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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 000000000..e39202e2b --- /dev/null +++ b/Documentation/w1/slaves/w1_therm.rst @@ -0,0 +1,139 @@ +====================== +Kernel driver w1_therm +====================== + +Supported chips: + + * Maxim ds18*20 based temperature sensors. + * Maxim ds1825 based temperature sensors. + * GXCAS GC20MH01 temperature sensor. + +Author: Evgeniy Polyakov <johnpol@2ka.mipt.ru> + + +Description +----------- + +w1_therm provides basic temperature conversion for ds18*20, ds28ea00, GX20MH01 +devices. + +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 entry ``w1_slave``. 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=. + +Alternatively, temperature can be read using ``temperature`` sysfs, it +returns only the temperature in millidegrees Centigrade. + +A bulk read of all devices on the bus could be done writing ``trigger`` +to ``therm_bulk_read`` entry at w1_bus_master level. This will +send the convert command to all devices on the bus, and if parasite +powered devices are detected on the bus (and strong pullup is enabled +in the module), it will drive the line high during the longer conversion +time required by parasited powered device on the line. Reading +``therm_bulk_read`` will return 0 if no bulk conversion pending, +-1 if at least one sensor still in conversion, 1 if conversion is complete +but at least one sensor value has not been read yet. Result temperature is +then accessed by reading the ``temperature`` entry of each device, which +may return empty if conversion is still in progress. Note that if a bulk +read is sent but one sensor is not read immediately, the next access to +``temperature`` on this device will return the temperature measured at the +time of issue of the bulk read command (not the current temperature). + +A strong pullup will be applied during the conversion if required. + +``conv_time`` is used to get current conversion time (read), and +adjust it (write). A temperature conversion time depends on the device type and +it's current resolution. Default conversion time is set by the driver according +to the device datasheet. A conversion time for many original device clones +deviate from datasheet specs. There are three options: 1) manually set the +correct conversion time by writing a value in milliseconds to ``conv_time``; 2) +auto measure and set a conversion time by writing ``1`` to +``conv_time``; 3) use ``features`` to enable poll for conversion +completion. Options 2, 3 can't be used in parasite power mode. To get back to +the default conversion time write ``0`` to ``conv_time``. + +Writing a resolution value (in bits) to ``w1_slave`` will change the +precision of the sensor for the next readings. Allowed resolutions are defined by +the sensor. Resolution is reset when the sensor gets power-cycled. + +To store the current resolution in EEPROM, write ``0`` to ``w1_slave``. +Since the EEPROM has a limited amount of writes (>50k), this command should be +used wisely. + +Alternatively, resolution can be read or written using the dedicated +``resolution`` entry on each device, if supported by the sensor. + +Some non-genuine DS18B20 chips are fixed in 12-bit mode only, so the actual +resolution is read back from the chip and verified. + +Note: Changing the resolution reverts the conversion time to default. + +The write-only sysfs entry ``eeprom`` is an alternative for EEPROM operations. +Write ``save`` to save device RAM to EEPROM. Write ``restore`` to restore EEPROM +data in device RAM. + +``ext_power`` entry allows checking the power state of each device. Reads +``0`` if the device is parasite powered, ``1`` if the device is externally powered. + +Sysfs ``alarms`` allow read or write TH and TL (Temperature High an Low) alarms. +Values shall be space separated and in the device range (typical -55 degC +to 125 degC). Values are integer as they are store in a 8bit register in +the device. Lowest value is automatically put to TL. Once set, alarms could +be search at master level. + +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``. The file will contain a single line with an integer value +representing the device index in the bus starting at 0. + +``features`` sysfs entry controls optional driver settings per device. +Insufficient power in parasite mode, line noise and insufficient conversion +time may lead to conversion failure. Original DS18B20 and some clones allow for +detection of invalid conversion. Write bit mask ``1`` to ``features`` to enable +checking the conversion success. If byte 6 of scratchpad memory is 0xC after +conversion and temperature reads 85.00 (powerup value) or 127.94 (insufficient +power), the driver returns a conversion error. Bit mask ``2`` enables poll for +conversion completion (normal power only) by generating read cycles on the bus +after conversion starts. In parasite power mode this feature is not available. +Feature bit masks may be combined (OR). More details in +Documentation/ABI/testing/sysfs-driver-w1_therm + +GX20MH01 device shares family number 0x28 with DS18*20. The device is generally +compatible with DS18B20. Added are lowest 2\ :sup:`-5`, 2\ :sup:`-6` temperature +bits in Config register; R2 bit in Config register enabling 13 and 14 bit +resolutions. The device is powered up in 14-bit resolution mode. The conversion +times specified in the datasheet are too low and have to be increased. The +device supports driver features ``1`` and ``2``. |