Functional Description
178 Datasheet
5.13.9.2 PIC Reserved Bits
Many bits within the PIC are reserved, and must have certain values written in order for
the PIC to operate properly. Therefore, there is no need to return these values in ALT
access mode. When reading PIC registers from 20h and A0h, the reserved bits shall
return the values listed in Table 5-37.
5.13.9.3 Read Only Registers with Write Paths in ALT Access Mode
The registers described in Table 5-38 have write paths to them in ALT 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 ALT
access mode and the current address/count register is written.
5.13.10 System Power Supplies, Planes, and Signals
5.13.10.1 Power Plane Control with SLP_S3#,
SLP_S4#, SLP_S5#, SLP_A# and SLP_LAN#
The SLP_S3# output signal can be used to cut power to the system core supply, since it
only goes active for the Suspend-to-RAM state (typically mapped to ACPI S3). Power
must be maintained to the PCH suspend well, and to any other circuits that need to
generate Wake signals from the Suspend-to-RAM state. During S3 (Suspend-to-RAM)
all signals attached to powered down plans will be tri-stated or driven low, unless they
are pulled using a pull-up resistor.
Cutting power to the core may be done using the power supply, or by external FETs on
the motherboard.
The SLP_S4# or 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 using the power supply,
or by external FETs on the motherboard.
Table 5-37. PIC Reserved Bits Return Values
PIC Reserved Bits Value Returned
ICW2(2:0) 000
ICW4(7:5) 000
ICW4(3:2) 00
ICW4(0) 0
OCW2(4:3) 00
OCW3(7) 0
OCW3(5) Reflects bit 6
OCW3(4:3) 01
Table 5-38. Register Write Accesses in ALT Access Mode
I/O Address Register Write Value
08h DMA Status Register for Channels 0–3
D0h DMA Status Register for Channels 4–7
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