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Thermal Specifications and Design Considerations

When calculating a temperature based on thermal diode measurements, a number of

parameters must be either measured or assumed. Most devices measure the diode

ideality and assume a series resistance and ideality trim value, although some are

capable of also measuring the series resistance. Calculating the temperature is then

accomplished using the equations listed under Table 30. In most temperature sensing

devices, an expected value for the diode ideality is designed-in to the temperature

calculation equation. If the designer of the temperature sensing device assumes a

perfect diode the ideality value (also called ntrim) will be 1.000. Given that most diodes

are not perfect, the designers usually select a ntrim value that more closely matches the

behavior of the diodes in the processor. If the processors diode ideality deviates from

that of ntrim, each calculated temperature will be offset by a fixed amount. This

temperature offset can be calculated with the equation:

Terror(nf) = Tmeasured X (1 – nactual/ntrim)

Where Terror(nf) is the offset in degrees C, Tmeasured is in Kelvin, nactual is the measured

ideality of the diode, and ntrim is the diode ideality assumed by the temperature

sensing device.

To improve the accuracy of diode based temperature measurements, a new register

containing Thermal Diode Offset data has been added to the Celeron D processor.

During manufacturing, each processors thermal diode will be evaluated for its behavior

relative to a theoretical diode. Using the equation above, the temperature error created

by the difference between ntrim and the actual ideality of the particular processor will

be calculated. This Thermal Diode Offset value will be programmed in to the new diode

correction MSR and when added to the Thermal Diode Base value can be used to

correct temperatures read by diode based temperature sensing devices.

If the ntrim value used to calculate the Thermal Diode Offset differs from the ntrim value

used in a temperature sensing device, the Terror(nf) may not be accurate. If desired, the

Thermal Diode Offset can be adjusted by calculating nactual and then recalculating the

offset using the actual ntrim as defined in the temperature sensor manufacturers'

datasheet.

The Diode_Base value and ntrim used to calculate the Diode_Correction_Offset are

listed in Table 32.

§ §

Table 32.

Thermal “Diode” ntrim and Diode_Correction_Offset

Symbol

Parameter

Unit

ntrim

Diode ideality used to calculate Diode_Offset

1.008

Diode_Base

Diode Base

Table 33.

Thermal Diode Interface

Signal Name

Land Number

Signal Description

THERMDA

AL1

diode anode

THERMDC

AK1

diode cathode

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