My testbench only shows don't cares for my inputs

Question

When I test my dataflow module, all my inputs come out as don't cares. I'm not sure where the problem lies. Could it be my equations or my testbench itself?

Here is my code.

Dataflow:

module TollSystem(weekDay,rushHour,lateNight,highTraffic,lowRate,mediumRate,highRate);

input weekDay,rushHour,lateNight,highTraffic;

output lowRate,mediumRate,highRate;

assign #6 lowRate = ~weekDay&~rushHour&lateNight&~highTraffic | 
weekDay&~rushHour&lateNight&~highTraffic;

assign #6 mediumRate = (~rushHour&~lateNight&~highTraffic) | (~weekDay&~lateNight&~highTraffic) | 
(rushHour&lateNight);

assign #6 highRate = (weekDay&~lateNight&rushHour) | 
(~lateNight&highTraffic|~rushHour&lateNight&highTraffic);

endmodule

TestBench:

module TollSystemtest();

reg weekDay,rushHour,lateNight,highTraffic;

wire lowRate,mediumRate,highRate;

TollSystem TSys(weekDay,rushHour,lateNight,highTraffic,lowRate,mediumRate,highRate);

initial
begin

 weekDay=0;rushHour=0;lateNight=0;highTraffic=0;
 weekDay=0;rushHour=0;lateNight=0;highTraffic=1;
 weekDay=0;rushHour=0;lateNight=1;highTraffic=0;
 weekDay=0;rushHour=0;lateNight=1;highTraffic=1;
 weekDay=0;rushHour=1;lateNight=0;highTraffic=0;
 weekDay=0;rushHour=1;lateNight=0;highTraffic=1;
 weekDay=0;rushHour=1;lateNight=1;highTraffic=0;
 weekDay=0;rushHour=1;lateNight=1;highTraffic=1;
 weekDay=1;rushHour=0;lateNight=0;highTraffic=0;
 weekDay=1;rushHour=0;lateNight=0;highTraffic=1;
 weekDay=1;rushHour=0;lateNight=1;highTraffic=0;
 weekDay=1;rushHour=0;lateNight=1;highTraffic=1;
 weekDay=1;rushHour=1;lateNight=0;highTraffic=0;
 weekDay=1;rushHour=1;lateNight=0;highTraffic=1;
 weekDay=1;rushHour=1;lateNight=1;highTraffic=0;
 weekDay=1;rushHour=1;lateNight=1;highTraffic=1;



$finish();
end
endmodule

input/output results:

[1

Answer 1

When I run your code, the simulation ends at time 0. Since no time elapses, it is no surprise that your inputs are reported as x/z. You need to add delays in your testbench. For example:

initial
begin
         weekDay=0;rushHour=0;lateNight=0;highTraffic=0;
    #20; weekDay=0;rushHour=0;lateNight=0;highTraffic=1;
    #20; weekDay=0;rushHour=0;lateNight=1;highTraffic=0;
    #20; weekDay=0;rushHour=0;lateNight=1;highTraffic=1;
    #20; weekDay=0;rushHour=1;lateNight=0;highTraffic=0;
    #20; weekDay=0;rushHour=1;lateNight=0;highTraffic=1;
    #20; weekDay=0;rushHour=1;lateNight=1;highTraffic=0;
    #20; weekDay=0;rushHour=1;lateNight=1;highTraffic=1;
    #20; weekDay=1;rushHour=0;lateNight=0;highTraffic=0;
    #20; weekDay=1;rushHour=0;lateNight=0;highTraffic=1;
    #20; weekDay=1;rushHour=0;lateNight=1;highTraffic=0;
    #20; weekDay=1;rushHour=0;lateNight=1;highTraffic=1;
    #20; weekDay=1;rushHour=1;lateNight=0;highTraffic=0;
    #20; weekDay=1;rushHour=1;lateNight=0;highTraffic=1;
    #20; weekDay=1;rushHour=1;lateNight=1;highTraffic=0;
    #20; weekDay=1;rushHour=1;lateNight=1;highTraffic=1;
    $finish();
end

Now, when I run a simulation and look at waveforms, I see all signals go to known values.

I added a delay of #20, but you should change that to something more meaningful to your design.