BUS INTERFACE UNIT

Figure 3-24 shows a typical 82C59A interface example. Bus ready must be provided to terminate both bus cycles in the interrupt acknowledge sequence.

NOTE

Due to an internal condition, external ready is ignored if the device is configured in Cascade mode and the Peripheral Control Block (PCB) is located at 0000H in I/O space. In this case, wait states cannot be added to interrupt acknowledge bus cycles. However, you can add wait states to interrupt acknowledge cycles if the PCB is located at any other address.

3.5.3.1System Design Considerations

Although ALE is generated for both bus cycles, the BIU does not drive valid address information. Actually, all address bits except A19:16 float during the time ALE becomes active (on both 8- and 16-bit bus devices). Address-decoding circuitry must be disabled for Interrupt Acknowledge bus cycles to prevent erroneous operation.

Processor

 

82C59A

INTA0

INTA

 

INT0

INT

IR0

RD

RD

IR7

WR

WR

 

PCS0

CS

 

LA1

A0

D7:0

 

 

AD7:0

 

 

 

 

A1065-0A

Figure 3-24. Typical 82C59A Interface

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Intel 80C188XL, 80C186XL user manual Processor 82C59A, INTA0 Inta INT0 IR0 IR7 PCS0 LA1, System Design Considerations