module stimulus;
  wire [3:0] max,med,min;
  reg [3:0] a,b,c;
  reg cin;
  sorting_three three(max,med,min,a,b,c,cin);

  initial begin

The above code giving correct result after Simulation. When I try to view RTL schematic. Its is giving error like:

ERROR:Xst - "stimulus.v" line 22: Module  has no port.

Total memory usage is 123976 kilobytes

Number of errors   :    1 (   0 filtered)
Number of warnings :    0 (   0 filtered)
Number of infos    :    0 (   0 filtered)

Process "Synthesis" failed

How to define port of stimulus( it is test bench)? I think wire and reg is sufficient to define input and output port. If it is not then what correction required to get correct RTL schematic.

  • 1
    \$\begingroup\$ You shouldn't be synthesizing a testbench anyway. A testbench module is meant to be used in simulation only, to represent parts of the application that are outside the module(s) being tested. Think about it -- a module that has no outputs synthesizes to zero logic, no matter how complex the code inside might be. \$\endgroup\$
    – Dave Tweed
    Sep 1, 2014 at 14:06

2 Answers 2


All modules need a list of ports, for example

module mymodule(port_a, port_b);

For a test bench module this list is empty, but it still needs to be there:

module stimulus();
  • 1
    \$\begingroup\$ Not completely correct. You can have module stimulus; without the (). \$\endgroup\$
    – dave_59
    Sep 2, 2014 at 15:22

The problem is whatever tool you are using to draw an RTL schematic won't accept a module that isn't 100% synthesizable. A synthesizable RTL block must have input and output ports.


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