Thermal Specifications
6.2.6
Tcontrol and Fan Speed Reduction
Tcontrol is a temperature specification based on a temperature reading from the
thermal diode. The value for Tcontrol will be calibrated in manufacturing and configured
for each processor. The Tcontrol value is set identically for both processor cores. The
Tcontrol temperature for a given processor can be obtained by reading the
IA32_TEMPERATURE_TARGET MSR in the processor. The Tcontrol value that is read
from the IA32_TEMPERATURE_TARGET MSR must be converted from Hexadecimal to
Decimal and added to a base value of 60° C. The value of Tcontrol may vary from 0x00h
to 0x1Eh.
When Tdiode is above Tcontrol, then TCASE must be at or below TCASE_MAX as defined by
the thermal profile. (Refer to Figure 6-1, Figure 6-2 and Figure 6-3 ; Table 6-2,
Table 6-3, Table 6-5, Table 6-6 and Table 6-8). Otherwise, the processor temperature
can be maintained at or below Tcontrol.
6.2.7
Thermal Diode
The Dual-Core Intel Xeon Processor 5000 series incorporates an on-die PNP transistor
whose base emitter junction is used as a thermal “diode”, one per core, with its
collector shorted to Ground. A thermal sensor located on the system board may
monitor the die temperature of the processor for thermal management and fan speed
control. Table 6-9, Table 6-11 and Table 6-12 provide the “diode” parameters and
interface specifications. Two different sets of “diode” parameters are listed in Table 6-9
and Table 6-11. The Diode Model parameters (Table 6-9) apply to traditional thermal
sensors that use the Diode Equation to determine the processor temperature.
Transistor Model parameters (Table 6-11) have been added to support thermal sensors
that use the transistor equation method. The Transistor Model may provide more
accurate temperature measurements when the diode ideality factor is closer to the
maximum or minimum limits. This thermal “diode” is separate from the Thermal
Monitor’s thermal sensor and cannot be used to predict the behavior of the Thermal
Monitor.
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 by using the equations listed under Table 6-9. 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 an 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. The 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.
In order to improve the accuracy of diode based temperature measurements, a new
register (Tdiode_Offset) has been added to Dual-Core Intel Xeon Processor 5000 series
which will contain thermal diode characterization data. During manufacturing each
processor’s thermal diode will be evaluated for its behavior relative to a theoretical
diode. Using the equation above, the temperature error created by the difference
Dual-Core Intel® Xeon® Processor 5000 Series Datasheet
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