Distribution constraints

SystemVerilog
1

Hello,

In this distribution constraint, I am not getting 50% of time exactly 5 bits are set and remaining 50% of time number of bits set are not 5

/*
Write a constraint in systemverilog
----------------------------
1. In a given array N, 5 bits are set 50% of the time
2. 5 bits are not set remaining 50% of the time
*/


class AC;
  rand bit[9:0] N;
  constraint c1 { N dist { $countones(N)==5 := 50,  $countones(N)!=5 := 50 }; }  
endclass

module TB;
  AC ac;
  
  initial
  begin
    repeat(10)
    begin
      ac = new();
      ac.randomize();
      $display("N is %b", ac.N);
    end
  end
endmodule

N is 0000000000
N is 0000000000
N is 0000000001
N is 0000000000
N is 0000000001
N is 0000000001
N is 0000000000
N is 0000000001
N is 0000000000
N is 0000000001

Is there any problem with the code

Thanks,
JeffD

2

Try:

  constraint c1 { $countones(N)==5  dist { 1 := 50,  0 := 50 }; }
1 Like
3

The problem statement seems awkward. As an example, I think this meets the requirements:

constraint c1 { N dist { 'b11111 := 1, 0 := 1 }; }

But I’m pretty sure it’s not what was intended.

4

@aaronferrucci, The verbose problem statement is "50% of the time randomize() gets called, 5 bits must be randomly set to 1 out of the 10 bits in variable N. The other 50% of the time, some number of bits other that 5 bits must be set.

1 Like
6

hi @dave_59 ,
can you explain why the above code tried by @dvuvmsv doesn’t work?

//Q. Write a constraint for a 10 bit variable so that; → 10% of the time 1 bit in en is high → 10% of the time 2 bits in en are high … → 10% of the time all 10 bits in en are high.

class example;
    rand bit [9:0] arr;
    
    constraint arr_dist{
      arr dist{($countones(arr)==1) := 10, ($countones(arr)==2) := 10,$countones(arr)==3 := 10, $countones(arr)==4 := 10, $countones(arr)==5 := 10, $countones(arr)==6 := 10, $countones(arr)==7 := 10, $countones(arr)==8 := 10, $countones(arr)==9 := 10, $countones(arr)==10 := 10};
    }
     function void print();
       $display("value of array is %b",arr);
     // $display("value of max_sum is %d",max_sum);
    endfunction      
  endclass

I want to try generating the arr variable in above fashion, i’m also getting all 0 as output after randomization.

7

Think of a dist constraint as a set membership inside expression first. It is basically (arr inside {0,1}) because the result of the two == equality expressions can only be true ‘1’ or false ‘0’.