[Design Application]
The Next-Generation System Power Management
Part II Of A Three-Part Series On The Mobile Power Initiative Focuses On The Advanced Configuration And Power Interface.
In the first article of the series "Notebook Computer Makers Address Power Challenges" (Electronic Design, Dec. 15, 1997, p. 44), the historical trends concerning power were reviewed. Basically, the power dissipated in mobile-PC interiors has increased by 90% from 1994 to 1997. If the current mobile-PC power trends continue, the power will increase another 85% from 1997 to 1999. In comparison, battery capacity is projected to increase around 45% during the same time period.
If nothing is done to address rising platform power, batteries would have to be even larger to keep the same battery life we enjoy today. These power trends are making mobile-PC design more difficult, and requiring a system-level approach to solving these issues.
To address this problem, Intel Corp. has launched the Mobile Power Initiative. It is a cooperative program that unites PC industry leaders in addressing all of the areas that impact mobile-PC system power: the hardware (covered in the first article), the system power management (covered in this article), and application software (covered in a future article).
A Historical Perspective One of the first real breakthroughs in mobile power management occurred in 1989 with the introduction of Intel's SL technology (first seen on the 386SL, and still included in all of today's Intel Pentium processors with MMX technology). This technology created a new mode for the CPU called System Management Mode (SMM). SMM allows embedded code within the BIOS to slow down, suspend, or shut down part or all of the system platform, and even the CPU itself, thereby preserving and extending battery life.
At a system level, there are timers for every hardware device that needs to be power managed. These timers keep track of how long it has been since the device was last accessed. When this timer reaches a preset time (usually configured by the BIOS) the device is power managed (put into a lower power state). This technology worked very well in its time, but as systems became more complex, the timers would need to be programmed based on what was happening at the system level. This lead to the next advance in system power management.
In 1991, Intel and Microsoft introduced Advanced Power Management (APM) as a means of integrating the operating system (OS) into the power management loop. This was done to allow communication between the OS and the SL power management (PM) code embedded within the BIOS. APM creates an interface between the OS and the BIOS. When APM was first introduced, it contained interfaces to address all of the known problems with timer based power management. Unfortunately, as systems continued to evolve, new interfaces became necessary to handle the changing hardware.
It soon became apparent that the industry would need to either continually update these interfaces, adding cost, complexity, and backward compatibility issues to both the BIOS and the OS, or find another mechanism to advance system-level PM. To address this, a more general control of PM was required.
The OS already deals with this type of issue through device drivers. By adding a generalized description of the hardware to the system, the OS can incorporate the PM capability. There is, however, a benefit of moving PM to a higher level. The operating system has access to more information about what tasks are running and what the user is doing, therefore it's in a better position to decide which devices should be on or off. As a result, the birth of the Advanced Configuration and Power Interface (ACPI) specification took place in January 1997.
The ACPI specification was authored by Intel, Microsoft, and Toshiba, but includes contributions from independent hardware vendors (IHV's), independent software vendors (ISV's), and original equipment manufacturers (OEM's) to ensure it met the needs of the entire industry. ACPI evolves the existing collection of PM BIOS code, APM Advanced Programming Interfaces (API's), and plug-and-play (PnP) BIOS API's into a well-specified PM and configuration mechanism. It provides support for an orderly transition from existing (legacy) hardware to ACPI hardware, and it allows for both mechanisms to exist in a single machine. Newer systems will use the ACPI, while older OSs can continue to operate with the older mechanisms.
Currently only a limited number of PC's support PM (mainly mobile PCs and desktop "green PCs"). This inhibits application vendors from supporting or exploiting PM. Moving PM into the OS makes it available on every system on which the OS is installed. The level of power savings will vary from machine to machine, but the end users and application vendors will see the same PM mechanism on all OS-directed power management (OSPM) machines.
ACPI Overview ACPI creates mechanisms for control of three basic functions: platform PM, device PM, and platform configuration. To accomplish those functions, ACPI impacts the OS device-driver model, as well as hardware on a chip set and a device level. An example of the scope of the initiative and its impact on all aspects of system design is illustrated in Figure 1.
The hardware impact lies in the ACPI register space. There are some features of ACPI that are critical enough to create fixed register space that allows the OS to interact with the hardware at the fixed location without having to wait for the OS to be fully loaded. This ability becomes important both at initial boot and when exiting a sleeping state. This fixed register space is part of the ACPI register block (Fig. 1, again).
ACPI-compliant systems will now require ACPI-compliant chip sets that meet the following requirements: They must support the ACPI fixed registers, interrupt events generating System Control Interrupts (SCI's), a PM timer, a real-time clock wake-up alarm, and an ACPI-compliant power/sleep button.
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