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