Unsigned to signed conversion and its simulation in Modelsim

Altera_Forum · ‎10-31-2016

Hello dear friends,

I am having an unsigned number of the input and need to sign extend it and negate (2s representation) if in the specific condition occurs (say if sign=1, to simplify)

Here is my code in SystemVerilog:

module signed_unsigned (

sign,

clk,

data_in,

data_out);

input logic sign;

input logic clk;

input unsigned [7:0] data_in;

output signed [8:0] data_out;

always_ff @ (posedge clk) begin

if (sign)

data_out <= -data_in;

else

data_out <= data_in;

end

endmodule

It looks like it analyzed properly in Quartus (see RTL representation in the attachment). However, when I simulate in Modelsim it doesn't show the correct result.

In the testbench I have a clock generator and just assignments to the data_in. I expect to see 101 value for 0xFF, 156 for 0xAA, etc but nothing happens. the numbers are same

What can be wrong? I am very confused.

Cheers,

Altera_Forum · ‎10-31-2016

I will use signed(for unsigned addt one MSB at zero)

e.g. assuming 8 bit signed:

if...

data <= input;

else

data <= -input

end

Altera_Forum · ‎10-31-2016

This works for me after making one correction unrelated to the signed issue. data_out needs to be a variable in order to make procedural assignments to it. You have it declared as a wire. (you are also using Verilog-1995 style ports, which I updated)

module signed_unsigned (
  input logic sign,
  input logic clk,
  input unsigned  data_in,
  output logic signed  data_out
);
  always_ff @ (posedge clk) begin
  if (sign)
    data_out <= -data_in;
  else
    data_out <= data_in;
  end
endmodule

It would have helped to show the testbench you were using. Here is what I used.

module top;
   bit clk, sign;
      logic unsigned  data_in;
   logic signed  data_out;
 
  always# 5 clk++;
   
  initial begin
      sign=1;
      data_in = 8'hff;
   @(negedge clk)
     $displayh(data_in,,data_out);
      data_in = 8'hAA;
   @(negedge clk)
      $displayh(data_in,,data_out);
     sign=0;
      data_in = 8'hff;
   @(negedge clk)
     $displayh(data_in,,data_out);
      data_in = 8'hAA;
   @(negedge clk)
      $displayh(data_in,,data_out);
     $finish;      
end
   signed_unsigned su(.*);
   
endmodule : top

And the output I got

# Loading work.top(fast)
#  run -all
#  ff 101
#  aa 156
#  ff 0ff
#  aa 0aa
#  ** Note: $finish    : signed.sv(25)

Altera_Forum · ‎10-31-2016

Thank you, dave_59. It is working now! My testbench was a bit crooked too :)