BeginnerQuestion 76 of 187

What is a multisource clock tree, and why would a design need one instead of a traditional CTS-built tree?

From PDVerse PnR Interview Handbook, part of the pdVerse Mentor Guide

By Tabish Iqbal, who has 9+ years of VLSI Physical Design experience

Short Answer

A multisource clock tree (MSCTS) is a custom clock structure with more on-chip-variation tolerance and better cross-corner performance than a traditional tree. It consists of a global clock structure (root, global tree -- usually an H-tree -- mesh drivers, and the clock mesh) plus local subtrees driven either by predefined tap drivers (a regular MSCTS) or directly from multiple mesh points (a structural MSCTS).

Technical Reference DiagramWhat is a multisource clock tree, and why would a design need one instead of a traditional CTS-built tree?

Technical Explanation

  • A multisource clock tree is a custom clock structure built specifically for more OCV tolerance and better cross-corner performance than a traditional CTS-built tree.
  • It has two parts: a global clock structure (clock root, global tree -- usually an H-tree -- mesh drivers, and the clock mesh) and local subtrees.
  • Local subtrees are driven either by predefined tap drivers connected to the mesh (a regular multisource clock tree, built with synthesize_clock_trees/clock_opt) or directly from multiple mesh points (a structural multisource clock tree, which preserves a user-defined structure and is optimized by merging/splitting/sizing).
  • The two structural parts described are built with real commands: create_clock_straps constructs the mesh, and create_clock_drivers places the tap/mesh drivers that connect local subtrees into it.

Common Mistake

The Trap: Treating "multisource clock tree" as just a fancier name for a normal clock tree, without understanding it's a genuinely different structure (global mesh plus local subtrees) built for cross-corner robustness specifically.

Follow-up Question & Model Response

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

Candidate Model Response: Regular MSCTS has local subtrees built by the standard synthesize_clock_trees/clock_opt flow from predefined tap drivers; structural MSCTS has subtrees driven directly from mesh points, preserving a user-defined structure that's refined by merging/splitting/sizing instead of built fresh.

Practical Example

Tapeout Scenario: A very large, high-performance core with tight cross-corner skew requirements uses a multisource clock tree instead of a traditional Sum/Pi tree specifically because a single global mesh plus local subtrees tolerates on-chip variation better than one long traditional tree can.

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