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editing: vid

VID pin settings
--------------------

The VID (Voltage Identification) pins on sensor chips
are used to read the CPU Core voltage setting.

The VID setting can be controlled by jumpers on the board,
or, in newer motherboards, by settings in the BIOS.
On these newer boards, the BIOS programs some device's
pins which in turn controls a DC-DC Converter to set its
output to a certain voltage. These pins are also connected to
the sensor chip so that the VID setting can be read back
by applications.

There are between 4 and 8 VID pins (the more recent the CPU, the more
numerous the VID pins.) The VID codes are defined by Intel in documents
titled "VRM X.X DC-DC Converter Design Guidelines".
(VRM = Voltage Regulator Module), or
"Voltage Regulator-Down (VRD) X.X Design Guide".
These documents are available at http://developer.intel.com.
There are equivalent documents for AMD processors (look for a table
named "VID Code Voltages" in processor specifications.)

There are several different VRM document versions.
The common versions are as follows:

Document Version	Voltage Range	Increment	Processors
----------------	-------------	---------	----------
"2.4"			0.8 - 1.55	0.025V		AMD Opteron 24x

8.2 (8.1, 8.3)		1.30 - 2.05V    0.05V		PII, PIII, Celeron
			2.1 - 3.5V	0.10V

8.4			1.30 - 2.05V    0.05V		PIII, Celeron
							4 pins only

8.5			1.050 - 1.825V	0.05V		PIII-S Tualatin

9.0, (9.1)		1.100 - 1.850V	0.025V		P4, AMD Socket A

10.0			0.8375 - 1.6000 0.0125V		Desktop Socket 478

"2.4" is not an actual document version but simply a way to identify
AMD Opteron 24x processors.
Note that versions 8.1 - 8.4 are compatible.

Since Linux 2.6.9, the correct VRM version is selected automatically
based on the CPU brand and model. Thus you shouldn't have to care about
it. If you are using a brand new CPU and even the latest kernel complains
("Unknown VRM version of your CPU"), this means that the kernel must be
taught about this new CPU model. Please report to us in this case.

Remember, the VID pins on the sensor chips are inputs only.
You cannot use them to manipulate the actual processor core voltage.

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