
82801AA and 82801AB Datasheet
5-51
Functional Description
5.12.4.2
Entering/Exiting the C2 State
Causes of S0/C2 Entry:
From C0 state:
Software reads the Level 2 Register
From C1 state:
STPCLK# goes active. This can only occur if the processor does an autohalt
instruction just after reading the Level 2 register (so that the autohalt instruction completes
prior to the processor recognizing the STPCLK#).
Note that in going to some of the lower power states, the system may appear to pass through the C2
state.
Causes of S0/C2 Exit:
To C0 state:
A Break event occurs. The transition back to the C0 state is qualified by the Stop-
Grant cycle being observed by the ICH. If the break event occurs prior to the Stop-Grant cycle
(associated with the prior assertion of STPCLK#), STPCLK# stays active until the Stop-Grant
cycle is observed.
To C1 state:
If STPCLK# goes inactive. The C1 state persists until an interrupt (regular
interrupt, NMI, SMI#, or INIT#) occurs.
5.12.4.3
Throttling Using STPCLK#
Throttling is used to lower power consumption or reduce heat. The ICH asserts STPCLK# to
throttle the processor clock and the processor appears to temporarily enter a C2 state. After a
programmable time, the ICH 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:
1. 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.
2. 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 ends 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 C2 state, even if Thermal override occurs.