AMD SC1200 Power Management Programming, APM Support, CPU Power Management, Suspend Modulation

32579B Core Logic Module

6.2.10Power Management Programming

The power management resources provided by a com- bined GX1 module and Core Logic module based system supports a high efficiency power management implementa- tion. The following explanations pertain to a full-featured “notebook” power management system. The extent to which these resources are employed depends on the appli- cation and on the discretion of the system designer.

Power management resources can be grouped according to the function they enable or support. The major functions are as follows:

•APM Support

•CPU Power Management

—Suspend Modulation

—3V Suspend

—Save-to-Disk

•Peripheral Power Management

—Device Idle Timers and Traps

—General Purpose Timers

—ACPI Timer Register

—Power Management SMI Status Reporting Registers

Included in the following subsections are details regarding the registers used for configuring power management fea- tures. The majority of these registers are directly accessed through the PCI configuration register space designated as Function 0 (F0). However, included in the discussions are references to F1BARx+I/O Offset xxh. This refers to regis- ters accessed through base address registers in Function 1 (F1) at Index 10h (F1BAR0) and Index 40h (F1BAR1).

6.2.10.1 APM Support

Many notebook computers rely solely on an Advanced Power Management (APM) driver for enabling the operat- ing system to power-manage the CPU. APM provides sev- eral services which enhance the system power management; but in its current form, APM is imperfect for the following reasons:

•APM is an OS-specific driver, and may not be available for some operating systems.

•Application support is inconsistent. Some applications in foreground may prevent Idle calls.

•APM does not help with Suspend determination or peripheral power management.

The Core Logic module provides two entry points for APM support:

•Software CPU Suspend control via the CPU Suspend Command register (F0 Index AEh).

•Software SMI entry via the Software SMI register (F0 Index D0h). This allows the APM BIOS to be part of the SMI handler.

6.2.10.2 CPU Power Management

The three greatest power consumers in a system are the display, the hard drive, and the CPU. The power manage- ment of the first two is relatively straightforward and is dis- cussed in Section 6.2.10.3 "Peripheral Power Management" on page 164.

APM, if available, is used primarily by CPU power manage- ment since the operating system is most capable of report- ing the Idle condition. Additional resources provided by the Core Logic module supplement APM by monitoring exter- nal activity and power managing the CPU based on the system demands. The two processes for power managing the CPU are Suspend Modulation and 3V Suspend.

Suspend Modulation

Suspend Modulation works by asserting and de-asserting the internal SUSP# signal to the GX1 module for config- urable durations. When SUSP# is asserted to the GX1 module, it enters an Idle state during which time the power consumption is significantly reduced. Even though the PCI clock is still running, the GX1 module stops the clocks to its core when SUSP# is asserted. By modulating SUSP# a reduced frequency of operation is achieved.

The Suspend Modulation feature works by assuming that the GX1 module is Idle unless external activity indicates otherwise. This approach effectively slows down the GX1 module until external activity indicates a need to run at full speed, thereby reducing power consumption. This approach is the opposite of that taken by most power man- agement schemes in the industry, which run the system at full speed until a period of inactivity is detected, and then slows down. Suspend Modulation, the more aggressive approach, yields lower power consumption.

Suspend Modulation serves as the primary CPU power management mechanism when APM is not present. It also acts as a backup for situations where APM does not cor- rectly detect an Idle condition in the system.

To provide high-speed performance when needed, SUSP# modulation is temporarily disabled any time system activity is detected. When this happens, the GX1 module is “instantly” converted to full speed for a programmed dura- tion. System activities in the Core Logic module are asserted as: any unmasked IRQ, accessing Port 061h, any asserted SMI, and/or accessing the Video Processor mod- ule interface.

The graphics controller is integrated in the GX1 module. Therefore, the indication of video activity is sent to the Core Logic module via the serial link (see Section 6.2.2 "PSE- RIAL Interface" on page 143 for more information on serial link) and is automatically decoded. Video activity is defined as any access to the VGA register space, the VGA frame buffer, the graphics accelerator control registers and the configured graphics frame buffer.

162 AMD Geode™ SC1200/SC1201 Processor Data Book