From Concept to Implementation: Leveraging Procedures in VHDL

Piyush Gupta

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A popular language for creating and describing digital electrical systems is VHDL (VHSIC Hardware Description Language). The usage of procedures—reusable code blocks that contain a series of instructions or operations—is a key component of VHDL.

VHDL procedures offer a systematic, modular approach to design. They facilitate code reuse, increase the readability of the code, and streamline the entire design process. Engineers can make their code easier to comprehend and maintain by dividing it into specialized tasks and subroutines using procedures.

Sequential or concurrent VHDL operations can be distinguished. Concurrent methods simulate simultaneous operations inside a design, whereas sequential procedures execute a set of statements in a particular order.

The ability to parameterize and reuse VHDL routines across many designs is another benefit of genericity. Engineers can produce adaptable and versatile components that can meet a variety of design criteria by applying generic processes.

Throughout this blog, we will look at the various varieties of VHDL procedures, their syntax and structure, and their useful applications. We will cover instances like constructing counters and state machines to show the advantages of employing procedures in VHDL designs.

Understanding Procedures in VHDL:

What are the procedures?

In VHDL, a procedure is a subprogram that contains a series of operations. They can be called from other areas of the code and can contain input and output parameters as well as local variables. Procedures enable a modular design approach by enabling the encapsulation of particular functionality in a distinct unit. They encourage code reuse and improve the design’s readability and organization.

Differences between procedures and functions in VHDL:

While procedures and functions share similarities, they have some key differences in VHDL:

  • Procedures do not return values, while functions do.
  • Procedures can modify the values of their input parameters, while functions cannot.
  • Functions can be used within expressions, providing a way to calculate and return values, while procedures are typically used for their side effects or sequential operations.

Benefits of using procedures in VHDL design:

Incorporating procedures in VHDL designs offers several advantages:

Code modularity: Procedures allow complex functionality to be encapsulated into reusable blocks, promoting code modularity and making the design more organized.

Reusability: Procedures can be called from different parts of the code, promoting code reuse and reducing duplication.

Maintainability: Procedures enhance the readability and maintainability of the code by isolating specific operations into separate units, making it easier to understand and modify the design.

Simplicity: Procedures simplify the design process by breaking down complex tasks into smaller, manageable steps.

Encapsulation: Procedures provide a level of encapsulation, allowing designers to hide the implementation details and focus on the higher-level functionality.

Procedure Syntax and Structure:

  1. Procedure declaration and definition:

Procedures are declared using the procedure keyword, followed by the procedure name and a parameter list (if any). The procedure definition includes the procedure’s statements and operations.

  1. Input and output parameters in procedures:

Procedures can have input and output parameters to pass values between the calling code and the procedure. Input parameters are used to provide data to the procedure, while output parameters allow the procedure to return values to the calling code.

  1. Handling of local variables and signals within procedures:

Procedures can declare local variables and signals that are accessible only within the procedure’s scope. Local variables are used to store temporary values, while signals can be used for interprocess communication within the procedure.

Sequential and Concurrent Procedures:

  1. Sequential procedures and their execution in VHDL:

Sequential procedures are executed one after another, following a specific order defined by the designer. They are suitable for operations that need to be performed sequentially, such as data manipulation or state transitions.

  1. Concurrent procedures and their role in VHDL design:

Concurrent procedures are executed concurrently, meaning they can be executed simultaneously in parallel processes. They are useful for operations that can be performed independently, allowing for efficient use of hardware resources.

Examples illustrating sequential and concurrent procedures:

Sequential procedure example: A procedure that performs a bubble sort algorithm on an array of values, sorting them in ascending order.

Concurrent procedure example: A procedure that calculates the sum of two numbers concurrently, utilizing parallel processing.

Procedure Types in VHDL:

  1. Generic procedures and their usage:

Generic procedures allow designers to create procedures that can operate on different data types or handle varying numbers of parameters. They provide flexibility and reusability by allowing the same procedure code to be applied to different scenarios.

  1. Overloaded procedures for different parameter combinations:

Overloaded procedures enable multiple procedures with the same name but different parameter combinations. This allows for more intuitive and concise code by providing different versions of a procedure tailored to specific input parameter types or numbers.

  1. Recursive procedures and their application in VHDL:

Recursive procedures are procedures that call themselves within their own body. They are useful for solving problems that can be naturally expressed using recursive algorithms, such as tree traversals or mathematical computations. Recursive procedures provide an elegant solution to handle repetitive tasks or complex data structures.

Procedure Call and Execution:

  1. How to call a procedure in VHDL:

Procedures are called by using their name followed by the actual parameter values. The calling code provides the necessary input values, and the procedure executes its defined operations.

  1. Passing parameters to procedures:

Parameters can be passed to procedures by value or by reference. When passed by value, a copy of the parameter’s value is used within the procedure. When passed by reference, the procedure can directly modify the value of the parameter.

  1. Execution flow and control in procedures:

The execution of a procedure follows a sequential flow. The statements within the procedure are executed one by one unless control flow statements, such as conditionals or loops, alter the execution order. The procedure returns control to the calling code after completing its operations.

Examples and Applications:

  1. Implementing a counter using procedures in VHDL:

This example demonstrates how procedures can be used to create a reusable counter module in VHDL, allowing for easy instantiation and manipulation of counters in different parts of a design.

  1. Creating a state machine using procedures:

Procedures can be utilized to implement the behavior of a state machine, where each procedure represents a specific state and handles the transitions and operations associated with that state. This approach simplifies the design and enhances readability.

  1. Other practical examples demonstrating the use of procedures in VHDL:
  • Signal conditioning: Procedures can be used to perform signal conditioning tasks, such as filtering or amplification, before further processing in a VHDL design.
  • Arithmetic operations: Procedures can encapsulate arithmetic operations, such as addition, subtraction, or multiplication, allowing for reusable and modular mathematical functions.
  • Data processing: Procedures can be employed for various data processing tasks, such as data compression, error correction, or data encryption, enhancing the overall functionality of the VHDL design.

Best Practices for Using Procedures in VHDL:

  1. Guidelines for writing efficient and reusable procedures:
  • Keep procedures modular and focused on a specific task to promote reusability.
  • Use meaningful and descriptive names for procedures to enhance code readability.
  • Document procedures with comments to explain their purpose, inputs, and outputs.
  • Follow coding standards and style guidelines to maintain consistency in the design.
  1. Error handling and exception handling in procedures:
  • Implement proper error handlings mechanisms, such as checks for valid input ranges or error codes, to ensure the reliability and robustness of the procedures.
  • Utilize exception-handling techniques, such as try-catch blocks or signal propagation, to gracefully handle exceptional cases or error conditions within procedures.
  1. Testing and debugging procedures in VHDL designs:
  • Write test benches to verify the correctness and functionality of procedures.
  • Use simulation tools to simulate the procedures in various scenarios and validate their expected behavior.
  • Debug procedures using waveform viewers and print statements to track the execution flow and variable values during simulation.


In conclusion, procedures are essential components of VHDL designs because they provide code modularity, reusability, and maintainability. They help to make code more readable and organized, enabling designers to produce well-organized designs that are effective. The development process can be sped up and designers can create scalable and reliable digital systems by successfully implementing procedures. Using VHDL procedures to their full potential encourages effective and maintainable design approaches for digital electronics.


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