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VHDL Reference Guide - Generics

Declaration ---- used in ----> Entity


entity entity_name is
	generic (generic list);
	port      (port list);
end entity_name;

component component_name
	generic (generic_list);	
	port (port_list);
end component;

instance_label: component_name
	generic map (generic_association_list)
	port map      (port_association_list);

See LRM sections, and 9.6

Rules and Examples

Generics are a means of passing specific information into an entity. They do not have a mode (direction):
entity PARITY is
  generic (N : integer);
  port    (A : in std_ulogic_vector
                  (N-1 downto 0);
         ODD : out std_ulogic);

In the corresponding component declaration, the generics are also declared before the ports:
component PARITY
  generic (N : integer);
  port    (A : in  std_ulogic_vector
                    (N-1 downto 0);
         ODD : out std_ulogic);
end component;

An instance of a component with generics, has a generic map declared before the port map (note: there is no semicolon between them!). This allows a value to be set for the generic:
  generic map (N   => 8)
  port map    (A   => DATA_BYTE,
               ODD => PARITY_BYTE);

By declaring generics of type time, delays may be programmed on an instance-by-instance basis. Generics may be given a default value, in case a value is not supplied for all instances:
entity AN2_GENERIC is
   generic (DELAY: time := 1.0 ns);
   port    (A,B  : in std_ulogic;
              Z  : out std_ulogic);
architecture BEH of AN2_GENERIC is
   Z <= A and B after DELAY;
end A; 

The value of a generic may be read in either the entity or any of its architectures. It may even be passed into lower-level components. Default values for generics may be given in an entity declaration or in a component declaration. generics may be set (via a generic map) in an instantiation, or a configuration. The rules regarding different combinations of these are complex: see "VHDL" by Douglas Perry, page 218.

Synthesis Issues

Usually, only generics of type integer are supported. Values for generics in an entity declaration have to be supplied by the user to allow elaboration at the time of synthesis. generic map is usually ignored.

Whats New in '93

Generics have not changed in VHDL-93.