82801BA Datasheet, PDF (141/498 Pages) Intel Corporation – Intel 82801BA I/O Controller Hub 2 (ICH2) and Intel 82801BAM I/O Controller Hub 2 Mobile

Functional Description
5.12.6
Dynamic PCI Clock Control (82801BAM ICH2-M)
For the ICH2-M, the PCI clock can be dynamically controlled independent of any other low-
power state. This control is accomplished using the CLKRUN# protocol as described in the PCI
Mobile Design Guide, and is transparent to software.
The Dynamic PCI Clock control is handled using the following signals:
• CLKRUN#: Used by PCI and LPC peripherals to request the system PCI clock to run
• STP_PCI#: Used to stop the system PCI clock
Note: The 33 MHz clock to the ICH2-M is “free-running” and is not affected by the STP_PCI#
signal.
5.12.6.1
Conditions for Stopping the PCI Clock (82801BAM ICH2-M)
When there is a lack of PCI activity, the ICH2-M has the capability to stop the PCI clocks to
conserve power. “PCI activity” is defined as any activity that requires the PCI clock to be running.
Any of the following conditions indicates that it is NOT OK to stop the PCI clock:
• Cycles on PCI or LPC
• Cycles of any internal device that would need to go on the PCI bus
• Cycles using PC/PCI DMA
• SERIRQ activity
Behavioral Descripion
• When there is a lack of activity (as defined above) for 29 PCI clocks, the ICH2-M deassert
(drive high) CLKRUN# for 1 clock and then tri-state the signal.
5.12.6.2
Conditions for Maintaining the PCI Clock (82801BAM ICH2-M)
PCI master that wish to maintain the PCI clock running will observe the CLKRUN# signal
deasserted, and then must re-assert if (drive it low) within 3 clocks.
Behavioral Description
• When the ICH2-M has tri-stated the CLKRUN# signal after deasserting it, the ICH2-M then
checks to see if the signal has been re-asserted (externally).
• After observing the CLKRUN# signal asserted for 1 clock, the ICH2-M again starts asserting
the signal.
• If an internal device needs the PCI bus, the ICH2-M asserts the CLKRUN# signal.
5.12.6.3
Conditions for Stopping the PCI Clock (82801BAM ICH2-M)
Behavioral Description
• If no device re-asserts CLKRUN# once it has been deasserted for 3 clocks, the ICH2-M stops
the PCI clock by asserting the STP_PCI# signal to the clock synthesizer.
82801BA ICH2 and 82801BAM ICH2-M Datasheet
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