1page.title=Sensor stack 2@jd:body 3 4<!-- 5 Copyright 2015 The Android Open Source Project 6 7 Licensed under the Apache License, Version 2.0 (the "License"); 8 you may not use this file except in compliance with the License. 9 You may obtain a copy of the License at 10 11 http://www.apache.org/licenses/LICENSE-2.0 12 13 Unless required by applicable law or agreed to in writing, software 14 distributed under the License is distributed on an "AS IS" BASIS, 15 WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. 16 See the License for the specific language governing permissions and 17 limitations under the License. 18--> 19<div id="qv-wrapper"> 20 <div id="qv"> 21 <h2>In this document</h2> 22 <ol id="auto-toc"> 23 </ol> 24 </div> 25</div> 26 27<p>The figure below represents the Android sensor stack. Each component 28 communicates only with the components directly above and below it, though some 29 sensors can bypass the sensor hub when it is present. Control flows from the 30 applications down to the sensors, and data flows from the sensors up to the 31 applications.</p> 32<img src="images/ape_fwk_sensors.png" alt="Layers and owners of the Android sensor stack" /> 33<p class="img-caption"><strong>Figure 1.</strong> Layers of the Android sensor stack and their respective owners</p> 34 35<h2 id="sdk">SDK</h2> 36<p>Applications access sensors through the <a href="http://developer.android.com/reference/android/hardware/SensorManager.html">Sensors SDK (Software Development Kit) API</a>. The SDK contains functions to list available sensors and to register to a 37 sensor.</p> 38<p>When registering to a sensor, the application specifies its preferred sampling 39 frequency and its latency requirements.</p> 40<ul> 41 <li> For example, an application might register to the default accelerometer, 42 requesting events at 100Hz, and allowing events to be reported with a 1-second 43 latency. </li> 44 <li> The application will receive events from the accelerometer at a rate of at 45 least 100Hz, and possibly delayed up to 1 second. </li> 46</ul> 47<p>See the <a href="index.html#targeted_at_developers">developer documentation</a> for more information on the SDK.</p> 48<h2 id="framework">Framework</h2> 49<p>The framework is in charge of linking the several applications to the <a href="hal-interface.html">HAL</a>. The HAL itself is single-client. Without this multiplexing happening at the 50 framework level, only a single application could access each sensor at any 51 given time.</p> 52<ul> 53 <li> When a first application registers to a sensor, the framework sends a request 54 to the HAL to activate the sensor. </li> 55 <li> When additional applications register to the same sensor, the framework takes 56 into account requirements from each application and sends the updated requested 57 parameters to the HAL. 58 <ul> 59 <li> The <a href="hal-interface.html#sampling_period_ns">sampling frequency</a> will be the maximum of the requested sampling frequencies, meaning some 60 applications will receive events at a frequency higher than the one they 61 requested. </li> 62 <li> The <a href="hal-interface.html#max_report_latency_ns">maximum reporting latency</a> will be the minimum of the requested ones. If one application requests one 63 sensor with a maximum reporting latency of 0, all applications will receive the 64 events from this sensor in continuous mode even if some requested the sensor 65 with a non-zero maximum reporting latency. See <a href="batching.html">Batching</a> for more details. </li> 66 </ul> 67 </li> 68 <li> When the last application registered to one sensor unregisters from it, the 69 frameworks sends a request to the HAL to deactivate the sensor so power is not 70 consumed unnecessarily. </li> 71</ul> 72<h3 id="impact_of_multiplexing">Impact of multiplexing</h3> 73<p>This need for a multiplexing layer in the framework explains some design 74 decisions.</p> 75<ul> 76 <li> When an application requests a specific sampling frequency, there is no 77 guarantee that events won’t arrive at a faster rate. If another application 78 requested the same sensor at a faster rate, the first application will also 79 receive them at the fast rate. </li> 80 <li> The same lack of guarantee applies to the requested maximum reporting latency: 81 applications might receive events with much less latency than they requested. </li> 82 <li> Besides sampling frequency and maximum reporting latency, applications cannot 83 configure sensor parameters. 84 <ul> 85 <li> For example, imagine a physical sensor that can function both in “high 86 accuracy” and “low power” modes. </li> 87 <li> Only one of those two modes can be used on an Android device, because 88 otherwise, an application could request the high accuracy mode, and another one 89 a low power mode; there would be no way for the framework to satisfy both 90 applications. The framework must always be able to satisfy all its clients, so 91 this is not an option. </li> 92 </ul> 93 </li> 94 <li> There is no mechanism to send data down from the applications to the sensors or 95 their drivers. This ensures one application cannot modify the behavior of the 96 sensors, breaking other applications. </li> 97</ul> 98<h3 id="sensor_fusion">Sensor fusion</h3> 99<p>The Android framework provides a default implementation for some composite 100 sensors. When a <a href="sensor-types.html#gyroscope">gyroscope</a>, an <a href="sensor-types.html#accelerometer">accelerometer</a> and a <a href="sensor-types.html#magnetic_field_sensor">magnetometer</a> are present on a device, but no <a href="sensor-types.html#rotation_vector">rotation vector</a>, <a href="sensor-types.html#gravity">gravity</a> and <a href="sensor-types.html#linear_acceleration">linear acceleration</a> sensors are present, the framework implements those sensors so applications 101 can still use them.