sujeto1 February 2016

Simulation of Modelsim launching from Quartus doesn't work properly

This is the test bench

`timescale 1 ps/ 1 ps

module sum_fix_vlg_tst();

reg select;
reg [7:-8] valor_a;
reg [7:-8] valor_b;

// wires                                               
wire [8:-8]  result_fx;

sum_fix i1 (


$monitor ("valor_a = %b, valor_b = %b, result_fx = %b", valor_a, 

select = 1;
valor_a = 32'b0000000011111111;
valor_b = 32'b0000000011111111;


valor_a = 32'b1111111111111111;
valor_b = 32'b1111111111111111;


valor_a = 32'b1001100111001000;
valor_b = 32'b0001111000111101;



And this is the uut

`timescale 1 ps/ 1 ps
module sum_fix (valor_a,valor_b,result_fx,select);

input [7:-8] valor_a,valor_b;
output reg [8:-8] result_fx;
input select;

always@ (valor_a or valor_b)
if (select==1)
result_fx = valor_a + valor_b;
result_fx = valor_a - valor_b;


Compiled in Quartus well. Then I launch Modelsim like this: Tools > RTL simulation. It launch Modelsim but it only gives me a lot of zzzzzzzzzzzzz in input and xxxxxxxxxxxxxx in output without anything in the wave.

I want to say that I'm not sure if I added this testbench correctly to the project. I'm a beginer. What I did was: Assignments > Settings > Compile Test: click Test Benches > new, looked for the file and add, and ok. I'm not sure that this is the correct way, because it looks too complex. Please help on this.

Also, I comment that I initiate the test bench writter template in quartus, but I ended changing everything from the file, final result is the code above.


wilcroft February 2016

So, I tried your code on EDAPlayrgound (, and it looks fine to me. (You an give it a run for yourself, if you'd like).

You're seeing X's because at time 0, you haven't specified values for any of your inputs (since you delay 10 timesteps before assigning anything). Consider either removing the first #10, or putting some assignments before it.

(X = Unknown Value; Z = High Impedance, which means neither a 1 nor a 0 input).

As for setting it up in Quartus, the way you did it was correct.

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Asked in February 2016
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