Sensors¶

The sensor subsystem exposes an API to uniformly access sensor devices. Common operations are: reading data and executing code when specific conditions are met.

Basic Operation¶

Channels¶

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. Consult the API Reference for all supported channels, along with their description and units of measurement:

Values¶

Sensor devices return results as sensor_value. This representation avoids use of floating point values as they may not be supported on certain setups.

Fetching Values¶

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 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 sensor_sample_fetch() is only called once, as it reads and compensates data for both channels.

#define BME280 DT_INST(0, bosch_bme280)

#if DT_NODE_HAS_STATUS(BME280, okay)
#define BME280_LABEL DT_LABEL(BME280)
#else
#error Your devicetree has no enabled nodes with compatible "bosch,bme280"
#define BME280_LABEL "<none>"
#endif

void main(void)
{
	const struct device *dev = device_get_binding(BME280_LABEL);

	if (dev == NULL) {
		printk("No device \"%s\" found; did initialization fail?\n",
		       BME280_LABEL);
		return;
	} else {
		printk("Found device \"%s\"\n", BME280_LABEL);
	}

	while (1) {
		struct sensor_value temp, press, humidity;

		sensor_sample_fetch(dev);
		sensor_channel_get(dev, SENSOR_CHAN_AMBIENT_TEMP, &temp);
		sensor_channel_get(dev, SENSOR_CHAN_PRESS, &press);
		sensor_channel_get(dev, SENSOR_CHAN_HUMIDITY, &humidity);

		printk("temp: %d.%06d; press: %d.%06d; humidity: %d.%06d\n",
		      temp.val1, temp.val2, press.val1, press.val2,
		      humidity.val1, humidity.val2);

		k_sleep(K_MSEC(1000));
	}
}

The example assumes that the returned values have type sensor_value, which is the case for BME280. A real application supporting multiple sensors should inspect the type field of the temp and press values and use the other fields of the structure accordingly.

Configuration and Attributes¶

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.

Triggers¶

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 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 thread, so that data fetching operations are possible. A driver can spawn its own thread to fetch data, thus ensuring minimum latency. Alternatively, multiple sensor drivers can share a system-wide thread. The shared thread 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.

#define UCEL_PER_CEL 1000000
#define UCEL_PER_MCEL 1000
#define TEMP_INITIAL_CEL 25
#define TEMP_WINDOW_HALF_UCEL 500000

static const char *now_str(void)
{
	static char buf[16]; /* ...HH:MM:SS.MMM */
	uint32_t now = k_uptime_get_32();
	unsigned int ms = now % MSEC_PER_SEC;
	unsigned int s;
	unsigned int min;
	unsigned int h;

	now /= MSEC_PER_SEC;
	s = now % 60U;
	now /= 60U;
	min = now % 60U;
	now /= 60U;
	h = now;

	snprintf(buf, sizeof(buf), "%u:%02u:%02u.%03u",
		 h, min, s, ms);
	return buf;
}

#ifdef CONFIG_MCP9808_TRIGGER

static struct sensor_trigger trig;

static int set_window(const struct device *dev,
		      const struct sensor_value *temp)
{
	const int temp_ucel = temp->val1 * UCEL_PER_CEL + temp->val2;
	const int low_ucel = temp_ucel - TEMP_WINDOW_HALF_UCEL;
	const int high_ucel = temp_ucel + TEMP_WINDOW_HALF_UCEL;
	struct sensor_value val = {
		.val1 = low_ucel / UCEL_PER_CEL,
		.val2 = low_ucel % UCEL_PER_CEL,
	};
	int rc = sensor_attr_set(dev, SENSOR_CHAN_AMBIENT_TEMP,
				 SENSOR_ATTR_LOWER_THRESH, &val);
	if (rc == 0) {
		val.val1 = high_ucel / UCEL_PER_CEL,
		val.val2 = high_ucel % UCEL_PER_CEL,
		rc = sensor_attr_set(dev, SENSOR_CHAN_AMBIENT_TEMP,
				     SENSOR_ATTR_UPPER_THRESH, &val);
	}

	if (rc == 0) {
		printf("Alert on temp outside [%d, %d] milli-Celsius\n",
		       low_ucel / UCEL_PER_MCEL,
		       high_ucel / UCEL_PER_MCEL);
	}

	return rc;
}

static inline int set_window_ucel(const struct device *dev,
				  int temp_ucel)
{
	struct sensor_value val = {
		.val1 = temp_ucel / UCEL_PER_CEL,
		.val2 = temp_ucel % UCEL_PER_CEL,
	};

	return set_window(dev, &val);
}

static void trigger_handler(const struct device *dev,
			    struct sensor_trigger *trig)
{
	struct sensor_value temp;
	static size_t cnt;
	int rc;

