src/devices/sensors/index.jd

page.title=Sensors HAL overview
@jd:body

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    <h2>In this document</h2>
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<h2 id="intro">Introduction</h2>
<p><a href="http://developer.android.com/guide/topics/sensors/sensors_overview.html">Android 
  sensors</a> give applications access to a mobile device's underlying base sensor(s): 
  accelerometer, gyroscope, and magnetometer. Manufacturers develop the drivers 
  that define additional composite sensor types from those base sensors. For 
  instance, Android offers both calibrated and uncalibrated gyroscopes, a 
  geomagnetic rotation vector and a game rotation vector. This variety gives 
  developers some flexibility in tuning applications for battery life optimization 
  and accuracy.</p>
<p>The <a href="{@docRoot}devices/reference/sensors_8h_source.html">Sensors
Hardware Abstraction Layer (HAL) API</a> is the interface between the hardware drivers 
and the Android framework; the <a href="http://developer.android.com/reference/android/hardware/Sensor.html">Sensors Software Development Kit (SDK) 
  API</a> is the interface between the Android framework and the Java applications. Please note, 
  the Sensors HAL API described in this documentation is not identical to the 
  Sensors SDK API described on <a href="http://developer.android.com/reference/android/hardware/Sensor.html">developer.android.com</a>. 
  For example, some sensors that are deprecated in the SDK may still exist in the 
  HAL, and vice versa.</p>
<p>Similarly, audio recorders, Global Positioning System (GPS) devices, and 
  accessory (pluggable) sensors are not supported by the Android Sensors HAL API 
  described here. This API covers sensors that are physically part of the device 
  only. Please see the <a href="{@docRoot}devices/audio.html">Audio</a>, <a href="{@docRoot}devices/reference/gps_8h.html">Location </a><a href="{@docRoot}devices/reference/gps_8h.html">Strategies</a>, 
  and the <a href="{@docRoot}accessories/index.html">Accessories</a> section 
  for information on those devices.</p>
<p>Application framework<br/>
  At the application framework level is the app code, which utilizes the <a href="http://developer.android.com/reference/android/hardware/package-summary.html">android.hardware</a> APIs to interact with the sensors hardware. Internally, this code calls 
  corresponding JNI glue classes to access the native code that interacts with the 
  sensors hardware.</p>
<p>JNI<br/>
  The JNI code associated with <a href="http://developer.android.com/reference/android/hardware/package-summary.html">android.hardware</a> is located in the frameworks/base/core/jni/ directory. This code calls the lower 
  level native code to obtain access to the sensor hardware.</p>
<p>Native framework<br/>
  The native framework is defined in frameworks/native/ and provides a native 
  equivalent to the <a href="http://developer.android.com/reference/android/hardware/package-summary.html">android.hardware</a> package. The native framework calls the Binder IPC proxies to obtain access to 
  sensor-specific services.</p>
<p>Binder IPC<br/>
  The Binder IPC proxies facilitate communication over process boundaries.</p>
<p>HAL<br/>
  The Hardware Abstraction Layer (HAL) defines the standard interface that sensor 
  services call into and that you must implement to have your sensor hardware 
  function correctly. The sensor HAL interfaces are located in 
  hardware/libhardware/include/hardware.</p>
<p>Kernel Driver<br/>
  The sensors driver interacts with the hardware and your implementation of the 
  HAL. The HAL is driver-agnostic.</p>
<h3 id="axis-def">Sensor axis definition</h3>
<p>The sensor event values are expressed in a specific frame that is static 
  relative to the phone. This API is relative only to the NATURAL orientation of 
  the screen. In other words:</p>
<ul>
  <li>the axes are not swapped when the device's screen orientation changes.</li>
  <li>higher level services <em>may</em> perform this transformation.</li>
</ul>
<div class="figure" style="width:269px"> <img src="http://developer.android.com/images/axis_device.png" alt="Coordinate system relative to device for Sensor
    API" height="225" />
  <p class="img-caption"> <strong>Figure 1.</strong> Coordinate system (relative to a device) that's used by the Sensor
    API. </p>
</div>
<h3 id="accuracy">Accuracy</h3>
<p>The sensors included by the manufacturer must be accurate and precise to meet
the expectations of application developers. The sensors included in Android devices are 
  tested for sensor interaction and accuracy as part of the <a href="{@docRoot}compatibility/index.html">Android Compatibility 
    program</a> starting in the 
  Android 4.4 release. Testing will continue to be improved in future releases. 
