1page.title=Power consumption
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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<h2 id="low_power_sensors">Low-power sensors</h2>
28<p>Some sensor types are defined as being low power. Low-power sensors must
29  function at low power, with their processing done in the hardware. This means
30  they should not require the SoC to be running. Here are some low-power sensor
31  types:</p>
32<ul>
33  <li> Geomagnetic rotation vector </li>
34  <li> Significant motion </li>
35  <li> Step counter </li>
36  <li> Step detector </li>
37  <li> Tilt detector </li>
38</ul>
39<p>They are accompanied by a low-power (<img src="images/battery_icon.png"
40width="20" height="20" alt="Low power sensor" />) icon in the <a
41href="sensor-types.html#composite_sensor_type_summary">Composite sensor type
42summary</a> table.</p>
43<p>These sensor types cannot be implemented at high power as their primary benefit
44  is low battery use. These sensors are expected to be activated for very long
45  periods, possibly 24/7. It is better to not implement a low-power sensor at all
46  rather than implement it as high power, as it would cause dramatic battery
47  drain.</p>
48<p>Composite low-power sensor types, such as the step detector, must have their
49  processing conducted in the hardware.</p>
50<p>See the CDD for specific power requirements, and expect tests in CTS to
51  verify those power requirements.</p>
52<h2 id="power_measurement_process">Power measurement process</h2>
53<p>The power is measured at the battery. For values in milliWatts, we use the
54  nominal voltage of the battery, meaning a 1mA current at 4V must be counted as
55  4mW.</p>
56<p>The power is measured when the SoC is asleep, and averaged over a few seconds
57  of the SoC being asleep, so that periodic spikes in power from the sensor chips
58  are taken into account.</p>
59<p>For one-shot wake-up sensors, the power is measured while the sensor doesn’t
60  trigger (so it doesn’t wake the SoC up). Similarly, for other sensors, the
61  power is measured while the sensor data is stored in the hardware FIFO, so the
62  SoC is not woken up.</p>
63<p>The power normally is measured as a delta with when no sensor is activated.
64  When several sensors are activated, the delta in power must be no greater than
65  the sum of the power of each activated sensor. If an accelerometer consumes
66  0.5mA and a step detector consumes 0.5mA, then activating both at the same time
67  must consume less than 0.5+0.5=1mA.</p>
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