What scan and DFT information is needed for physical design?
From PDVerse PnR Interview Handbook, part of the pdVerse Mentor Guide
Short Answer
Scan chains exist to make internal registers reachable during manufacturing test — data shifts serially through them during scan-shift and exercises the circuit during capture. A scan-inserted netlist only tells you that registers are connected in a chain; SCANDEF is what actually supplies the chain's physical structure and the reordering constraints the physical-optimization flow is allowed to use.
Technical Explanation
- Scan chains exist to make internal registers reachable during manufacturing test — data shifts serially through them during scan-shift and exercises the circuit during capture.
- A scan-inserted netlist only tells you that registers are connected in a chain; SCANDEF is what actually supplies the chain's physical structure and the reordering constraints the physical-optimization flow is allowed to use.
- Test-mode clocks, enable values, and timing assumptions can genuinely differ from functional mode, so your timing setup has to represent both — moving scan registers physically changes the distance between consecutive chain elements, and reordering can shrink that distance and relieve congestion.
- Reordering isn't a free lunch, though — it has to respect the DFT methodology: fixed segments, clock-domain boundaries, and lockup-cell requirements. A geometrically shorter chain that breaks those rules isn't a valid transformation, so in an interview, explain what the normal handoff looks like and the consequences of skipping it — don't present bypassing missing SCANDEF as a routine shortcut.
Command Checks & Actions
set_early_data_check_policy -policy strict -check place.coarse.missing_scan_defMakes a missing scan definition a hard error instead of letting placement silently proceed differently, exactly the failure mode this question is testing awareness of.
create_placementAttempting placement without a valid scan def is what actually surfaces the missing-scan-def error the policy above is guarding against -- the policy only changes how loudly that failure is reported.
Common Mistake
The Trap: 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?"
Candidate Model Response: The test flow must use the final valid scan ordering and connectivity after physical optimization.
Practical Example
Tapeout Scenario: 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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