GX1 Datasheet, PDF (177/247 Pages) National Semiconductor (TI) – Processor Series Low Power Integrated x86 Solution

5.0 Power Management
Power consumption in a GX1 processor based system is
managed with the use of both of hardware and software.
The complete hardware solution is provided for only when
the GX1 processor is combined with a Geode I/O compan-
ion such as the CS5530.
The GX1 processor power consumption is managed prima-
rily through a sophisticated clock stop management tech-
nology. The GX1 processor also provides the hardware
enablers from which the complete power management
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 relatively
straightforward and is discussed in the CS5530 I/O com-
panion data book. Managing CPU power is more difficult
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 activity
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 support 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 appli-
cation 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 appli-
cation and the discretion of the system designer.
The GX1 processor and Geode I/O companion chips con-
tain advanced power management features for reducing
the power consumption of the processor in the system.
5.1 POWER MANAGEMENT FEATURES
The GX1 processor based system supports the following
power management features:
• GX1 processor hardware
— System Management Mode (SMM)
— Suspend-on-Halt
— CPU Suspend
— 3 Volt Suspend
— GX1 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 GX1 processor’s power management features.
Software support of power management is discussed in
this specification only when necessary to better explain the
GX1 processor’s power management features.
5.1.1 System Management Mode
The GX1 processor has an operation mode called System
Management Mode. This mode is generally entered when
the SMI# pin goes active. SMM is explained in Section 3.7
“System Management Mode” on page 83. If active power
management is desired, then the Geode I/O companion 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 GX1 processor with the system active. This fea-
ture allows the system to reduce power when the system’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 pro-
cessor core while the intergrated functions (graphics, mem-
ory controller, PCI controller) are still active. There is
absolutely no observational evidence that the processor
has changed operational behavior except for two things.
The GX1 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 GX1 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.
CPU Suspend can be used for Suspend Modulation. The
Geode I/O companion can be programmed to assert/deas-
sert SUSP# at a programmable frequency and duty cycle.
This has the effect of reducing the average frequency that
the processor is running and thus reduces power con-
sumption and performance. Certain processing activities
(SMI#, Interrupts, and VGA activity) can be monitored by
the Geode I/O companion to temporarily suspend, Sus-
pend Modulation for a programmable amount of time. Sus-
pend modulation programming is explained in detail in the
Geode I/O companion data books such as the CS5530.
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