When can a black box be intentional?
From PDVerse PnR Interview Handbook, part of the pdVerse Mentor Guide
Short Answer
A black box is only legitimate when it's a deliberate, methodology-sanctioned abstraction — not simply a missing reference library that nobody noticed. lef`/frame view) rather than full internal detail, because the internals aren't needed (or aren't allowed to be seen) at this stage of the flow.
Technical Explanation
- Even when intentional, a black box still needs a known, well-defined boundary — real pin names, real timing/power models, a tracked revision, and clear ownership (who maintains this block, who to ask if something looks wrong).
- Without that boundary information, a "black box" is functionally indistinguishable from a missing/broken handoff — the difference between intentional abstraction and an accidental gap is entirely in whether that boundary data actually exists and is documented.
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- If it's genuinely intentional, downstream flows (STA, LVS, physical implementation) should be using the appropriate abstract views consistently — a black box with no model at all simply cannot be timed or physically implemented correctly, intentional or not.
Visual Verification
Legitimate black boxes possess formal interface timing models (.lib), physical abstracts (LEF), and signed boundary waivers, distinguishing them from unresolved missing cell defects.
What To Check
- Warning sign: A missing macro model is labelled intentional without evidence of pins, timing boundaries, or ownership.
- Inspect: Start with one named object in the report and trace it to the netlist, library, or SDC statement that created it.
- Correct: Obtain the approved abstract views or return the handoff; record the intended abstraction and its interface limits.
Command Checks & Actions
- report_unbound: lists objects that did not resolve
- check_netlist: reports structural connectivity problems
Run the commands in order. Each line answers a separate part of the check.
Healthy, Suspicious & Hard-stop Results
- Expected: A black box is acceptable only when the methodology deliberately abstracts its internals and still provides a controlled interface for timing and implementation.
- 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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