GX1 Datasheet, PDF (178/247 Pages) National Semiconductor (TI) – Processor Series Low Power Integrated x86 Solution

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 can also be used for a crude 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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