hwmon: (adt7470) Add documentation

Kernel driver adt7470

=====================

Supported chips:

  * Analog Devices ADT7470

    Prefix: 'adt7470'

    Addresses scanned: I2C 0x2C, 0x2E, 0x2F

    Datasheet: Publicly available at the Analog Devices website

Author: Darrick J. Wong

Description

-----------

This driver implements support for the Analog Devices ADT7470 chip.  There may

be other chips that implement this interface.

The ADT7470 uses the 2-wire interface compatible with the SMBus 2.0

specification. Using an analog to digital converter it measures up to ten (10)

external temperatures. It has four (4) 16-bit counters for measuring fan speed.

There are four (4) PWM outputs that can be used to control fan speed.

A sophisticated control system for the PWM outputs is designed into the ADT7470

that allows fan speed to be adjusted automatically based on any of the ten

temperature sensors. Each PWM output is individually adjustable and

programmable. Once configured, the ADT7470 will adjust the PWM outputs in

response to the measured temperatures with further host intervention.  This

feature can also be disabled for manual control of the PWM's.

Each of the measured inputs (temperature, fan speed) has corresponding high/low

limit values. The ADT7470 will signal an ALARM if any measured value exceeds

either limit.

The ADT7470 DOES NOT sample all inputs continuously.  A single pin on the

ADT7470 is connected to a multitude of thermal diodes, but the chip must be

instructed explicitly to read the multitude of diodes.  If you want to use

automatic fan control mode, you must manually read any of the temperature

sensors or the fan control algorithm will not run.  The chip WILL NOT DO THIS

AUTOMATICALLY; this must be done from userspace.  This may be a bug in the chip

design, given that many other AD chips take care of this.  The driver will not

read the registers more often than once every 5 seconds.  Further,

configuration data is only read once per minute.

Special Features

----------------

The ADT7470 has a 8-bit ADC and is capable of measuring temperatures with 1

degC resolution.

The Analog Devices datasheet is very detailed and describes a procedure for

determining an optimal configuration for the automatic PWM control.

Configuration Notes

-------------------

Besides standard interfaces driver adds the following:

* PWM Control

* pwm#_auto_point1_pwm and pwm#_auto_point1_temp and

* pwm#_auto_point2_pwm and pwm#_auto_point2_temp -

point1: Set the pwm speed at a lower temperature bound.

point2: Set the pwm speed at a higher temperature bound.

The ADT7470 will scale the pwm between the lower and higher pwm speed when

the temperature is between the two temperature boundaries.  PWM values range

from 0 (off) to 255 (full speed).  Fan speed will be set to maximum when the

temperature sensor associated with the PWM control exceeds

pwm#_auto_point2_temp.

Notes

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As stated above, the temperature inputs must be read periodically from

userspace in order for the automatic pwm algorithm to run.