Datasheet 81
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
T
TM2
f
MAX
f
TM2
VID
VID
TM2
PROCHOT#
Time
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