What scan and DFT information is needed for physical design?
From PDVerse PnR Interview Handbook · pdVerse Mentor Guide
Short Answer
Physical design needs the scan connectivity and allowed reordering constraints when it optimizes scan chains. A scan-inserted netlist describes connections; SCANDEF supplies scan-chain structure and constraints used by the physical optimization flow.
Technical Explanation
Scan chains make internal registers accessible during manufacturing test. Scan shifting serially transfers data through registers; capture exercises the circuit for test. The test clocks, enable values, and timing assumptions can differ from normal operation, so the timing setup must represent the required test modes as well as functional mode. Moving scan registers changes the physical distance between consecutive scan elements. Reordering can reduce scan wire length and congestion, but it must respect the test methodology, fixed segments, clock domains, lockup requirements, and any prohibited changes. A shorter geometric chain is not automatically a valid DFT transformation. The supplied ICC2 guide describes reading SCANDEF with read_def and using the data in placement and clock optimization. It also documents explicit options for missing-SCANDEF behavior. For an interview, explain the normal required handoff and the implications of an exception; do not recommend bypassing missing information as routine setup. Check that chain names and register/pin references match the delivered netlist revision.
Common Mistake
Assuming the functional SDC completely covers scan shift and capture, or allowing unconstrained scan reordering across all registers.
Follow-up Question & Model Response
Why is updated scan information needed downstream? The test flow must use the final valid scan ordering and connectivity after physical optimization.
Practical Example
A netlist revision adds ten scan registers but the SCANDEF still describes the old chain. Even if import finishes, chain coverage can be incomplete. Compare annotated scan elements with the DFT handoff and resolve the mismatch before reordering.
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