Kernel driver pc87427
  =====================
  
  Supported chips:
    * National Semiconductor PC87427
      Prefix: 'pc87427'
      Addresses scanned: none, address read from Super I/O config space

      Datasheet: No longer available

  Author: Jean Delvare <jdelvare@suse.de>

  
  Thanks to Amir Habibi at Candelis for setting up a test system, and to
  Michael Kress for testing several iterations of this driver.
  
  
  Description
  -----------
  
  The National Semiconductor Super I/O chip includes complete hardware
  monitoring capabilities. It can monitor up to 18 voltages, 8 fans and

  6 temperature sensors. Only the fans and temperatures are supported at
  the moment, voltages aren't.

  This chip also has fan controlling features (up to 4 PWM outputs),
  which are partly supported by this driver.

  
  The driver assumes that no more than one chip is present, which seems
  reasonable.
  
  
  Fan Monitoring
  --------------
  
  Fan rotation speeds are reported as 14-bit values from a gated clock
  signal. Speeds down to 83 RPM can be measured.
  
  An alarm is triggered if the rotation speed drops below a programmable

  limit. Another alarm is triggered if the speed is too low to be measured

  (including stalled or missing fan).

  
  
  Fan Speed Control
  -----------------
  
  Fan speed can be controlled by PWM outputs. There are 4 possible modes:
  always off, always on, manual and automatic. The latter isn't supported
  by the driver: you can only return to that mode if it was the original
  setting, and the configuration interface is missing.

  
  
  Temperature Monitoring
  ----------------------
  
  The PC87427 relies on external sensors (following the SensorPath
  standard), so the resolution and range depend on the type of sensor
  connected. The integer part can be 8-bit or 9-bit, and can be signed or
  not. I couldn't find a way to figure out the external sensor data
  temperature format, so user-space adjustment (typically by a factor 2)
  may be required.