vcd Command
The vcd command generates simulation results in VCD format. This command enables you to dump specified instances to a VCD file, to name the VCD file, to start and stop the dump process, and other functions. See also Writing Activity Data of the Design.
Note  This command is case sensitive.
vcd (option)
dumpfile <file_name> Gives a name to VCD file.  Default file name is dump.vcd. Invokes Verilog $dumpfile directive.  
dumpvars -m <module_name> [-l <level>]Dumps the specified variables and their values to the VCD file.
-m <module_name> dumps the module of that name.  
0 - Level 0 causes a dump of all variables in the specified module and in all module instances below the specified module. The argument 0 applies only to subsequent arguments which specify module instances, and no to individual variables.
1 -Level 1 dumps all variable within the module specified by -m; it does not dump variables in any of the modules instantiated by the module specified by -m.
Invokes Verilog $dumpfile directive.
dumpoffTemporarily suspends the dumping process, and dumps all selected variables as an ’X’ value. Invokes Verilog $dumpoff directive.  
dumponResumes the dumping process after the dumpoff option has been invoked.  Dumps all selected values at the time dumpon is invoked. Invokes Verilog $dumpon directive.  
dumpallCreates a checkpoint in the VCD file that dumps the current value of all selected variables. Invokes Verilog $dumpall directive.  
dumplimit <file_size>Limits the size of the VCD file. file_size specifies the maximum size of the VCD file in bytes. When the size of VCD file reaches the limit, the dump process stops and a comment is inserted in the VCD file indicating that the dump limit was reached. Invokes Verilog $dumplimit directive.  
dumpflushEmpties the operating system’s VCD file buffer to ensure that all the data in that buffer is stored in the VCD file. After executing, dump process resumes as before so no value changes are lost. Invokes Verilog $dumpflush directive.  
The vcd command can be used as follows.
Following are the commands you would use to write the VCD simulation values of the module UUT to a VCD file after running simulation for 1000 ns.
Specify which file to write:
vcd dumpfile adder.vcd
Specify which module net activities to write:
vcd dumpvars -m /UUT
Run simulation for given time:
run 1000 ns
Dump the activity data to the vcd file.
vcd dumpflush

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