SGP40MOX(4)

NAME
sgp40mox - Driver for Sensirion SGP40 MOx gas sensor

SYNOPSIS
sgp40mox* at iic? addr 0x59

DESCRIPTION
The sgp40mox driver provides an air quality measurement from the SGP40 sensor via the envsys(4) framework. The sgp40mox addr argument selects the address at the iic(4) bus. The crc validity and temperature and %RH compensation can be changed through sysctl(8) nodes.

In order to calculate the VOC index, the volatile organic compounds index, which is the measure of air quality the sensor is polled once a second and the raw sensor value is fed into the Sensirion VOC algorithm. This VOC algorithm used in this driver is licensed under a 3 clause BSD license and was pulled from the Sensirion Github repository at https://github.com/Sensirion/embedded-sgp.

SYSCTL VARIABLES
The following sysctl(3) variables are provided:

hw.sgp40mox0.compensation.temperature
    This should be set to the temperature in Celsius of the environment that the sensor is in. The valid values are from -45 to 130 degrees Celsius.

hw.sgp40mox0.compensation.humidity
    This should be set to the %RH of the environment that the sensor is in. The valid values are from 0 to 100.

    For the best performance of the VOC algorithm it is important that the temperature and %RH compensation values be current and set using the sysctl(3) variables mentioned above. This data will need to be pulled from another source, such as a another sensor in the environment that the SGP40 is in.

hw.sgp40mox0.ignorecrc
    If set, the crc calculation will be ignored on the calls to the chip for the purposes of measurement.

hw.sgp40mox0.debug
    If the driver is compiled with SGP40_DEBUG, this node will appear and can be used to set the debugging level.

hw.sgp40mox0.readattempts
    To read the air quality metric from the chip requires that the command be sent, then a delay must be observed before a read can be done to get the values back. The delays are documented in the data sheet for the chip. The driver will attempt to read back the values readattempts number of times. The default is 10 which should be more than enough for most purposes.

SEE ALSO
envsys(4), iic(4), envstat(8), sysctl(8)

HISTORY
The sgp40mox driver first appeared in NetBSD 10.0.

AUTHORS
-nosplit The sgp40mox driver was written by Brad Spencer <brad@anduin.eldar.org>.

BUGS
The driver does not make complete use of the VOC algorithm. In particular, there is no need to restart the algorithm from scratch if there is a stoppage of polling for less than 10 minutes. The driver does not have the ability to determine that, and therefore assumes that the sensor is completely cold each time the driver attaches to the chip.

The temperature and humidity compensation could be allowed to contain fractional degrees Celsius and %RH. The driver only supports setting whole numbers for either of those.