69 lines
3.1 KiB
HTML
69 lines
3.1 KiB
HTML
<html devsite>
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<head>
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<title>Power consumption</title>
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<meta name="project_path" value="/_project.yaml" />
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<meta name="book_path" value="/_book.yaml" />
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</head>
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<body>
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<!--
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Copyright 2017 The Android Open Source Project
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Licensed under the Apache License, Version 2.0 (the "License");
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you may not use this file except in compliance with the License.
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You may obtain a copy of the License at
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http://www.apache.org/licenses/LICENSE-2.0
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Unless required by applicable law or agreed to in writing, software
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distributed under the License is distributed on an "AS IS" BASIS,
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WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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See the License for the specific language governing permissions and
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limitations under the License.
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-->
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<h2 id="low_power_sensors">Low-power sensors</h2>
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<p>Some sensor types are defined as being low power. Low-power sensors must
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function at low power, with their processing done in the hardware. This means
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they should not require the SoC to be running. Here are some low-power sensor
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types:</p>
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<ul>
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<li> Geomagnetic rotation vector </li>
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<li> Significant motion </li>
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<li> Step counter </li>
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<li> Step detector </li>
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<li> Tilt detector </li>
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</ul>
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<p>They are accompanied by a low-power (<img src="images/battery_icon.png"
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width="20" height="20" alt="Low power sensor" />) icon in the <a
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href="sensor-types.html#composite_sensor_type_summary">Composite sensor type
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summary</a> table.</p>
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<p>These sensor types cannot be implemented at high power as their primary benefit
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is low battery use. These sensors are expected to be activated for very long
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periods, possibly 24/7. It is better to not implement a low-power sensor at all
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rather than implement it as high power, as it would cause dramatic battery
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drain.</p>
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<p>Composite low-power sensor types, such as the step detector, must have their
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processing conducted in the hardware.</p>
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<p>See the CDD for specific power requirements, and expect tests in CTS to
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verify those power requirements.</p>
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<h2 id="power_measurement_process">Power measurement process</h2>
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<p>The power is measured at the battery. For values in milliWatts, we use the
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nominal voltage of the battery, meaning a 1mA current at 4V must be counted as
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4mW.</p>
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<p>The power is measured when the SoC is asleep, and averaged over a few seconds
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of the SoC being asleep, so that periodic spikes in power from the sensor chips
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are taken into account.</p>
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<p>For one-shot wake-up sensors, the power is measured while the sensor doesn’t
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trigger (so it doesn’t wake the SoC up). Similarly, for other sensors, the
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power is measured while the sensor data is stored in the hardware FIFO, so the
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SoC is not woken up.</p>
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<p>The power normally is measured as a delta with when no sensor is activated.
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When several sensors are activated, the delta in power must be no greater than
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the sum of the power of each activated sensor. If an accelerometer consumes
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0.5mA and a step detector consumes 0.5mA, then activating both at the same time
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must consume less than 0.5+0.5=1mA.</p>
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</body>
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</html>
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