How to use native I2C on Linux (SMBUS2)

Related Config File Sections
hardware:

MPF can control I2C devices on Linux using the Python smbus2_asyncio extension.

1. Install the smbus2_asyncio extension

Install smbus2_asyncio via pip:

pip3 install smbus2_asyncio

2. Figure out which bus to use

  • Some boards such as the Raspberry Pi have native I2C buses. Figure out which bus to use and make sure MPF has sufficient permissions to use it (Alternatively, you can also controll the I2C on the RPi remotely using the RPi platform).
  • You can build an adapter to tap I2C out of a spare VGA, DVI or HDMI port: http://www.instructables.com/id/Worlds-Cheapest-I2C-I-Squared-C-Adapter/
  • Commercial USB-I2C adapters exist but are usually very expensive
  • You can build your own USB-I2C adapter. Hardware can be bought ready-made for less than 10 bucks. Atiny85 based boards can be bought at Adafruit as Trinket (and elsewhere just google it).

This is an adafruit trinket used as USB-I2C adapter for an MMA8451-based accelerometer:

../../_images/mma8451-i2c-usb-accelerometer.jpg

3. Connect your hardware

Connect the hardware to the bus. This will be at least SDA, SCL and ground. Usually, you have to power your device somehow and in a lot of cases this power can be provided from the controller.

4. Set your I2C devices to use the “smbus2” platform

Next you need to configure I2C in MPF to use the smbus2 platform. By default, all types of devices are assumed to be using the same platform that you have set in the hardware: of your machine config file. So if your platform is set to fast, MPF assumes your I2C devices are connected to a FAST controller, and if your platform is set to p3_roc, MPF assumes your I2C devices are connected to the P3-Roc board.

To configure MPF to use native I2C, you can add an entry to the hardware: section of your machine config to tell it to override the default platform for your I2C devices and to instead use the smbus2 platform, like this:

hardware:
    i2c: smbus2

See the Mixing-and-Matching hardware platforms guide for more information about setting device-specific default platforms versus overriding the platform for individual devices.

5. Understanding I2C numbering

When using I2C addresses in I2C devices smbus2 will interpret those as bus-address. If you only provide an address it will use bus 0. On Linux bus 0 will ususally be /dev/i2c-0, 1 will be /dev/i2c-1 and so on.

6. Add udev rules if you have multiple i2c devices

If you have more than one i2c device connected to your PC via USB you can assign a name to your ports based on the USB port they are connected to.

First identify the port of your I2C hardware. Usually it should be /dev/i2c0 or /dev/i2c1.

Then run udevadm info on your port:

udevadm inf /dev/i2c0

This will show you the DEVPATH. Now replace the last part i2cX with an asterisk and add an udev rules like this in /etc/udev/rules.d/i2c.rules:

SUBSYSTEM=="i2c-dev", ACTION=="add", DEVPATH=="/devices/pci0000:00/0000:00:14.0/usb1/1-3/1-3.1/1-3.1:1.0/*", SYMLINK+="i2c-front", GROUP="adm", MODE="0660

After a reboot you should get a /dev/i2c-front device if you connect an i2c device to that specific USB port. You can use that port in your config.