Rand behaviour using randc

UVM
, ,
1

HI All,

This is the code to get rand behaviour using randc keyword.

module p1;
  class c1;
    randc bit [2:0] addr;
    function void post_randomize();
      $display("unique value %0d",addr);
    endfunction
  endclass
  c1 obj;
  initial begin
    repeat (8) begin
    obj = new;
    obj.randomize();
    end
  end
endmodule

The output of the code contains some repeated values since I am constructing obj eight times.

I am curious to find/know, what is the alternative way to get the same rand output by using randc.
Please suggest another alternative method without constructing objects eight times, I would like to get rand behavior from randc.

Thank You,

2

In reply to Thirumalesh Kumar:


 class c1;
    randc bit [2:0] addr;
    rand  bit [2:0] addr1;    
    constraint EQUAL { addr1 == addr ; }
 endclass
 c1 obj;
  initial begin
    obj = new;
    repeat (8) begin
    if( !obj.randomize() ) $display("Randomization fails !!");
    end
  end
3

In reply to ABD_91:

Please find the working code below

module top;
  class c1;
    randc bit [2:0] addr;
    function void post_randomize();
      $display("unique value %0d",addr);
    endfunction
  endclass
  c1 obj;
  initial begin
    obj = new;
    repeat (9) begin
    void'(obj.randomize());
    end
  end
endmodule

I’m constructing the object once. Afterwards I’m iterating the randomization.

4

In reply to chr_sue:

I’m constructing the object once. Afterwards I’m iterating the randomization.

This doesn’t meet the requirement of “rand behaviour using randc”
Both of our solutions don’t meet the requirement :(

5

HI ABD_91 and chr_sue, Thank you for the reply.

Above code will not give the solutin and I am sorry if I would have asked in the wrong way.

Here is the requirement: using randc, I want to get normal random values which may repeat.
For example: 0 1 2 2 7 6 5 5

I do not want see all possible values like : 0 1 2 3 4 5 6 7

How to get random numbers with some are repeated using randc?

Thank You,

6

In reply to Thirumalesh Kumar:
I believe this is impossible. The SV LRM says
“Variables declared with the randc keyword are random-cyclic variables that cycle through all the values in a random permutation of their declared range. The basic idea is that randc randomly iterates over all the values in the range and that no value is repeated
within an iteration.”

7

In reply to Thirumalesh Kumar:

The reason you appear to be getting rand behavior from a randc member is because you are constructing a new object each time. Each time you construct an object, it gets seeded by the next random state value (RNG) from the process constructing the object. That starts a new random cyclic series of numbers which has a 1 in 8 chance of repeating the start of the previous series.

To get the behavior you are looking for without constructing a new object, you would have to manually re-seed the object using obj.srandom() before calling obj.randomize() each time. That would break the random stability of obj, but that is the best answer you are going to get.

8

In reply to dave_59:

Dave,
I did try the srandom() approach:


 initial begin
    obj = new();
    repeat (9) begin    
      void'(obj.randomize());
      obj.srandom($urandom()); // Is this correct  ?
    end
  end

I still observe randc behavior