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Functional Description
5-52
82801AA and 82801AB Datasheet
5.12.4.4
Transition Rules Among S0/Cx and Throttling States
The following priority rules and assumptions apply among the various S0/Cx and throttling states:
Entry to any S0/Cx state is mutually exclusive with entry to any S1-S5 state. This is because
the processor can only perform one register access at a time and Sleep states have higher
priority than thermal throttling.
When the SLP_EN bit is set (system going to a sleep state (S1-S5), the THTL_EN bit can be
internally treated as being disabled (no throttling while going to sleep state). Note that thermal
throttling (based on THRM# signal) cannot be disabled in an S0 state. However, once the
SLP_EN bit is set, the thermal throttling is shut off (since STPCLK# is active in S1-S5 states).
If the THTL_EN bit is set, and a Level 2 read then occurs, the system should immediately go
and stay in a C2 state until a break event occurs. A Level 2 read has higher priority than the
software initiated throttling or thermal throttling. If Thermal Override is causing throttling,
and a Level 2 read then occurs, the system stays in a C2 state until a break event occurs. A
Level 2 read has higher priority than the Thermal Override. However, if THRM# is still active
after a break event occurs, the system goes back to the C0 state and STPCLK# throttles (due to
the THRM# signal).
After an exit from a C2 state (due to a Break event), and if the THTL_EN bit is still set, or if a
Thermal Override is still occurring, the system continues to throttle STPCLK#. Depending on
the time of break event, the first transition on STPCLK# active can be delayed by up to one
period.
If in the C2 state after C1 (Halt), a break event returns the system to C1 state. If a break event
was an interrupt, then the INTR is sent to the processor and system transitions from C1 to C0
state.
5.12.5
Sleep States
5.12.5.1
Sleep State Overview
The ICH directly supports different sleep states (S1-S5), which are entered by setting the SLP_EN
bit, or due to a Power Button Override. The entry to the Sleep states are based on several
assumptions:
Entry to a Cx state is mutually exclusive with entry to a Sleep state. This is because the
processor can only perform one register access at a time. A request to Sleep always has higher
priority than throttling.
Prior to setting the SLP_EN bit, the software turns off processor-controlled throttling. Note
that thermal throttling cannot be disabled, but setting the SLP_EN bit disables thermal
throttling (since S1-S5 sleep state has higher priority).
Upon exit from the ICH-controlled Sleep states, the WAK_STS bit is set to 1.
5.12.5.2
Initiating Sleep State
Sleep states (S1-S5) are initiated by:
Setting the desired type in the SLP_TYP field and setting the SLP_EN bit.
Pressing the PWRBTN# Signal for more than 4 seconds to cause a Power Button Override
event.