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

Thermal Specifications
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Intel
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Pentium
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4 Processor Thermal Design Guide 35
An ACPI register, performance counter registers, status bits in model specific register (MSR), and
the PROCHOT# output pin are available to monitor and control the Thermal Monitor.
Figure 17. Concept for Clocks under Thermal Monitor Control
PROCHOT#
Resultant
internal clock
Normal clock
Internal clock
Duty cycle
control
3.4.3 Bi-Directional PROCHOT#
The Pentium 4 processor with 512-KB L2 cache on 0.13 micron process also implements a bi-
directional PROCHOT# capability to allow systems to protect Voltage Regulators (VRs) 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 feature is intended to offer thermal protection for VRs designed to
handle maximum sustain current instead of maximum theoretical current. PROCHOT# should be
asserted (pulled low) externally to activate the TCC when the VR thermal limits are reached,
thereby allowing the VR to cool down with reduced processor power consumption. Systems
should still provide proper cooling for the VRs, and rely on bi-directional PROCHOT# only as a
backup in case of system cooling failure.
3.4.4 Operation and Configuration
To maintain compatibility with previous generations of processors, which have no integrated
thermal logic, the thermal control circuit portion of Thermal Monitor is disabled by default.
During the power-on process, the BIOS must enable the thermal control circuit; or a software
driver may do this after the operating system has booted.
The thermal control circuit feature can be configured and monitored in a number of ways. OEMs
are expected to enable the thermal control circuit while using various registers and outputs to
monitor the processor thermal status. The thermal control circuit is enabled by BIOS setting a bit
in an MSR (model specific register). Enabling the thermal control circuit allows the processor to
maintain a safe operating temperature without the need for special software drivers or interrupt
handling routines. When the thermal control circuit has been enabled, processor power
consumption will be reduced within a few hundred clock cycles after the thermal sensor detects a
high temperature (i.e., within a few hundred clock cycles of PROCHOT# assertion). The thermal
control circuit and PROCHOT# go inactive once the temperature has been brought back down
below the thermal trip point, although a small hysteresis (~1 °C) has been included to prevent