Thermal Management Logic and Thermal Monitor Feature
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Thermal/Mechanical Design Guide 35
For information regarding THERMTRIP#, refer to the processor datasheet and to Section
4.2.8 of
this Thermal Design Guidelines.
4.2.6 Operating System and Application Software
Considerations
The Thermal Monitor feature and its thermal control circuit work seamlessly with ACPI
compliant operating systems. The Thermal Monitor feature is transparent to application software
since the processor bus snooping, ACPI timer, and interrupts are active at all times.
4.2.7 On-Die Thermal Diode
There are two independent thermal sensing devices in the Pentium 4 processor in the 775–land
LGA package. One is the on-die thermal diode and the other is in the temperature sensor used for
the Thermal Monitor and for THERMTRIP#. The Thermal Monitor’s temperature sensor and the
on-die thermal diode are independent and physically isolated devices. Circuit constraints and
performance requirements prevent the Thermal Monitor’s temperature sensor and the on-die
thermal diode from being located at the same place on the silicon. The temperature distribution
across the die may result in significant temperature differences between the on-die thermal diode
and the Thermal Monitor’s temperature sensor. This temperature variability across the die is
highly dependent on the application being run. As a result, it is not possible to predict the
activation of the thermal control circuit by monitoring the on-die thermal diode.
System integrators should note that there is no defined correlation between the on-die thermal
diode and the processor case temperature. The temperature distribution across the die is affected
by the power being dissipated, type of activity the processor is performing (e.g., integer or
floating point intensive), and the leakage current. The dynamic and independent nature of these
effects makes it difficult to provide a meaningful correlation for the processor population.
System integrators that plan on using the thermal diode for system or component level fan control
to optimize acoustics need to refer to Acoustic Fan Control, Section
6.1.
4.2.7.1 Reading the On-Die Thermal Diode Interface
The on-die thermal diode is accessible from a pair of pins on the processor. The fan speed
controller remote thermal sense signals should be connected to these pins per the vendor’s
recommended layout guidelines.
Table 1. Thermal Diode Interface
Pin Name Pin Number Pin Description
THERMDA
AL1 Diode anode
THERMDC
AK1 Diode anode
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