</p> 102<p>The default implementation does not have access to all the data that other 103 implementations have access to, and it must run on the SoC, so it is not as 104 accurate nor as power efficient as other implementations can be. As much as 105 possible, device manufacturers should define their own fused sensors (rotation 106 vector, gravity and linear acceleration, as well as newer composite sensors like 107 the <a href="sensor-types.html#game_rotation_vector">game rotation vector</a>) rather than rely on this default implementation. Device manufacturers can 108 also request sensor chip vendors to provide them with an implementation.</p> 109<p>The default sensor fusion implementation is not being maintained and 110 might cause devices relying on it to fail CTS.</p> 111<h3 id="under_the_hood">Under the Hood</h3> 112<p>This section is provided as background information for those maintaining the 113 Android Open Source Project (AOSP) framework code. It is not relevant for 114 hardware manufacturers.</p> 115<h4 id="jni">JNI</h4> 116<p>The framework uses a Java Native Interface (JNI) associated with <a href="http://developer.android.com/reference/android/hardware/package-summary.html">android.hardware</a> and located in the <code>frameworks/base/core/jni/</code> directory. This code calls the 117 lower level native code to obtain access to the sensor hardware.</p> 118<h4 id="native_framework">Native framework</h4> 119<p>The native framework is defined in <code>frameworks/native/</code> and provides a native 120 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 121 sensor-specific services.</p> 122<h4 id="binder_ipc">Binder IPC</h4> 123<p>The Binder IPC proxies facilitate communication over process boundaries.</p> 124<h2 id="hal">HAL</h2> 125<p>The Sensors Hardware Abstraction Layer (HAL) API is the interface between the 126 hardware drivers and the Android framework. It consists of one HAL interface 127 sensors.h and one HAL implementation we refer to as sensors.cpp.</p> 128<p>The interface is defined by Android and AOSP contributors, and the 129 implementation is provided by the manufacturer of the device.</p> 130<p>The sensor HAL interface is located in <code>hardware/libhardware/include/hardware</code>. 131 See <a href="{@docRoot}devices/halref/sensors_8h.html">sensors.h</a> for additional details.</p> 132<h3 id="release_cycle">Release cycle</h3> 133<p>The HAL implementation specifies what version of the HAL interface it 134 implements by setting <code>your_poll_device.common.version</code>. The existing HAL 135 interface versions are defined in sensors.h, and functionality is tied to those 136 versions.</p> 137<p>The Android framework currently supports versions 1.0 and 1.3, but 1.0 will 138 soon not be supported anymore. This documentation describes the behavior of version 139 1.3, to which all devices should upgrade. For details on how to upgrade to 140 1.3, see <a href="versioning.html">HAL version deprecation</a>.</p> 141<h2 id="kernel_driver">Kernel driver</h2> 142<p>The sensor drivers interact with the physical devices. In some cases, the HAL 143 implementation and the drivers are the same software entity. In other cases, 144 the hardware integrator requests sensor chip manufacturers to provide the 145 drivers, but they are the ones writing the HAL implementation.</p> 146<p>In all cases, HAL implementation and kernel drivers are the responsibility of 147 the hardware manufacturers, and Android does not provide preferred approaches to 148 write them.</p> 149<h2 id="sensor_hub">Sensor hub</h2> 150<p>The sensor stack of a device can optionally include a sensor hub, useful to 151 perform some low-level computation at low power while the SoC can be in a 152 suspend mode. For example, step counting or sensor fusion can be performed on 153 those chips. It is also a good place to implement sensor batching, adding 154 hardware FIFOs for the sensor events. See <a 155href="batching.html">Batching</a> for more information.</p> 156<p>How the sensor hub is materialized depends on the architecture. It is sometimes 157 a separate chip, and sometimes included on the same chip as the SoC. Important 158 characteristics of the sensor hub is that it should contain sufficient memory 159 for batching and consume very little power to enable implementation of the low 160 power Android sensors. Some sensor hubs contain a microcontroller for generic 161 computation, and hardware accelerators to enable very low power computation for 162 low power sensors.</p> 163<p>How the sensor hub is architectured and how it communicates with the sensors 164 and the SoC (I2C bus, SPI bus, …) is not specified by Android, but it should aim 165 at minimizing overall power use.</p> 166<p>One option that appears to have a significant impact on implementation 167 simplicity is having two interrupt lines going from the sensor hub to the SoC: 168 one for wake-up interrupts (for wake-up sensors), and the other for non-wake-up 169 interrupts (for non-wake-up sensors).</p> 170<h2 id="sensors">Sensors</h2> 171<p>Those are the physical MEMs chips making the measurements. In many cases, 172 several physical sensors are present on the same chip. For example, some chips 173 include an accelerometer, a gyroscope and a magnetometer. (Such chips are often 174 called 9-axis chips, as each sensor provides data over 3 axes.)</p> 175<p>Some of those chips also contain some logic to perform usual computations such 176 as motion detection, step detection and 9-axis sensor fusion.</p> 177<p>Although the CDD power and accuracy requirements and recommendations target the 178 Android sensor and not the physical sensors, those requirements impact the 179 choice of physical sensors. For example, the accuracy requirement on the game 180 rotation vector has implications on the required accuracy for the physical 181 gyroscope. It is up to the device manufacturer to derive the requirements for 182 physical sensors.</p> 183
