NAME

     ow_temp — Dallas Semiconductor 1-Wire Temperature sensor

SYNOPSIS

     device ow_temp

DESCRIPTION

     The ow_temp module supports many of the 1-Wire temperature sensors.

     The sensor is read periodically and the results returned via a sysctl(3) as described below.

HARDWARE

     These temperature sensors are supported by the ow_temp driver:
     DS1820    1-Wire Digital Thermometer
     DS18S20   High-Precision 1-Wire Digital Thermometer
     DS18B20   Programmable Resolution 1-Wire Digital Thermometer
     DS1822    Econo 1-Wire Digital Thermometer
     DS1825    Programmable Resolution 1-Wire Digital Thermometer with 4-bit ID
     MAX31820  1-Wire, Parasite-Power, Ambient Temperature Sensor

     The driver supports Family codes 0x10, 0x22, 0x28, and 0x3b.

SYSCTL

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

     temperature
                 The last temperature read, in milli-Kelvin.

     badcrc      The number of CRC errors in reading the temperature form the device.  Some CRC
                 errors are to be expected.  High rates of CRC errors, however, generally
                 indicate a noisy environment, cabling issues, or too many devices on the bus.

     badread     The number of times a non-CRC error was encountered reading the temperature from
                 the card.  This type of error is very rare.

     reading_interval
                 The time, in ticks, between successive reads of the sensor.

     parasite    This item is non-zero when the device is connected using its parasitic power
                 mode.  It can also indicate a wiring error.

     Temperatures are reported in milli-Kelvin, even though the absolute accuracy is around 0.2
     degrees for the good devices and around 1 degree for cheaper devices.  The devices report in
     steps of 0.0625 degrees.  The driver preserves the precision of the device's measurements in
     its sysctl(8) reports.  These devices often have a much higher relative accuracy and
     repeatability than their absolute accuracy.  This makes them well suited for control loops
     that strive for stability and become possible if the full precision is preserved.

SEE ALSO

     ow(4), owc(4), sysctl(8), owll(9), own(9)

LEGAL

     1-Wire is a registered trademark of Maxim Integrated Products, Inc.

HISTORY

     The ow_temp driver first appeared in FreeBSD 11.0.

AUTHORS

     The ow_temp device driver and this manual page were written by Warner Losh.

BUGS

     The parasitic mode of the devices does not work.  It requires support from the owc(4) driver
     that is unimplemented.

     The ID bits from the DS1825 are not recognized or reported.

     The type of the device is not reported via sysctl(8).

     Alarm mode is not supported.  It is not possible to set the low and high alarm temperatures.

     There is no way to write to the EEPROM.

     “Convert Temperature” requests are sent directly to the device.  There is no way to use the
     broadcast ability of the 1-Wire bus to do all the conversions in parallel.

     It is not possible to set the precision on those devices that support it.

     The time to convert is fixed at 1 second, even though some devices are faster.

     There is no character device to supply a stream of readings to a program.  Programs
     interested in the temperature must poll the sysctl to get the temperature.