82371AB (PIIX4)
E
178
4/9/97 2:23 PM PIIX4aDS
INTEL CONFIDENTIAL
(until publication date)
PRELIMINARY
8.7.
Serial Interrupts
PIIX4 supports a serial IRQ scheme. This allows a single signal to be used to report ISA-style interrupt requests.
Typically, it will be used in a mobile environment by docking bridges or Cardbus controllers.
Because more than one device may need to share the single serial IRQ signal, an Open Collector signaling
scheme is used. Timing is based on the PCI Clock. If the PCI Clock is inactive when a device needs to signal an
interrupt, the CLKRUN# signal must first be asserted by the device to restart the PCI Clock. The serial IRQ
configuration is handled via the PCI configuration space. No other registers are associated with the scheme.
8.7.1.
PROTOCOL
Serial interrupt information is transferred using three types of frames: a Start frame, one or more IRQ Data
frames, and one Stop frame. There are also two modes of operation: Quiet Mode and Continuous Mode.
Quiet (Active) Mode
To indicate an interrupt, the peripheral brings the SERIRQ signal active for one clock, and then tri-states the
signal. This brings all the state machines from IDLE to the ACTIVE states.
PIIX4 then takes control of the SERIRQ signal by driving it low on the next clock, and continues driving it low for
3–7 clocks more (programmable). Thus, the total number of clocks low will be 4–8. After those clocks, PIIX4
drives SERIRQ high for one clock and then tri-state the signal.
Continuous (Idle) Mode
In this mode, PIIX4 initiates the Start frame, rather than the peripherals. Typically, this is done to update IRQ
status (acknowledges). PIIX4 drives SERIRQ low for 4–8 clocks. This is the default mode after reset, and can be
used to enter the Quiet mode.
Data Frame
Once the Start frame has been initiated, all of the serial interrupt peripherals must start counting frames based
on the rising edge of SERIRQ. Each of the IRQ/DATA frames has exactly three phases of one clock each: a
Sample phase, a Recovery phase, and a Turn-around phase.
During the Sample phase, the device drives SERIRQ low if the corresponding interrupt signal should be active. If
the corresponding interrupt is inactive, then the devices should not drive the SERIRQ signal. It will remain high
due to pull-up resistors. During the other two phases (Turn around and Recovery), no device should drive the
SERIRQ signal. The IRQ/DATA frames have a specific order and usage, as shown in Table 26.
If an SMI# is activated on frame 3, PIIX4 drives its EXTSMI# signal active. This then generates an SMI# to the
microprocessor, if enabled.