
SiS5595 PCI System I/O Chipset
Preliminary V2.0 Nov. 2, 1998 14 Silicon Integrated Systems Corporation
SIRQ# is already in a serial IRQ cycle and the IRQ/Data transition can be delivered in
the serial IRQ cycle.
2)
Continuous (Idle) Mode:
Only the SiS5595 can initiate a Start Frame to update SIRQ#
line information. All other serial IRQ agents become passive and may not initiate a Start
Frame. SIRQ# will be driven low for four to eight clocks by the SiS5595. There are two
functions in this mode. It can be used to stop or idle the SIRQ# or the SiS5595 can
operate SIRQ# in a continuous mode by initiating a Start Frame at the end of every
Stop Frame. A serial IRQ mode transition can only occur during the Stop Frame. Upon
reset, the Serial IRQ bus is defaulted to continuous mode, therefore only the SiS5595
can initiate the first Start Frame. Slave must continuously sample the Stop Frames
pulse width to determine the next serial IRQ cycle's mode.
3.1.4.3 IRQ/Data Frame
Once a Start Frame has been initiated, all serial IRQ devices must detect the rising edge of
the Start pulse and start counting IRQ/Data Frames from there. There are three clock
phases for each IRQ/Data Frame: Sample Phase, Recovery Phase, and Turn-around Phase.
During the Sample phase the serial IRQ device must drive the SIRQ# low, if and only if, its
last detected IRQ/Data value was low. If its detected IRQ/Data value is high, SIRQ# must be
left tri-stated. During the Recovery phase, a serial IRQ device will drive SIRQ# back high if it
has driven the SIRQ# low in the previous clock. During the Turn-around phase all serial IRQ
devices must be tri-stated. All serial IRQ devices will drive SIRQ# low at the appropriate
sample point regardless of which device initiated the sample activity, if its associated
IRQ/Data line is low.
The Sample phase for each IRQ/Data follows the low to high transition of the Start Frame
pulse by a number of clocks equal to the IRQ/Data Frame times three, minus one. (e.g., the
IRQ5 sample clock is the sixth IRQ/Data frame, (6x3)-1=17th clock after the rising edge of
the Start Pulse).
Table 3.1-1 IRQs Mapping in Serial IRQ Periods
SERIAL IRQ SAMPLING PERIODS
IRQ/Data Frame
1
2
3
4
5
6
7
8
9
10
11
12
13
Signal Sampled
Reserved
IRQ1
SMI#
IRQ3
IRQ4
IRQ5
IRQ6
IRQ7
IRQ8#
IRQ9
IRQ10
IRQ11
IRQ12
# of clocks past Start
2
5
8
11
14
17
20
23
26
29
32
35
38