The sensor subsystem exposes an API to uniformly access sensor devices. Common operations are: reading data and executing code when specific conditions are met.
Fundamentally, a channel is a quantity that a sensor device can measure.
Sensors can have multiple channels, either to represent different axes of the same physical property (e.g. acceleration); or because they can measure different properties altogether (ambient temperature, pressure and humidity). Complex sensors cover both cases, so a single device can expose three acceleration channels and a temperature one.
It is imperative that all sensors that support a given channel express results in the same unit of measurement. The following is a list of all supported channels, along with their description and units of measurement:
.. doxygenenum:: sensor_channel
Sensor devices return results as :c:type:`struct sensor_value`. This representation avoids use of floating point values as they may not be supported on certain setups.
.. doxygenstruct:: sensor_value
:members:
Getting a reading from a sensor requires two operations. First, an
application instructs the driver to fetch a sample of all its channels.
Then, individual channels may be read. In the case of channels with
multiple axes, they can be read in a single operation by supplying
the corresponding _XYZ channel type and a buffer of 3
:c:type:`struct sensor_value` objects. This approach ensures consistency
of channels between reads and efficiency of communication by issuing a
single transaction on the underlying bus.
Below is an example illustrating the usage of the BME280 sensor, which measures ambient temperature and atmospheric pressure. Note that :c:func:`sensor_sample_fetch` is only called once, as it reads and compensates data for both channels.
.. literalinclude:: ../../samples/sensor/bme280/src/main.c :language: c :lines: 12- :linenos:
The example assumes that the returned values have type :c:type:`struct sensor_value`, which is the case for BME280. A real application supporting multiple sensors should inspect the :c:data:`type` field of the :c:data:`temp` and :c:data:`press` values and use the other fields of the structure accordingly.
Setting the communication bus and address is considered the most basic configuration for sensor devices. This setting is done at compile time, via the configuration menu. If the sensor supports interrupts, the interrupt lines and triggering parameters described below are also configured at compile time.
Alongside these communication parameters, sensor chips typically expose multiple parameters that control the accuracy and frequency of measurement. In compliance with Zephyr's design goals, most of these values are statically configured at compile time.
However, certain parameters could require runtime configuration, for example, threshold values for interrupts. These values are configured via attributes. The example in the following section showcases a sensor with an interrupt line that is triggered when the temperature crosses a threshold. The threshold is configured at runtime using an attribute.
:dfn:`Triggers` in Zephyr refer to the interrupt lines of the sensor chips. Many sensor chips support one or more triggers. Some examples of triggers include: new data is ready for reading, a channel value has crossed a threshold, or the device has sensed motion.
To configure a trigger, an application needs to supply a :c:type:`struct sensor_trigger` and a handler function. The structure contains the trigger type and the channel on which the trigger must be configured.
Because most sensors are connected via SPI or I2C busses, it is not possible to communicate with them from the interrupt execution context. The execution of the trigger handler is deferred to a fiber, so that data fetching operations are possible. A driver can spawn its own fiber to fetch data, thus ensuring minimum latency. Alternatively, multiple sensor drivers can share a system-wide fiber. The shared fiber approach increases the latency of handling interrupts but uses less memory. You can configure which approach to follow for each driver. Most drivers can entirely disable triggers resulting in a smaller footprint.
The following example contains a trigger fired whenever temperature crosses the 26 degree Celsius threshold. It also samples the temperature every second. A real application would ideally disable periodic sampling in the interest of saving power. Since the application has direct access to the kernel config symbols, no trigger is registered when triggering was disabled by the driver's configuration.
.. literalinclude:: ../../samples/sensor/mcp9808/src/main.c :language: c :lines: 12- :linenos: