68 lines
2.9 KiB
Plaintext
68 lines
2.9 KiB
Plaintext
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.
|