Configuration Specification

Description:

A configuration specification is a construct that defines which entity and architecture is used in place of the instances of a single component during elaboration.
Each component instantiation refers to some design entity (entity/architecture pair) and the association is specified by a configuration specification.
Component specification appears in the declarative part of the unit, where the instances are used.
This way components can be configured within the architecture which instances them without using a separate configuration declaration.

Configuration specifications are inflexible because changing the configuration requires editing the architecture containing the configuration.
It is usually better to use separate configuration declarations. When the ports and generics in the component declaration are not equal to their counterparts in the entity declaration,
one can use an explicit notification on how the ports and generics in the entity should be bound to ports and generics of the component instance. The generic map and port map are used for this purpose.

The configuration specification is typically placed at the beginning of a VHDL design file, before the entity declaration.

Syntax:


        configuration configuration_name of entity_name is
            use configuration ibrary_name.configuration_name;
          end configuration_name;

Example:


        configuration my_config of top_entity is
            use configuration work.mux_config;
        end my_config;

      entity TopEntity is
        port (
          sel : in std_logic;
          a, b : in std_logic;
          y : out std_logic
        );
      end entity TopEntity;

      architecture top_arch of TopEntity is
        component Mux is
          port (
            sel : in std_logic;
            a, b : in std_logic;
            y : out std_logic
          );
        end component;
      begin
        mux_inst : Mux
          port map (
            sel => sel,
            a => a,
            b => b,
            y => y
          );
      end architecture top_arch;
In this example: The configuration specification uses the mux_config configuration, which was defined earlier.
By using the configuration specification, you can ensure that the appropriate configuration is used for the TopEntity design unit, without having to explicitly reference the configuration in the entity or architecture definitions.

Notes: