Exceptions Priority with -reset_path Example - 2022.2 English

Vivado Design Suite User Guide: Using Constraints (UG903)
Document ID
UG903
Release Date
2022-11-02
Version
2022.2 English
>	set_false_path -from [get_clocks clkA] -to [get_clocks clkB]
>	set_max_delay 1 -from [get_clocks clkA] -to [get_clocks clkB] -reset_path

The paths between clocks clkA and clkB are covered by the Max Delay with a path requirement of 1ns. The Max Delay is defined with the same arguments for -from/-to and specifies -reset_path, which overrides the False Path.

>	set_false_path -from [get_clocks clkA] -to [get_clocks clkB]
>	set_max_delay 1 -from [get_pins reg0/CLK] -to [get_pins reg1/D] -reset_path

The paths between reg0/CLK and reg1/D are covered by the False Path since that constraint has a higher precedence over the Max Delay. The Max Delay doesn't override the False Path despite the -reset_path as it is not defined with the same arguments for -from/-to.

Recommended: You must avoid using several timing exceptions on the same paths, so that the timing analysis results are not dependent on priority rules, and it is easier to validate the effect of your constraints.

It is recommended that you validate the timing exceptions with the report_exceptions command. This command provides insight on which timing exceptions are overriden or ignored. For more information, refer to Vivado Design Suite User Guide: Design Analysis and Closure Techniques (UG906).

If a string instead of an object is passed to the constraint, the Tcl interpreter uses the following sequence to determine which object matches the string:

  1. port
  2. pin
  3. cell
  4. net

The search is not exhaustive. As soon as objects of a certain type match the string pattern, they are returned, even though objects of another type down the list might also match the same pattern.