generating random numbers in verilog

Question

I ran into this issue while writing a test bench for a project. I found how to use $random in verilog but for some reason it produces strange results for me. I have 2 16 bit regs called A and B, I tested it with random number between 0-10 every second output is a value which is the max value that you can have in 16 bits. I wonder if anyone else had had similar issue and can offer help. Thanks

reg[15:0]a;
reg[15:0]b;

 integer seed,i,j;
 initial begin 


   for (i=0; i<6; i=i+1)
    begin
      a=$random%10; 
      #100;
      b=$random%20;
    $display("A %d, B: %d",a,b);    
   end 
$finish;

`$random`, `$urandom`, `$srandom` and `$urandom_range()` are some of the system tasks for generating random numbers. `$urandom_range` generates random numbers in a given range. `$srandom` for seed specific random number generation. — sharvil111, Dec 01 '15 at 03:47

e19293001 · Answer 1 · 2015-12-01T03:06:10.473

output is a value which is the max value that you can have in 16 bits.

$random produces signed random numbers. Negative numbers will appear to be large numbers when you print it as unsigned numbers.

The code that you posted might appear something like below and this is not within the range from 0 to 10.

A     8, B: 65517
A 65527, B: 65527
A     7, B:    17
A 65535, B: 65522
A     1, B:     9
A     8, B:    17

As a solution, use $urandom to generate unsigned random numbers that you expect between 0 to 10 and 0 to 20.

Here is a modification from the code that you posted

module test;
   initial begin
      reg[15:0]a;
      reg [15:0] b;

      integer    seed,i,j;
      for (i=0; i<6; i=i+1)
        begin
           a=$urandom%10; 
           #100;
           b=$urandom%20;
           $display("A %d, B: %d",a,b);    
        end 
      $finish;
   end
endmodule

The code above might produce an output something like:

A     8, B:     0
A     3, B:    10
A     7, B:    11
A     6, B:     3
A     2, B:     7
A     1, B:    10

Which I guess is what you expect.

The code can be found and run here.

generating random numbers in verilog

1 Answers1