xref: /freebsd/share/man/man4/ow_temp.4 (revision f86e60008bdf690b61af2c18e98ee791ca91433f)

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.Dd July 20, 2015
.Dt OW_TEMP 4
.Os
.Sh NAME
.Nm ow_temp
.Nd Dallas Semiconductor 1-Wire Temperature sensor
.Sh SYNOPSIS
.Cd device ow_temp
.Sh DESCRIPTION
The
.Nm
module supports many of the 1-Wire temperature sensors.
.Pp
The sensor is read periodically and the results returned via a
.Xr sysctl 3
as described below.
.Sh HARDWARE
These temperature sensors are supported by the
.Nm
driver:
.Bl -tag -width MAX31820 -compact
.It DS1820
1-Wire Digital Thermometer
.It DS18S20
High-Precision 1-Wire Digital Thermometer
.It DS18B20
Programmable Resolution 1-Wire Digital Thermometer
.It DS1822
Econo 1-Wire Digital Thermometer
.It DS1825
Programmable Resolution 1-Wire Digital Thermometer with 4-bit ID
.It MAX31820
1-Wire, Parasite-Power, Ambient Temperature Sensor
.El
.Pp
The driver supports Family codes 0x10, 0x22, 0x28, and 0x3b.
.Sh SYSCTL
The
.Nm
driver reports data via
.Xr sysctl 8
entries in the device's node in the
.Xr sysctl 8
tree:
.Bl -tag -width xxxxxxxxxx
.It temperature
The last temperature read, in milli-Kelvin.
.It 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.
.It badread
The number of times a non-CRC error was encountered reading the temperature
from the card.
This type of error is very rare.
.It reading_interval
The time, in ticks, between successive reads of the sensor.
.It parasite
This item is non-zero when the device is connected using its parasitic
power mode.
It can also indicate a wiring error.
.El
.Pp
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
.Xr 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.
.Sh SEE ALSO
.Xr ow 4 ,
.Xr owc 4 ,
.Xr sysctl 8 ,
.Xr owll 9 ,
.Xr own 9
.Sh LEGAL
.Tn 1-Wire
is a registered trademark of Maxim Integrated Products, Inc.
.Sh HISTORY
The
.Nm
driver first appeared in
.Fx 11.0 .
.Sh AUTHORS
The
.Nm
device driver and this manual page were written by
.An Warner Losh .
.Sh BUGS
The parasitic mode of the devices does not work.
It requires support from the
.Xr owc 4
driver that is unimplemented.
.Pp
The ID bits from the
.Em DS1825
are not recognized or reported.
.Pp
The type of the device is not reported via
.Xr sysctl 8 .
.Pp
Alarm mode is not supported.
It is not possible to set the low and high alarm temperatures.
.Pp
There is no way to write to the EEPROM.
.Pp
.Dq 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.
.Pp
It is not possible to set the precision on those devices that support
it.
.Pp
The time to convert is fixed at 1 second, even though some devices are
faster.
.Pp
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.