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Thermal Specifications and Design Considerations

The PROCHOT# signal is asserted when a high temperature situation is detected, regardless of

whether or not Thermal Monitor or Thermal Monitor 2 is enabled.

It should be noted that the Thermal Monitor 2 TCC can not be activated via the on demand mode.

The Thermal Monitor TCC, however, can be activated through the use of the on demand mode.

5.2.3

On-Demand Mode

The Pentium 4 processor in the 775-land package 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 4 processor in the 775-land package 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.

Figure 5-4. Thermal Monitor 2 Frequency and Voltage Ordering

VID

Frequency

Temperature

TM2

MAX

TM2

VID

TM2

PROCHOT#

Time

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