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4.2.3
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 boot process, the BIOS must enable the Thermal Control Circuit.
Note: Thermal Monitor must be enabled to ensure proper processor operation.
The Thermal Control Circuit feature can be configured and monitored in a number of ways. OEMs
are required 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 the BIOS setting a
bit in an MSR (model specific register). Enabling the Thermal Control Circuit allows the processor
to attempt 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. PROCHOT# assertion. The Thermal Control Circuit and PROCHOT#
transition to inactive once the temperature has been reduced below the thermal trip point, although
a small time-based hysteresis has been included to prevent 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.
The power reduction mechanism of thermal monitor can also be activated manually using an
on-demand mode. Refer to
4.2.4
On-Demand Mode
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 thermal solution investigations or for performance
implication studies. When using the MSRs to activate the on-demand clock modulation 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 s. 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 1/4 (25%) is selected, the clock-on time would be reduced to approximately 1 µs
[on time (1 µs) ÷ total cycle time (3 + 1) µs = 1/4 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) = 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.
®
®
Intel
Celeron
D Processor in the 775-Land LGA Package Thermal Design Guide25
303730
Order #
Thermal Management Logic and Thermal Monitor
Section 4.2.4
for details on this feature.
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