hwmon: (adt7475) Voltage attenuators can be bypassed

[deliverable/linux.git] / Documentation / hwmon / adt7475

Kernel driver adt7475
=====================

Supported chips:
  * Analog Devices ADT7473
    Prefix: 'adt7473'
    Addresses scanned: I2C 0x2C, 0x2D, 0x2E
    Datasheet: Publicly available at the On Semiconductors website
  * Analog Devices ADT7475
    Prefix: 'adt7475'
    Addresses scanned: I2C 0x2E
    Datasheet: Publicly available at the On Semiconductors website
  * Analog Devices ADT7490
    Prefix: 'adt7490'
    Addresses scanned: I2C 0x2C, 0x2D, 0x2E
    Datasheet: Publicly available at the On Semiconductors website

Authors:
	Jordan Crouse
	Hans de Goede
	Darrick J. Wong (documentation)
	Jean Delvare


Description
-----------

This driver implements support for the Analog Devices ADT7473, ADT7475 and
ADT7490 chip family. The ADT7473 and ADT7475 differ only in minor details.
The ADT7490 has additional features, including extra voltage measurement
inputs and PECI support. All the supported chips will be collectively
designed by the name "ADT747x" in the rest of this document.

The ADT747x uses the 2-wire interface compatible with the SMBus 2.0
specification. Using an analog to digital converter it measures three (3)
temperatures and two (2) or more voltages. It has four (4) 16-bit counters
for measuring fan speed. There are three (3) PWM outputs that can be used
to control fan speed.

A sophisticated control system for the PWM outputs is designed into the
ADT747x that allows fan speed to be adjusted automatically based on any of the
three temperature sensors. Each PWM output is individually adjustable and
programmable. Once configured, the ADT747x will adjust the PWM outputs in
response to the measured temperatures without further host intervention.
This feature can also be disabled for manual control of the PWM's.

Each of the measured inputs (voltage, temperature, fan speed) has
corresponding high/low limit values. The ADT747x will signal an ALARM if
any measured value exceeds either limit.

The ADT747x samples all inputs continuously. The driver will not read
the registers more often than once every other second. Further,
configuration data is only read once per minute.

Chip Differences Summary
------------------------

ADT7473:
  * 2 voltage inputs
  * system acoustics optimizations (not implemented)

ADT7475:
  * 2 voltage inputs

ADT7490:
  * 6 voltage inputs
  * 1 Imon input (not implemented)
  * PECI support (not implemented)
  * 2 GPIO pins (not implemented)
  * system acoustics optimizations (not implemented)

Special Features
----------------

The ADT747x has a 10-bit ADC and can therefore measure temperatures
with a resolution of 0.25 degree Celsius. Temperature readings can be
configured either for two's complement format or "Offset 64" format,
wherein 64 is subtracted from the raw value to get the temperature value.

The datasheet is very detailed and describes a procedure for determining
an optimal configuration for the automatic PWM control.

Fan Speed Control
-----------------

The driver exposes two trip points per PWM channel.

point1: Set the PWM speed at the lower temperature bound
point2: Set the PWM speed at the higher temperature bound

The ADT747x will scale the PWM linearly between the lower and higher PWM
speed when the temperature is between the two temperature boundaries.
Temperature boundaries are associated to temperature channels rather than
PWM outputs, and a given PWM output can be controlled by several temperature
channels. As a result, the ADT747x may compute more than one PWM value
for a channel at a given time, in which case the maximum value (fastest
fan speed) is applied. PWM values range from 0 (off) to 255 (full speed).

Fan speed may be set to maximum when the temperature sensor associated with
the PWM control exceeds temp#_max.

Notes
-----

The nVidia binary driver presents an ADT7473 chip via an on-card i2c bus.
Unfortunately, they fail to set the i2c adapter class, so this driver may
fail to find the chip until the nvidia driver is patched.