  See the <em>Sensors</em> section of the Android Compatibility Definition Document (CDD) 
  for the exact requirements.</p>
<h3 id="power">Power consumption</h3>
<p>Some defined sensor are higher power than others. Others are lower power by 
  design and should be implemented as such with their processing done in the 
  hardware. This means they should not require the application processor to be 
  running. Here are the low-power sensors:</p>
<ul>
  <li><a href="{@docRoot}devices/sensors/composite_sensors.html#Magnetometer">Geomagnetic rotation vector</a></li>
  <li><a href="{@docRoot}devices/sensors/composite_sensors.html#Significant">Significant motion</a></li>
  <li><a href="{@docRoot}devices/sensors/composite_sensors.html#counter">Step counter</a></li>
  <li><a href="{@docRoot}devices/sensors/composite_sensors.html#detector">Step detector</a></li>
</ul>
<p>They are accompanied by a low-power <img src="images/battery_icon.png"
alt="low-power sensors"/>
  icon in the <a href="{@docRoot}devices/sensors/composite_sensors.html#summary">Sensor summary</a> table. </p>
<p>These sensor types cannot be implemented at high power as their primary benefit 
  is low battery use. It is better to not implement a low-power sensor at all 
  rather than implement it as high power.</p>
<p>Composite low-power sensor types, such as the step detector, must have their 
  processing conducted in the hardware; power use is much lower than if done in 
  the software. Power use is low on small microprocessors and even lower still on 
  application-specific integrated circuits (ASICs). A hardware implementation of 
  composite sensor types can also make use of more raw sensor data and a better 
  synchronization between sensors.</p>
<h3 id="release">HAL release cycle</h3>
<p>Functionality is tied to versions of the API. Android maintains two versions of 
  the Sensors HAL API per release. For instance, if version 1 was the latest and 
  version 1.1 is released, the version prior to 1 will no longer be supported upon 
  that release. Only the two latest versions of the Sensors HAL API are supported.</p>
<h2 id="interaction">Interaction</h2>
<h3 id="concurrent">Concurrent running</h3>
<p>Android sensors must work independently of one another. Activating one sensor 
  shall not deactivate another sensor. Activating one shall not reduce the rate of 
  another. This is a key element of compatibility testing.</p>
<h3 id="suspend">Interaction with suspend mode</h3>
<p>Unless otherwise noted, an enabled sensor shall not prevent the system on a chip 
  (SoC) from going into suspend mode. It is the responsibility of applications to keep a 
  partial <a href="http://developer.android.com/reference/android/os/PowerManager.WakeLock.html">wake 
    lock</a> should they wish to receive sensor events while the screen is off. While in 
  suspend mode, and unless otherwise noted (<a
href="{@docRoot}devices/sensors/batching.html">batch</a> mode 
  and sensor particularities), enabled sensors' events are lost.</p>
<p>Note that conceptually, the sensor itself is not deactivated while in suspend 
  mode. Instead, the data it returns is missing. The oldest data is dropped to 
  accommodate the latest data. As soon as the SoC gets out of suspend mode, 
  operations resume as normal.</p>
<p>Most applications should either hold a wake lock to ensure the system doesn't go 
  to suspend, or unregister from the sensors when they do not need them, unless 
  batch mode is active. When batching, sensors must continue to fill their 
  internal FIFO. (See the documentation of <a
href="{@docRoot}devices/sensors/batching.html">batch</a> mode 
  to learn how suspend interacts with batch mode.)</p>
<p>Wake-up sensors are a notable exception to the above. Wake-up sensors must
wake up the SoC to deliver events. They must still let the SoC go into suspend
mode, but must also wake it up when an event is triggered.</p>
<h3 id="fusion">Sensor fusion and virtual sensors</h3>
<p>Many composite sensor types are or can be implemented as virtual sensors from 
  underlying base sensors on the device. Examples of composite sensors types 
  include the rotation vector sensor, orientation sensor, step detector and step 
  counter.</p>
<p>From the point of view of this API, these virtual sensors <strong>must</strong> appear as 
  real, individual sensors. It is the responsibility of the driver and HAL to make 
  sure this is the case.</p>
<p>In particular, all sensors must be able to function concurrently. For example, 
  if defining both an accelerometer and a step counter, then both must be able to 
  work concurrently.</p>
<h3 id="hal">HAL interface</h3>
<p>These are the common sensor calls expected at the HAL level:</p>
<ol>
  <li><em>getSensorList()</em> - Gets the list of all sensors.</li>
  <li><em>activate()</em> - Starts or stops the specified sensor.</li>
  <li><em>batch()</em> - Sets parameters to group event data collection and optimize power use.</li>
  <li><em>setDelay()</em> - Sets the event's period in 
