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82801AA and 82801AB Datasheet
5-37
Functional Description
5.9
Serial Interrupt (D31:F0)
The ICH supports a serial IRQ scheme. This allows a single signal to be used to report interrupt
requests. The signal used to transmit this information is shared between the host, the ICH, and all
peripherals that support serial interrupts. The signal line, SERIRQ, is synchronous to PCI clock,
and follows the sustained tri-state protocol that is used by all PCI signals. This means that if a
device has driven SERIRQ low, it first drives it high synchronous to PCI clock and releases it the
following PCI clock. The serial IRQ protocol defines this sustained tri-state signaling in the
following fashion:
S - Sample Phase.
Signal driven low
R - Recovery Phase.
Signal driven high
T - Turn-around Phase.
Signal released
The ICH supports a message for 21 serial interrupts. These represent the 15 ISA interrupts
(IRQ[0,1, 2:15), the four PCI interrupts, and the control signals SMI# and IOCHK#. The serial IRQ
protocol does not support the additional APIC interrupts (20:23).
5.9.1
Start Frame
The serial IRQ protocol has two modes of operation which affect the start frame. These two modes
are: Continuous, where the ICH 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 ICH 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 ICH 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 ICH 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.
5.9.2
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.
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