src/devices/sensors/report-modes.jd

page.title=Reporting modes
@jd:body

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    <h2>In this document</h2>
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<p>Sensors can generate events in different ways called reporting modes; each
  sensor type has one and only one reporting mode associated with it. Four
  reporting modes exist.</p>
<h2 id="continuous">Continuous</h2>
<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
  reporting mode are <a href="sensor-types.html#accelerometer">accelerometers</a> and <a href="sensor-types.html#gyroscope">gyroscopes</a>.</p>
<h2 id="on-change">On-change</h2>
<p>Events are generated only if the measured values have changed. Activating the
  sensor at the HAL level (calling <code>activate(..., enable=1)</code> on it) also triggers
  an event, meaning the HAL must return an event immediately when an on-change
  sensor is activated. Example sensors using the on-change reporting mode are the
  step counter, proximity, and heart rate sensor types.</p>
<p>The <a href="hal-interface.html#sampling_period_ns">sampling_period_ns</a>
  parameter passed to the <code>batch</code> function is used to set the minimum
  time between consecutive events, meaning an event should not be generated until
  sampling_period_ns nanoseconds elapsed since the last event, even if the value
  changed since then. If the value changed, an event must be generated as soon as
  <code>sampling_period_ns</code> has elapsed since the last event.</p>
<p>For example, suppose:</p>
<ul>
  <li> We activate the step counter with <code>sampling_period_ns = 10 * 10^9</code> (10 seconds). </li>
  <li> And walk for 55 seconds, then stand still for one minute. </li>
  <li> Then the events will be generated about every 10 seconds during the first
    minute (including at time t=0 because of the activation of the sensor, and t=60
    seconds), for a total of seven events, and no event will be generated in the second
    minute because the value of the step count didn’t change after t=60 seconds. </li>
</ul>
<h2 id="one-shot">One-shot</h2>
<p>Upon detection of an event, the sensor deactivates itself and then sends a
  single event through the HAL. Order matters to avoid race conditions. (The
  sensor must be deactivated before the event is reported through the HAL). No
  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>
<p>One-shot sensors are sometimes referred to as trigger sensors.</p>
<p>The <code>sampling_period_ns</code> and <code>max_report_latency_ns</code>
  parameters passed to the <code>batch</code> function are ignored. Events from
  one-shot events cannot be stored in hardware FIFOs; the events must be
  reported as soon as they are generated.</p>
<h2 id="special">Special</h2>
<p>See the individual <a href="sensor-types.html">sensor type descriptions</a>
for details on when the events are generated.</p>