IntermediateQuestion 258 of 142Source PDF page undefined

What's the actual structural difference between a regular and a structural multisource clock tree?

From PDVerse PnR Interview Handbook · pdVerse Mentor Guide

By Tabish Iqbal · 9+ Years VLSI Physical Design Experience

Short Answer

A regular MSCTS has local subtrees driven by predefined tap drivers connected to the mesh, built with the normal synthesize_clock_trees/clock_opt flow. A structural MSCTS has subtrees driven directly from multiple mesh points, preserving a user-defined structure that's optimized by merging/splitting/sizing rather than built fresh by the standard CTS algorithm -- the choice determines whether you're letting the tool build subtrees or preserving a structure you already designed.

Technical Reference DiagramWhat's the actual structural difference between a regular and a structural multisource clock tree?
What's the actual structural difference between a regular and a structural multisource clock tree?, illustrating the physical design concept.

Technical Explanation

  • A regular multisource clock tree has local subtrees driven by predefined tap drivers connected to the mesh, and those subtrees are built with the normal synthesize_clock_trees/clock_opt flow.
  • A structural multisource clock tree has subtrees driven directly from multiple mesh points, preserving a user-defined structure that gets optimized by merging/splitting/sizing rather than built fresh by the standard algorithm.
  • The practical choice: regular MSCTS lets the tool build subtrees for you from tap drivers; structural MSCTS preserves a structure you already designed and only refines it.
  • The two variants are built with different real commands: synthesize_clock_trees (or clock_opt) for the regular variant, versus synthesize_multisource_clock_subtrees for the structural variant.

Common Mistake

The Trap: Assuming both variants are built the same way by the standard CTS algorithm, when structural MSCTS specifically preserves a pre-existing user structure instead of building fresh.

Follow-up Question & Model Response

"Why would a designer choose structural MSCTS over regular when regular is the more automated option?"

Candidate Model Response: When a team has already hand-designed a subtree structure for known EM-critical or timing-critical regions and needs CTS to preserve and refine it, rather than replace it with an automatically-built structure from tap drivers.

Practical Example

Debug Scenario: A design team hand-designed a subtree structure for known EM-critical regions and needs CTS to respect it rather than rebuild it -- structural MSCTS is the right choice specifically because it preserves that structure through merging/splitting/sizing instead of replacing it.

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