si70xxtemp —
Driver for Silicon Labs SI7013/SI7020/SI7021, HTU21D and SHT21
  sensor chip via I2C bus
si70xxtemp* at iic? addr 0x40
The si70xxtemp driver provides measurements from the
  SI7013/SI7020/SI7021 humidity/temperature sensors via the
  envsys(4) framework. The
  si70xxtemp addr argument selects
  the address at the iic(4) bus. The
  resolution, heater control and crc validity can be changed through
  sysctl(8) nodes.
The following sysctl(3) variables
  are provided:
  - hw.si70xxtemp0.resolutions
- Lists the resolutions supported by the driver and chip.
- hw.si70xxtemp0.resolution
- Set the resolution, or number of bits, used for %RH and temperature. Use
      one of the strings listed in hw.si70xxtemp.resolutions.
- hw.si70xxtemp0.ignorecrc
- If set, the crc calculation for %RH and temperature will be ignored.
- hw.si70xxtemp0.vddok
- If 1, the chip is getting enough power.
- hw.si70xxtemp0.heateron
- Turn the heater on and off.
- hw.si70xxtemp0.heaterstrength
- From 1 to 6, the amount of energy put into the heater. The higher the
      number, the more power used.
    Some HTU21D chips do not support a heater register. These
        chips are detected and the heater features of the driver will be
        disabled. 
- hw.si70xxtemp0.debug
- If the driver is compiled with SI70XX_DEBUG, this
      node will appear and can be used to set the debugging level.
- hw.si70xxtemp0.readattempts
- To read %RH or temperature the driver uses a No Hold Master command. This
      command needs to be sent to the device, a wait must then occur and then
      another read command is sent to read back the values. Depending on the
      resolution, and other factors, the wait time varies. The driver will
      attempt to read back the values readattempts number of times. The default
      is 40 which should be enough for most purposes. There is an initial wait
      of 10,500 microseconds followed by a additional 1,000 microseconds per
      read attempt.
Thesi70xxtemp driver first appeared in
  NetBSD 8.0.