Figure 12.2 shows a timing example of VIC port operation. In this example IRQC is received followed by IRQB having a higher priority. The waveforms in Figure 12.2 show an asynchronous relationship between CLKIN and HCLK, and the delays marked Sync cater for the delay of the synchronizers. When this interface is used synchronously, these delays are reduced to being a single cycle of the receiving clock.
Figure 12.2 illustrates the basic handshake mechanism that operates between the processor and a PL192 VIC:
An IRQC interrupt request occurs causing the PL192 VIC to set the processor nIRQ input.
The processor samples the nIRQ input LOW and initiates an interrupt entry sequence.
Another IRQB interrupt request of higher priority than IRQC occurs.
Between B3 and B4, the processor decides that the pending interrupt is an IRQ rather than a FIQ and asserts the IRQACK signal.
At B4 the VIC samples IRQACK HIGH and starts generating IRQADDRV. The VIC can still change IRQADDR to the IRQB vector address while IRQADDRV is LOW.
At B6 the VIC asserts IRQADDRV while IRQADDR is set to the IRQB vector address. IRQADDR is held until the processor acknowledges it has sampled it, even if a higher priority interrupt is received while the VIC is waiting.
Around B8 the processor samples the value of the IRQADDR input bus and deasserts IRQACK.
When the VIC samples IRQACK LOW, it stacks the priority of the IRQB interrupt and deasserts IRQADDRV. It also deasserts nIRQ if there are no higher priority interrupts pending.
When the processor samples IRQADDRV LOW, it knows it can sample the nIRQ input again. Therefore, if the VIC requires some time for deasserting nIRQ, it must ensure that IRQADDRV stays HIGH until nIRQ has been deasserted.
The clearing of the interrupt is handled in software by the interrupt handling routine. This enables multiple interrupt sources to share a single interrupt priority. In addition, the interrupt handling routine must communicate to the VIC that the interrupt currently being handled is complete, using the memory-mapped or coprocessor-mapped interface, to enable the interrupt masking to be unwound.