Intel Pentium 4 Processor with 512-KB L2 Cache on 0.13 Micron Process Thermal Design Guidelines

Thermal Specifications
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36 Intel
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Pentium
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4 Processor Thermal Design Guide
multiple PROCHOT# transitions around the trip point. External hardware can monitor
PROCHOT# and generate an interrupt whenever there is a transition from active-to-inactive or
inactive-to-active. PROCHOT# can also be configured to generate an internal interrupt which
would initiate an OEM supplied interrupt service routine. Regardless of the configuration selected,
PROCHOT# will always indicate the thermal status of the processor.
For testing purposes, the thermal control circuit may also be activated by setting bits in the ACPI
MSRs. The MSRs may be set based on a particular system event (e.g., an interrupt generated after
a system event), or may be set at any time through the operating system or custom driver control
thus forcing the thermal control circuit on. This is referred to as “on-demand” mode. Activating
the thermal control circuit may be useful for cooling solution investigations or for performance
implication studies. When using the MSRs to activate the Thermal Monitor feature, the duty cycle
is configurable in steps of 12.5%, from 12.5% to 87.5%.
For any duty cycle, the maximum time period the clocks are disabled is ~3 µs. This time period is
frequency dependent, and decreases as frequency increases. To achieve different duty cycles, the
length of time that the clocks are disabled remains constant, and the time period that the clocks are
enabled is adjusted to achieve the desired ratio. For example, if the clock disable period is 3 µs,
and a duty cycle of ¼ (25%) is selected, the clock on time would be reduced to approximately 1 µs
[on time (1 µs) ÷ total cycle time (3 + 1) µs = ¼ duty cycle]. Similarly, for a duty cycle of 7/8
(87.5%), the clock on time would be extended to 21 µs [21 ÷ (21 + 3) µs = 7/8 duty cycle].
In a high temperature situation, if the thermal control circuit and ACPI MSRs (automatic and on-
demand modes) are used simultaneously, the fixed duty cycle determined by automatic mode
would take precedence.
3.4.5 System Considerations
The Thermal Monitor feature may be used in a variety of ways, depending upon the system design
requirements and capabilities. Intel requires the thermal control circuit to be enabled for all Intel
Pentium 4 processor with 512-KB L2 cache on 0.13 micron process based systems. At a
minimum, the thermal control circuit supplies an added level of protection against processor over-
temperature failure.
To minimize the cost of a processor thermal solution, system designers are encouraged to take
advantage of the Thermal Monitor feature capability. The Thermal Monitor feature allows
processor thermal solutions to design to the thermal design power (TDP) target, as opposed to
maximum processor power consumption. Designing to the lower TDP target results in a lower
thermal solution cost, while still maintaining a level of processor performance that is virtually
indistinguishable from systems designed to manage maximum power dissipation levels. Each
application program has its own unique power profile, although the profile has some variability
due to loop decisions, I/O activity and interrupts. In general, compute intensive applications with a
high cache hit rate dissipate more processor power than applications that are I/O intensive or have
low cache hit rates.
The processor thermal design power (TDP) is based on measurements of processor power
consumption while running various high power applications. This data is used to determine those
applications that are interesting from a power perspective. These applications are then evaluated in
a controlled thermal environment to determine their sensitivity to activation of the thermal control
circuit. This data is used to derive the TDP targets published in the processor datasheet.