A configuration is a construct that defines how component instances in a given block are bound to design entities in order to describe how design entities are put together to form a complete design.

Simplified Syntax

configuration configuration_name of entity_name is

-- configuration declarations

for architecture_name

for instance_label:component_name

use entity library_name.entity_name(arch_name);

end for;

-- other for clauses

end for;

end [configuration] [configuration_name];

configuration configuration_name of entity_name is

-- configuration declarations

for architecture_name

for instance_label:component_name

use configuration library_name.config_name;

end for;

-- other for clauses

end for;

end [configuration] [configuration_name];

Description

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. If for some reasons, however, it is appropriate (or desired) to postpone (or defer) such association until later, configuration declaration can be used for specifying such deferred component specifications.

The configuration declaration starts with the configuration name and then it is associated to a given design entity. Declarative part of the configuration may contain use clauses, attribute specifications and group declarations. The main part of the configuration declaration contains so called block configuration. It indicates which architecture will be used with the entity specified earlier, as well as which configuration elements will be used in the component instantiation. This declaration may contain other blocks' configurations, allowing this way to specify hierarchical structures. Such a configuration can be called hierarchical, while a configuration without hierarchy can be called simple.

A simple configuration contains reference to only one architecture body (Example 1).

Hierarchical configurations, on the other hand, allow to nest block configurations (Example 2). This mechanism allows binding component instantiation statements with the design entities down the hierarchy.

When the ports and generics in component declaration do not match with their counterparts in entity declaration, so called binding indication can be applied. Simply speaking this is 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 clauses are used for this purpose. This technique is used in Example 1. In practice, however, it is recommended to match the generics and ports of components and respective entities as this improves readability.

Two main binding methods that can be applied in configuration specifications: binding of entities and binding configurations. They are illustrated in Examples 1 and 3, respectively.

Examples

Example 1

entity INVERTER is
  generic (PropTime : TIME := 5 ns);
  port ( IN1 : in BIT; OUT1 : out BIT);
end INVERTER;
architecture STRUCT_I of INVERTER is
begin
  OUT1 <= not IN1 after PropTime;
end STRUCT_I;
entity TEST_INV is end TEST_INV;
architecture STRUCT_T of TEST_INV is
signal S1, S2 : BIT := '1';
-- INV_COMP component declaration:
component INV_COMP is
  generic (TimeH : TIME);
  port ( IN_A : in BIT; OUT_A : out BIT );
end component;
begin
  -- instantiation of INV_COMP component:
  LH:INV_COMP generic map (10 ns) port map (S1,S2);
end STRUCT_T;
configuration CONFIG_TINV of TEST_INV is
for STRUCT_T -- indicates architecture body of TEST_INV
  -- indicates design entity for LH instantiation statement:
  for LH : INV_COMP
    use entity WORK.INVERTER (STRUCT_I)
    -- indicates generic and port aspects:
    generic map (PropTime => TimeH)
    port map (IN1 => IN_A, OUT1 => OUT_A);
  end for;
end for ;
end CONFIG_TINV;

The CONFIG_TINV configuration declaration can be used as an example of the basic configuration declaration. There is only one block configuration in the configuration declaration. This block contains a component declaration INV_COMP. In the component instantiation statement LH, the design entity INVERTER is assigned to INV_COMP component.

There is one block configuration in the CONFIG_TINV configuration declaration, it indicates that STRUCT_T architecture body will be used. INV_COMP component configuration appears in the block configuration. The CONFIG_TINV configuration for the TEST_INV design entity allows associating LG component: INV_COMP with INVERTER design entity and its STRUCT_1 architecture body.

Figure 1. Example of configuration declaration

Example 2

-- block configuration for architecture body STRUCT:
for STRUCT
-- component configuration specified in architecture body STRUCT:
  for SPEECH_CPU: SPEECH
    use entity SP_LIB.DIG_REC(ISD)
    generic map (TimeRec => 20 sec);
      -- block configuration for architecture body ISD of DIG_REC:
      for ISD
      -- component configuration specified in architecture body ISD:
        for AD_CONV : ADC_1 -- relates to instance AD_CONV of ADC_1
          use entity ANALOG_DEV.ADC;
        end for; -- for AD_CONV
      end for; -- for ISD
  end for; -- for SPEECH_CPU
end for; -- for STRUCT

The block configuration, indicating architecture body STRUCT, appears in the configuration declaration. Next, in the block configuration there is the component configuration SPEECH, which also contains block configuration IST. The block configuration IST configures the architecture body IST which contain an instantiation statement with label AD_CONV. The entire block is hierarchically specified here.

Example 3

configuration Conf_Test of Test is
  for STRUCTURE_T
    for T_1 : DEC use configuration CONF_E;
    end for;
  end for;
end configuration Conf_Test;

In this example, the configuration declaration of design entity EXAMPLE is used. It binds EXAMPLE design entity to DEC component by using its configuration CONF_E as an entity aspect for T_1 component instance in the body architecture STRUCTURE_T.

Figure 2. Example of configuration declaration

Important Notes

Configuration assigns one and only one architecture to a given entity.
Synthesis tools do generally not support configurations.
For a configuration of some design entity, both the entity and the configuration must be declared in the same library.