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QWIIC, STEMMA and Grove Add-ons in Linux

Note

This article is under construction.

I’m creating a place for me to start taking notes on how to load drivers for I2C devices (mostly), but also other Grove add-ons.

Todo

Create a simple drawing of BeaglePlay connecting to an external add-on with an interesting device on it.

For simplicity sake, I’ll use these definitions

  • add-on: the QWIIC, STEMMA (QT) or Grove add-on separate from your Linux computer

  • device: the “smart” IC on the add-on to which we will interface from your Linux computer

  • board: the Linux single board computer with the embedded interface controller you are using

  • module: a kernel module that might contain the driver

Using I2C with Linux drivers

Linux has a ton of drivers for I2C devices. We just need a few parameters to load them.

Using a Linux I2C kernel driver module can be super simple, like in the below example for monitoring a digital light sensor.

cd /dev/bone/i2c/2
echo tsl2561 0x29 > new_device
watch -n0 cat "2-0029/iio:device0/in_illuminance0_input"

Once you issue this, your screen continuously refresh with luminance values from the add-on sensor.

In the above example, /dev/bone/i2c/2 comes from which I2C controller we are using on the board and there are specific pins on the board where you can access it. On BeagleBone boards, there is often a symbolic link to the controller based upon the cape expansion header pins being used. See I2C for the cape expansion header pin assignments.

tsl2561 is the name of the driver we want to load and 0x29 is the address of the device on the I2C bus. If you want to know about I2C device addresses, the Sparkfun I2C tutorial isn’t a bad place to start. The new_device virtual file is documented in the Linux kernel documentation on instantiating I2C devices.

On the last line, watch is a program that will repeatedly run the command that follows. The -n0 sets the refresh rate. The program cat will share the contents of the file 2-0029/iio:device0/in_illuminance0_input.

2-0029/iio:device0/in_illuminance0_input is not a file on a disk, but output directly from the driver. The leading 2 in 2-0029 represents the I2C controller index. The 0029 represents the device I2C address. Most small sensor and actuator drivers will show up as Industrial I/O (IIO) devices. New IIO devices get incrementing indexes. In this case, iio:device0 is the first IIO device driver loaded. Finally, in_illuminance0_input comes from the SYSFS application binary interface for this type of device, a light sensor. The Linux kernel ABI documentation for sysfs-bus-iio provides the definition of available data often provided by light sensor drivers.

What:         /sys/.../iio:deviceX/in_illuminance_input
What:         /sys/.../iio:deviceX/in_illuminance_raw
What:         /sys/.../iio:deviceX/in_illuminanceY_input
What:         /sys/.../iio:deviceX/in_illuminanceY_raw
What:         /sys/.../iio:deviceX/in_illuminanceY_mean_raw
What:         /sys/.../iio:deviceX/in_illuminance_ir_raw
What:         /sys/.../iio:deviceX/in_illuminance_clear_raw
KernelVersion:        3.4
Contact:      linux-iio@vger.kernel.org
Description:
              Illuminance measurement, units after application of scale
              and offset are lux.

Read further to discover how to find these bits of magic text used above.

The generic steps are fairly simple:

  1. Identify driver name and address

  2. Ensure driver is enabled in kernel build

  3. Identify I2C signals on board and controller in Linux

  4. Ensure pinmux set to I2C

  5. Ensure add-on connection is good

  6. Issue Linux command to load driver

  7. Identify and utilize interface provided by driver

Driver name

One resource that is very helpful is the list that Vaishnav put together for supporting Mikroelektronika Click add-ons. This list of Click add-ons with driver information can help a lot with matching a device to the driver name, device address, and kernel configuration setting.

Note

Documentation for your particular add-on might indicate a different device address than is configured on Click add-ons.

I’m not aware of a trivial way of discovering the mapping that Vaishnav created outside of looking at the kernel sources. As an example, let’s look at the Grove Digital Light Sensor add-on which is documented to utilize a TSL2561.

Searching through the kernel sources, we can find the driver code at drivers/iio/light/tsl2563.c. There is a list of driver names in a i2c_device_id table:

static const struct i2c_device_id tsl2563_id[] = {
      { "tsl2560", 0 },
      { "tsl2561", 1 },
      { "tsl2562", 2 },
      { "tsl2563", 3 },
      {}
};

Important

Don’t miss that the driver, tsl2561 , is actually part of a superset driver, tsl2563 . This can make things a bit trickier to find, so you have to look within the text of the driver source, not just the filenames.

Kernel configuration

I2C signals and controller

Pinmuxing

Wiring

Load driver

Interface

Finding I2C add-on modules

Note

There are some great resources out there:

Pitfalls

Not all I2C devices with drivers in the Linux kernel can be loaded this way. The most common reason is that the device driver expects an interrupt signal or other GPIO along with the I2C communication. In these cases, a device tree overlay or driver modification may be necessary.