1 <html devsite> 2 <head> 3 <title>Power consumption</title> 4 <meta name="project_path" value="/_project.yaml" /> 5 <meta name="book_path" value="/_book.yaml" /> 6 </head> 7 <body> 8 <!-- 9 Copyright 2017 The Android Open Source Project 10 11 Licensed under the Apache License, Version 2.0 (the "License"); 12 you may not use this file except in compliance with the License. 13 You may obtain a copy of the License at 14 15 http://www.apache.org/licenses/LICENSE-2.0 16 17 Unless required by applicable law or agreed to in writing, software 18 distributed under the License is distributed on an "AS IS" BASIS, 19 WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. 20 See the License for the specific language governing permissions and 21 limitations under the License. 22 --> 23 24 25 26 <h2 id="low_power_sensors">Low-power sensors</h2> 27 <p>Some sensor types are defined as being low power. Low-power sensors must 28 function at low power, with their processing done in the hardware. This means 29 they should not require the SoC to be running. Here are some low-power sensor 30 types:</p> 31 <ul> 32 <li> Geomagnetic rotation vector </li> 33 <li> Significant motion </li> 34 <li> Step counter </li> 35 <li> Step detector </li> 36 <li> Tilt detector </li> 37 </ul> 38 <p>They are accompanied by a low-power (<img src="images/battery_icon.png" 39 width="20" height="20" alt="Low power sensor" />) icon in the <a 40 href="sensor-types.html#composite_sensor_type_summary">Composite sensor type 41 summary</a> table.</p> 42 <p>These sensor types cannot be implemented at high power as their primary benefit 43 is low battery use. These sensors are expected to be activated for very long 44 periods, possibly 24/7. It is better to not implement a low-power sensor at all 45 rather than implement it as high power, as it would cause dramatic battery 46 drain.</p> 47 <p>Composite low-power sensor types, such as the step detector, must have their 48 processing conducted in the hardware.</p> 49 <p>See the CDD for specific power requirements, and expect tests in CTS to 50 verify those power requirements.</p> 51 <h2 id="power_measurement_process">Power measurement process</h2> 52 <p>The power is measured at the battery. For values in milliWatts, we use the 53 nominal voltage of the battery, meaning a 1mA current at 4V must be counted as 54 4mW.</p> 55 <p>The power is measured when the SoC is asleep, and averaged over a few seconds 56 of the SoC being asleep, so that periodic spikes in power from the sensor chips 57 are taken into account.</p> 58 <p>For one-shot wake-up sensors, the power is measured while the sensor doesnt 59 trigger (so it doesnt wake the SoC up). Similarly, for other sensors, the 60 power is measured while the sensor data is stored in the hardware FIFO, so the 61 SoC is not woken up.</p> 62 <p>The power normally is measured as a delta with when no sensor is activated. 63 When several sensors are activated, the delta in power must be no greater than 64 the sum of the power of each activated sensor. If an accelerometer consumes 65 0.5mA and a step detector consumes 0.5mA, then activating both at the same time 66 must consume less than 0.5+0.5=1mA.</p> 67 68 </body> 69 </html> 70
