How should I/O guides and place_io be used?
From PDVerse PnR Interview Handbook · pdVerse Mentor Guide
Short Answer
I/O guides reserve legal regions for I/O driver cells, while constraints describe package, protocol, power, and matching intent. place_io applies those rules to the intended guides.
Technical Explanation
An I/O guide is a permitted region, not proof that the package connection or signal order is correct. Separate wire-bond and flip-chip needs. Review clocks, resets, differential signals, buses, analogue-sensitive interfaces, power/ground distribution, corners, and breaks.
Formula Or Decision Rule
Decision rule: package feasibility and on-die dataflow must both support the placement.
What To Check
• Warning sign: Pads are legal in guides but bus order creates crossings or power distribution is unbalanced. • Inspect: choose one affected region, macro, row, pin, path, or net and trace the physical cause. • Correct: Correct constraints or guide geometry, rerun placement, and review connectivity and package alignment.
Command Checks & Actions
• place_io -io_guide <io_guide_list>: places I/O driver cells in constrained guides
• report_design -floorplan: reports the floorplan state understood by the tool
Run only the commands needed for this question. Save the report with the floorplan version and analysis context.Healthy, Suspicious & Hard-stop Results
• Healthy: I/O guides reserve legal regions for I/O driver cells, while constraints describe package, protocol, power, and matching intent. place_io applies those rules to the intended guides. • Suspicious: Pads are legal in guides but bus order creates crossings or power distribution is unbalanced. • Hard stop: required legality or physical feasibility is missing or unexplained. Correct constraints or guide geometry, rerun placement, and review connectivity and package alignment.
Common Mistake
Do not hide the symptom with an arbitrary utilisation, halo, channel, blockage, pin move, or die-size change. Pads are legal in guides but bus order creates crossings or power distribution is unbalanced.
What The Interviewer Is Testing
The interviewer is testing whether you can connect “How should I/O guides and place_io be used?” to measurable physical evidence and an owned correction.
Follow-up Question & Model Response
Follow-up probe: What would make you change your decision? Model response: “I would change it if the same controlled rerun shows that pads are legal in guides but bus order creates crossings or power distribution is unbalanced. I would use the named objects and reports to decide whether to iterate, waive, or return the issue.”
Practical Example
Symptom: Pads are legal in guides but bus order creates crossings or power distribution is unbalanced. Corrective path: Correct constraints or guide geometry, rerun placement, and review connectivity and package alignment.
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