AR# 54909


Vivado Timing - How can I verify if my timing exception constraints are correctly applied?


I am adding timing exception constraints for some paths.

How can I verify if these constraints are correctly applied and affect timing analysis as expected?


You can verify the timing exception constraints in a Synthesized or Implemented design in the following ways:

Solution #1: Run report_exceptions command in the Tcl console.
Solution #2: Run the "report_timing -from -to" command in the Tcl console to directly query the path(s) to check.

You can verify if the timing exception takes effect by checking the "Slack" or "Timing Exception" in the returned report.

Below is an example.


set_false_path -from [get_pins iSpiSerDes/regSpiCsB_fall_reg/C] -to [get_ports outSpiCsB]

Solution #1:

Returned report:
Position  From                                                                  Through       To                                   Setup          Hold            Status             
----------  ---------------------------------------------------------   --------------   -----------------------------   --------------  --------------  -----------------  
21           [get_pins iSpiSerDes/regSpiCsB_fall_reg/C]    *                  [get_ports outSpiCsB]    false           false             

Solution #2:
report_timing -from [get_pins iSpiSerDes/regSpiCsB_fall_reg/C] -to [get_ports outSpiCsB]

Returned report:
Slack:                    inf
  Source:                 iSpiSerDes/regSpiCsB_fall_reg/C
                            (falling edge-triggered cell FDRE clocked by clk_out1_clk_wiz_0  {rise@0.000ns fall@7.500ns period=15.001ns})
  Destination:            outSpiCsB
                            (output port clocked by cclk  {rise@0.000ns fall@7.500ns period=15.001ns})
  Clock Path Skew:        2.067ns (DCD - SCD + CPR)
  Clock Uncertainty:      0.190ns  ((TSJ^2 + DJ^2)^1/2) / 2 + PE
  Timing Exception:       False Path

The "inf" Slack indicates a false path slack.

The "Timing Exception" shows the exception type of this path.

AR# 54909
Date 02/27/2015
Status Active
Type General Article
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