	++cnt;
	rc = sensor_sample_fetch(dev);
	if (rc != 0) {
		printf("sensor_sample_fetch error: %d\n", rc);
		return;
	}
	rc = sensor_channel_get(dev, SENSOR_CHAN_AMBIENT_TEMP, &temp);
	if (rc != 0) {
		printf("sensor_channel_get error: %d\n", rc);
		return;
	}

	printf("trigger fired %u, temp %g deg C\n", cnt,
	       sensor_value_to_double(&temp));
	set_window(dev, &temp);
}
#endif

void main(void)
{
	const char *const devname = DT_LABEL(DT_INST(0, microchip_mcp9808));
	const struct device *dev = device_get_binding(devname);
	int rc;

	if (dev == NULL) {
		printf("Device %s not found.\n", devname);
		return;
	}

#ifdef CONFIG_MCP9808_TRIGGER
	rc = set_window_ucel(dev, TEMP_INITIAL_CEL * UCEL_PER_CEL);
	if (rc == 0) {
		trig.type = SENSOR_TRIG_THRESHOLD;
		trig.chan = SENSOR_CHAN_AMBIENT_TEMP;
		rc = sensor_trigger_set(dev, &trig, trigger_handler);
	}

	if (rc != 0) {
		printf("Trigger set failed: %d\n", rc);
		return;
	}
	printk("Trigger set got %d\n", rc);
#endif

	while (1) {
		struct sensor_value temp;

		rc = sensor_sample_fetch(dev);
		if (rc != 0) {
			printf("sensor_sample_fetch error: %d\n", rc);
			break;
		}

		rc = sensor_channel_get(dev, SENSOR_CHAN_AMBIENT_TEMP, &temp);
		if (rc != 0) {
			printf("sensor_channel_get error: %d\n", rc);
			break;
		}

		printf("%s: %g C\n", now_str(),
		       sensor_value_to_double(&temp));

		k_sleep(K_SECONDS(2));
	}
}

API Reference¶

group sensor_interface

Sensor Interface.

Defines

SENSOR_G¶: The value of gravitational constant in micro m/s^2.

SENSOR_PI¶: The value of constant PI in micros.

Typedefs

typedef void (*sensor_trigger_handler_t)(const struct device *dev, struct sensor_trigger *trigger)¶

Callback API upon firing of a trigger.

Parameters

dev: Pointer to the sensor device
trigger: The trigger

typedef int (*sensor_attr_set_t)(const struct device *dev, enum sensor_channel chan, enum sensor_attribute attr, const struct sensor_value *val)¶

Callback API upon setting a sensor’s attributes.

See sensor_attr_set() for argument description

typedef int (*sensor_attr_get_t)(const struct device *dev, enum sensor_channel chan, enum sensor_attribute attr, struct sensor_value *val)¶

Callback API upon getting a sensor’s attributes.

See sensor_attr_get() for argument description

typedef int (*sensor_trigger_set_t)(const struct device *dev, const struct sensor_trigger *trig, sensor_trigger_handler_t handler)¶

Callback API for setting a sensor’s trigger and handler.

See sensor_trigger_set() for argument description

typedef int (*sensor_sample_fetch_t)(const struct device *dev, enum sensor_channel chan)¶

Callback API for fetching data from a sensor.

See sensor_sample_fetch() for argument description

typedef int (*sensor_channel_get_t)(const struct device *dev, enum sensor_channel chan, struct sensor_value *val)¶

Callback API for getting a reading from a sensor.

See sensor_channel_get() for argument description

Enums

enum sensor_channel¶

Sensor channels.

Values:

enumerator SENSOR_CHAN_ACCEL_X¶: Acceleration on the X axis, in m/s^2.

enumerator SENSOR_CHAN_ACCEL_Y¶: Acceleration on the Y axis, in m/s^2.

enumerator SENSOR_CHAN_ACCEL_Z¶: Acceleration on the Z axis, in m/s^2.

enumerator SENSOR_CHAN_ACCEL_XYZ¶: Acceleration on the X, Y and Z axes.

enumerator SENSOR_CHAN_GYRO_X¶: Angular velocity around the X axis, in radians/s.

enumerator SENSOR_CHAN_GYRO_Y¶: Angular velocity around the Y axis, in radians/s.

enumerator SENSOR_CHAN_GYRO_Z¶: Angular velocity around the Z axis, in radians/s.

enumerator SENSOR_CHAN_GYRO_XYZ¶: Angular velocity around the X, Y and Z axes.

