Geode GXLV Processor Series Low Power Integrated x86 Solutions

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Geode GXLV Processor Series5.0 Power ManagementPower consumption in a GXLV processor based system ismanaged with the use of both of hardware and software.The complete hardware solution is provided for only whenthe GXLV processor is combined with a Geode I/O companionsuch as the CS5530.The GXLV processor power consumption is managed primarilythrough a sophisticated clock stop managementtechnology. The GXLV processor also provides the hardwareenablers from which the complete power managementsolution depends on.Typically the three greatest power consumers in a batterypowered device are the display, the hard drive (if it hasone) and the CPU. Managing power for the first two is relativelystraightforward and is discussed in the CS5530 I/Ocompanion data book. Managing CPU power is more difficultsince effective use of the clock stop technologyrequires effective detection of inactivity, both at a systemlevel and at a code processing level.Basically two methods are supported to manage powerduring periods of inactivity. The first method, called activitybased power management allows the hardware in theGeode I/O companion to monitor activity to certaindevices in the system and if a period of inactivity occurstake some form of power conservation action. Thismethod does not require OS support because this supportis handled by SMM software. Simple monitoring ofexternal activity is imperfect as well as inefficient. Thesecond method, called passive power management,requires the OS to take the active role in managing power.National supports two application programming interfaces(APIs) to enable power management by the OS:Advanced Power Management (APM) and AdvancedConfiguration and Power Interface (ACPI). These twomethods can be used independent of one another or theycan be used together. The extent to which theseresources are employed depends on the application andthe discretion of the system designer.The GXLV processor and Geode I/O companion chipscontain advanced power management features for reducingthe power consumption of the processor in the system.5.1 POWER MANAGEMENT FEATURESThe GXLV processor based system supports the followingpower management features:• GXLV processor hardware- System Management Mode (SMM)- Suspend-on-Halt- CPU Suspend- 3 Volt Suspend- GXLV Processor Serial Bus• Geode I/O companion hardware:- I/O activity monitoring- SMI generation- CPU Suspend control- Suspend Modulation- 3 Volt Suspend- ACPI hardware• Software:- API for APM aware OS- API for ACPI aware OS- PM VSA for not PM aware OS’sGeode I/O companion power management support is discussedin this specification only when necessary to betterexplain the GXLV processor’s power management features.Software support of power management is discussed inthis specification only when necessary to better explainthe GXLV processor’s power management features.5.1.1 System Management ModeThe GXLV processor has an operation mode called SystemManagement Mode. This mode is generally enteredwhen the SMI# pin goes active. SMM is explained in Section3.7 “System Management Mode” on page 83. If activepower management is desired, then the Geode I/O companionis programmed at boot time to activate SMMthrough the SMI# pin due to specific I/O inactivity.SMM is also used in the passive power managementmethod, however, it is limited to supporting specific APIcalls such as entering sleep modes.5.1.2 Suspend-on-HaltSuspend-on-Halt is the most effective power reducing featureof the GXLV processor with the system active. Thisfeatureallowsthesystemtoreducepowerwhenthesystem’sOS becomes idle without producing any delay whenthe system’s OS becomes active.When entered, Suspend-on-Halt stops the clock to theprocessor core while the intergrated functions (graphics,memory controller, PCI controller) are still active. There isabsolutely no observational evidence that the processorhas changed operational behavior except for two things.The GXLV draws significantly less core power and theSUSPA# pin is active while in this state.5.1.3 CPU SuspendCPU Suspend is a hardware initiated power managementstate. The SUSP# pin is asserted by external hardwaresuch as an Geode I/O companion. The GXLV processorasserts the SUSPA# pin to indicate that the processor hasentered CPU Suspend. This state is similar to Suspendon-Haltexcept for its entry and exit method. SUSP# activecauses the processor to enter the state and SUSP# inactivecauses its exit. The power savings is identical to Suspend-on-Halt.Also, as in Suspend-on-Halt, the processorwill temporally disable CPU Suspend when there is PCImaster activity.www.national.com 176 Revision 1.3
