The 8259A interrupt-acknowledge sequence may require idle timing between INTA̅ cycles to satisfy PIC speed and cascade-address delays. Under the specified interface convention, three idle states Ti are allowed, matching the final key.
Formula / Key Relation
INTA̅ sequence: acknowledge cycles separated by required bus timing/idle states
Detailed Explanation
Concept & Reasoning
Cascaded 8259A systems use interrupt-acknowledge bus cycles to identify the requesting level and supply the vector information. Timing margins are inserted so internal priority resolution and cascade signals become valid before the next acknowledge phase. This is a hardware-interface timing fact specific to the 8259A/processor bus combination.
How to Solve It in the Exam
8259A timing questions are datasheet/protocol facts; don't derive them from CPU frequency alone.
Important Exam Point
Keep cascade delay in mind when counting idle states.
Related Revision Path
This question sits in the revision path Microprocessors and Interfacing → 8259A PIC → Interrupt Acknowledge .
Quick Trick
8259A timing questions are datasheet/protocol facts; don't derive them from CPU frequency alone.
Common Mistake
Assuming back-to-back INTA̅ cycles always require zero idle states.
Exam Tip
Keep cascade delay in mind when counting idle states.
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