Sensor

Component Type: Input

Status: Implemented

Overview

CIOSensor is a generic IO object for reading data from hardware sensors integrated via the NuttX sensor framework. It supports a variety of sensor types, including accelerometers, gyroscopes, and environmental sensors.

Sensors use the same scalar representation selected for the NuttX sensor framework: DTYPE_FLOAT with CONFIG_SENSORS_USE_FLOAT or DTYPE_B16 with CONFIG_SENSORS_USE_B16. The dimension of the data (e.g., 3 for a 3-axis accelerometer) is automatically determined based on the sensor type.

Default Units

Dawn uses the default NuttX sensor framework units for all sensor-backed IO. These are the units published by the /dev/uorb/sensor_* devices and defined by the NuttX sensor structures.

Internal Dawn components should treat sensor values as already being in these units. Unit conversion belongs at the protocol boundary: a protocol that publishes a different external representation must convert from the Dawn default unit to its protocol-specific unit. This keeps the output unit of internal processing predictable, including the simple case where a sensor IO is connected directly to a protocol endpoint.

The default units used by Dawn are:

Dawn Type	Default Dawn Unit
accel	float or b16, meters per second squared
mag	float or b16, microtesla
gyro	float or b16, radians per second
light	float or b16, lux
baro	float or b16, hectopascal
prox	float or b16, centimeters
humid	float or b16, percent
temp	float or b16, degrees Celsius
atemp	float or b16, degrees Celsius
rgb	float or b16, percent
ir	float or b16, lux
uv	float or b16, UV index
gas	float or b16, kilohms
gnss	float, 5-vector: latitude, longitude (degrees), altitude (metres), ground speed (m/s), course (degrees)
gnss_time	uint64, UTC time in seconds since the Unix epoch
gnss_info	float, 5-vector: horizontal/vertical accuracy (metres), HDOP, PDOP, VDOP
gnss_sats	uint32, number of satellites used in the fix

GNSS does not fit the generic float-vector sensor model (it carries non-float fields and the useful values are not contiguous), so the four gnss* subtypes are handled by a dedicated class, dawn::CIOSensorGnss, which overloads the sensor IO. They all read the same sensor_gnss device, and each assembles a logical group of fields from the NuttX sensor_gnss event with the proper data type:

• gnss – position and velocity, as [latitude, longitude, altitude, ground_speed, course];

• gnss_time – the modem UTC time (the kernel value is microseconds; it is converted to seconds since the epoch, suitable for an LwM2M Time resource);

• gnss_info – fix quality, as [eph, epv, hdop, pdop, vdop];

• gnss_sats – the satellite count.

Use a vecsplit program to break the multi-element gnss / gnss_info vectors into per-field scalars (for example to feed an LwM2M Location object). The scalar gnss_time / gnss_sats IOs read -ENODATA between fixes; pair them with a latest program so a fetch-only protocol always reads the last value (the init value until the first fix) instead of an error.

Implementation

The component interacts with the NuttX sensor framework through the /dev/uorb/sensor_* character devices. It uses the poll() mechanism to provide asynchronous notifications when new samples are available.

The following table shows the mapping between Dawn sensor types and the underlying NuttX sensor structures:

Dawn Type	NuttX Path	NuttX Structure
accel	sensor_accel	struct sensor_accel
mag	sensor_mag	struct sensor_mag
gyro	sensor_gyro	struct sensor_gyro
light	sensor_light	struct sensor_light
baro	sensor_baro	struct sensor_baro
prox	sensor_prox	struct sensor_prox
humid	sensor_humi	struct sensor_humi
temp	sensor_temp	struct sensor_temp
atemp	sensor_ambient_temp	struct sensor_temp
rgb	sensor_rgb	struct sensor_rgb
ir	sensor_ir	struct sensor_ir
uv	sensor_uv	struct sensor_uv
gas	sensor_gas	struct sensor_gas
gnss	sensor_gnss	struct sensor_gnss
gnss_time	sensor_gnss	struct sensor_gnss
gnss_info	sensor_gnss	struct sensor_gnss
gnss_sats	sensor_gnss	struct sensor_gnss

Configuration

Kconfig

• CONFIG_DAWN_IO_SENSOR: enables sensor-backed IO objects.

• CONFIG_SENSORS_USE_FLOAT: NuttX publishes sensor values as float; Dawn sensor descriptors must use dtype: float.

• CONFIG_SENSORS_USE_B16: NuttX publishes sensor values as signed 16.16 fixed-point b16_t; Dawn sensor descriptors must use dtype: b16.

YAML

ios:
  - id: temp1
    type: sensor
    subtype: temp
    dtype: float
    config:
      devno: 0

The subtype determines the sensor class, and devno matches the instance number in the OS (e.g., /dev/uorb/sensor_temp0).

External Control

• ControlIO: not supported

• TriggerIO: not supported

Brainstorming & Future Ideas

Configuration items not yet implemented:

• Interval

• Orientation

• Scale

• Filters

• Calibration data

• Threshold

• Gain (polarisation)

• Enable/disable

• Unit

• Alerts

Doxygen

• dawn::CIOSensor