Configuration Specification

Used In

  • Architecture

Reference Manual

VHDL-93:
  • Section 5.2

Add information about what a configuration is and what it’s used for

Syntax 

for instance_label: component_name
    use entity
        library_name.entity_name(architecture_name);

for instance_label: component_name
    use configuration library_name.config_name;

Rules and Examples

Using a configuration specification, components may be configured within an architecture which instances them, rather than using a separate configuration declaration design unit. This is less flexible, as the architecture has to be re-analyzed to change the configuration.

Component instances may be individually configured:

architecture STR of XA is

    component HALFADD
        port(
            A,B : in bit;
            SUM, CARRY : out bit
        );
    end component;

    component ORGATE
        port(
            A,B : in bit;
            Z : out bit
        );
    end component;

    for U1 : HALFADD use entity work.HA(BEHAVE);
    for U2 : ORGATE use entity work.OG(BEHAVE);

begin

    U1: HALFADD port map ( A, B, S, C);
    U2: ORGATE port map ( A, B, Y);

end STR;

The keyword all may be used to refer to all instances of a component:

for all: FULLADDER use entity
    work.FULLADD(STRUCTURAL);

The keyword others may also be used to refer to all instances not explicitly mentioned.

If the port names on an entity do not match those on the component declaration, a port map may be included in the configuration:

for all:HALFADD use entity
    work.HALFADD(BEHAVE)
    port map (
        X => A,
        Y => B,
        S => SUM,
        C => CARRY
    );

In the absence of an explicit configuration for any part of a model, default binding will occur. For each unbound instance of every component, an entity will be selected whose name, port names and port types etc. match those in the corresponding component declaration. Where an entity has more than one architecture, the last analyzed architecture will be used.

An entity-architecture pair may be directly instantiated, i.e. a component need not be declared. This is more compact, but does not allow the flexibility of configuration.

DIRECT: entity HA_ENTITY(HA_ARCH) port map( A, B, S, C);

A configuration specification for a component (or instance) may legally be overridden by a configuration declaration for the same item.

Synthesis Issues

Configuration is not usually supported by synthesis tools. The user usually has to ensure that component and entity names and ports match, and that only one architecture per entity is analyzed.

See Also