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I2C

Introduction​

The Inter-Integrated Circuit (I2C), sometimes called the Two-Wire Interface, is a serial interface used to connect multiple devices quickly and easily to controllers and processors such as the Omega2.

Some examples of I2C devices include:

  • Sensors, such as temperature, humidity, and current.
  • Actuators, such as buzzers and lights.
  • Controllers, such as motors and relays.

The Omega2 has one hardware I2C bus.

Context​

Communication with the I2C is performed over 2 data lanes, each given their own pin on the Omega2:

  • Clock (SCL) – Signals when data is being transferred.
  • Data (SDA) – Carries the data to be transferred.

The I2C bus uses a master-worker architecture:

  • There are many workers on an I2C bus.
  • Bus masters are devices that are in control of when and to whom they send and receive data.
    • Masters send commands which include the address of the worker who should receive it.
    • When using I2C with the Omega2, the Omega2 is configured to be the only bus master.
  • Bus workers are devices that respond to masters when they receive a command addressed to them.
    • Each worker is identified with a hexadecimal address (e.g. 0x27).
    • Workers will safely ignore commands not addressed to them.
  • Masters and workers operate in either of two modes:
    • Transmit – sending data.
    • Receive – receiving data.

Hardware​

The Omega2 uses a hardware-based I2C controller:

  • The logic level is 3.3V.
  • Can operate in host-mode only.
  • Has two baud rates.
    • Standard mode - 100kbps (default)
    • Fast mode - 400kbps (also supported)

The I2C pins are highlighted on the Omega2/2S diagrams below.

omega2-pinout i2c-pins

Software​

The Omega2 I2C bus interface is available at /dev/i2c-0.

Interacting with the I2C bus​

The following features are available on the I2C bus.

Command line​

Use the i2c-tools command line program to communicate with any connected I2C device.

tip

The command line tools are useful for basic testing of I2C devices with your Omega2.

Installation instructions:

To use the command line tools you’ll first need to update your packages and install the i2c-tools using Omega’s package manager opkg.

To update your list of packages:

opkg update

Next, you’ll install the i2c-tools package:

opkg install i2c-tools

Now you’re ready to work with the command line tools.

Usage instructions:

For more information on how to use the command line tools, see https://linuxhint.com/i2c-linux-utilities/.

C/C++​

Onion recommends using the libi2c library when programming with C/C++.

For more information on how to use the library, see the source code at: https://git.kernel.org/pub/scm/utils/i2c-tools/i2c-tools.git/tree/.

Python​

Onion recommends using the python3-smbus module when programming with Python.

Installation instructions:

If you haven’t already done so, you’ll need to update your list of packages using Omega’s package manager opkg.

To update your list of packages:

opkg update

Next, you’ll install the pthon3-smbus module:

opkg install python3-smbus

And now you’re all set to use python3-smbus in your programs.

Usage instructions:

For more information on how to use the python3-smbus module, see the source code at: https://git.kernel.org/pub/scm/utils/i2c-tools/i2c-tools.git/tree/py-smbus/smbusmodule.c .

tip

In the linked documentation, the fromsmbus2 import SMBus import statement is used. Omega2 users must instead use fromsmbus import in their SMBus import statement.

Additional resources​