How do you debug unresolved references or missing library cells?
From PDVerse PnR Interview Handbook · pdVerse Mentor Guide
Short Answer
An unresolved reference means an instantiated design or cell cannot be bound to an appropriate definition. Debug the intended reference and library setup before altering the circuit to make the error disappear.
Technical Explanation
First identify the exact instance and referenced cell or module in the diagnostic. Determine whether it is a standard cell, a hard macro, a hierarchical block, or an intentional black box. Inspect the current reference-library list and paths, then verify that the expected library actually contains the required name and views. Search-path success alone does not prove the correct release was selected. Common causes include omitted macro libraries, a netlist mapped to a different standard-cell family, renamed modules, incomplete hierarchy, and stale or incompatible library revisions. Also check pin/interface consistency: finding a cell with the same name but different pins is not a valid repair. If reference setup changes, use the documented linking or rebinding flow and recheck affected objects. After repair, confirm the unresolved-reference report and compare design counts with the handoff. Rerun relevant timing, physical, and power-intent checks because a newly resolved model can change their results. Document intentional exceptions with an owner and purpose; otherwise a black box can hide missing logic or timing.
Common Mistake
Replacing unknown cells with arbitrary buffers or suppressing link warnings to proceed with placement.
Follow-up Question & Model Response
Why can a library-list change affect already linked instances? Binding and reference ordering can change which definitions are used; inspect the documented rebind behavior and resulting models.
Practical Example
The netlist instantiates SRAM_1K32 but only SRAM_51232 is present in the macro library. Renaming the netlist reference to the available macro changes the design specification. Obtain or regenerate the correct model instead.
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