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Just like static variables in a class, constraints can be declared as static. A static constraint is shared across all the class instances.

Constraints are affected by the static keyword only if they are turned on and off using constraint_mode() method. When a non-static constraint is turned off using this method, the constraint is turned off in that particular instance of the class which calls the method. But, when a static constraint is turned off and on using this method, the constraint is turned off and on in all the instances of the class.

A constraint block can be declared as static by including the static keyword in its definition.

Syntax

 
class [class_name];
  ...
 
  static constraint [constraint_name] [definition]
endclass
 

Next, we'll compare non-static constraints with static constraints and see how they differ.

Non-static Constraints

Constraints are by default non-static and hence a separate copy exists for each class instance. In the example shown below, we have a class called ABC that has two normal constraints. The first one constrains a to a value greater than 5 and the second one constrains it to a value less than 12.

 
class ABC;
  rand bit [3:0]  a;
 
  // Both are non-static constraints 
  constraint c1 { a > 5; }
  constraint c2 { a < 12; }
endclass
 
module tb;
  initial begin
    ABC obj1 = new;
    ABC obj2 = new;
    for (int i = 0; i < 5; i++) begin
      obj1.randomize();
      obj2.randomize();
      $display ("obj1.a = %0d, obj2.a = %0d", obj1.a, obj2.a);
    end
  end
endmodule
 

See that both constraints are active in both the class instances obj1 and obj2. Both objects successfully constrain the variable a to a value in between 5 and 12.

Simulation Log
ncsim> run
obj1.a = 9, obj2.a = 6
obj1.a = 7, obj2.a = 11
obj1.a = 6, obj2.a = 6
obj1.a = 9, obj2.a = 11
obj1.a = 6, obj2.a = 9
ncsim: *W,RNQUIE: Simulation is complete.

Click to try this example in a simulator!   

Static Constraints

Let us turn off a non-static constraint and compare it with the results when a static constraint is turned off.

Turn off non-static constraint

In this case, we'll set the second constraint called c2 as static.

 
class ABC;
  rand bit [3:0]  a;
 
  // "c1" is non-static, but "c2" is static
  constraint c1 { a > 5; }
  static constraint c2 { a < 12; }
endclass
 
module tb;
  initial begin
    ABC obj1 = new;
    ABC obj2 = new;
 
    // Turn off non-static constraint
    obj1.c1.constraint_mode(0);
 
    for (int i = 0; i < 5; i++) begin
      obj1.randomize();
      obj2.randomize();
      $display ("obj1.a = %0d, obj2.a = %0d", obj1.a, obj2.a);
    end
  end
endmodule
 

When the non-static constraint c1 was turned off using constraint_mode(), obj1 produced values outside the range specified by c1 constraint and matches our expectation.

Simulation Log
ncsim> run
obj1.a = 3, obj2.a = 6
obj1.a = 7, obj2.a = 11
obj1.a = 6, obj2.a = 6
obj1.a = 9, obj2.a = 11
obj1.a = 6, obj2.a = 9
ncsim: *W,RNQUIE: Simulation is complete.

Click to try this example in a simulator!   

Turn off static constraint

In this case, the static constraint called c2 will be turned off. We expect both the object instances obj1 and obj2 to be affected by this.

 
class ABC;
  rand bit [3:0]  a;
 
  // "c1" is non-static, but "c2" is static
  constraint c1 { a > 5; }
  static constraint c2 { a < 12; }
endclass
 
module tb;
  initial begin
    ABC obj1 = new;
    ABC obj2 = new;
 
    // Turn non-static constraint
    obj1.c2.constraint_mode(0);
 
    for (int i = 0; i < 5; i++) begin
      obj1.randomize();
      obj2.randomize();
      $display ("obj1.a = %0d, obj2.a = %0d", obj1.a, obj2.a);
    end
  end
endmodule
 

Note that both objects of the same class have the constraint c2 turned off as expected. Both variables have values of a outside the constraint range.

Simulation Log
ncsim> run
obj1.a = 15, obj2.a = 12
obj1.a = 9, obj2.a = 15
obj1.a = 14, obj2.a = 6
obj1.a = 11, obj2.a = 11
obj1.a = 12, obj2.a = 11
ncsim: *W,RNQUIE: Simulation is complete.

Click to try this example in a simulator!   

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