On-Demand Mode; Prochot# Signal Pin; Datasheet - Intel SL8J6 - Pentium 4 Processor Datasheet

Thermal Specifications and Design Considerations
5.2.2

On-Demand Mode

The 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 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 Pin

An external signal, PROCHOT# (processor hot), is asserted when the processor die 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.
The processor implements a bi-directional PROCHOT# capability to allow system designs to
protect various components from over-temperature situations. The PROCHOT# signal is bi-
directional in that it can either signal when the processor has reached its maximum operating
temperature or be driven from an external source to activate the TCC. The ability to activate the
TCC via PROCHOT# can provide a means for thermal protection of system components.
One application is the thermal protection of voltage regulators (VR). System designers can create a
circuit to monitor the VR temperature and activate the TCC when the temperature limit of the VR
is reached. By asserting PROCHOT# (pulled-low) and activating the TCC, the VR can cool down
as a result of reduced processor power consumption. Bi-directional PROCHOT# can allow VR
thermal designs to target maximum sustained current instead of maximum current. Systems should
still provide proper cooling for the VR, and rely on bi-directional PROCHOT# only as a backup in
case of system cooling failure. The system thermal design should allow the power delivery
circuitry to operate within its temperature specification even while the processor is operating at its
Thermal Design Power. With a properly designed and characterized thermal solution, it is
anticipated that bi-directional PROCHOT# would only be asserted for very short periods of time
when running the most power intensive applications. An under-designed thermal solution that is
not able to prevent excessive assertion of PROCHOT# in the anticipated ambient environment may
cause a noticeable performance loss. Refer to the Intel
Platform Design Guide and the Voltage Regulator-Down (VRD) 10.0 Design Guidelines for
Desktop Socket 478 for details on implementing the bi-directional PROCHOT# feature.
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865G/865GV/865PE/865P Chipset

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