1 page.title=Reporting modes 2 @jd:body 3 4 <!-- 5 Copyright 2014 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>Sensors can generate events in different ways called reporting modes; each 28 sensor type has one and only one reporting mode associated with it. Four 29 reporting modes exist.</p> 30 <h2 id="continuous">Continuous</h2> 31 <p>Events are generated at a constant rate defined by the <a href="hal-interface.html#sampling_period_ns">sampling_period_ns</a> parameter passed to the <code>batch</code> function. Example sensors using the continuous 32 reporting mode are <a href="sensor-types.html#accelerometer">accelerometers</a> and <a href="sensor-types.html#gyroscope">gyroscopes</a>.</p> 33 <h2 id="on-change">On-change</h2> 34 <p>Events are generated only if the measured values have changed. Activating the 35 sensor at the HAL level (calling <code>activate(..., enable=1)</code> on it) also triggers 36 an event, meaning the HAL must return an event immediately when an on-change 37 sensor is activated. Example sensors using the on-change reporting mode are the 38 step counter, proximity, and heart rate sensor types.</p> 39 <p>The <a href="hal-interface.html#sampling_period_ns">sampling_period_ns</a> 40 parameter passed to the <code>batch</code> function is used to set the minimum 41 time between consecutive events, meaning an event should not be generated until 42 sampling_period_ns nanoseconds elapsed since the last event, even if the value 43 changed since then. If the value changed, an event must be generated as soon as 44 <code>sampling_period_ns</code> has elapsed since the last event.</p> 45 <p>For example, suppose:</p> 46 <ul> 47 <li> We activate the step counter with <code>sampling_period_ns = 10 * 10^9</code> (10 seconds). </li> 48 <li> And walk for 55 seconds, then stand still for one minute. </li> 49 <li> Then the events will be generated about every 10 seconds during the first 50 minute (including at time t=0 because of the activation of the sensor, and t=60 51 seconds), for a total of seven events, and no event will be generated in the second 52 minute because the value of the step count didnt change after t=60 seconds. </li> 53 </ul> 54 <h2 id="one-shot">One-shot</h2> 55 <p>Upon detection of an event, the sensor deactivates itself and then sends a 56 single event through the HAL. Order matters to avoid race conditions. (The 57 sensor must be deactivated before the event is reported through the HAL). No 58 other event is sent until the sensor is reactivated. <a href="sensor-types.html#significant_motion">Significant motion</a> is an example of this kind of sensor.</p> 59 <p>One-shot sensors are sometimes referred to as trigger sensors.</p> 60 <p>The <code>sampling_period_ns</code> and <code>max_report_latency_ns</code> 61 parameters passed to the <code>batch</code> function are ignored. Events from 62 one-shot events cannot be stored in hardware FIFOs; the events must be 63 reported as soon as they are generated.</p> 64 <h2 id="special">Special</h2> 65 <p>See the individual <a href="sensor-types.html">sensor type descriptions</a> 66 for details on when the events are generated.</p> 67
