Thermal Monitor controls the processor temperature by modulating the internal processor
core clocks. The processor clocks are modulated when the TCC is activated. Thermal
Monitor uses two modes to activate the TCC: Automatic mode and OnDemand mode.
Automatic mode is required for the processor to operate within specifications and
must first be enabled via BIOS. Once automatic mode is enabled, the TCC will activate
only when the internal die temperature is very near the temperature limits of the
processor. When TCC 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 50%
duty cycle. Clocks will not be off for more than 3 µs when TCC is active. Cycle times are
processor speed dependent and will decrease linearly 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 the trip point. Once the
temperature has returned to a noncritical level, and the hysteresis timer has expired,
modulation ceases and TCC goes inactive. Processor performance will be decrease by
~50% when the TCC is active (assuming a 50% duty cycle), however, with a properly
designed and characterised thermal solution the TCC most likely will only be activated
briefly when the system is near maximum temperature and during the most power
intensive applications.
For automatic mode, the 50% duty cycle is factory configured and cannot be modified.
Also, automatic mode does not require any additional hardware, software drivers or
interrupt handling routines.
The TCC may also be activated via OnDemand mode. If bit 4 of the ACPI Thermal
Monitor Control Register is written to a "1" the TCC will be activated immediately,
independent of the processor temperature. When using OnDemand mode to activate the
TCC, the duty cycle of the clock modulation is programmable via bits 3:1 of the same
ACPI Thermal Monitor Control Register. In automatic mode, the duty cycle is fixed at 50%
on, 50% off, however in OnDemand 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. OnDemand mode may
be used at the same time automatic mode is enabled, however, if the system tries to
enable the TCC via OnDemand mode at the same time automatic mode is enabled AND
a high temperature condition exists, the 50% duty cycle of the automatic mode will
override the duty cycle selected by the OnDemand mode.
An external signal, PROCHOT# (processor hot) is asserted at any time the TCC is active
(either in automatic or OnDemand mode). Bus snooping and interrupt latching are also
active while the TCC is active. The temperature at which the thermal control circuit
activates is not user configurable and is not software visible.
Besides the thermal sensor and thermal control circuit, the Thermal Monitor feature also
includes one ACPI register, one performance counter register, three model specific
registers (MSR), and one I/O pin (PROCHOT#). All are available to monitor and control
the state of the Thermal Monitor feature. Thermal Monitor can be configured to generate
an interrupt upon the assertion or deassertion of PROCHOT# (i.e. upon the activation/
deactivation of TCC).
If automatic mode is disabled the processor will be operating out of specification and
cannot be guaranteed to provide reliable results. Regardless of enabling of the automatic
or OnDemand modes, in the event of a catastrophic cooling failure, the processor will
automatically shut down when the silicon has reached a temperature of approximately
135 °C. At this point the system bus signal THERMTRIP# will go active and stay active
until RESET# has been initiated. THERMTRIP# activation is independent of processor
activity and does not generate any bus cycles. If THERMTRIP# is asserted, processor
core voltage (Vcc) must be removed within the timeframe defined in Table 16.