set_external_delay - 2024.1 English

Vivado Design Suite Tcl Command Reference Guide (UG835)

Document ID
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2024.1 English

Set external delay


set_external_delay ‑from <args> ‑to <args> [‑min] [‑max] [‑add] [‑quiet]
    [‑verbose] <delay_value>


Name Description
-from Output port
-to Input port
[-min] Specifies minimum delay
[-max] Specifies maximum delay
[-add] Add to existing external delay
[-quiet] Ignore command errors
[-verbose] Suspend message limits during command execution
<delay_value> External (feedback) delay value


XDC, Timing


Tip: The XDC > Timing Constraints language templates and the Timing Constraints Wizard in the Vivado™ IDE offer timing diagrams and additional details around defining specific timing constraints. You can refer to these sources for additional information.

Sets the external (feedback) delay in nanoseconds (ns) between an output and input port. The external delay is used in the calculation of the PLL/MMCM compensation delay for PLLs/MMCMs with external feedback.

A min or max value can be specified. By default the value specified applies to both min (hold) and max (setup) compensation delays.

The command returns the defined delay.


-from <arg> - (Required) The output port name.

-to <arg> - (Required) The input port name.

-min - (Optional) Specifies the delay_value is a minimum delay value for hold time analysis.

-max - (Optional) Specifies the delay_value is a maximum delay value for setup analysis.

-add - (Optional) Add the specified delay to the existing external delay value.

-quiet - (Optional) Execute the command quietly, returning no messages from the command. The command also returns TCL_OK regardless of any errors encountered during execution.
Note: Any errors encountered on the command-line, while launching the command, will be returned. Only errors occurring inside the command will be trapped.
-verbose - (Optional) Temporarily override any message limits and return all messages from this command.
Note: Message limits can be defined with the set_msg_config command.

<delay_value> - (Required) The external delay value specified as nanoseconds (ns). The default value is 0.


The following example sets the external feedback delay to 1.0 ns between the port ClkOut and ClkFb:

set_external_delay -from [get_ports ClkOut] -to [get_ports ClkFb] 1.0