Thermal Specifications and Design Considerations
5.2
Processor Thermal Features
5.2.1
Thermal Monitor
The Thermal Monitor feature helps control the processor temperature by activating the TCC when
the processor silicon reaches its maximum operating temperature. The TCC reduces processor
power consumption as needed by modulating (starting and stopping) the internal processor core
clocks. The Thermal Monitor feature must be enabled for the processor to be operating
within specifications. The temperature at which Thermal Monitor activates the thermal control
circuit is not user configurable and is not software visible. Bus traffic is snooped in the normal
manner, and interrupt requests are latched (and serviced during the time that the clocks are on)
while the TCC is active.
When the Thermal Monitor feature is enabled, and a high temperature situation exists (i.e., TCC is
active), the clocks will be modulated by alternately turning the clocks off and on at a duty cycle
specific to the processor (typically 30–50%). Clocks often will not be off for more than
3.0 microseconds when the TCC is active. Cycle times are processor speed dependent and will
decrease as processor core frequencies increase. A small amount of hysteresis has been included to
prevent rapid active/inactive transitions of the TCC when the processor temperature is near its
maximum operating temperature. Once the temperature has dropped below the maximum
operating temperature, and the hysteresis timer has expired, the TCC goes inactive and clock
modulation ceases.
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 applicable thermal design guide
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 Celeron 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 Celeron 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%
Datasheet
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INTEL [ INTEL ]
