To support separate compilation, extern declarations of a module can be used to declare the ports on a module without defining the module itself. An extern module declaration consists of the keyword extern followed by the module name and the list of ports for the module.

  1 extern module counter (input clk,enable,reset, 
  2   output logic [3:0] data);
  3 
  4 //+++++++++++++++++++++++++++++++++++++++++++++++++
  5 // Extern Module
  6 //+++++++++++++++++++++++++++++++++++++++++++++++++
  7 module extern_module();
  8 
  9 logic clk = 0;
 10 always  #1  clk++;
 11 logic enable, reset;
 12 wire [3:0] data;
 13 
 14 counter U(clk,enable,reset,data);
 15 
 16 initial begin
 17   $monitor("@%0dns reset %b enable %b data %b",
 18      $time,reset,enable,data);
 19   reset <= 1;
 20    #10  reset <= 0;
 21    #1  enable <= 1;
 22    #10  enable <= 0;
 23    #4  $finish;
 24 end
 25 
 26 endmodule

  1 //=================================================
  2 // Module declration 
  3 //=================================================
  4 module counter(input clk,enable,reset, 
  5 output logic [3:0] data);
  6 
  7   always @ (posedge clk)
  8     if (reset) data <= 0;
  9     else if (enable) data ++;
 10 
 11 endmodule

 @0ns reset 1 enable x data xxxx
 @1ns reset 1 enable x data 0000
 @10ns reset 0 enable x data 0000
 @11ns reset 0 enable 1 data 0000
 @13ns reset 0 enable 1 data 0001
 @15ns reset 0 enable 1 data 0010
 @17ns reset 0 enable 1 data 0011
 @19ns reset 0 enable 1 data 0100
 @21ns reset 0 enable 0 data 0101

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