Why can a syntactically valid SDC still be wrong?
From PDVerse PnR Interview Handbook · pdVerse Mentor Guide
Short Answer
Syntax validation proves commands can be parsed; it does not prove objects resolve, values model the interface, or constraints enter the correct mode/scenario.
Technical Explanation
Syntax asks whether a sentence follows the language rules. Meaning asks whether that sentence selects the intended clocks, ports, pins, modes, and values.
What To Check
• Warning sign: read_sdc -syntax_only passes but applied constraints select empty collections or the wrong clocks. • Inspect: Start with one named object in the report and trace it to the netlist, library, or SDC statement that created it. • Correct: Apply the SDC in the intended context, review messages and reports, and repair the owning constraint source.
Command Checks & Actions
• read_sdc -syntax_only <file.sdc>: parses or applies the timing constraints
• read_sdc <file.sdc>: parses or applies the timing constraints
Run the commands in order. Each line answers a separate part of the check.Healthy, Suspicious & Hard-stop Results
• Expected: Syntax validation proves commands can be parsed; it does not prove objects resolve, values model the interface, or constraints enter the correct mode/scenario. • Investigate: read_sdc -syntax_only passes but applied constraints select empty collections or the wrong clocks. • Stop before floorplanning when required logic or timing coverage is missing or unexplained. Apply the SDC in the intended context, review messages and reports, and repair the owning constraint source.
Common Mistake
Do not assume the check passed just because ICC2 continued: read_sdc -syntax_only passes but applied constraints select empty collections or the wrong clocks. 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: read_sdc -syntax_only passes but applied constraints select empty collections or the wrong clocks.
Practical Example
Symptom: read_sdc -syntax_only passes but applied constraints select empty collections or the wrong clocks. Corrective path: Apply the SDC in the intended context, review messages and reports, and repair the owning constraint source.
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