GXM Datasheet, PDF (174/244 Pages) National Semiconductor (TI) – Geode™ GXm Processor Integrated x86 Solution with MMX Support

6.0 Power Management
The power management resources provided by a com-
bined Geode GXm processor and CS5530-based system
have been designed to support a full-featured notebook
implementation. The extent to which these resources are
employed depends on the application and the discretion
of the system designer.
The three greatest power consumers in a notebook sys-
tem are the display, the hard drive and the CPU. Manag-
ing power for the first two is relatively straightforward and
is discussed in the Geode CS5530 I/O companion speci-
fication. Managing CPU power can be more difficult since
detecting inactive (Idle) states by monitoring external
activity is imperfect as well as inefficient.
The GXm processor and CS5530 I/O companion chip
contain the most advanced power management features
for reducing the power consumption of the processor in
the system while delivering the highest performance in
any mobile processor. The GXm processor supports the
following CPU power management features:
• APM Support
• CPU Suspend Command Registers (CS5530)
• Suspend Modulation
• 3 Volt Suspend
• GXm Integrated Processor Serial Bus
6.1 APM SUPPORT
Many notebook computers rely solely on the APM
(Advanced Power Management) driver for DOS, Windows
3.1 and Windows 95 operating systems to manage power
to the CPU. APM provides several services that enhance
the system power management by determining when the
CPU is idle. For the CPU, APM is theoretically the best
approach but there are some drawbacks.
1. APM is an OS-specific driver which may not be avail-
able for some operating systems.
2. Application support is inconsistent. Some applica-
tions in foreground may prevent idle calls.
The components for APM support are:
• Software CPU Suspend control via the CS5530 CPU
Suspend Command Register (ACh).
• Software SMI entry via the Software SMI Register
(D0h). This allows the APM BIOS to be part of the SMI
handler.
6.2 CPU SUSPEND COMMAND REGISTERS
Power management system software can invoke the
SUSP#/SUSPA# protocol with the “CPU Suspend Com-
mand” and the “Suspend Notebook Command” registers
in the CS5530. If the SUSP#/SUSPA# protocol is invoked,
all pending SMIs are serviced and SMI# is deasserted.
Then SUSP# is asserted by the CS5530 and, subse-
quently, SUSPA# is returned by the GXm processor.
When a condition that ends the “Suspend” state exists,
SMI# is re-asserted. At this point, if the PLL in the GXm
processor has not been stopped, then SUSP# is deas-
serted. SUSP# is never deasserted until SUSPA# has
been sampled active (low).
Note:
The SMI# pin is a unidirectional line from the
CS5530 to the GXm processor. It is active low.
When SMI is initiated from a normal mode, the
SMI# pin is asserted low and is held low until the
SMI source is cleared. At that time, SMI# is de-
asserted.
6.3 SUSPEND MODULATION
The hardware provided to support the GXm processor’s
power management works by assuming that the GXm
processor is Idle and reducing power until activity is
detected. Most power management schemes in the indus-
try run the system at full speed until a period of inactivity
is detected. National Semiconductor’s more aggressive
approach yields lower power consumption. When activity
is detected, the GXm processor is instantly converted to
full speed for a programmed duration. This is called Sus-
pend Modulation.
Suspend Modulation acts as backup for cases where
APM doesn’t correctly detect an Idle condition in the sys-
tem. As long as it is enabled, it will only become active in
the background. The “Suspend Modulation Enable Regis-
ter” in the CS5530 enables the Suspend Modulation fea-
ture.
The “Suspend Modulation ON Count Register” in the
CS5530 is an 8-bit counter that represents the number of
32 µs intervals that the SUSP# pin will be asserted to the
GXm processor. This counter, together with the “Suspend
Modulation OFF Count Register” and the IRQ/Video
Speedup Registers, performs the Suspend Modulation
function for GXm processor’s power management. The
ratio of the on count to the off count sets up an effective
(emulated) clock frequency, allowing the power manager
in the system to reduce the GXm processor’s power con-
sumption.
6.4 3-VOLT SUSPEND MODE
The GXm processor and CS5530 support stopping the
processor and system clocks using the 3-Volt Suspend
Mode. If configured (refer to the CS5530 specification),
the CS5530 asserts the SUSP_3V pin after the
SUSP#/SUSPA# handshake. SUSP_3V is intended to be
connected to the output enable of a clock synthesizer or
buffer chip so that the clocks to the GXm processor
(SYSCLK), the CS5530 (PCI_CLK), and other system
devices are stopped. The SUSP_3V pin is asserted on
any write to the CS5530’s “CPU Suspend Command Reg-
ister” or “Suspend Notebook Command Register” with bit
0 of the “Clock Stop Control Register” set.
The GXm processor has two low-power Suspend modes.
The mode implemented is determined by bit 0 in the PM
Clock Stop Control Register. One mode (bit 0 clear) turns
off the internal clocks to everything except the internal dis-
play and memory controllers, thereby keeping the display
active. The second mode, which is lower power, turns off
all internal clocks generated from SYSCLK. This mode is
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