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178

Revision 1.0

Power Management (Continued)

5.1.3.1

Suspend Modulation for Thermal

Management

The best use of Suspend Modulation is for thermal man-

agement. The Geode I/O companion monitors the temper-

ature of the system and/or CPU and asserts the SMI# pin,

if the system or CPU gets too hot. The power management

SMM handler enables Suspend Modulation. When the tem-

perature drops to a certain point the Geode I/O companion

again asserts the SMI# pin. The power management SMM

handler disables Suspend Modulation and normal opera-

tion resumes. A significant side effect of Suspend Modula-

tion is a lowering of system performance while in this state.

The system design must take this into account. If the sys-

tem exceeds temperature limits only in extreme conditions

then thermal management by use of Suspend Modulation

can be easily and effectively used to reduce system cost by

eliminating fans and possibility heatsinks. However, if maxi-

mum performance is required in all conditions then Sus-

pend Modulation should not be used.

5.1.3.2

Suspend Modulation for Power Management

Suspend modulation canalsobeusedfor acrude method

of power management. The Geode I/O companion moni-

tors I/O activity and when that monitoring indicates inactiv-

ity, the Geode /O companion asserts the SMI# pin. The

power management SMM handler enables Suspend Modu-

lation. When I/O activity picks up, the SMI# pin is asserted

again and the power management SMM handler exits Sus-

pend Modulation and normal operation resumes.

5.1.4

3 Volt Suspend

3 Volt Suspend is identical to CPU Suspend with the addi-

tion of setting CLK_STP in the PM_CNTRL_CSTP register

(Table 5-2 on page 183), and turning off the graphics pipe-

line (set GX_BASE+8304h[0] = 0) before the assertion of

SUSP#. If CLK_STP is set and the graphics pipeline is still

active then the SUSP# will be ignored and 3 Volt Suspend

will not be entered. As 3 Volt Suspend is being entered, the

memory controller puts the SDRAMS in self refresh mode.

At this point, all internal clocks in the GX1 processor are

stopped. Once SUSPA# has gone active, SYSCLK input

pin can be stopped. While in this state the GX1 processor

will not respond to anything except the deassertion of

SUSP# as long as SYSCLK has been restarted.

5.1.5

GX1 Processor Serial Bus

The power management logic of the GX1 processor pro-

vides the Geode I/O companion with information regarding

the GX1 processor productivity. If the GX1 processor is

determined to be relatively inactive, the GX1 processor

power consumption can be greatly reduced by entering the

Suspend Modulation mode.

Although the majority of the system power management

logic is implemented in the Geode I/O companion, a small

amount of logic is required within the GX1 processor to

provide information from the graphics controller that is not

externally visible otherwise. The GX1 processor imple-

ments a simple serial communications mechanism to trans-

mit the CPU status to the Geode I/O companion. The GX1

processor accumulates CPU events in a 8-bit register, “PM

Serial Packet" register (GX_BASE+850Ch), that is serially

transmitted out of the GX1 processor every 1 to 10 µs. The

transmission frequency is set with bits [4:3] of the “PM

Serial Packet Control" register. These register formats are

given in Table 5-2 on page 183.

5.1.6

Advanced Power Management (APM) Support

Many battery powered devices rely solely on the APM

(Advanced Power Management) driver for DOS, Windows

95/98, and other 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 is not available for all

operating systems.

• Application support is inconsistent. Some applications in

foreground may prevent idle calls.

The components for APM support are:

• Software CPU Suspend control via the Geode I/O

companion CPU Suspend Command register.

• Software SMI entry via the Software SMI register. This

allows the APM BIOS to be part of the SMM handler.

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