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AR# 57056

Vivado Constraints - Warning:[Vivado 12-627] No clocks matched 'xxxx'


I am receiving warning messages like the following in Vivado Design Suite:

 [Vivado 12-627] No clocks matched 'clk'. [<constraint_file_name>.xdc:1]
 [Vivado 12-626] No clocks found. Please use 'create_clock' or 'create_generated_clock' command to create clocks. [<constraint_file_name>.xdc:1]

The following is an example constraint that is causing these warnings:

set_input_delay -clock [get_clocks clk] -max 4.0 [get_ports da_in]

In addition to the above two warnings, there is a Critical Warning given on this "set_input_delay" constraint.

 [Vivado 12-1387] No valid object(s) found for set_input_delay constraint with option '-clock [get_clocks clk]'. [<constraint_file_name>.xdc:1]


These messages indicate that the clock object that was queried in the "get_clocks" command could not be found when this constraint was processed. 

A clock object needs to be defined by "create_clock" or "create_generated_clock" before it is referenced in any other constraints or commands.

Below are possible causes of this issue:


1. The clock object does not exist in the design. 

You can run "report_clocks" in the Synthesized design or Implemented design to check if the clock is defined.
If the clock is not listed by "report_clocks":

  • Add "create_clock" or "create_generated_clock" to define the clock.
  • If you already have "create_clock" or "create_generated_clock" for this clock, check the Warning or Critical Warning messages and see if the constraint is ignored due to any syntax or usage error, or is overriden by another constraint.
    An example issue of this can be found in (Xilinx Answer 53805).
2. The clock object exists in the Synthesized or Implemented design, but it is defined after this constraint is processed.

  • If "report_clocks" returns this clock object and the constraint can be executed successfully in the Tcl Console in Synthesized or Implemented design, then this is a constraint processing order issue.
    You need to make sure that the "create_clock" or "create_generated_clock" constraint is processed before the constraint in question.
    You can run the "report_compile_order -constraints" command to check the processing order of the constraints files.
    If the order is not expected, set the PROCESSING_ORDER property of the constraints file to adjust it.
For more information on PROCESSING_ORDER, please refer to (UG903) Vivado Design Suite User Guide: Using Constraints.


For more information on "report_clocks" or "report_compile_order", please refer to (UG835) Vivado Design Suite Tcl Command Reference Guide.


AR# 57056
Date 05/11/2015
Status Active
Type General Article
  • Vivado Design Suite
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