src/devices/sensors/index.jd

page.title=Sensors
@jd:body

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    <h2>In this document</h2>
    <ol id="auto-toc">
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    <h2 id="what_are_“android_sensors”">What are Android sensors?</h2>
    <p>Android sensors give applications access to a mobile device's underlying
      physical sensors. They are data-providing virtual devices defined by the
      implementation of <a
      href="{@docRoot}devices/halref/sensors_8h.html">sensors.h</a>,
      the sensor Hardware Abstraction Layer (HAL).</p>
    <ul>
      <li> Those virtual devices provide data coming from a set of physical sensors:
        accelerometers, gyroscopes, magnetometers, barometer, humidity, pressure,
        light, proximity and heart rate sensors.</li>
      <li> Notably, camera, fingerprint sensor, microphone and touch screen are currently
        not in the list of physical devices providing data through “Android sensors.”
        They have their own reporting mechanism.</li>
      <li> The separation is arbitrary, but in general, Android sensors provide lower
        bandwidth data. For example, “100hz x 3 channels” for an accelerometer versus
        “25hz x 8 MP x 3 channels” for a camera or “44kHz x 1 channel” for a
        microphone.</li>
    </ul>
    <p>How the different physical sensors are connected to the system on chip
       (SoC) is not defined by Android.</p>
    <ul>
      <li> Often, sensor chips are connected to the SoC through a <a href="sensor-stack.html#sensor_hub">sensor hub</a>, allowing some low-power monitoring and processing of the data. </li>
      <li> Often, Inter-Integrated Circuit (I2C) or Serial Peripheral Interface
        (SPI) is used as the transport mechanism. </li>
      <li> To reduce power consumption, some architectures are hierarchical, with some
	minimal processing being done in the application-specific integrated
	circuit (ASIC - like motion detection on the accelerometer chip), and
        more is done in a microcontroller (like step detection
        in a sensor hub). </li>
      <li> It is up to the device manufacturer to choose an architecture based on
	accuracy, power, price and package-size characteristics. See <a
        href="sensor-stack.html">Sensor stack</a> for more information. </li>
      <li> Batching capabilities are an important consideration for power optimization.
        See <a href="batching.html">Batching</a> for more information. </li>
    </ul>
    <p>Each Android sensor has a “type” representing how the sensor behaves and what
      data it provides.</p>
    <ul>
      <li> The official Android <a href="sensor-types.html">Sensor types</a> are defined in <a href="{@docRoot}devices/halref/sensors_8h.html">sensors.h</a> under the names SENSOR_TYPE_…
        <ul>
          <li> The vast majority of sensors have an official sensor type. </li>
          <li> Those types are documented in the Android SDK. </li>
	  <li> Behavior of sensors with those types are tested in the Android
               Compatibility Test Suite (CTS). </li>
        </ul>
      </li>
      <li> If a manufacturer integrates a new kind of sensor on an Android device, the
        manufacturer can define its own temporary type to refer to it.
        <ul>
          <li> Those types are undocumented, so application developers are unlikely to use
            them, either because they don’t know about them, or know that they are rarely
            present (only on some devices from this specific manufacturer). </li>
          <li> They are not tested by CTS. </li>
	  <li> Once Android defines an official sensor type for this kind of
	       sensor, manufacturers must stop using their own temporary type
	       and use the official type instead. This way, the sensor will be
               used by more application developers. </li>
        </ul>
      </li>
      <li> The list of all sensors present on the device is reported by the HAL
        implementation.
        <ul>
          <li> There can be several sensors of the same type. For example, two proximity
            sensors or two accelerometers. </li>
          <li> The vast majority of applications request only a single sensor of a given type.
            For example, an application requesting the default accelerometer will get the
            first accelerometer in the list. </li>
          <li> Sensors are often defined by <a href="suspend-mode.html#wake-up_sensors">wake-up</a> and <a href="suspend-mode.html#non-wake-up_sensors">non-wake-up</a> pairs, both sensors sharing the same type, but differing by their wake-up
            characteristic. </li>
        </ul>
      </li>
    </ul>
<p>Android sensors provide data as a series of sensor events.</p>
      <p> Each <a href="hal-interface.html#sensors_event_t">event</a> contains:</p>
        <ul>
          <li> a handle to the sensor that generated it </li>
          <li> the timestamp at which the event was detected or measured </li>
          <li> and some data </li>
        </ul>
      <p>The interpretation of the reported data depends on the sensor type.
          See the <a href="sensor-types.html">sensor type</a> definitions for details on
          what data is reported for each sensor type.</p>

<h2 id="existing_documentation2">Existing documentation</h2>
    <h3 id="targeted_at_developers">Targeted at developers</h3>
    <ul>
      <li> Overview
        <ul>
          <li><a href="https://developer.android.com/guide/topics/sensors/sensors_overview.html"> https://developer.android.com/guide/topics/sensors/sensors_overview.html </a></li>
        </ul>
      </li>
      <li> SDK reference
        <ul>
          <li> <a href="https://developer.android.com/reference/android/hardware/SensorManager.html">https://developer.android.com/reference/android/hardware/SensorManager.html</a></li>
          <li><a href="https://developer.android.com/reference/android/hardware/SensorEventListener.html"> https://developer.android.com/reference/android/hardware/SensorEventListener.html</a></li>
          <li> <a href="https://developer.android.com/reference/android/hardware/SensorEvent.html">https://developer.android.com/reference/android/hardware/SensorEvent.html</a></li>
          <li><a href="https://developer.android.com/reference/android/hardware/Sensor.html"> https://developer.android.com/reference/android/hardware/Sensor.html</a></li>
        </ul>
      </li>
      <li> StackOverflow and tutorial websites
        <ul>
          <li> Because sensors documentation was sometimes lacking, developers resorted to Q&amp;A
            websites like StackOverflow to find answers. </li>
          <li> Some tutorial websites exist as well, but do not cover the latest features like
            batching, significant motion and game rotation vectors. </li>
          <li> The answers over there are not always right, and show where more documentation
            is needed. </li>
        </ul>
      </li>
    </ul>
<h3 id="targeted_at_manufacturers_public">Targeted at manufacturers</h3>
    <ul>
      <li> Overview
        <ul>
	  <li>This <a href="{@docRoot}devices/sensors/index.html">Sensors</a>
            page and its sub-pages. </li>
        </ul>
      </li>
      <li> Hardware abstraction layer (HAL)
        <ul>
          <li> <a href="{@docRoot}devices/halref/sensors_8h_source.html">https://source.android.com/devices/halref/sensors_8h_source.html</a></li>
          <li> Also known as “sensors.h” </li>
          <li> The source of truth. First document to be updated when new features are
            developed. </li>
        </ul>
      </li>
      <li> Android CDD (Compatibility Definition Document)
        <ul>
          <li><a href="{@docRoot}compatibility/android-cdd.pdf">https://source.android.com/compatibility/android-cdd.pdf</a></li>
          <li> See sections relative to sensors. </li>
          <li> The CDD is lenient, so satisfying the CDD requirements is not enough to ensure
            high quality sensors. </li>
        </ul>
      </li>
    </ul>