Intel 80C188XL, 80C186XL user manual 8.5.2Interrupt Vectoring in Slave Mode

INTERRUPT CONTROL UNIT

8.5.2Interrupt Vectoring in Slave Mode

In Slave mode, the external 8259A module acts as the master interrupt controller. Therefore, in- terrupt acknowledge cycles are required for every interrupt, including those from integrated pe- ripherals. During the first interrupt acknowledge cycle, the external 8259A determines which slave interrupt controller has the highest priority interrupt request. It then drives that slave’s ad- dress onto its CAS2:0 pins (Figure 8-20). External logic must decode the correct slave address from the CAS2:0 pins to drive the SELECT pin.

T1 T2 T3 T4 TI TI T1 T2 T3 T4

CLKOUT

S2:0 INTA INTA

INTA0

SELECT

LOCK

CAS2:0 Slave Cascade Address From 8259A

NOTES:

1.INT1/SELECT has the SELECT function in slave mode.

2.INT2/INTA0 has the INTA0 function in slave mode.

3.Cascade address is driven by the external 8259A.

4.SELECT must be driven before phase 2 of T2 of the second INTA.

5.SELECT read by processor.

6.ALE is generated for each INTA.

7.RD is inactive.

A1199-A0

Figure 8-20. Interrupt Vectoring in Slave Mode

The SELECT pin is the slave-select input to the Interrupt Control Unit. During the second inter- rupt acknowledge cycle, the highest-priority slave interrupt controller transfers the interrupt type of its highest priority interrupt to the CPU. If the Interrupt Control Unit is the highest-priority slave, it passes the interrupt type to the CPU internally; however, the interrupt acknowledge cycle still must occur for the benefit of the external 8259A module.

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