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w1_therm

w1_therm 2.1 KB
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  1. Kernel driver w1_therm
    2. 

  2. ====================
    3. 

  3. 

  4. Supported chips:
    5. 

  5.   * Maxim ds18*20 based temperature sensors.
    6. 

  6.   * Maxim ds1825 based temperature sensors.
    7. 

  7. 

  8. Author: Evgeniy Polyakov <johnpol@2ka.mipt.ru>
    9. 

  9. 

  10. 

  11. Description
    12. 

  12. -----------
    13. 

  13. 

  14. w1_therm provides basic temperature conversion for ds18*20 devices.
    15. 

  15. supported family codes:
    16. 

  16. W1_THERM_DS18S20	0x10
    17. 

  17. W1_THERM_DS1822		0x22
    18. 

  18. W1_THERM_DS18B20	0x28
    19. 

  19. W1_THERM_DS1825		0x3B
    20. 

  20. 

  21. Support is provided through the sysfs w1_slave file.  Each open and
    22. 

  22. read sequence will initiate a temperature conversion then provide two
    23. 

  23. lines of ASCII output.  The first line contains the nine hex bytes
    24. 

  24. read along with a calculated crc value and YES or NO if it matched.
    25. 

  25. If the crc matched the returned values are retained.  The second line
    26. 

  26. displays the retained values along with a temperature in millidegrees
    27. 

  27. Centigrade after t=.
    28. 

  28. 

  29. Parasite powered devices are limited to one slave performing a
    30. 

  30. temperature conversion at a time.  If none of the devices are parasite
    31. 

  31. powered it would be possible to convert all the devices at the same
    32. 

  32. time and then go back to read individual sensors.  That isn't
    33. 

  33. currently supported.  The driver also doesn't support reduced
    34. 

  34. precision (which would also reduce the conversion time).
    35. 

  35. 

  36. The module parameter strong_pullup can be set to 0 to disable the
    37. 

  37. strong pullup, 1 to enable autodetection or 2 to force strong pullup.
    38. 

  38. In case of autodetection, the driver will use the "READ POWER SUPPLY"
    39. 

  39. command to check if there are pariste powered devices on the bus.
    40. 

  40. If so, it will activate the master's strong pullup.
    41. 

  41. In case the detection of parasite devices using this command fails
    42. 

  42. (seems to be the case with some DS18S20) the strong pullup can
    43. 

  43. be force-enabled.
    44. 

  44. If the strong pullup is enabled, the master's strong pullup will be
    45. 

  45. driven when the conversion is taking place, provided the master driver
    46. 

  46. does support the strong pullup (or it falls back to a pullup
    47. 

  47. resistor).  The DS18b20 temperature sensor specification lists a
    48. 

  48. maximum current draw of 1.5mA and that a 5k pullup resistor is not
    49. 

  49. sufficient.  The strong pullup is designed to provide the additional
    50. 

  50. current required.
    51.