start_vcd - 2024.1 English

Vivado Design Suite Tcl Command Reference Guide (UG835)

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2024.1 English

Start capturing VCD output (equivalent of $dumpon verilog system task). This can be used after a stop_vcd TCL command has stopped VCD generation started by open_vcd


start_vcd [‑ports] [‑file <arg>] [‑quiet] [‑verbose]


Name Description
[-ports] Control extended vcd dump file.
[-file] Specify the file where ports have to start dumping . Default: Starts all dumpport files
[-quiet] Ignore command errors
[-verbose] Suspend message limits during command execution



The start_vcd command specifies that the tool start writing Value Change Dump (VCD) information into the specified VCD object. This Tcl command models the behavior of the Verilog $dumpon system task.

Important: You must execute the open_vcd command before using the start_vcd command.

Nothing is returned by this command.


-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.


The following example starts the writing of HDL signals into the current VCD file: