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Generic

Used In

  • Entity
  • Component

Reference Manual

VHDL-93:
  • Section 1.1.1.1
  • Section 5.2.1.2
  • Section 9.6

Syntax

entity entity_name is
    generic (
        -- generic list
    );
    port (
        -- port list
    );
end entity_name;
entity component_name is
    generic (
        -- generic list
    );
    port (
        -- port list
    );
end component_name;
instance_label: component_name
    generic map (
        -- generic_association_list
    )
    port map (
        -- port_association_list
    );

Rules and Examples

Generics are a means of passing specific information into an entity. They do not have a mode as they can only go into an entity or component:

entity PARITY is
    generic (
        N : integer
    );
    port (
        A : in std_ulogic_vector(N-1 downto 0),
        ODD : out std_ulogic
    );
end PARITY;

In the corresponding component declaration, the generics are also declared before the ports:

component PARITY is
    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:

U1: PARITY
    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
    );
end AN2_GENERIC;

architecture BEH of AN2_GENERIC is

begin

    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.

See Also