linux/Documentation/hwmon/lm78
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   1Kernel driver lm78
   2==================
   3
   4Supported chips:
   5  * National Semiconductor LM78 / LM78-J
   6    Prefix: 'lm78'
   7    Addresses scanned: I2C 0x28 - 0x2f, ISA 0x290 (8 I/O ports)
   8    Datasheet: Publicly available at the National Semiconductor website
   9               http://www.national.com/
  10  * National Semiconductor LM79
  11    Prefix: 'lm79'
  12    Addresses scanned: I2C 0x28 - 0x2f, ISA 0x290 (8 I/O ports)
  13    Datasheet: Publicly available at the National Semiconductor website
  14               http://www.national.com/
  15
  16Author: Frodo Looijaard <frodol@dds.nl>
  17
  18Description
  19-----------
  20
  21This driver implements support for the National Semiconductor LM78, LM78-J
  22and LM79. They are described as 'Microprocessor System Hardware Monitors'.
  23
  24There is almost no difference between the three supported chips. Functionally,
  25the LM78 and LM78-J are exactly identical. The LM79 has one more VID line,
  26which is used to report the lower voltages newer Pentium processors use.
  27From here on, LM7* means either of these three types.
  28
  29The LM7* implements one temperature sensor, three fan rotation speed sensors,
  30seven voltage sensors, VID lines, alarms, and some miscellaneous stuff.
  31
  32Temperatures are measured in degrees Celsius. An alarm is triggered once
  33when the Overtemperature Shutdown limit is crossed; it is triggered again
  34as soon as it drops below the Hysteresis value. A more useful behavior
  35can be found by setting the Hysteresis value to +127 degrees Celsius; in
  36this case, alarms are issued during all the time when the actual temperature
  37is above the Overtemperature Shutdown value. Measurements are guaranteed
  38between -55 and +125 degrees, with a resolution of 1 degree.
  39
  40Fan rotation speeds are reported in RPM (rotations per minute). An alarm is
  41triggered if the rotation speed has dropped below a programmable limit. Fan
  42readings can be divided by a programmable divider (1, 2, 4 or 8) to give
  43the readings more range or accuracy. Not all RPM values can accurately be
  44represented, so some rounding is done. With a divider of 2, the lowest
  45representable value is around 2600 RPM.
  46
  47Voltage sensors (also known as IN sensors) report their values in volts.
  48An alarm is triggered if the voltage has crossed a programmable minimum
  49or maximum limit. Note that minimum in this case always means 'closest to
  50zero'; this is important for negative voltage measurements. All voltage
  51inputs can measure voltages between 0 and 4.08 volts, with a resolution
  52of 0.016 volt.
  53
  54The VID lines encode the core voltage value: the voltage level your processor
  55should work with. This is hardcoded by the mainboard and/or processor itself.
  56It is a value in volts. When it is unconnected, you will often find the
  57value 3.50 V here.
  58
  59If an alarm triggers, it will remain triggered until the hardware register
  60is read at least once. This means that the cause for the alarm may
  61already have disappeared! Note that in the current implementation, all
  62hardware registers are read whenever any data is read (unless it is less
  63than 1.5 seconds since the last update). This means that you can easily
  64miss once-only alarms.
  65
  66The LM7* only updates its values each 1.5 seconds; reading it more often
  67will do no harm, but will return 'old' values.
  68
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