SHT4X

Contributed by Matteo Golin.

The SHT4x is a family of temperature and humidity sensors created by Sensirion which operates over I2C. They include a small heating element.

The driver provided allows interfacing with the sensor over I2C. It has been tested against the SHT41.

Application Programming Interface

The header file for the SHT4X driver interface can be included using:

# include <nuttx/sensors/sht4x.h>

The SHT4x registration function allows the driver to be registered as a POSIX character driver.

The standard POSIX read() operation will return the temperature and humidity measurements in plain-text, which is useful when debugging/testing the driver using cat from the shell.

The write() operation is not implemented for this sensor.

Specific operations the sensor offers can be performed via the POSIX ioctl operation. The supported commands are:

SNIOC_RESET

SNIOC_WHO_AM_I

SNIOC_READ_RAW_DATA

SNIOC_MEASURE

SNIOC_READ_CONVERT_DATA

SNIOC_HEAT

SNIOC_CONFIGURE

SNIOC_RESET: This will perform the SHT4X’s soft reset command.

err = ioctl(sensor, SNIOC_RESET);
if (err) {
  fprintf(stderr, "SNIOC_RESET: %s\n", strerror(errno));
} else {
  puts("RESET success!");
}

SNIOC_WHO_AM_I

This command reads the serial number of the SHT4X sensor. The serial number is returned in the argument to the command, which must be a uint32_t pointer.

uint32_t serialno = 0;
err = ioctl(sensor, SNIOC_WHO_AM_I, &serialno);

SNIOC_READ_RAW_DATA

This command allows the caller to read the raw data returned from the sensor, without the driver performing any calculation to convert it into familiar units (i.e. degrees Celsius for temperature).

The argument to this command must be a pointer to a struct sht4x_raw_data_s structure. The raw data will be returned here.

struct sht4x_raw_data_s raw;
err = ioctl(sensor, SNIOC_READ_RAW_DATA, &raw);

SNIOC_MEASURE

This command will measure temperature and humidity, and return it in familiar units to the user. Temperature will be in degrees (Fahrenheit or Celsius depends on the Kconfig options selected during compilation) and humidity will be %RH.

The argument to this command must be a pointer to a struct sht4x_conv_data_s. This is where the converted data will be returned.

struct sht4x_conv_data_s data;
err = ioctl(sensor, SNIOC_MEASURE, &data);

SNIOC_READ_CONVERT_DATA

Same as SNIOC_MEASURE.

SNIOC_HEAT

This command will instruct the SHT4X to turn on its heater unit for the specified time. Afterwards, a measurement of temperature and humidity is taken, and the converted data is returned to the caller.

The argument to this command must be a pointer to a struct sht4x_conv_data_s. This is where the converted data will be returned. The temperature field of the struct must contain a value from the enum sht4x_heater_e, which will indicate the duration the heater is on and the power used.

Heating commands are not allowed more than once per second to avoid damaging the sensor. If a command is issued before this one second cool-down period is over, EAGAIN is returned.

struct sht4x_conv_data_s data;
data.temp = SHT4X_HEATER_200MW_1;
err = ioctl(sensor, SNIOC_HEAT, &data);

SNIOC_CONFIGURE

This command allows the caller to configure the precision of the SHT4X sensor used by subsequent measurement commands. By default, the sensor starts at high precision.

The argument to this command is one of the values in enum sht4x_precision_e.

err = ioctl(sensor, SNIOC_CONFIGURE, SHT4X_PREC_LOW);