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What is a component in VHDL? How do I do a component declaration and instantiation?

3 Answers

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VHDL Component Declaration

Quick Syntax

Here's how you do a component declaration:
component MY_MODULE
clk : in std_logic;
reset : in std_logic;
input_en : in std_logic;
output : out std_logic);
end component;

However, since VHDL-93, you no longer have to do component declarations, and in fact, it's recommended that you don't in most situations (read why below). Let me show you the recommended way that the pro's use next.

Assuming you compiled your entity in the library work, here is how you directly instantiate your entity in another source file like your top level:
MODULE1 : entity work.my_module
port map (
clk => clk,
reset => reset,
input_en => module1_input_en,
output => module1_output);
Note that with this method, component declarations are not needed.


The purpose of the component declraration in VHDL-87 was to let the tools know what the port pins are. However, that information is already known in the design source file as the entity. Therefore, in VHDL-93, they started allowing you to directly instantiate an entity, bypassing the need for component declarations altogether.

Please note that older text books and internet articles have not been updated with the VHDL language changes that have taken place over the past 30+ years since VHDL-87. As noted, in the old days, you had to do component declarations and port maps so that the tools could find all of your source files. That means you had the same ports in 3 places: design source file, component declaration, and component instantiation.

However, with VHDL-93, the language woke up to the crys of many engineers and realized that having the same kind of code in three places causes all sorts of headaches and a maintenance nightmare.

So they came up with a better way where you can directly instantiate the specific entity without having to use component declarations. That way you only need the source file ports, and then wherever it's being instantiated, you simply reference it with something like this: MODULE1 : entity work.my_module, where work is the library that it was compiled into. This is the method I'm using in this answer post.

Even after all of these years, you still see people doing the old way which is crazy. The biggest sign of someone looking like a newbie in VHDL is if they do component declarations. It went out of fashion long ago with the pros. It's not just a fashionable thing, any port map changes requires another change if component declarations are used, which is cumbersome.

Why use component declarations? If you have mixed language designs, like VHDL and Verilog, then you will need to do component declarations for the Verilog source files in your design. That way the tools know what the pins are and can marry up the port maps with the Verilog modules.

Best Practices

1. Only use component declarations for mixed language designs, i.e. to declare a Verilog module. Otherwise, directly instantiate an entity by referring to it's compile library and name.

2. Make sure you get those pesky commas and semicolons right and close parenthesis, unless you enjoy many compile failures.
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by (500 points)

A component declaration is a way of "letting the current architecture know" of an entity described in a different location.

Only the interfaces of the component is declared and it's usually done inside a package or an architecture.
After a component has been declared it can be instantiated in the architecture.

This allows implementing hierarchical design with code re-use.

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by (500 points)

This is the declaration of a virtual design entity interface. This object may be used in a component instantiation statement to bind design entity architecture that either signifies the interface or is defined by a configuration declaration.

This is a great way of abstracting a design and configuring a design's hierarchy with intent for reuse. It is good when a team knows how to use it and reaches agreement on it. But in my experience, that often isn't the case. Example:

  component counter is
generic (
D_WIDTH : integer := 8
port (
reset_n_i : in std_logic
clk_i : in std_logic;
q_o : out std_logic_vector(D_WIDTH-1 downto 0);
end component;

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