enumerator SENSOR_CHAN_MAGN_X¶: Magnetic field on the X axis, in Gauss.

enumerator SENSOR_CHAN_MAGN_Y¶: Magnetic field on the Y axis, in Gauss.

enumerator SENSOR_CHAN_MAGN_Z¶: Magnetic field on the Z axis, in Gauss.

enumerator SENSOR_CHAN_MAGN_XYZ¶: Magnetic field on the X, Y and Z axes.

enumerator SENSOR_CHAN_DIE_TEMP¶: Device die temperature in degrees Celsius.

enumerator SENSOR_CHAN_AMBIENT_TEMP¶: Ambient temperature in degrees Celsius.

enumerator SENSOR_CHAN_PRESS¶: Pressure in kilopascal.

enumerator SENSOR_CHAN_PROX¶: Proximity. Adimensional. A value of 1 indicates that an object is close.

enumerator SENSOR_CHAN_HUMIDITY¶: Humidity, in percent.

enumerator SENSOR_CHAN_LIGHT¶: Illuminance in visible spectrum, in lux.

enumerator SENSOR_CHAN_IR¶: Illuminance in infra-red spectrum, in lux.

enumerator SENSOR_CHAN_RED¶: Illuminance in red spectrum, in lux.

enumerator SENSOR_CHAN_GREEN¶: Illuminance in green spectrum, in lux.

enumerator SENSOR_CHAN_BLUE¶: Illuminance in blue spectrum, in lux.

enumerator SENSOR_CHAN_ALTITUDE¶: Altitude, in meters

enumerator SENSOR_CHAN_PM_1_0¶: 1.0 micro-meters Particulate Matter, in ug/m^3

enumerator SENSOR_CHAN_PM_2_5¶: 2.5 micro-meters Particulate Matter, in ug/m^3

enumerator SENSOR_CHAN_PM_10¶: 10 micro-meters Particulate Matter, in ug/m^3

enumerator SENSOR_CHAN_DISTANCE¶: Distance. From sensor to target, in meters

enumerator SENSOR_CHAN_CO2¶: CO2 level, in parts per million (ppm)

enumerator SENSOR_CHAN_VOC¶: VOC level, in parts per billion (ppb)

enumerator SENSOR_CHAN_GAS_RES¶: Gas sensor resistance in ohms.

enumerator SENSOR_CHAN_VOLTAGE¶: Voltage, in volts

enumerator SENSOR_CHAN_CURRENT¶: Current, in amps

enumerator SENSOR_CHAN_RESISTANCE¶: Resistance , in Ohm

enumerator SENSOR_CHAN_ROTATION¶: Angular rotation, in degrees

enumerator SENSOR_CHAN_POS_DX¶: Position change on the X axis, in points.

enumerator SENSOR_CHAN_POS_DY¶: Position change on the Y axis, in points.

enumerator SENSOR_CHAN_POS_DZ¶: Position change on the Z axis, in points.

enumerator SENSOR_CHAN_RPM¶: Revolutions per minute, in RPM.

enumerator SENSOR_CHAN_GAUGE_VOLTAGE¶: Voltage, in volts

enumerator SENSOR_CHAN_GAUGE_AVG_CURRENT¶: Average current, in amps

enumerator SENSOR_CHAN_GAUGE_STDBY_CURRENT¶: Standy current, in amps

enumerator SENSOR_CHAN_GAUGE_MAX_LOAD_CURRENT¶: Max load current, in amps

enumerator SENSOR_CHAN_GAUGE_TEMP¶: Gauge temperature

enumerator SENSOR_CHAN_GAUGE_STATE_OF_CHARGE¶: State of charge measurement in %

enumerator SENSOR_CHAN_GAUGE_FULL_CHARGE_CAPACITY¶: Full Charge Capacity in mAh

enumerator SENSOR_CHAN_GAUGE_REMAINING_CHARGE_CAPACITY¶: Remaining Charge Capacity in mAh

enumerator SENSOR_CHAN_GAUGE_NOM_AVAIL_CAPACITY¶: Nominal Available Capacity in mAh

enumerator SENSOR_CHAN_GAUGE_FULL_AVAIL_CAPACITY¶: Full Available Capacity in mAh

enumerator SENSOR_CHAN_GAUGE_AVG_POWER¶: Average power in mW

enumerator SENSOR_CHAN_GAUGE_STATE_OF_HEALTH¶: State of health measurement in %

enumerator SENSOR_CHAN_GAUGE_TIME_TO_EMPTY¶: Time to empty in minutes

enumerator SENSOR_CHAN_GAUGE_TIME_TO_FULL¶: Time to full in minutes

enumerator SENSOR_CHAN_GAUGE_CYCLE_COUNT¶: Cycle count (total number of charge/discharge cycles)

enumerator SENSOR_CHAN_GAUGE_DESIGN_VOLTAGE¶: Design voltage of cell in V (max voltage)

enumerator SENSOR_CHAN_GAUGE_DESIRED_VOLTAGE¶: Desired voltage of cell in V (nominal voltage)

enumerator SENSOR_CHAN_GAUGE_DESIRED_CHARGING_CURRENT¶: Desired charging current in mA

enumerator SENSOR_CHAN_ALL¶: All channels.

