What is Procedure in VHDL: Definition and Usage

In VHDL, a procedure is a reusable block of code that performs a specific task and can be called from other parts of the design. It helps organize code by grouping statements together, similar to a recipe you follow step-by-step. Procedures can have input and output parameters to exchange data.

How It Works

A procedure in VHDL is like a small helper that you write once and use many times. Imagine you have a recipe for making a sandwich. Instead of writing the steps every time you want a sandwich, you just say "make sandwich" and follow the recipe. Similarly, a procedure groups a set of instructions that perform a task.

When you call a procedure, the program jumps to that block of code, runs the instructions, and then returns to where it left off. Procedures can take inputs (ingredients) and give outputs (finished sandwich), allowing flexible use in different situations. This helps keep your VHDL code clean, organized, and easier to understand.

Example

This example shows a simple procedure that adds two numbers and returns the result. It is called from a process to demonstrate how procedures work in VHDL.

vhdl

library ieee;
use ieee.std_logic_1164.all;
use ieee.numeric_std.all;

entity procedure_example is
end procedure_example;

architecture behavior of procedure_example is

  -- Procedure to add two integers and return the sum
  procedure add_numbers(
    a : in integer;
    b : in integer;
    result : out integer
  ) is
  begin
    result := a + b;
  end add_numbers;

  signal sum_result : integer := 0;

begin

  process
  begin
    add_numbers(5, 7, sum_result);
    report "Sum is " & integer'image(sum_result);
    wait;
  end process;

end behavior;

Output

Sum is 12

When to Use

Use procedures in VHDL when you have a task that repeats multiple times or when you want to break complex logic into smaller, manageable parts. For example, if you need to perform the same calculation in different places, a procedure avoids rewriting the same code.

Procedures are useful in testbenches for stimulus generation, in designs for arithmetic operations, or any scenario where modular, reusable code improves clarity and maintenance. They help reduce errors and make your design easier to update.

Key Points

• A procedure groups statements to perform a specific task.
• It can have input and output parameters to exchange data.
• Procedures help make VHDL code modular and reusable.
• They are called from processes or other parts of the design.
• Procedures do not return a value like functions but can modify outputs via parameters.

Key Takeaways

A procedure in VHDL is a reusable block of code that performs a task without returning a value.

Procedures help organize and simplify your design by grouping related statements.

Use procedures to avoid repeating code and to make your design easier to maintain.

Procedures can have input and output parameters to pass data in and out.

They are called from processes or other parts of your VHDL code.