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IRQSER Period 14 is used to transfer IRQ13. Logical devices 0 (FDC), 3 (Par Port), 4 (Ser
Port 1), 5 (Ser Port 2), 6 (RTC), and 7 (KBD) shall have IRQ13 as a choice for their primary
interrupt.
Stop Cycle Control
Once all IRQ/Data Frames have completed the Host Controller will terminate IRQSER activity by
initiating a Stop Frame. Only the Host Controller can initiate the Stop Frame. A Stop Frame is
indicated when the IRQSER is low for two or three clocks. If the Stop Frame’s low time is two clocks
then the next IRQSER Cycle’s sampled mode is the Quiet mode; and any IRQSER device may
initiate a Start Frame in the second clock or more after the rising edge of the Stop Frame’s pulse. If
the Stop Frame’s low time is three clocks then the next IRQSER Cycle’s sampled mode is the
Continuos mode; and only the Host Controller may initiate a Start Frame in the second clock or more
after the rising edge of the Stop Frame’s pulse.
Latency
Latency for IRQ/Data updates over the IRQSER bus in bridge-less systems with the minimum
IRQ/Data Frames of seventeen, will range up to 96 clocks (3.84uS with a 25MHz PCI Bus or 2.88uS
with a 33MHz PCI Bus). If one or more PCI to PCI Bridge is added to a system, the latency for
IRQ/Data updates from the secondary or tertiary buses will be a few clocks longer for synchronous
buses, and approximately double for asynchronous buses.
EOI/ISR Read Latency
Any serialized IRQ scheme has a potential implementation issue related to IRQ latency. IRQ latency
could cause an EOI or ISR Read to precede an IRQ transition that it should have followed. This could
cause a system fault.
The host interrupt controller is responsible for ensuring that these latency
issues are mitigated. The recommended solution is to delay EOIs and ISR Reads to the interrupt
controller by the same amount as the IRQSER Cycle latency in order to ensure that these events do
not occur out of order.
AC/DC Specification Issue
All IRQSER agents must drive / sample IRQSER synchronously related to the rising edge of PCI bus
clock. IRQSER (SIRQ) pin uses the electrical specification of PCI bus. Electrical parameters will
follow PCI spec. section 4, sustained tri-state.
Reset and Initialization
The IRQSER bus uses nPCIRST as its reset signal (nPCIRST is equivalent to using nRESET_OUT)
and follows the PCI bus reset mechanism. The IRQSER pin is tri-stated by all agents while nPCIRST
is active. With reset, IRQSER Slaves and Bridges are put into the (continuous) IDLE mode. The Host
Controller is responsible for starting the initial IRQSER Cycle to collect system’s IRQ/Data default
values. The system then follows with the Continuous/Quiet mode protocol (Stop Frame pulse width)
for subsequent IRQSER Cycles. It is Host Controller’s responsibility to provide the default values to
8259’s and other system logic before the first IRQSER Cycle is performed. For IRQSER system
suspend, insertion, or removal application, the Host controller should be programmed into Continuous
(IDLE) mode first. This is to guarantee IRQSER bus is in IDLE state before the system configuration
changes.