SystemVerilog Static Constraints

Just like static variables in a class, constraints can be declared static. A static constraint is shared across every instance of the class — but only in one specific sense: how it responds to being turned on or off.

Constraints are affected by the static keyword only through constraint_mode(). When a non-static constraint is turned off, it's turned off only for the specific instance that made the call. When a static constraint is turned off, it's turned off for every instance of the class at once.

13 min read | Intermediate Level

What You'll Learn

• What makes a static constraint different from the default (non-static) kind
• Why constraint_mode() only affects one instance for a non-static constraint, but every instance for a static one
• How to verify the difference concretely, using one class with both a static and a non-static constraint side by side

Syntax

A constraint block is declared static by including the static keyword in its definition:

class [class_name];
	...

	static constraint [constraint_name] [definition]
endclass

Non-static Constraints

Constraints are non-static by default, so a separate copy exists for each class instance. Here, both constraints on a are ordinary, non-static blocks:

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

Both constraints are active in both instances, so both consistently land in the range (5, 12):

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.

Static Constraints

Now let's turn off a non-static constraint and compare it against turning off a static one, using the exact same class with one of each.

Turn Off a Non-static Constraint

Here, c1 stays non-static, but c2 is declared static. Only obj1's copy of c1 is disabled:

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

Output

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.

obj1 produces a value (3) outside c1's a > 5 range — confirming c1 is genuinely off for that instance. obj2's values all stay within (5, 12) throughout, showing its own copy of c1 was never touched.

Turn Off a Static Constraint

This time, the static constraint c2 is disabled — through obj1's handle, but since c2 is static, both instances should be affected:

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 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

Output

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.

Both objects show values exceeding 11 (15, 14, 12 on obj1; 12, 15 on obj2), confirming c2 is genuinely off for both — even though the call was made through obj1's handle alone. c1 (still active, still non-static) continues to keep every value above 5 on both sides.

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