor division, today NXP Semiconductors, and used for attaching low-speed peripherals to a motherboard, embedded system, cellphone, or other digital electronic devices.

Several competitors, such as Siemens AG (later Infineon Technologies AG, now Intel mobile communications), NEC, Texas Instruments, STMicroelectronics (formerly SGS-Thomson), Motorola (later Freescale), and Intersil, have introduced compatible I²C products to the market since the mid-1990s.

Since October 10, 2006, no licensing fees are required to implement the I²C protocol. However, fees are still required to obtain I²C slave addresses allocated by NXP.

Interesting remark
SMBus, defined by Intel in 1995, is a subset of I²C that defines the protocols more strictly. One purpose of SMBus is to promote robustness and interoperability. Accordingly, modern I²C systems incorporate policies and rules from SMBus, sometimes supporting both I²C and SMBus, requiring only minimal reconfiguration.

I²C in the Erle-Brain

The educational drone Erle includes 3 I²C busses available under /dev: /dev/i2c-0, /dev/i2c-1 and /dev/i2c-2.

/dev/i2c-2 is by default not activated. In order to activate it you should type:

echo BB-I2C1 > $SLOTS

You can verify that it has been activated typing:

cat $SLOTS

Inspecting I²C

A good tool to inspect the I²C bus is the i2cdetect bash command:

i2cdetect -r 1
WARNING! This program can confuse your I2C bus, cause data loss and worse!
I will probe file /dev/i2c-1 using read byte commands.
I will probe address range 0x03-0x77.
Continue? [Y/n]
     0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f
00:          -- -- -- -- -- -- -- -- -- -- -- -- --
10: -- -- -- 13 -- -- -- -- -- -- -- -- -- -- -- --
20: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
30: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
40: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
50: -- -- -- -- UU UU UU UU -- -- -- -- -- -- -- --
60: -- -- -- -- -- -- -- -- 68 -- -- -- -- -- -- --
70: -- -- -- -- -- -- -- --

This command sends a ping to the 0x03-0x77 range (in hexadecimal) and records the answers in every address thereby it's able to figure out if there are any devices. Particularly we can see that the addresses 0x13 and 0x68 answered which correspond to the IR and IMU sensors respectively.

Instantiate I²C devices from the userspace

The Linux kernel also allows to manually configure I²C devices through a sysfs interface:

In general, the kernel should know which I2C devices are connected and what addresses they live at. However, in certain cases, it does not, so a sysfs interface was added to let the user provide the information. This interface is made of 2 attribute files which are created in every I2C bus directory: new_device and delete_device. Both files are write only and you must write the right parameters to them in order to properly instantiate, respectively delete, an I2C device.

File new_device takes 2 parameters: the name of the I2C device (a string) and the address of the I2C device (a number, typically expressed in hexadecimal starting with 0x, but can also be expressed in decimal.)

File delete_device takes a single parameter: the address of the I2C device. As no two devices can live at the same address on a given I2C segment, the address is sufficient to uniquely identify the device to be deleted.

Example: echo eeprom 0x50 > /sys/bus/i2c/devices/i2c-3/new_device

While this interface should only be used when in-kernel device declaration can't be done, there is a variety of cases where it can be helpful:

  • The I2C driver usually detects devices (method 3 above) but the bus segment your device lives on doesn't have the proper class bit set and thus detection doesn't trigger.
  • The I2C driver usually detects devices, but your device lives at an unexpected address.
  • The I2C driver usually detects devices, but your device is not detected, either because the detection routine is too strict, or because your device is not officially supported yet but you know it is compatible.
  • You are developing a driver on a test board, where you soldered the I2C device yourself.

This interface is a replacement for the force_* module parameters some I2C drivers implement. Being implemented in i2c-core rather than in each device driver individually, it is much more efficient, and also has the advantage that you do not have to reload the driver to change a setting. You can also instantiate the device before the driver is loaded or even available, and you don't need to know what driver the device needs.

Considering this, we could instantiate a sensor (hih6130) connected to /dev/i2c-1 and with address 0x27 doing:

echo hih6130 0x27 > /sys/bus/i2c/devices/i2c-1/new_device

After that, the device will be available under /sys/bus/i2c/drivers/hih6130/1-0027.

To remove the device you can use:

echo 0x27 > /sys/bus/i2c/devices/i2c-1/delete_device


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