linux/Documentation/w1/slaves/w1_therm
Ben Sen 0a19f129d7 w1: add ability to set (SRAM) and store (EEPROM) configuration for temp sensors like DS18B20
Since many temperature sensors come "preconfigured" with a lower
precision, people are stuck at that precision when running on a kernel
based device (unlike the Dallas 1Wire library for e.g. Arduino, which
supports writing the configuration/scratchpad). This patch adds write
support for the scratchpad/precision registers via w1_slave sysfs.

Signed-off-by: Ben Sen <0.x29a.0@gmail.com>
Acked-by: Evgeniy Polyakov <zbr@ioremap.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
2016-05-01 14:37:49 -07:00

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Kernel driver w1_therm
====================
Supported chips:
* Maxim ds18*20 based temperature sensors.
* Maxim ds1825 based temperature sensors.
Author: Evgeniy Polyakov <johnpol@2ka.mipt.ru>
Description
-----------
w1_therm provides basic temperature conversion for ds18*20 devices, and the
ds28ea00 device.
supported family codes:
W1_THERM_DS18S20 0x10
W1_THERM_DS1822 0x22
W1_THERM_DS18B20 0x28
W1_THERM_DS1825 0x3B
W1_THERM_DS28EA00 0x42
Support is provided through the sysfs w1_slave file. Each open and
read sequence will initiate a temperature conversion then provide two
lines of ASCII output. The first line contains the nine hex bytes
read along with a calculated crc value and YES or NO if it matched.
If the crc matched the returned values are retained. The second line
displays the retained values along with a temperature in millidegrees
Centigrade after t=.
Parasite powered devices are limited to one slave performing a
temperature conversion at a time. If none of the devices are parasite
powered it would be possible to convert all the devices at the same
time and then go back to read individual sensors. That isn't
currently supported. The driver also doesn't support reduced
precision (which would also reduce the conversion time) when reading values.
Writing a value between 9 and 12 to the sysfs w1_slave file will change the
precision of the sensor for the next readings. This value is in (volatile)
SRAM, so it is reset when the sensor gets power-cycled.
To store the current precision configuration into EEPROM, the value 0
has to be written to the sysfs w1_slave file. Since the EEPROM has a limited
amount of writes (>50k), this command should be used wisely.
The module parameter strong_pullup can be set to 0 to disable the
strong pullup, 1 to enable autodetection or 2 to force strong pullup.
In case of autodetection, the driver will use the "READ POWER SUPPLY"
command to check if there are pariste powered devices on the bus.
If so, it will activate the master's strong pullup.
In case the detection of parasite devices using this command fails
(seems to be the case with some DS18S20) the strong pullup can
be force-enabled.
If the strong pullup is enabled, the master's strong pullup will be
driven when the conversion is taking place, provided the master driver
does support the strong pullup (or it falls back to a pullup
resistor). The DS18b20 temperature sensor specification lists a
maximum current draw of 1.5mA and that a 5k pullup resistor is not
sufficient. The strong pullup is designed to provide the additional
current required.
The DS28EA00 provides an additional two pins for implementing a sequence
detection algorithm. This feature allows you to determine the physical
location of the chip in the 1-wire bus without needing pre-existing
knowledge of the bus ordering. Support is provided through the sysfs
w1_seq file. The file will contain a single line with an integer value
representing the device index in the bus starting at 0.