enumerator SENSOR_CHAN_COMMON_COUNT¶: Number of all common sensor channels.

enumerator SENSOR_CHAN_PRIV_START¶: This and higher values are sensor specific. Refer to the sensor header file.

enumerator SENSOR_CHAN_MAX¶: Maximum value describing a sensor channel type.

enum sensor_trigger_type¶

Sensor trigger types.

Values:

enumerator SENSOR_TRIG_TIMER¶: Timer-based trigger, useful when the sensor does not have an interrupt line.

enumerator SENSOR_TRIG_DATA_READY¶: Trigger fires whenever new data is ready.

enumerator SENSOR_TRIG_DELTA¶: Trigger fires when the selected channel varies significantly. This includes any-motion detection when the channel is acceleration or gyro. If detection is based on slope between successive channel readings, the slope threshold is configured via the SENSOR_ATTR_SLOPE_TH and SENSOR_ATTR_SLOPE_DUR attributes.

enumerator SENSOR_TRIG_NEAR_FAR¶: Trigger fires when a near/far event is detected.

enumerator SENSOR_TRIG_THRESHOLD¶: Trigger fires when channel reading transitions configured thresholds. The thresholds are configured via the SENSOR_ATTR_LOWER_THRESH and SENSOR_ATTR_UPPER_THRESH attributes.

enumerator SENSOR_TRIG_TAP¶: Trigger fires when a single tap is detected.

enumerator SENSOR_TRIG_DOUBLE_TAP¶: Trigger fires when a double tap is detected.

enumerator SENSOR_TRIG_FREEFALL¶: Trigger fires when a free fall is detected.

enumerator SENSOR_TRIG_COMMON_COUNT¶: Number of all common sensor triggers.

enumerator SENSOR_TRIG_PRIV_START¶: This and higher values are sensor specific. Refer to the sensor header file.

enumerator SENSOR_TRIG_MAX¶: Maximum value describing a sensor trigger type.

enum sensor_attribute¶

Sensor attribute types.

Values:

enumerator SENSOR_ATTR_SAMPLING_FREQUENCY¶: Sensor sampling frequency, i.e. how many times a second the sensor takes a measurement.

enumerator SENSOR_ATTR_LOWER_THRESH¶: Lower threshold for trigger.

enumerator SENSOR_ATTR_UPPER_THRESH¶: Upper threshold for trigger.

enumerator SENSOR_ATTR_SLOPE_TH¶: Threshold for any-motion (slope) trigger.

enumerator SENSOR_ATTR_SLOPE_DUR¶: Duration for which the slope values needs to be outside the threshold for the trigger to fire.

enumerator SENSOR_ATTR_OVERSAMPLING¶: Oversampling factor

enumerator SENSOR_ATTR_FULL_SCALE¶: Sensor range, in SI units.

enumerator SENSOR_ATTR_OFFSET¶: The sensor value returned will be altered by the amount indicated by offset: final_value = sensor_value + offset.

enumerator SENSOR_ATTR_CALIB_TARGET¶: Calibration target. This will be used by the internal chip’s algorithms to calibrate itself on a certain axis, or all of them.

enumerator SENSOR_ATTR_COMMON_COUNT¶: Number of all common sensor attributes.

enumerator SENSOR_ATTR_PRIV_START¶: This and higher values are sensor specific. Refer to the sensor header file.

enumerator SENSOR_ATTR_MAX¶: Maximum value describing a sensor attribute type.

Functions

int sensor_attr_set(const struct device *dev, enum sensor_channel chan, enum sensor_attribute attr, const struct sensor_value *val)¶

Set an attribute for a sensor.

Return

0 if successful, negative errno code if failure.

Parameters

dev: Pointer to the sensor device
chan: The channel the attribute belongs to, if any. Some attributes may only be set for all channels of a device, depending on device capabilities.
attr: The attribute to set
val: The value to set the attribute to

int sensor_attr_get(const struct device *dev, enum sensor_channel chan, enum sensor_attribute attr, struct sensor_value *val)¶

Get an attribute for a sensor.

