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Clock domain crossing

Tell me

A clock domain crossing (CDC) is a path whose launching flip-flop and capturing flip-flop are clocked by different clocks, so the data moves from one clock's timing to another's.

Lens's suggestion rules include a hint for path groups whose names match async_*_default: verify the CDC intent and the false, multicycle and async constraints.

Show me

Lens has no dedicated CDC panel. Crossings surface as suggestions in the Timing Explorer for the selected clock, and the report that feeds them is that clock's own setup or hold timing report.

  • Timing Review tab, Per-Clock Breakdown: switch to By Path Group to list results by path group, where you can look for groups whose names match async_*_default.
  • Select a path-group row, then use Digital Soul's Diagnose Selected Path Group. It explains what that group contains: its WNS, TNS and violations, its share of the run, the clock families inside it and representative failing paths. Setup and hold stay separate.
  • Timing Explorer, Suggestions: rule-based recommendations for the selected path. The suggestion rules, the path-group hint among them, drive this panel and the rule chips in the Path Budget Waterfall header, which carries one chip per rule that fired. Click a suggestion chip to open the full rule as a deterministic card in the Digital Soul rail.
  • Scenario Analyzer: patterns across scenarios and clocks, such as a specific clock domain failing across all hold corners.

Teach me

Lens detects crossings itself, from the paths. When you open a clock in the Timing Explorer, Lens scans that clock's report for paths whose Startpoint is clocked by one clock and whose Endpoint is clocked by another, and raises a Cross-domain path detected suggestion for the first two it finds, naming the clock and the launch and capture clocks, and asking you to verify that the crossing is intentional and covered by set_clock_groups, set_false_path or a synchronizer.

Lens does not read your constraints, so it cannot tell an intended crossing from an accidental one. A path your timing tool already excludes, for example by a false-path constraint, never appears in the report and so is never seen by Lens; a crossing that does appear is timed and listed like any other path, and the suggestion is only a prompt to check its intent.

The hint comes from the Path-Group Guidance suggestion rule, a contextual hint chosen by path group.

The rule fires for any path group whose name contains both async and default, so PrimeTime's built-in async_default group matches, as does any async_*_default group of your own.

  • A path group is the report's grouping of paths, never a clock, even when it has the same name as a clock.
  • Lens does not parse SDC files for timing, and never writes them.

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Updated 19 October 2026