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27 Chapter 3
3.12 Interrupt Setting
When the interrupt occurs from MCX312, the interrupt signal of
MCX312 will be changed from high to low. Because the PCI bus inter-
rupt is high level sensitive, the PCI-1220U inverse the signal and latch the
signal to adapt the PCI bus INTA. The Fig- 3.17 shows the interrupt
structure of the PCI-1220U. We suggest users who want to program their
own interrupt service routine (ISR) should follow the procedures:
Step 1: When interrupt occurs. (Hardware)
Step 2: Program will jump to ISR. (Software)
Step 3: In ISR program the first thing have to do is clear interrupt for pre-
venting hanging up the PCI bus.
Step 4: In ISR program the last thing have to do is read nRR3 of MCX312
for accepting next interrupt occurs.
Figure 3.17: Circuit Diagram of Interrupt Setting