What should be reviewed after read_sdc applies a file?
From PDVerse PnR Interview Handbook, part of the pdVerse Mentor Guide
Short Answer
Parsing success is the first gate, not the finish line — an SDC file with zero syntax errors can still leave the design under-constrained or misconstrained. Review the errors and warnings first, then dig into the object-resolution messages specifically — those tell you whether every get_pins/get_clocks-style selector actually found real objects, or silently matched nothing.
Technical Explanation
- Check for overridden constraints too — a later command in the file (or a later file) can quietly replace an earlier constraint without any error being raised.
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What To Check
- Warning sign: The SDC applies with empty selectors or replaces an earlier clock unexpectedly.
- Inspect: Compare one named object across the related reports; the same object should tell a consistent story.
- Correct: Fix the source SDC or load context, reopen a clean checkpoint, and reapply once.
Command Checks & Actions
- read_sdc <file.sdc>: parses or applies the timing constraints
- report_clocks: shows clock definitions and relationships
- report_exceptions: shows timing exceptions and their scope
Run the commands in order. Each line answers a separate part of the check.
Healthy, Suspicious & Hard-stop Results
- Expected: Review errors, warnings, object-resolution messages, overridden constraints, current context, and resulting reports; parsing success is only the first gate.
- Stop before floorplanning when required logic or timing coverage is missing or unexplained.
Common Mistake
The Trap: Do not assume the check passed just because ICC2 continued. Fix or narrowly justify the named objects, then rerun the same command.
What The Interviewer Is Testing
Be ready to explain why this matters before floorplanning.
Follow-up Question & Model Response
Model response: “I would save the report, inspect one affected object in the correct block and scenario, and make or request this correction. ”
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