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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.

APM is an OS-specific driver which may not be avail-

able for some operating systems.

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

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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