HTU21(4)               FreeBSD Kernel Interfaces Manual               HTU21(4)

NAME
     htu21 - driver for HTU21D and compatible temperature and humidity sensors

SYNOPSIS
     To compile this driver into the kernel, place the following lines in your
     kernel configuration file:

           device htu21
           device iicbus

     Alternatively, to load the driver as a module at boot time, place the
     following line in loader.conf(5):

           htu21_load="YES"

DESCRIPTION
     The htu21 driver provides temperature and relative humidity readings over
     I2C bus for the supported sensors:
     •   HTU21D
     •   SHT21
     •   Si7021

     The htu21 driver reports data via sysctl(8) entries in the device's node
     in the sysctl(8) tree:

     temperature  The temperature, in hundredths of Kelvin.

     humidity     The relative humidity, in hundredths of a percent.

     crc_errors   The number of CRC errors in reading the measurements from
                  the device.

     power        The good power indication.  This can be useful with battery
                  powered sensors.

     heater       The built-in heater control.  The heater can be used for
                  testing and for recovery from saturation after high
                  humidity.

     hold_bus     Whether the sensor should hold SCL low while performing the
                  measurement.  Normally, the sensor releases the bus and
                  NACKs all accessed until the measurement is completed.  The
                  hold mode may be useful in mult-master environments.

     On an FDT(4) based system the following properties must be set:

     compatible  Must be set to "meas,htu21".

     reg         The I2C address of htu21.  Although, it is hard-wired to 0x40
                 (7-bit) on all supported sensors.

     The DTS part for a htu21 device usually looks like:

     / {

             ...
             htu21d {
                     compatible = "meas,htu21";
                     reg = <0x40>;
             };
     };

SEE ALSO
     fdt(4), iicbus(4), sysctl(8)

HISTORY
     The htu21 driver and this manual page was written by Andriy Gapon
     <avg@FreeBSD.org>.

BUGS
     There is no way to control the measurement resolution.

     Some sensor variants do not provide a serial number or use an
     incompatible format.  The htu21 driver does not distinguish those
     variants and may complain about incorrect serial number checksum.

FreeBSD 13.1-RELEASE-p6        January 19, 2021        FreeBSD 13.1-RELEASE-p6