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How Is It Different From Hard Constraints?

SystemVerilog Constraints

The same code with a hard range constraint, so randomize() with { data == 2; } conflicts and randomization fails.

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testbench.sv
//--------------------------------------------------------------
// Author  : Admin, ChipVerify
// Website : www.chipverify.com
//--------------------------------------------------------------

class ABC;
  rand bit [3:0] data;

  constraint c_data { data >= 4;
                     data <= 12; }
endclass

module tb;
  ABC abc;
  
  initial begin
    abc = new;
    for (int i = 0; i < 5; i++) begin
      abc.randomize() with { data == 2; };
      $display ("abc = 0x%0h", abc.data);
    end
  end
endmodule



/*
Simulation Log:
---------------

ncsim> run
      abc.randomize() with { data == 2; };
                  |
ncsim: *W,SVRNDF (./testbench.sv,19|18): The randomize method call failed. The unique id of the failed randomize call is 0.
Observed simulation time : 0 FS + 0
ncsim: *W,RNDOCS: These constraints contribute to the set of conflicting constraints:

  constraint c_data { data >= 4; (./testbench.sv,9)
      abc.randomize() with { data == 2; }; (./testbench.sv,19)
ncsim: *W,RNDOCS: These variables contribute to the set of conflicting constraints:

rand variables:
       data [./testbench.sv, 7]

abc = 0x0
      abc.randomize() with { data == 2; };
                  |
ncsim: *W,SVRNDF (./testbench.sv,19|18): The randomize method call failed. The unique id of the failed randomize call is 1.
Observed simulation time : 0 FS + 0
ncsim: *W,RNDOCS: These constraints contribute to the set of conflicting constraints:

  constraint c_data { data >= 4; (./testbench.sv,9)
      abc.randomize() with { data == 2; }; (./testbench.sv,19)
ncsim: *W,RNDOCS: These variables contribute to the set of conflicting constraints:

rand variables:
       data [./testbench.sv, 7]

abc = 0x0
      abc.randomize() with { data == 2; };
                  |
ncsim: *W,SVRNDF (./testbench.sv,19|18): The randomize method call failed. The unique id of the failed randomize call is 2.
Observed simulation time : 0 FS + 0
ncsim: *W,RNDOCS: These constraints contribute to the set of conflicting constraints:

  constraint c_data { data >= 4; (./testbench.sv,9)
      abc.randomize() with { data == 2; }; (./testbench.sv,19)
ncsim: *W,RNDOCS: These variables contribute to the set of conflicting constraints:

rand variables:
       data [./testbench.sv, 7]

abc = 0x0
      abc.randomize() with { data == 2; };
                  |
ncsim: *W,SVRNDF (./testbench.sv,19|18): The randomize method call failed. The unique id of the failed randomize call is 3.
Observed simulation time : 0 FS + 0
ncsim: *W,RNDOCS: These constraints contribute to the set of conflicting constraints:

  constraint c_data { data >= 4; (./testbench.sv,9)
      abc.randomize() with { data == 2; }; (./testbench.sv,19)
ncsim: *W,RNDOCS: These variables contribute to the set of conflicting constraints:

rand variables:
       data [./testbench.sv, 7]

abc = 0x0
      abc.randomize() with { data == 2; };
                  |
ncsim: *W,SVRNDF (./testbench.sv,19|18): The randomize method call failed. The unique id of the failed randomize call is 4.
Observed simulation time : 0 FS + 0
ncsim: *W,RNDOCS: These constraints contribute to the set of conflicting constraints:

  constraint c_data { data >= 4; (./testbench.sv,9)
      abc.randomize() with { data == 2; }; (./testbench.sv,19)
ncsim: *W,RNDOCS: These variables contribute to the set of conflicting constraints:

rand variables:
       data [./testbench.sv, 7]

abc = 0x0
ncsim: *W,RNQUIE: Simulation is complete.
ncsim> exit
*/
design.sv
// Code your design here
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