LM64 datasheet(27/29 Pages) NSC

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3.0 Application Notes (Continued)

For example, if a processor manufacturer specifies a ±0.1%

variation in

η from part to part. As an example, assume that

a temperature sensor has an accuracy specification of ±1˚C

at room temperature of 25˚C. The resulting accuracy will be:

ACC = ±1˚C+(±0.1% of 298˚K) = ±1.3˚C

The additional inaccuracy in the temperature measurement

caused by

η, can be eliminated if each temperature sensor is

calibrated with the remote diode that it will be paired with.

Refer to the processor datasheet for the non-ideality factor.

3.3.2 Compensating for Diode Non-Ideality

In order to compensate for the errors introduced by non-

ideality, the temperature sensor is calibrated for a particular

processor. National Semiconductor temperature sensors are

always calibrated to the typical non-ideality of a particular

processor type.

The LM64 is calibrated for a MMBT3904 diode-connected

transistor.

When a temperature sensor, calibrated for a specific type of

processor is used with a different processor type or a given

processor type has a non-ideality that strays form the typical

value, errors are introduced.

Temperature errors associated with non-ideality may be in-

troduced in a specific temperature range of concern through

the use of the Temperature Offset Registers 11

HEX and

HEX.

The

user

encouraged

send

e-mail

hardware.monitor.team@nsc.com to further request infor-

mation on our recommended setting of the offset register for

different processor types.

3.4 COMPUTING RPM OF THE FAN FROM THE TACH

COUNT

The Tach Count Registers 46

HEX and 47HEX count the num-

ber of periods of the 90 kHz tachometer clock in the LM64 for

the tachometer input from the fan assuming a 2 pulse per

revolution fan tachometer, such as the fans supplied with the

Pentium 4 boxed processors. The RPM of the fan can be

computed from the Tach Count Registers 46

HEX and 47HEX.

This can best be shown through an example.

Example:

Given: the fan used has a tachometer output with 2 per

revolution.

Let:

HEX = Decimal (11 x 16) + 15 = 191

and

HEX = Decimal (7 x 256) = 1792.

The total Tach Count, in decimal, is 191 + 1792 = 1983.

The RPM is computed using the formula

where

f = 1 for 2 pulses/rev fan tachometer output;

f = 2 for 1 pulse/rev fan tachometer output, and

f = 2 / 3 for 3 pulses/rev fan tachometer output

For our example

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