Functional Description
5-72
Intel
82801BA ICH2 Datasheet
5.12.5
Dynamic Processor Clock Control
ICH2 has extensive control for dynamically starting and stopping system clocks. The clock control
is used for transitions among the various S0/Cx states and processor throttling. Each dynamic clock
control method is described in this section. The various Sleep states may also perform types of non-
dynamic clock control.
The ICH2 supports the ACPI C0, C1 and C2 states.
The dynamic processor clock control is handled using the following signals:
STPCLK#: Used to halt processor instruction stream.
CPUSLP#: Must be asserted prior to STP_CPU# (in Stop Grant mode)
The C1 state is entered based on the processor performing an autohalt instruction. The C2 state is
entered based on the processor reading the Level 2 register in the ICH2.
A C1 or C2 state ends due to a break event. Based on the break event, the ICH2 returns the system
to C0 state.
Table 5-41
lists the possible break events from C2. The break events from C1 are
indicated in the processor’s datasheet.
5.12.5.1
Throttling Using STPCLK#
Throttling is used to lower power consumption or reduce heat. The ICH2 asserts STPCLK# to
throttle the processor clock and the processor appears to temporarily enter a C2 state. After a
programmable time, the ICH2 deasserts STPCLK# and the processor appears to return to the C0
state. This allows the processor to operate at reduced average power, with a corresponding
decrease in performance. Two methods are included to start throttling:
Software enables a timer with a programmable duty cycle. The duty cycle is set by the
THTL_DTY field and the throttling is enabled using the THTL_EN field. This is known as
Manual Throttling. The period is fixed to be in the non-audible range, due to the nature of
switching power supplies.
A Thermal Override condition (THRM# signal active for >2 seconds) occurs that
unconditionally forces throttling, independent of the THTL_EN bit. The throttling due to
Thermal Override has a separate duty cycle (THRM_DTY) which may vary by field and
system. The Thermal Override condition will end when THRM# goes inactive.
Throttling due to the THRM# signal has higher priority than the software initiated throttling.
Throttling does not occur when the system is in a C2 state, even if Thermal override occurs.
Table 5-41. Break Events
Event
Breaks from
Comment
Any unmasked interrupt goes
active
C2
IRQ[0:15] when using the 8259s, IRQ[0:23] for I/O APIC.
Since SCI is an interrupt, any SCI will also be a break event.
Any internal event that will
cause an NMI or SMI#
C2
Many possible sources
Any internal event that will
cause INIT# to go active
C2
Could be indicated by the keyboard controller via the RCIN
input signal.
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