Defaults Statement (AHDL)

The Defaults Statement allows you to specify default values for variables used in Truth Table, If Then, and Case Statements. Since active-high signals automatically default to GND, Defaults Statements are required only for active-low signals.

Note:

You should not confuse default values for variables with default values for ports that are assigned in the Subdesign Section.
Defaults Statements must follow the rules described in Defaults Statement Rules.

The following example shows a Defaults Statement:

BEGIN
   DEFAULTS
      a = VCC;
   END DEFAULTS;
   IF y & z THEN
      a = GND;  % a is active low %
   END IF;
END;

The Defaults Statement has the following characteristics:

It is enclosed by the keywords DEFAULTS and END DEFAULTS and ends with a semicolon (;).
The body of the Defaults Statement consists of one or more Boolean equations that assign constant values to variables. In the example shown above, the Defaults Statement assigns the default value VCC to the variable a.
Each equation ends with a semicolon (;).
The Defaults Statement is activated if a variable that follows it is undefined for certain conditions. In the example shown above, the variable a is undefined when y or z is a logical low, so the equation (a = VCC) in the Defaults Statement is activated.