SystemVerilog Disable Constraints

All constraints are by default enabled and will be considered by the SystemVerilog constraint solver during randomization. A disabled constraint is not considered during randomization.

Constraints can be enabled or disabled by constraint_mode().

Syntax

constraint_mode() can be called both as a task and as a function.

When called as a task, the method does not return anything. The task is supplied with an input argument to either turn on or off the given constraint. When called as a function, the method returns the current state of the given constraint.

// Called as a task
class_obj.const_name.constraint_mode(0); 			// Turn off the constraint
class_obj.const_name.constraint_mode(1); 			// Turn on the constraint
	
// Called as a function
status = class_obj.const_name.constraint_mode(); 	// status is an int variable to hold return value

constraint_mode() is a built-in method and cannot be overriden !

Method

The following table shows what each value of the argument input indicates when constraint_mode is called as a task.

Example

In the following example, we'll see what effect constraint_mode() has on its constraints.

class Fruits;
  rand bit[3:0]  num; 				// Declare a 4-bit variable that can be randomized
  
  constraint c_num { num > 4;  		// Constraint is by default enabled, and applied
                    num < 9; }; 	// during randomization giving num a value between 4 and 9
endclass

module tb;
  initial begin
    Fruits f = new ();
    
	// 1. Print value of num before randomization
    $display ("Before randomization num = %0d", f.num); 	
    
    // 2. Call "constraint_mode" as a function, the return type gives status of constraint
    if (f.c_num.constraint_mode ())
      $display ("Constraint c_num is enabled");
    else
      $display ("Constraint c_num is disabled");
      
    // 3. Randomize the class object
    f.randomize ();
    
    // 4. Display value of num after randomization
    $display ("After randomization num = %0d", f.num);    
  end
endmodule

ncsim> run
Before randomization num = 0
Constraint c_num is active
After randomization num = 8
ncsim: *W,RNQUIE: Simulation is complete.

Now, let's try disabling the constraint using constraint_mode() before attempting to randomize the variable.

module tb;
  initial begin
    Fruits f = new ();
    $display ("Before randomization num = %0d", f.num); 	
    
    // Disable constraint 
    f.c_num.constraint_mode(0);
    
    if (f.c_num.constraint_mode ())
      $display ("Constraint c_num is enabled");
    else
      $display ("Constraint c_num is disabled");
      
    // Randomize the variable and display
    f.randomize ();
    $display ("After randomization num = %0d", f.num);    
  end
endmodule

ncsim> run
Before randomization num = 0
Constraint c_num is disabled
After randomization num = 15
ncsim: *W,RNQUIE: Simulation is complete.

Note that turning off the constraint made the solver choose any value that the variable supported, instead of confining the values to the range specified in the constraint.

If constraint_mode() method is called on a constraint that does not exist, it will result in a compiler error.

	module tb;
		initial begin
			Fruits f = new();
			f.c_does_not_exist.constraint_mode(1);
		end
	endmodule

	f.c_does_not_exist.constraint_mode (1);
                     |
ncvlog: *E,NOTCLM (testbench.sv,11|21): c_does_not_exist is not a class item.