// Signal Tracing example
//  Imagine you want to know what drives b
//  Start at step 1 ----+
//                      |
//                      V

module top (a,b);

output b;           // 1)  click on b to see what drives it
                    //   once you've clicked follow the instructions
                    //   at the top to the window

input  a;           // 9)  finally a is an input

sub1 s1(
    .c(a),          // 8) now we're back here, click on a

    .d(b));         // 2) b is driven by output d of sub1, so...
                    //     click on .d to see what drives this output
                    //     (only outputs can be clicked on in instantiations)

endmodule




module sub1 (c,d);

input  c;           //  7) c is an input, click on it to see what drives it
output d;

sub2 s1(
    .e(c),          //  6) e is driven by c , click on c to see what
                    //      drives it

    .f(d));         //  3) output d of sub1 is driven by output f of sub2
                    //    click on .f to see what drives output f

endmodule




module sub2 (e,f);

input  e;           // 5) e is an input to the module. As the module
                    //   is only instantiated once you can click on
                    //   e to go up to the line that drives e
output f;   

always @(e)
    begin
        f = !e;     // 4) Output f is driven by !e.
    end             //    As e is on the right hand side of the expression
                    //    you can click on e to find out what drives it.
endmodule