    nanoseconds for a given sensor.</li>
  <li><em>flush()</em> - Flush adds an event to the end of the 
    &quot;batch mode&quot; FIFO for the specified sensor and flushes the FIFO.</li>
  <li><em>poll()</em> - Returns an array of sensor data. </li>
</ol>
<p>Please note, the implementation must be thread safe and allow these values to be 
  called from different threads.</p>
<h4 id="getSensorList">getSensorList(sensor_type)</h4>
<p>Provide the list of sensors implemented by the HAL for the given sensor type. </p>
<p>Developers may then make multiple calls to get sensors of different types or use 
  Sensor.TYPE_ALL to get all the sensors. See getSensorList() defined on 
  developer.android.com for more details.</p>
<h4 id="activate">activate(sensor, true/false)</h4>
<pre>
            int (*activate)(struct sensors_poll_device_t *dev,
                    int handle, int enabled);</pre>
<p>Activates or deactivates the sensor with the specified handle. Handles must be 
  higher than SENSORS_HANDLE_BASE and must be unique. A handle identifies a given 
  sensor. The handle is used to activate and/or deactivate sensors. In this 
  version of the API, there can only be 256 handles.</p>
<p>The handle is the handle of the sensor to change. The enabled argument is set to 
  1 to enable or 0 to disable the sensor.</p>
<p>Unless otherwise noted in the individual sensor type descriptions, an activated 
  sensor never prevents the SoC from going into suspend mode; that is, the HAL 
  shall not hold a partial wake lock on behalf of applications.<br/>
  <br/>
  One-shot sensors deactivate themselves automatically upon receiving an event, 
  and they must still accept to be deactivated through a call to activate(..., 
  ..., 0).<br/>
  <br/>
  If &quot;enabled&quot; is 1 and the sensor is already activated, this function is a no-op 
  and succeeds. If &quot;enabled&quot; is 0 and the sensor is already deactivated, this 
  function is a no-op and succeeds. This returns 0 on success and a negative errno 
  code otherwise.</p>
<h4 id="batch">batch(sensor, batching parameters)</h4>
<pre>
            int (*batch)(struct sensors_poll_device_1* dev,
                   int handle, int flags, int64_t period_ns, int64_t timeout);
</pre>
<p>Sets parameters to group event data collection and reduce power use. Batching 
  can enable significant power savings by allowing the application processor to 
  sleep rather than awake for each notification. Instead, these notifications can 
  be grouped and processed together. See the <a
href="{@docRoot}devices/sensors/batching.html">Batching</a> section for details.</p>
<h4 id="setDelay">setDelay(sensor, delay)</h4>
<pre>
            int (*setDelay)(struct sensors_poll_device_t *dev,
                    int handle, int64_t period_ns);
</pre>
<p>Sets the event's period in nanoseconds for a given sensor. What the period_ns 
  parameter means depends on the specified sensor's trigger mode:</p>
<ul>
  <li>Continuous: setDelay() sets the sampling rate.</li>
  <li>On-change: setDelay() limits the delivery rate of events.</li>
  <li>One-shot: setDelay() is ignored. It has no effect.</li>
  <li>Special: See specific sensor type descriptions.</li>
</ul>
<p>For continuous and on-change sensors, if the requested value is less than sensor_t::minDelay, then it's silently clamped to sensor_t::minDelay unless 
  sensor_t::minDelay is 0, in which case it is clamped to &gt;= 1ms:<br/>
  @return 0 if successful, &lt; 0 on error</p>
<h4 id="flush">flush()</h4>
<pre>
            int (*flush)(struct sensors_poll_device_1* dev, int handle);
</pre>
<p>Flush adds a META_DATA_FLUSH_COMPLETE event (sensors_event_meta_data_t) to the 
  end of the &quot;batch mode&quot; FIFO for the specified sensor and flushes the FIFO; 
  those events are delivered as usual (i.e.: as if the batch timeout had expired) 
  and removed from the FIFO.<br/>
  <br/>
  The flush happens asynchronously (i.e.: this function must return immediately). 
  If the implementation uses a single FIFO for several sensors, that FIFO is 
  flushed and the META_DATA_FLUSH_COMPLETE event is added only for the specified 
  sensor.<br/>
  <br/>
  If the specified sensor wasn't in batch mode, flush succeeds and promptly sends 
  a META_DATA_FLUSH_COMPLETE event for that sensor.</p>
<p>If the FIFO was empty at the time of the call, flush returns 0 (success) and 
  promptly sends a META_DATA_FLUSH_COMPLETE event for that sensor.<br/>
  <br/>
  If the specified sensor wasn't enabled, flush returns -EINVAL. return 0 on 
  success, negative errno code otherwise.</p>
<h4 id="poll">poll()</h4>
<pre>            int (*poll)(struct sensors_poll_device_t *dev,
                    sensors_event_t* data, int count);</pre>
<p>Returns an array of sensor data. This function must block until events are 
  available. It will return the number of events read on success, or -errno in 
  case of an error.</p>
<p>The number of events returned in data must be less or equal to the &quot;count&quot; 
  argument. This function shall never return 0 (no event).</p>