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AR# 55908

Vivado Timing - What does unexpandable_clocks mean in the check_timing report?


While I run the command "check_timing", there are some unexpandable_clocks in the report.

What does this mean?


The unexpandable_clocks section of the check timing report reports clock sets in which

1) The period is not expandable with respect to each other and
2) there is at least 1 path between the clock sets.

A clock set is unexpandable if no common multiples are found within 1000 cycles between the source and destination.

If you are seeing some unexpandable_clocks then the requirement between the clock pairs cannot be determined by the tool.

You will need to apply Timing Exception constraints to the clock pairs to set false paths or else explicitly give the correct requirement for analysis.

Note: The unexpandable_clocks section in the check_timing report only detects and reports the unexpandable clocks but does not check if the clock pairs are correctly constrained.

You will still see the unexpandable_clocks even if you already added the correct Timing Exception constraints.

To check if the clock pairs are correct constrained, run report_clock_interactions and check the "Path Req (WNS)" and "Path Req (WHS)" columns - "Unexpanded" means no Timing Exceptions are added.
AR# 55908
Date 01/21/2015
Status Active
Type General Article
  • Vivado Design Suite