Thermal Specifications and Design Considerations
With a properly designed and characterized thermal solution, it is anticipated that the TCC would
only be activated for very short periods of time when running the most power intensive
applications. The processor performance impact due to these brief periods of TCC activation is
expected to be so minor that it would be immeasurable. An under-designed thermal solution that is
not able to prevent excessive activation of the TCC in the anticipated ambient environment may
cause a noticeable performance loss, and in some cases may result in a TC that exceeds the
specified maximum temperature and may affect the long-term reliability of the processor. In
addition, a thermal solution that is significantly under-designed may not be capable of cooling the
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processor even when the TCC is active continuously. Refer to the Intel Pentium 4 Processor on
90 nm Process in the 775-Land Package Thermal Design Guidelines for information on designing
a thermal solution.
The duty cycle for the TCC, when activated by the Thermal Monitor, is factory configured and
cannot be modified. The Thermal Monitor does not require any additional hardware, software
drivers, or interrupt handling routines.
5.2.2
Thermal Monitor 2
The Pentium 4 processor in the 775-land package also supports a power management capability
known as Thermal Monitor 2. This mechanism provides an efficient mechanism for limiting the
processor temperature by reducing power consumption within the processor.
When Thermal Monitor 2 is enabled, and a high temperature situation is detected, the enhanced
Thermal Control Circuit (TCC) will be activated. This enhanced TCC causes the processor to
adjust its operating frequency (bus multiplier) and input voltage (VID). This combination of
reduced frequency and VID results in a decrease in processor power consumption.
A processor enabled for Thermal Monitor 2 includes two operating points, each consisting of a
specific operating frequency and voltage. The first point represents the normal operating conditions
for the processor.
The second point consists of both a lower operating frequency and voltage. When the enhanced
TCC is activated, the processor automatically transitions to the new frequency. This transition
occurs very rapidly (on the order of 5 µs). During the frequency transition, the processor is unable
to service any bus requests, and consequently, all bus traffic is blocked. Edge-triggered interrupts
will be latched and kept pending until the processor resumes operation at the new frequency.
Once the new operating frequency is engaged, the processor will transition to the new core
operating voltage by issuing a new VID code to the voltage regulator. The voltage regulator must
support VID transitions in order to support Thermal Monitor 2. During the voltage change, it will
be necessary to transition through multiple VID codes to reach the target operating voltage. Each
step will be one VID table entry (i.e., 12.5 mV steps). The processor continues to execute
instructions during the voltage transition. Operation at this lower voltage reduces both the dynamic
and leakage power consumption of the processor, providing a reduction in power consumption at a
minimum performance impact.
Once the processor has sufficiently cooled, and a minimum activation time has expired, the
operating frequency and voltage transition back to the normal system operating point. Transition of
the VID code will occur first, to insure proper operation once the processor reaches its normal
operating frequency. Refer to Figure 5-4 for an illustration of this ordering.
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