Advanced Notebook I/O for ISA or LPC Designs
Datasheet
SMSC SIO10N268
Page 141
Rev. 0.5 (03-24-05)
DATASHEET
SER_IRQ PERIOD
SIGNAL SAMPLED
# OF CLOCKS PAST START
16
IRQ15
47
The SER_IRQ data frame supports IRQ2 from a logical device on Period 3, which can be used for the
System Management Interrupt (nSMI). When using Period 3 for IRQ2 the user should mask off the SMI
via the SMI Enable Register. Likewise, when using Period 3 for nSMI the user should not configure any
logical devices as using IRQ2.
SER_IRQ Period 14 is used to transfer IRQ13. Logical devices FDC, Parallel Port, Serial Port 1, Serial
Port 2, Serial Port 3, Serial Port 4, and WDT have IRQ13 as a choice for their primary interrupt.
The SMI is enabled onto the SMI frame of the Serial IRQ via bit 6 of SMI Enable Register 2 and onto the
IO_SMI# pin via bit 7 of the SMI Enable Register 2.
The following devices may be mapped into the Serial IRQ stream.
FDC
Parallel Port
Serial Port 1
Serial Port 2
Serial Port 3
Serial Port 4
WDT
8.14.1.3 Stop Cycle Control
Once all IRQ/Data Frames have completed the Host Controller will terminate SER_IRQ activity by initiating
a Stop Frame. Only the Host Controller can initiate the Stop Frame. A Stop Frame is indicated when the
SER_IRQ is low for two or three clocks. If the Stop Frame’s low time is two clocks then the next SER_IRQ
Cycle’s sampled mode is the Quiet mode; and any SER_IRQ 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 SER_IRQ 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.
8.14.1.4 Latency
Latency for IRQ/Data updates over the SER_IRQ bus in bridge-less systems with the minimum Host
supported IRQ/Data Frames of seventeen, will range up to 96 clocks (3.84
S 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.
8.14.1.5 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 SER_IRQ Cycle latency in order to ensure that these events do not occur out of
order.