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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:

TACC = ±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.

8.1.3.2 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. Texas Instruments 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 introduced in a specific temperature range of concern

through the use of the Temperature Offset Registers 11HEX and 12HEX.

The user is encouraged to send an e-mail to hardware.monitor.team@nsc.com to further request information

on our recommended setting of the offset register for different processor types.

8.1.4 Computing RPM of the Fan from the TACH Count

The Tach Count Registers 46HEX and 47HEX count the number 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 46HEX and 47HEX. This can best be shown through an example.

Example:

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

Let:

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

The RPM is computed using the formula

)

000

400

Decimal

Count

Tach

Total

RPM

Fan

(6)

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

RPM

Fan

2723

1983

000

400

(7)

www.ti.com

LM64

SNAS207B – MAY 2004 – REVISED JANUARY 2024

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