In SystemVerilog, constraints are enabled by default and are always considered by the constraint solver during randomization. However, you may want to disable specific constraints at times. When a constraint is disabled, it will be ignored during randomization. The method used to enable or disable constraints is called constraint_mode().

What is constraint_mode()?

The constraint_mode() function allows you to enable or disable constraints. It can be used in two ways:

1. As a task: It doesn’t return any value. Instead, it modifies the state of the constraint.
   • class_obj.const_name.constraint_mode(0); to turn off the constraint.
   • class_obj.const_name.constraint_mode(1); to turn on the constraint.
2. As a function: It returns the current state of the constraint, either enabled or disabled.
   • status = class_obj.const_name.constraint_mode(); where status holds the return value, which indicates whether the constraint is enabled (1) or disabled (0).

Note: The constraint_mode() method is built-in and cannot be overridden.

constraint_mode() Method Values:

Example: Using constraint_mode() to Enable/Disable Constraints

Let’s look at an example to understand how constraint_mode() works with constraints.

class Fruits;
  rand bit[3:0] num;  // Declare a 4-bit variable that can be randomized

  constraint c_num { 
    num > 4;  // Constraint ensures num is greater than 4
    num < 9;  // Constraint ensures num is less than 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

Simulation Log:

Before randomization num = 0
Constraint c_num is enabled
After randomization num = 8

In this example, the constraint c_num is enabled by default, and when randomization occurs, the value of num is between 4 and 9.

Example: Disabling Constraints Using constraint_mode()

Now, let’s see what happens when we disable the constraint using constraint_mode().

module tb;
  initial begin
    Fruits f = new ();
    $display ("Before randomization num = %0d", f.num);

    // Disable constraint
    f.c_num.constraint_mode(0);

    // 3. Check if constraint is disabled
    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

Simulation Log:

Before randomization num = 0
Constraint c_num is disabled
After randomization num = 15

Here, we disabled the constraint before randomization. As a result, the solver randomly picks any value for num, not restricted by the original constraints.

Handling Errors with Non-Existing Constraints

If constraint_mode() is called on a non-existing constraint, a compiler error will occur. Here’s an example:

module tb;
  initial begin
    Fruits f = new();
    f.c_does_not_exist.constraint_mode(1);  // This will cause a compiler error
  end
endmodule

Simulation Log:

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

This error occurs because c_does_not_exist is not a valid constraint in the Fruits class.

Conclusion

• Constraints are enabled by default in SystemVerilog and are considered during randomization.
• The constraint_mode() method helps you enable or disable specific constraints during randomization.
• If you disable a constraint, the solver will choose values without considering that constraint.
• Make sure you call constraint_mode() only on valid constraints, as trying to call it on non-existing ones will result in an error.

By controlling constraints dynamically with constraint_mode(), you can have more flexibility over the randomization process in SystemVerilog.

Comparison of Enabling and Disabling Constraints