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diff --git a/Documentation/w1/slaves/w1_therm.rst b/Documentation/w1/slaves/w1_therm.rst new file mode 100644 index 000000000000..90531c340a07 --- /dev/null +++ b/Documentation/w1/slaves/w1_therm.rst @@ -0,0 +1,74 @@ +====================== +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. |