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.
Events are generated at a constant rate defined by the sampling_period_ns parameter passed to the
batch function. Example sensors using the continuous
reporting mode are accelerometers and gyroscopes.
Events are generated only if the measured values have changed. Activating the
sensor at the HAL level (calling
activate(..., enable=1) 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.
parameter passed to the
batch 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
sampling_period_ns has elapsed since the last event.
For example, suppose:
- We activate the step counter with
sampling_period_ns = 10 * 10^9(10 seconds).
- And walk for 55 seconds, then stand still for one minute.
- 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.
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. Significant motion is an example of this kind of sensor.
One-shot sensors are sometimes referred to as trigger sensors.
parameters passed to the
batch 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.
See the individual sensor type descriptions for details on when the events are generated.