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