Return

0 if successful, negative errno code if failure.

Parameters

dev: Pointer to the sensor device
chan: The channel the attribute belongs to, if any. Some attributes may only be set for all channels of a device, depending on device capabilities.
attr: The attribute to get
val: Pointer to where to store the attribute

static inline int sensor_trigger_set(const struct device *dev, struct sensor_trigger *trig, sensor_trigger_handler_t handler)¶

Activate a sensor’s trigger and set the trigger handler.

The handler will be called from a thread, so I2C or SPI operations are safe. However, the thread’s stack is limited and defined by the driver. It is currently up to the caller to ensure that the handler does not overflow the stack.

This API is not permitted for user threads.

Return

0 if successful, negative errno code if failure.

Parameters

dev: Pointer to the sensor device
trig: The trigger to activate
handler: The function that should be called when the trigger fires

int sensor_sample_fetch(const struct device *dev)¶

Fetch a sample from the sensor and store it in an internal driver buffer.

Read all of a sensor’s active channels and, if necessary, perform any additional operations necessary to make the values useful. The user may then get individual channel values by calling sensor_channel_get.

Since the function communicates with the sensor device, it is unsafe to call it in an ISR if the device is connected via I2C or SPI.

Return

0 if successful, negative errno code if failure.

Parameters

dev: Pointer to the sensor device

int sensor_sample_fetch_chan(const struct device *dev, enum sensor_channel type)¶

Fetch a sample from the sensor and store it in an internal driver buffer.

Read and compute compensation for one type of sensor data (magnetometer, accelerometer, etc). The user may then get individual channel values by calling sensor_channel_get.

This is mostly implemented by multi function devices enabling reading at different sampling rates.

Since the function communicates with the sensor device, it is unsafe to call it in an ISR if the device is connected via I2C or SPI.

Return

0 if successful, negative errno code if failure.

Parameters

dev: Pointer to the sensor device
type: The channel that needs updated

int sensor_channel_get(const struct device *dev, enum sensor_channel chan, struct sensor_value *val)¶

Get a reading from a sensor device.

Return a useful value for a particular channel, from the driver’s internal data. Before calling this function, a sample must be obtained by calling sensor_sample_fetch or sensor_sample_fetch_chan. It is guaranteed that two subsequent calls of this function for the same channels will yield the same value, if sensor_sample_fetch or sensor_sample_fetch_chan has not been called in the meantime.

For vectorial data samples you can request all axes in just one call by passing the specific channel with _XYZ suffix. The sample will be returned at val[0], val[1] and val[2] (X, Y and Z in that order).

Return

0 if successful, negative errno code if failure.

Parameters

dev: Pointer to the sensor device
chan: The channel to read
val: Where to store the value

static inline int32_t sensor_ms2_to_g(const struct sensor_value *ms2)¶

Helper function to convert acceleration from m/s^2 to Gs.

Return

The converted value, in Gs.

Parameters

ms2: A pointer to a sensor_value struct holding the acceleration, in m/s^2.

static inline void sensor_g_to_ms2(int32_t g, struct sensor_value *ms2)¶

Helper function to convert acceleration from Gs to m/s^2.

Parameters

g: The G value to be converted.
ms2: A pointer to a sensor_value struct, where the result is stored.

static inline int32_t sensor_rad_to_degrees(const struct sensor_value *rad)¶

Helper function for converting radians to degrees.

Return

The converted value, in degrees.

Parameters

rad: A pointer to a sensor_value struct, holding the value in radians.

static inline void sensor_degrees_to_rad(int32_t d, struct sensor_value *rad)¶

Helper function for converting degrees to radians.

Parameters

d: The value (in degrees) to be converted.
rad: A pointer to a sensor_value struct, where the result is stored.

static inline double sensor_value_to_double(struct sensor_value *val)¶

Helper function for converting struct sensor_value to double.

Return

The converted value.

Parameters

val: A pointer to a sensor_value struct.

struct sensor_value¶

#include <sensor.h>

Representation of a sensor readout value.

The value is represented as having an integer and a fractional part, and can be obtained using the formula val1 + val2 * 10^(-6). Negative values also adhere to the above formula, but may need special attention. Here are some examples of the value representation:

 0.5: val1 =  0, val2 =  500000
-0.5: val1 =  0, val2 = -500000
-1.0: val1 = -1, val2 =  0
-1.5: val1 = -1, val2 = -500000

struct sensor_trigger¶: #include <sensor.h>
Sensor trigger spec.

struct sensor_driver_api¶: #include <sensor.h>