Power Management (Continued)
CPU Suspend can be used for Suspend Modulation. The
Geode I/O companion can be programmed to
assert/deassert 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 consumption and performance. Certain processing
activities (SMI#, Interrupts, and VGA activity) can be mon-
itored by the Geode I/O companion to temporarily sus-
pend, Suspend Modulation for a programmable amount of
time. Suspend modulation programming is explained in
detail in the Geode I/O companion data books such as the
CS5530.
5.1.5 GXLV Processor Serial Bus
The power management logic of the GXLV processor pro-
vides the Geode I/O companion with information regard-
ing the GXLV processor productivity. If the GXLV
processor is determined to be relatively inactive, the
GXLV 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 GXLV processor to
provide information from the graphics controller that is not
externally visible otherwise. The GXLV processor imple-
ments a simple serial communications mechanism to
transmit the CPU status to the Geode I/O companion. The
GXLV processor accumulates CPU events in a 8-bit regis-
ter, “PM Serial Packet Register” (GX_BASE+850Ch), that
is serially transmitted out of the GXLV processor every 1
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 tem-
perature of the system and/or CPU and asserts the SMI#
pin, if the system or CPU gets too hot. The power man-
agement SMM handler enables Suspend Modulation.
When the temperature 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 operation resumes. A significant side effect of
Suspend Modulation is a lowering of system performance
while in this state. The system design must take this into
account. If the system 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 maximum performance is required
in all conditions then Suspend Modulation should not be
used.
µ
to 10 s. 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 starting on page 181.
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.
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 inac-
tivity, the Geode /O companion asserts the SMI# pin. The
power management SMM handler enables Suspend Mod-
ulation. When I/O activity picks up, the SMI# pin is
asserted again and the power management SMM handler
exits Suspend Modulation and normal operation resumes.
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.
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 Regis-
ter (Table 5-2 on page 181), and turning off the graphics
pipeline (set GX_BASE+8304h[0] = 0) before the asser-
tion of SUSP#. If CLK_STP is set and the graphics pipe-
line 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
GXLV processor are stopped. Once SUSPA# has gone
active, SYSCLK input pin can be stopped. While in this
state the GXLV processor will not respond to anything
except the deassertion of SUSP# as long as SYSCLK has
been restarted.
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