A critical path detours around a macro despite acceptable centre-to-centre distance. What should you inspect?

Short Answer

This is a classic trap: the straight-line, center-to-center distance between two macros can look perfectly reasonable while the actual legal route between their real pins is much longer — because timing follows routable geometry, not a ruler line drawn between abstract macro centers. Start with the real pin locations, not the macro centers — if the relevant pins sit on the far sides of each macro relative to each other, the effective route is already longer than the naive center-to-center number suggested.

Technical Explanation

• Check the actual route corridor available — is there a legal, unblocked path connecting those two pins, or does the router have to work around placement/routing blockages that the center-distance metric never accounted for?
• Check voltage-area boundaries along the path — crossing into a different voltage area can force a detour (or a feedthrough/level-shifter insertion) that a pure-distance metric would completely miss.
• Consider whether a feedthrough option exists through an intervening block that could shorten the effective path — if one's available and wasn't used, that's worth investigating as a fix.
• Check hierarchy crossings and local congestion along the candidate route — a technically shorter path through a heavily congested region may not be what the tool actually chose, and understanding why it didn't choose it tells you what's really constraining the path.
• Look for missed buffering opportunities too — sometimes the "detour" exists specifically because the direct path lacked legal sites for the repeaters the signal needed.
• The methodology fix: compare the full estimated path length (not center distance) and the floorplan's actual route opportunity between both the current floorplan and any alternative you're considering, using identical constraints for a fair comparison.

Formula Or Decision Rule

Decision rule: reduce the legal route and preserve access, not merely the geometric distance.

What To Check

• Warning sign: The macro is moved closer while its pins still face away.
• Inspect: choose one affected region, macro, row, pin, path, or net and trace the physical cause.
• Correct: Reorient or reposition it, open the correct corridor or feedthrough, and rerun timing plus congestion.

Healthy, Suspicious & Hard-stop Results

• Healthy: Inspect actual macro pin sides, route corridor, blockages, voltage areas, feedthrough options, hierarchy crossings, congestion, and buffering sites. Centre distance can hide a long legal path.
• Hard stop: required legality or physical feasibility is missing or unexplained.

What The Interviewer Is Testing

The interviewer is testing whether you can connect “A critical path detours around a macro despite acceptable centre-to-centre distance. ” to measurable physical evidence and an owned correction.

Follow-up Question & Model Response

"* Candidate Model Response: Model response: “I would change it if the same controlled rerun shows that the macro is moved closer while its pins still face away. ”