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Functional Description
5-60
82801AA and 82801AB Datasheet
5.12.8.3
Read Only Registers with Write Paths in Alternate Access Mode
The registers described in
Table 5-46
have write paths to them in alternate access mode. Software
restores these values after returning from a powered down state. These registers must be handled
special by software. When in normal mode, writing to the base address/count register also writes to
the current address/count register. Therefore, the base address/count must be written first, then the
part is put into alternate access mode and the current address/count register is written.
5.12.9
System Power Supplies, Planes, and Signals
Power Plane Control with SLP_S3# and SLP_S5#
The SLP_S3# output signal can be used to cut power to the system core supply, since it only goes
active for the STR state (typically mapped to ACPI S3). Power must be maintained to the ICH
Resume Well, and to any other circuits that need to generate Wake signals from the STR state.
Cutting power to the core may be done via the power supply, or by external FETs to the
motherboard. The SLP_S5# output signal can be used to cut power to the system core supply, as
well as power to the system memory, since the context of the system is saved on the disk. Cutting
power to the memory may be done via the power supply, or by external FETs to the motherboard.
PWROK Signal
The PWROK input should go active based on the core supply voltages becoming valid. PWROK
should go high at least 16 ms after the power is guaranteed valid.
Note:
Traditional designs have a reset button logically AND’d with the PWROK signal from the power
supply and the processor’s voltage regulator module. If this is done with the ICH, the
PWROK_FLR bit is set. The ICH treats this internally as if the RSMRST# signal had gone active.
However, it is not treated as a full power failure. If PWROK goes inactive and then active (but
RSMRST# stays high), the ICH will reboot (regardless of the state of the AFTERG3 bit). If the
RSMRST# signal also goes low before PWROK goes high, this is a full power failure, and the
reboot policy is controlled by the AFTERG3 bit.
Table 5-46. Register Write Accesses in Alternate Access Mode
I/O Address
Register Write Value
08h
DMA Status Register for channel’s 0-3.
D0h
DMA Status Register for channel’s 4-7.