Why can a syntactically valid SDC still be wrong?
From PDVerse PnR Interview Handbook, part of the pdVerse Mentor Guide
Short Answer
Syntax just asks: does this command follow the language's grammar? Meaning asks something completely different: does it actually select the clocks, ports, pins, modes, and values you intended? A file can parse cleanly and still be functionally wrong — for example, a port name that changed during synthesis means the command runs without error but silently selects nothing.
Technical Explanation
- That's why passing syntax validation is not the finish line — always read the resulting clocks, endpoints, and exceptions the tool actually produced from your constraints, not just whether it accepted the file.
Visual Verification
An SDC constraint can be 100% syntactically valid in Tcl but semantically useless if its target object collection returns zero matched ports or pins during design binding.
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.
- 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.
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