Android sensor HAL

AN4503

18/26

DocID026411 Rev 2

4

Android sensor HAL

4.1

Overview

The Android sensor HAL is the library which provides the links from kernel-space drivers to

the Android sensor service and Android SensorManager.

4.1.1

Sensor libraries

These libraries are dynamic; they take data in /dev/input/event and make them available to

the upper layer. This is done through the SensorManager class, sensor service class and

the sensor HAL.

The ServiceManager of the Android Framework checks the path /system/lib/hw to see if

some of the following dynamic libs are present:

 sensors.default.so

 sensors.<TARGET_BOARD_PLATFORM>.so

4.2

Files

The environmental sensors currently under consideration are:

 HTS221: relative humidity + temperature

 LPS25: pressure + temperature

 UVIS25: Ultraviolet

At this stage, the path and names are hard-coded; specifically: “lps25h”, “hts221” and

“uvis25”, as provided by the underlying kernel drivers. The path is instead found by the

libsensors.

The library is written in the C++ language using the object-oriented approach. For each

sensor, there is a custom class file: HumSensor.cpp, PressSensor.cpp and UVSensor.cpp,

which extends the common base class (SensorBase.cpp).

4.3

How to build and install the Android sensor HAL

To build the libsensors.so in the correct environment starting from the sources files, the

package must be compiled and added to the actual sensor HAL library, by following the

instructions below:

Copy the sensor HAL zipped package into the relevant android sources path, usually

located in:

 [Root Android Sources]/device/[vendor name]/[boardname]/

Untar it "tar -xzvf libsensors_env.tar.gz", or unzip in case it is provided in the .zip format

Before building the library, initialize the Android environment:

 [RAS]$ source build/envsetup.sh

 [RAS]$ lunch [target board]

Compile the library.