145 lines
7.0 KiB
ReStructuredText
145 lines
7.0 KiB
ReStructuredText
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======================
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Kernel driver w1_therm
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======================
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Supported chips:
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* Maxim ds18*20 based temperature sensors.
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* Maxim ds1825 based temperature sensors.
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* GXCAS GX20MH01 temperature sensor.
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* Maxim MAX31850 thermoelement interface.
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Author: Evgeniy Polyakov <johnpol@2ka.mipt.ru>
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Description
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-----------
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w1_therm provides basic temperature conversion for ds18*20, ds28ea00, GX20MH01
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and MAX31850 devices.
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Supported family codes:
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==================== ====
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W1_THERM_DS18S20 0x10
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W1_THERM_DS1822 0x22
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W1_THERM_DS18B20 0x28
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W1_THERM_DS1825 0x3B
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W1_THERM_DS28EA00 0x42
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==================== ====
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Support is provided through the sysfs entry ``w1_slave``. Each open and
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read sequence will initiate a temperature conversion, then provide two
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lines of ASCII output. The first line contains the nine hex bytes
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read along with a calculated crc value and YES or NO if it matched.
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If the crc matched the returned values are retained. The second line
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displays the retained values along with a temperature in millidegrees
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Centigrade after t=.
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Alternatively, temperature can be read using ``temperature`` sysfs, it
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returns only the temperature in millidegrees Centigrade.
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A bulk read of all devices on the bus could be done writing ``trigger``
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to ``therm_bulk_read`` entry at w1_bus_master level. This will
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send the convert command to all devices on the bus, and if parasite
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powered devices are detected on the bus (and strong pullup is enabled
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in the module), it will drive the line high during the longer conversion
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time required by parasited powered device on the line. Reading
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``therm_bulk_read`` will return 0 if no bulk conversion pending,
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-1 if at least one sensor still in conversion, 1 if conversion is complete
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but at least one sensor value has not been read yet. Result temperature is
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then accessed by reading the ``temperature`` entry of each device, which
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may return empty if conversion is still in progress. Note that if a bulk
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read is sent but one sensor is not read immediately, the next access to
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``temperature`` on this device will return the temperature measured at the
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time of issue of the bulk read command (not the current temperature).
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A strong pullup will be applied during the conversion if required.
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``conv_time`` is used to get current conversion time (read), and
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adjust it (write). A temperature conversion time depends on the device type and
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it's current resolution. Default conversion time is set by the driver according
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to the device datasheet. A conversion time for many original device clones
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deviate from datasheet specs. There are three options: 1) manually set the
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correct conversion time by writing a value in milliseconds to ``conv_time``; 2)
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auto measure and set a conversion time by writing ``1`` to
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``conv_time``; 3) use ``features`` to enable poll for conversion
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completion. Options 2, 3 can't be used in parasite power mode. To get back to
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the default conversion time write ``0`` to ``conv_time``.
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Writing a resolution value (in bits) to ``w1_slave`` will change the
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precision of the sensor for the next readings. Allowed resolutions are defined by
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the sensor. Resolution is reset when the sensor gets power-cycled.
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To store the current resolution in EEPROM, write ``0`` to ``w1_slave``.
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Since the EEPROM has a limited amount of writes (>50k), this command should be
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used wisely.
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Alternatively, resolution can be read or written using the dedicated
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``resolution`` entry on each device, if supported by the sensor.
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Some non-genuine DS18B20 chips are fixed in 12-bit mode only, so the actual
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resolution is read back from the chip and verified.
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Note: Changing the resolution reverts the conversion time to default.
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The write-only sysfs entry ``eeprom_cmd`` is an alternative for EEPROM operations.
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Write ``save`` to save device RAM to EEPROM. Write ``restore`` to restore EEPROM
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data in device RAM.
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``ext_power`` entry allows checking the power state of each device. Reads
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``0`` if the device is parasite powered, ``1`` if the device is externally powered.
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Sysfs ``alarms`` allow read or write TH and TL (Temperature High an Low) alarms.
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Values shall be space separated and in the device range (typical -55 degC
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to 125 degC). Values are integer as they are store in a 8bit register in
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the device. Lowest value is automatically put to TL. Once set, alarms could
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be search at master level.
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The module parameter strong_pullup can be set to 0 to disable the
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strong pullup, 1 to enable autodetection or 2 to force strong pullup.
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In case of autodetection, the driver will use the "READ POWER SUPPLY"
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command to check if there are pariste powered devices on the bus.
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If so, it will activate the master's strong pullup.
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In case the detection of parasite devices using this command fails
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(seems to be the case with some DS18S20) the strong pullup can
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be force-enabled.
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If the strong pullup is enabled, the master's strong pullup will be
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driven when the conversion is taking place, provided the master driver
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does support the strong pullup (or it falls back to a pullup
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resistor). The DS18b20 temperature sensor specification lists a
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maximum current draw of 1.5mA and that a 5k pullup resistor is not
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sufficient. The strong pullup is designed to provide the additional
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current required.
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The DS28EA00 provides an additional two pins for implementing a sequence
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detection algorithm. This feature allows you to determine the physical
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location of the chip in the 1-wire bus without needing pre-existing
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knowledge of the bus ordering. Support is provided through the sysfs
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``w1_seq``. The file will contain a single line with an integer value
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representing the device index in the bus starting at 0.
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``features`` sysfs entry controls optional driver settings per device.
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Insufficient power in parasite mode, line noise and insufficient conversion
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time may lead to conversion failure. Original DS18B20 and some clones allow for
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detection of invalid conversion. Write bit mask ``1`` to ``features`` to enable
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checking the conversion success. If byte 6 of scratchpad memory is 0xC after
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conversion and temperature reads 85.00 (powerup value) or 127.94 (insufficient
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power), the driver returns a conversion error. Bit mask ``2`` enables poll for
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conversion completion (normal power only) by generating read cycles on the bus
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after conversion starts. In parasite power mode this feature is not available.
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Feature bit masks may be combined (OR). More details in
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Documentation/ABI/testing/sysfs-driver-w1_therm
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GX20MH01 device shares family number 0x28 with DS18*20. The device is generally
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compatible with DS18B20. Added are lowest 2\ :sup:`-5`, 2\ :sup:`-6` temperature
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bits in Config register; R2 bit in Config register enabling 13 and 14 bit
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resolutions. The device is powered up in 14-bit resolution mode. The conversion
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times specified in the datasheet are too low and have to be increased. The
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device supports driver features ``1`` and ``2``.
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MAX31850 device shares family number 0x3B with DS1825. The device is generally
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compatible with DS1825. The higher 4 bits of Config register read all 1,
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indicating 15, but the device is always operating in 14-bit resolution mode.
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