80 Datasheet
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
processor even when the TCC is active continuously. Refer to the Intel® Pentium® D Processor
and Intel® Pentium® Processor Extreme Edition 840 Thermal and Mechanical 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 On-Demand Mode
The Pentium D processor provides an auxiliary mechanism that allows system software to force the
processor to reduce its power consumption. This mechanism is referred to as "On-Demand" mode
and is distinct from the Thermal Monitor feature. On-Demand mode is intended as a means to
reduce system level power consumption. Systems using the Pentium D processor must not rely on
software usage of this mechanism to limit the processor temperature.
If bit 4 of the ACPI P_CNT Control Register (located in the processor IA32_THERM_CONTROL
MSR) is written to a '1', the processor will immediately reduce its power consumption via
modulation (starting and stopping) of the internal core clock, independent of the processor
temperature. When using On-Demand mode, the duty cycle of the clock modulation is
programmable via bits 3:1 of the same ACPI P_CNT Control Register. In On-Demand mode, the
duty cycle can be programmed from 12.5% on/ 87.5% off, to 87.5% on/12.5% off in 12.5%
increments. On-Demand mode may be used in conjunction with the Thermal Monitor. If the system
tries to enable On-Demand mode at the same time the TCC is engaged, the factory configured duty
cycle of the TCC will override the duty cycle selected by the On-Demand mode.
5.2.3 PROCHOT# Signal
An external signal, PROCHOT# (processor hot), is asserted when the processor core temperature
has reached its maximum operating temperature. If the Thermal Monitor is enabled (note that the
Thermal Monitor must be enabled for the processor to be operating within specification), the TCC
will be active when PROCHOT# is asserted. The processor can be configured to generate an
interrupt upon the assertion or de-assertion of PROCHOT#. Refer to the Intel Architecture
Software Developer's Manuals for specific register and programming details.
For the Pentium D processor, PROCHOT# can be configured via BIOS as an output or a bi-
directional signal. As an output, PROCHOT# (Processor Hot) will go active when the processor
temperature monitoring sensor detects that one or both cores has reached its maximum safe
operating temperature. This indicates that the processor Thermal Control Circuit (TCC) has been
activated, if enabled. As an input, assertion of PROCHOT# by the system will activate the TCC, if
enabled, for both cores. The TCC will remain active until the system de-asserts PROCHOT#.
If PROCHOT# is configured as an output only, the FORCEPR# signal can be driven from an
external source to activate the TCC. This will prevent one core from asserting the PROCHOT#
signal of the other core and unnecessarily activating the TCC of that core. Refer to Section 5.2.4
for details on the FORCEPR# signal.