What is Configuration in VHDL: Definition and Usage

In VHDL, a configuration is a construct that specifies how components in a design are connected to their implementations. It allows you to select which architecture of an entity to use and how components are bound, giving control over the design hierarchy without changing the source code.

How It Works

Think of a VHDL configuration like a recipe card for assembling a device. The device (entity) can have multiple versions (architectures), and the configuration tells the compiler which version to use and how to connect smaller parts (components) inside it.

Normally, VHDL picks the architecture automatically, but with a configuration, you explicitly say which architecture to use and how to link components. This is useful when you want to test different designs or swap parts without changing the main code.

Example

This example shows an entity with two architectures and a configuration that selects one architecture.

vhdl

library IEEE;
use IEEE.STD_LOGIC_1164.ALL;

entity MyDevice is
  Port ( A : in STD_LOGIC;
         B : out STD_LOGIC);
end MyDevice;

architecture Behavioral of MyDevice is
begin
  B <= A;
end Behavioral;

architecture Alternative of MyDevice is
begin
  B <= not A;
end Alternative;

configuration MyDeviceConfig of MyDevice is
  for Alternative
  end for;
end MyDeviceConfig;

When to Use

Use configurations when you want to control which architecture or component implementation is used without editing the main design files. This is helpful in large projects where multiple versions of a module exist, or when testing different implementations.

For example, in a project with a standard and a low-power version of a circuit, configurations let you switch between them easily. Also, configurations help in simulation and synthesis tools to bind components explicitly.

Key Points

• Configurations specify which architecture of an entity to use.
• They control component bindings explicitly.
• They allow design flexibility without changing source code.
• Useful for testing, simulation, and managing multiple implementations.

Key Takeaways

A VHDL configuration selects the architecture and component bindings for an entity.

It provides control over design hierarchy without modifying source code.

Configurations are useful for testing different implementations and managing complex designs.

They help tools know exactly which parts to use during simulation and synthesis.