Inertial Module

The Librem 5 development board is equipped with a collection of sensors provided by the LSM9DS1 inertial module, including a accelerometer, gyroscope and magnetometer. The system-level interfaces to this peripheral are described below.

See this guide for basic information about accessing sensors from user space.

Overview

The sensor hardware is accessed via I²C bus 2 by a kernel module which exports data from the sensors as files in the /sys directory structure. Daemons and other middleware components, such as iio-sensor-proxy access these files to present information and services to applications that require sensor data.

Device Files

The accelerometer, gyroscope and magnetometer are exposed to user space in the sys file system, exported via the /sys directory.

Each sensor device is exposed as a subdirectory of /sys/bus/iio/devices with a name following the scheme iio:device<n> where <n> is the numeric index of the device.

The index of any given sensor may not strictly defined in advance, or may be subject to change, so it may be useful to inspect the name of each device. This is exported in the name file in each device directory.

Each device directory also contains files that can be used to read the properties of the sensor. These are described in the Linux kernel sysfs interface to IIO devices file in the kernel documentation.

I²C Bus

The inertial module exposes a magnetometer via address 0x1e on I²C bus 2, and both an accelerometer and gyroscope via address 0x6a on the same bus. Each device contains a set of registers that can be accessed by their addresses in that device.

Normally, when the kernel modules are loaded, the I²C addresses are not available to user space programs. However, when the modules are not loaded, you can use the i2cget and i2cset tools to interact with them.

The i2cdetect tool can be used to check for availability of the sensors on the I²C bus. The following shows the output when the devices are available:

purism@pureos:~$ sudo i2cdetect -y 1

     0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f
00:          -- -- -- -- -- -- -- -- -- -- -- -- --
10: -- -- -- -- -- -- -- -- -- -- -- -- -- -- 1e --
20: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
30: -- -- -- -- -- -- -- -- -- -- -- -- -- -- UU --
40: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
50: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
60: 60 -- -- -- -- -- -- -- -- -- UU -- -- -- -- --
70: -- -- -- -- -- -- -- --

If they are in use, UU will be shown instead of the device addresses.