When should macros be edge-biased versus placed internally?

Short Answer

There's no universal right answer here — it genuinely depends on where the macro's pins, its logical neighbors, and its routing needs actually point, so the decision has to be made from the connectivity data, not a rule of thumb. Edge placement tends to simplify external access (bringing signals in/out from I/O, or connecting to neighboring blocks) and keeps the central core area free and continuous for standard-cell rows — good when the macro mostly talks to the outside world.

Technical Explanation

• Internal placement can shorten central dataflow paths when the macro is heavily used by logic scattered across the whole core, but it comes at a real cost: more channel fragmentation and more blockage interactions around the macro's boundary, since standard cells now have to route around it from all sides instead of just one.
• Compare macro pin sides, timing-critical path direction, row continuity, power/ground access, and escape routing capacity for both options before deciding — this is a data-driven placement study, not aesthetics.
• One thing to actively avoid: don't push so many macros to the edges that you effectively wall off the entire core boundary — that turns your I/O and routing access into a bottleneck of its own.

Formula Or Decision Rule

Decision rule: choose the strategy with the stronger combined timing, routability, and change-risk evidence.

What To Check

• Warning sign: All macros are pushed to edges by habit, creating long internal buses or corner congestion.
• Inspect: choose one affected region, macro, row, pin, path, or net and trace the physical cause.
• Correct: Test an alternative with the same area and measurement method, then document the winning tradeoff.

Healthy, Suspicious & Hard-stop Results

• Healthy: Edge placement can simplify external access and preserve central rows; internal placement can shorten central dataflow but creates more channel and blockage interactions.
• Hard stop: required legality or physical feasibility is missing or unexplained.

What The Interviewer Is Testing

” 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 all macros are pushed to edges by habit, creating long internal buses or corner congestion. ”