GXLV Datasheet, PDF (176/247 Pages) National Semiconductor (TI) – Geode™ GXLV Processor Series Low Power Integrated x86 Solutions

5.0 Power Management
Power consumption in a GXLV processor based system is
managed with the use of both of hardware and software.
The complete hardware solution is provided for only when
the GXLV processor is combined with a Geode I/O com-
panion such as the CS5530.
The GXLV processor power consumption is managed pri-
marily through a sophisticated clock stop management
technology. The GXLV processor also provides the hard-
ware enablers from which the complete power manage-
ment solution depends on.
Typically the three greatest power consumers in a battery
powered device are the display, the hard drive (if it has
one) and the CPU. Managing power for the first two is rel-
atively straightforward and is discussed in the CS5530 I/O
companion data book. Managing CPU power is more diffi-
cult since effective use of the clock stop technology
requires effective detection of inactivity, both at a system
level and at a code processing level.
Basically two methods are supported to manage power
during periods of inactivity. The first method, called activ-
ity based power management allows the hardware in the
Geode I/O companion to monitor activity to certain
devices in the system and if a period of inactivity occurs
take some form of power conservation action. This
method does not require OS support because this sup-
port is handled by SMM software. Simple monitoring of
external activity is imperfect as well as inefficient. The
second method, called passive power management,
requires the OS to take the active role in managing power.
National supports two application programming interfaces
(APIs) to enable power management by the OS:
Advanced Power Management (APM) and Advanced
Configuration and Power Interface (ACPI). These two
methods can be used independent of one another or they
can be used together. The extent to which these
resources are employed depends on the application and
the discretion of the system designer.
The GXLV processor and Geode I/O companion chips
contain advanced power management features for reduc-
ing the power consumption of the processor in the sys-
tem.
5.1 POWER MANAGEMENT FEATURES
The GXLV processor based system supports the following
power management features:
• GXLV processor hardware
- System Management Mode (SMM)
- Suspend-on-Halt
- CPU Suspend
- 3 Volt Suspend
- GXLV Processor Serial Bus
• Geode I/O companion hardware:
- I/O activity monitoring
- SMI generation
- CPU Suspend control
- Suspend Modulation
- 3 Volt Suspend
- ACPI hardware
• Software:
- API for APM aware OS
- API for ACPI aware OS
- PM VSA for not PM aware OS’s
Geode I/O companion power management support is dis-
cussed in this specification only when necessary to better
explain the GXLV processor’s power management fea-
tures.
Software support of power management is discussed in
this specification only when necessary to better explain
the GXLV processor’s power management features.
5.1.1 System Management Mode
The GXLV processor has an operation mode called Sys-
tem Management Mode. This mode is generally entered
when the SMI# pin goes active. SMM is explained in Sec-
tion 3.7 “System Management Mode” on page 83. If active
power management is desired, then the Geode I/O com-
panion is programmed at boot time to activate SMM
through the SMI# pin due to specific I/O inactivity.
SMM is also used in the passive power management
method, however, it is limited to supporting specific API
calls such as entering sleep modes.
5.1.2 Suspend-on-Halt
Suspend-on-Halt is the most effective power reducing fea-
ture of the GXLV processor with the system active. This
feature allows the system to reduce power when the sys-
tem’s OS becomes idle without producing any delay when
the system’s OS becomes active.
When entered, Suspend-on-Halt stops the clock to the
processor core while the intergrated functions (graphics,
memory controller, PCI controller) are still active. There is
absolutely no observational evidence that the processor
has changed operational behavior except for two things.
The GXLV draws significantly less core power and the
SUSPA# pin is active while in this state.
5.1.3 CPU Suspend
CPU Suspend is a hardware initiated power management
state. The SUSP# pin is asserted by external hardware
such as an Geode I/O companion. The GXLV processor
asserts the SUSPA# pin to indicate that the processor has
entered CPU Suspend. This state is similar to Suspend-
on-Halt except for its entry and exit method. SUSP# active
causes the processor to enter the state and SUSP# inac-
tive causes its exit. The power savings is identical to Sus-
pend-on-Halt. Also, as in Suspend-on-Halt, the processor
will temporally disable CPU Suspend when there is PCI
master activity.
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