Power Management (Continued)CPU Suspend can be used for Suspend Modulation. TheGeode I/O companion can be programmed toassert/deassert SUSP# at a programmable frequencyand duty cycle. This has the effect of reducing the averagefrequency that the processor is running and thus reducespower consumption and performance. Certain processingactivities (SMI#, Interrupts, and VGA activity) can be monitoredby the Geode I/O companion to temporarily suspend,Suspend Modulation for a programmable amount oftime. Suspend modulation programming is explained indetail in the Geode I/O companion data books such as theCS5530.5.1.3.1 Suspend Modulation for ThermalManagementThe best use of Suspend Modulation is for thermal management.The Geode I/O companion monitors the temperatureof the system and/or CPU and asserts the SMI#pin, if the system or CPU gets too hot. The power managementSMM handler enables Suspend Modulation.When the temperature drops to a certain point the GeodeI/O companion again asserts the SMI# pin. The powermanagement SMM handler disables Suspend Modulationand normal operation resumes. A significant side effect ofSuspend Modulation is a lowering of system performancewhile in this state. The system design must take this intoaccount. If the system exceeds temperature limits only inextreme conditions then thermal management by use ofSuspend Modulation can be easily and effectively used toreduce system cost by eliminating fans and possibilityheatsinks. However, if maximum performance is requiredin all conditions then Suspend Modulation should not beused.5.1.3.2 Suspend Modulation for Power ManagementSuspend modulation can also be used for a crude methodof power management. The Geode I/O companion monitorsI/O activity and when that monitoring indicates inactivity,the Geode /O companion asserts the SMI# pin. Thepower management SMM handler enables Suspend Modulation.When I/O activity picks up, the SMI# pin isasserted again and the power management SMM handlerexits Suspend Modulation and normal operation resumes.5.1.4 3 Volt Suspend3 Volt Suspend is identical to CPU Suspend with the additionof setting CLK_STP in the PM_CNTRL_CSTP Register(Table 5-2 on page 181), and turning off the graphicspipeline (set GX_BASE+8304h[0] = 0) before the assertionof SUSP#. If CLK_STP is set and the graphics pipelineis still active then the SUSP# will be ignored and 3Volt Suspend will not be entered. As 3 Volt Suspend isbeing entered, the memory controller puts the SDRAMSin self refresh mode. At this point, all internal clocks in theGXLV processor are stopped. Once SUSPA# has goneactive, SYSCLK input pin can be stopped. While in thisstate the GXLV processor will not respond to anythingexcept the deassertion of SUSP# as long as SYSCLK hasbeen restarted.5.1.5 GXLV Processor Serial BusThe power management logic of the GXLV processor providesthe Geode I/O companion with information regardingthe GXLV processor productivity. If the GXLVprocessor is determined to be relatively inactive, theGXLV processor power consumption can be greatlyreduced by entering the Suspend Modulation mode.Although the majority of the system power managementlogic is implemented in the Geode I/O companion, a smallamount of logic is required within the GXLV processor toprovide information from the graphics controller that is notexternally visible otherwise. The GXLV processor implementsa simple serial communications mechanism totransmit the CPU status to the Geode I/O companion. TheGXLV processor accumulates CPU events in a 8-bit register,“PM Serial Packet Register” (GX_BASE+850Ch), thatis serially transmitted out of the GXLV processor every 1to 10 µs. The transmission frequency is set with bits [4:3]of the “PM Serial Packet Control Register”. These registerformats are given in Table 5-2 starting on page 181.5.1.6 Advanced Power Management (APM) SupportMany battery powered devices rely solely on the APM(Advanced Power Management) driver for DOS, Windows95/98, and other operating systems to manage power tothe CPU. APM provides several services that enhance thesystem power management by determining when theCPU is idle. For the CPU, APM is theoretically the bestapproach but there are some drawbacks.• APM is an OS-specific driver which is not available forall operating systems.• Application support is inconsistent. Some applicationsin foreground may prevent idle calls.The components for APM support are:• Software CPU Suspend control via the Geode I/Ocompanion CPU Suspend Command Register.• Software SMI entry via the Software SMI Register. Thisallows the APM BIOS to be part of the SMM handler.Geode GXLV Processor SeriesRevision 1.3 177 www.national.com
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Geode GXLV Processor Series Low Pow
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