What this chapter covers

Earlier chapters defined constraint types. This chapter is how you manage the whole set: SDC organization, check_timing completeness, consistency across flow stages and hierarchy, top-down budgeting, and remaining commands (derates, units, object selection). Constraint quality is a process, not a file that “looks full.”

Every signoff review should include lint, coverage, exception lists, and a diff against synthesis/P&R SDC. Hierarchical chips need budgets that add up at the top.

Worked example

A practical loop: read SDC → check_timing → unconstrained endpoints report → exception report → compare clocks and I/O against the spec → only then read WNS. If synthesis used a looser false path than signoff, the netlist was optimized against a different intent.

A common misconception

Treating a green WNS as proof of complete constraints. Another is maintaining five slightly different SDCs with no owner, so exceptions rot. Wildcard object collections that shrink after a rename apply to nothing.

Sample interview answers

What does check_timing catch that WNS does not? Missing clocks, unconstrained endpoints, unresolved generated clocks, loops, and other constraint holes that never appear as a failing path.

Why compare SDC across synthesis and signoff? If exceptions or clocks differ, the netlist was built under one intent and signed off under another.

What is a healthy exception policy? Every false path and MCP has an owner, a reason, a tight collection, and a report that proves it did not go wider than intended.

The paid chapter includes lint checklists, precedence notes, miscellaneous SDC commands, revision/interview sheets, and the 80-question close-out for the SDC handbook.

Library: SDC handbook · STA: Configuring STA · Related: check_timing, STA questions

💡 Key Technical Topics Covered

  • PrimeTime check_timing Error Debugging
  • SDC Constraint Precedence Rules
  • Hierarchical SDC File Organization
  • Constraint Signoff & Audit Checklist

🎯 VLSI Physical Design Interview Questions Addressed

  • What are the SDC constraint priority rules when multiple exceptions overlap?
  • How do you check for unconstrained endpoints using check_timing?
  • What is the signoff checklist for validating SDC files before layout?