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.
Continuous
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.
On-change
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.
The
sampling_period_ns
parameter passed to the batch
function is used to set the
minimum time between consecutive events, meaning an event shouldn't 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). - We walk for 55 seconds, then stand still for one minute.
- The events are generated about every 10 seconds during the first
minute (including at time
t=0
because of the activation of the sensor, andt=60
seconds), for a total of seven events. No event is generated in the second minute because the value of the step count didn’t change aftert=60
seconds.
One-shot
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.
The sampling_period_ns
and max_report_latency_ns
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.
Special
See the individual sensor type descriptions for details on when the events are generated.