82801BA Datasheet, PDF (156/498 Pages) Intel Corporation – Intel 82801BA I/O Controller Hub 2 (ICH2) and Intel 82801BAM I/O Controller Hub 2 Mobile

Functional Description
5.12.14
Legacy Power Management Theory of Operation
Instead of relying on ACPI software, legacy power management uses BIOS and various hardware
mechanisms. ICH2 has a greatly simplified method for legacy power management compared with
previous generations (e.g., PIIX4).
The scheme relies on the concept of detecting when individual subsystems are idle, detecting when
the whole system is idle, and detecting when accesses are attempted to idle subsystems.
However, the operating system is assumed to be at least APM enabled. Without APM calls, there is
no quick way to know when the system is idle between keystrokes. The ICH2 does not support the
burst modes found in previous components (e.g., PIIX4).
5.12.14.1 Desktop APM Power Management (82801BA ICH2 only)
The ICH2 has a timer that, when enabled by the 1MIN_EN bit in the SMI Control and Enable
Register, generates an SMI# once per minute. The SMI handler can check for system activity by
reading the DEVACT_STS register. If none of the system bits are set, the SMI handler can
increment a software counter. When the counter reaches a sufficient number of consecutive
minutes with no activity, the SMI handler can then put the system into a lower power state.
If there is activity, various bits in the DEVACT_STS register are set. Software clears the bits by
writing a 1 to the bit position.
The DEVACT_STS Register allows for monitoring various internal devices, or Super I/O devices
(SP, PP, FDC) on LPC or PCI, keyboard controller accesses, or audio functions on LPC or PCI.
Other PCI activity can be monitored by checking the PCI interrupts.
5.12.14.2 Mobile APM Power Management (82801BAM ICH2-M only)
In mobile systems, there are additional requirements associated with device power management.
To handle this, the ICH2-M has specific SMI# traps available. The following algorithm is used:
1. The periodic SMI# timer checks if a device is idle for the require time. If so, it puts to the
device into a low-power states and sets the associated SMI# trap.
2. When software (not the SMI# handler) attempts to access the device, a trap occurs (the cycle
doesn’t really go to the device and an SMI# is generated).
3. The SMI# handler turns on the device and turns off the trap
The SMI# handler exits with an I/O restart. This allows the original software to continue.
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82801BA ICH2 and 82801BAM ICH2-M Datasheet