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68 lines
2.9 KiB
Plaintext
68 lines
2.9 KiB
Plaintext
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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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 devices, and the
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ds28ea00 device.
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supported family codes:
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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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Support is provided through the sysfs w1_slave file. 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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Parasite powered devices are limited to one slave performing a
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temperature conversion at a time. If none of the devices are parasite
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powered it would be possible to convert all the devices at the same
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time and then go back to read individual sensors. That isn't
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currently supported. The driver also doesn't support reduced
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precision (which would also reduce the conversion time) when reading values.
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Writing a value between 9 and 12 to the sysfs w1_slave file will change the
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precision of the sensor for the next readings. This value is in (volatile)
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SRAM, so it is reset when the sensor gets power-cycled.
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To store the current precision configuration into EEPROM, the value 0
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has to be written to the sysfs w1_slave file. Since the EEPROM has a limited
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amount of writes (>50k), this command should be used wisely.
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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 file. 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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