82801FB Datasheet, PDF (142/786 Pages) Intel Corporation – Intel I/O Controller Hub 6 (ICH6) Family

Functional Description
5.11.1
Start Frame
The serial IRQ protocol has two modes of operation which affect the start frame. These two modes
are: Continuous, where the ICH6 is solely responsible for generating the start frame; and Quiet,
where a serial IRQ peripheral is responsible for beginning the start frame.
The mode that must first be entered when enabling the serial IRQ protocol is continuous mode. In
this mode, the ICH6 asserts the start frame. This start frame is 4, 6, or 8 PCI clocks wide based
upon the Serial IRQ Control Register, bits 1:0 at 64h in Device 31:Function 0 configuration space.
This is a polling mode.
When the serial IRQ stream enters quiet mode (signaled in the Stop Frame), the SERIRQ line
remains inactive and pulled up between the Stop and Start Frame until a peripheral drives the
SERIRQ signal low. The ICH6 senses the line low and continues to drive it low for the remainder
of the Start Frame. Since the first PCI clock of the start frame was driven by the peripheral in this
mode, the ICH6 drives the SERIRQ line low for 1 PCI clock less than in continuous mode. This
mode of operation allows for a quiet, and therefore lower power, operation.
5.11.2
Data Frames
Once the Start frame has been initiated, all of the SERIRQ peripherals must start counting frames
based on the rising edge of SERIRQ. Each of the IRQ/DATA frames has exactly 3 phases of 1
clock each:
• Sample Phase. During this phase, the SERIRQ device drives SERIRQ low if the
corresponding interrupt signal is low. If the corresponding interrupt is high, then the SERIRQ
devices tri-state the SERIRQ signal. The SERIRQ line remains high due to pull-up resistors
(there is no internal pull-up resistor on this signal, an external pull-up resistor is required). A
low level during the IRQ0–1 and IRQ2–15 frames indicates that an active-high ISA interrupt is
not being requested, but a low level during the PCI INT[A:D], SMI#, and IOCHK# frame
indicates that an active-low interrupt is being requested.
• Recovery Phase. During this phase, the device drives the SERIRQ line high if in the Sample
Phase it was driven low. If it was not driven in the sample phase, it is tri-stated in this phase.
• Turn-around Phase. The device tri-states the SERIRQ line
5.11.3 Stop Frame
After all data frames, a Stop Frame is driven by the ICH6. The SERIRQ signal is driven low by the
ICH6 for 2 or 3 PCI clocks. The number of clocks is determined by the SERIRQ configuration
register. The number of clocks determines the next mode:
Table 5-18. Stop Frame Explanation
Stop Frame Width
2 PCI clocks
3 PCI clocks
Next Mode
Quiet Mode. Any SERIRQ device may initiate a Start Frame
Continuous Mode. Only the host (Intel^® ICH6) may initiate a Start Frame
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Intel^® I/O Controller Hub 6 (ICH6) Family Datasheet