How should I/O guides and place_io be used?
From PDVerse PnR Interview Handbook, part of the 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
- Hard stop: required legality or physical feasibility is missing or unexplained.
Common Mistake
The Trap: Do not hide the symptom with an arbitrary utilisation, halo, channel, blockage, pin move, or die-size change.
What The Interviewer Is Testing
” to measurable physical evidence and an owned correction.
Follow-up Question & Model Response
"* Candidate Model Response: 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. ”
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