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Software Selectable IRQs
There are eight IRQ numbers which can be selected by software along with a Master Interrupt
Enable as follows:
Master Enable
Since the interrupt is programmable, the card powers-up with all interrupt driving circuits tri-
stated. During software initialization of the card, both channels of the ESCC should be set up
first, then the desired interrupt should be programmed and finally the Master Interrupt Enable bit
set [I/O (7).6]. At this point the chosen IRQ signal will be driven inactive (low). When the SCC
generates an interrupt, the selected IRQ will be driven active (high) by the card. When the
interrupt has been serviced, the IRQ signal will return to the inactive state. (The IRQ signal is
never tri-stated during this process).
On most PC/104 buses the IRQ signals are positive edge activated with a pull-up resistor on the
IRQ signal (usually located on the PC/104 CPU module). Therefore, it is possible to generate a
"phantom" interrupt when the Master Interrupt Enable bit is turned off. During the shutdown of
the card (when a software driver is stopped or disabled by the operating system), the interrupt
should first be disconnected/disabled from the operating system and then the Master Interrupt
Enable bit turned off. A false interrupt can be prevented in this way.
Shared IRQ (Revision C and proceeding)
The Selected IRQ can be shared between multiple ComSync/104 cards. This shared IRQ
mechanism is designed using the guidelines in the PC/104 Specification V 2.5, appendix C. To
enable Shared IRQ, turn On the Shared IRQ Enable bit I/O (8).0. When this bit is On, the IRQ
signal will be tri-stated when it is inactive and driven high when active. A pull-down resistor is
required on the IRQ signal to allow it to operate correctly (See IRQ Pull-Down).
IRQ Pull-Down (Revision C and proceeding)
As noted in the Shared IRQ section, the chosen IRQ requires a pull-down resistor (1K) in order
to operate correctly. There can be only one pull-down resistor for each shared IRQ. To enable
the pull-down, turn On the pull-down Enable bit, I/O (8).1. This activates a 1K pull-down (to
GND) resistor on the programmed IRQ. Remember if you have two or more cards sharing an
IRQ, only one of them should have the pull-down enabled.
Table 4: IRQs
Setting
I/O (7).6
I/O (4).3 » 5
1
1
1
1
1
1
1
1
IRQ Selected
(Enabled)
0
3
1
5
2
7
3
9
4
10
5
11
6
12
7
15
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