Architectures in VHDL Programming Language

Introduction to Architectures in VHDL Programming Language

Welcome to this blog post about Architectures in VHDL Programming Language! If you̵

7;re looking to learn how to design and model digital systems, you’ve come to the right place. VHDL (VHSIC Hardware Description Language) is a widely used language for hardware design that allows you to describe the behavior and structure of digital circuits with precision. In this post, I’ll introduce the concept of architectures in VHDL, explain their purpose, and discuss how they relate to entities. By the end, you’ll have a solid understanding of VHDL architectures and be ready to explore more advanced topics. Let’s dive in!

What is Architectures in VHDL Programming Language?

In VHDL (VHSIC Hardware Description Language), architecture refers to the section of the code where you describe the actual behavior or internal implementation of a digital system. While the entity defines the design’s interface by specifying its inputs and outputs (ports), the architecture explains how the design works internally and processes the input signals to produce the output.

VHDL allows you to associate each entity with one or more architectures, providing flexibility in defining different implementations for the same interface. This feature proves useful for defining both the structural composition (using components) and the behavioral description (using processes) of a circuit.

Key Aspects of Architecture in VHDL:

1. Structure of Architecture Block

The architecture block follows the entity declaration and starts with the architecture keyword. It consists of two major parts:

• Declarative Section: This is where signals, components, constants, and other design elements used within the architecture are declared.
• Statement Section: This contains the actual implementation, such as concurrent statements or processes, defining how the inputs are processed to generate the outputs.

Basic syntax:

architecture <architecture_name> of <entity_name> is
  -- Declarative section
begin
  -- Statement section (behavioral description)
end <architecture_name>;

2. Multiple Architectures

An entity can have multiple architectures, each providing a different implementation of the design. This can be helpful for testing or prototyping different approaches. For example, one architecture could describe the system’s behavior, while another defines a more detailed structural implementation.

3. Behavioral vs. Structural Descriptions

• Behavioral Architecture: This type describes how the entity behaves in response to its inputs, usually using VHDL processes, signal assignments, or sequential logic. It focuses on what the circuit does rather than how it is built.
• Structural Architecture: This type defines the internal components and how they are connected, focusing on how the circuit is built using predefined components or other entities.

4. Process Blocks

Process Blocks Within the architecture, you can have one or more processes that define how signals are updated based on input changes. Processes are sequential blocks that execute whenever an event occurs on the signals they are sensitive to. This is common in behavioral architectures.

5. Concurrency

One of the main features of VHDL is that most statements in the architecture (outside of processes) are concurrent. This means they are evaluated and executed simultaneously, which matches the real-world behavior of digital circuits, where multiple components operate at the same time.

Example of an Architecture in VHDL

Below is a simple example that shows how an architecture works in VHDL. The entity defines the interface (inputs and outputs), while the architecture describes the behavior of the system.

-- Entity declaration
entity AND_Gate is
    Port ( A : in  STD_LOGIC;
           B : in  STD_LOGIC;
           Y : out  STD_LOGIC);
end AND_Gate;

-- Architecture defining the behavior
architecture Behavioral of AND_Gate is
begin
    -- Concurrent signal assignment
    Y <= A and B;
end Behavioral;

• In this example:
  • The entity AND_Gate defines two inputs (A and B) and one output (Y).
  • The architecture Behavioral describes how the output Y is produced by performing a logical AND operation on inputs A and B.

Types of Architectures

1. Behavioral Architecture:

Describes the behavior of a system using concurrent or sequential statements. It’s typically used when focusing on how the circuit behaves functionally.

Example of behavioral description:

architecture Behavioral of AND_Gate is
begin
  Y <= A and B; -- Behavior described using logical operation
end Behavioral;

2. Structural Architecture:

Describes the system in terms of interconnected components or sub-modules. It’s typically used when focusing on how the circuit is physically implemented.

Example of structural description:

architecture Structural of AND_Gate is
    component NAND_Gate
        Port ( A, B : in STD_LOGIC;
               Y    : out STD_LOGIC);
    end component;

    signal NAND_Output : STD_LOGIC;

begin
    NAND1: NAND_Gate port map (A => A, B => B, Y => NAND_Output);
    NAND2: NAND_Gate port map (A => NAND_Output, B => NAND_Output, Y => Y);
end Structural;

In this example, instead of directly describing the behavior of the AND gate, the architecture Structural uses two NAND gates to create the AND gate.

Why do we need Architectures in VHDL Programming Language?

In VHDL (VHSIC Hardware Description Language), architectures are essential because they define the internal functionality and behavior of digital circuits. While entities specify the external interface of a component, architectures describe how that component operates. Here’s why architectures are necessary in VHDL:

1. Separation of Interface and Implementation

The primary reason we need architectures is to separate the interface from the implementation. An entity describes the ports (inputs and outputs) of a digital component, while the architecture defines the logic that connects these ports internally. This separation allows for:

• Modular design: Different architectures can be applied to the same entity, promoting code reusability and flexibility.
• Clear structure: By having a clear distinction between the interface and the internal functionality, it is easier to manage complex designs and make modifications without altering the interface.

2. Support for Multiple Implementations

An entity can have multiple architectures, each offering a different implementation of the same interface. This is useful when:

• Prototyping: You can create different versions of a design (e.g., behavioral or structural) and test which implementation works best.
• Optimization: You can swap out architectures for the same entity to meet specific design requirements, such as performance, power consumption, or area optimization.

For instance, you might use one architecture for simulation (behavioral description) and another for synthesis (structural description).

3. Behavioral and Structural Design

VHDL supports both behavioral and structural designs within architectures, allowing designers to:

• Describe behavior: Behavioral architectures define how the system operates in terms of logical functions, signal assignments, and processes. This is typically used during the early stages of design to model the functionality.
• Describe structure: Structural architectures describe how the system is physically built using interconnected components, which is crucial for the synthesis and implementation stages in hardware.

This flexibility ensures that designers can represent their systems at different levels of abstraction.

4. Concurrency and Parallelism

Architectures allow the description of concurrent operations, which is essential in hardware design. Digital circuits operate in parallel, and VHDL architectures enable designers to model this parallelism naturally.

• Multiple components or operations can be executed concurrently within an architecture, reflecting the real behavior of hardware systems.

5. Process Blocks for Sequential Logic

While VHDL allows for concurrent operations, it also supports sequential logic through the use of process blocks within architectures. This feature is vital for modeling circuits with clocked elements, such as flip-flops and registers.

• Architectures give the flexibility to mix both concurrent and sequential logic, enabling the design of complex digital systems, including state machines and pipelined architectures.

6. Hierarchical Design

Architectures enable hierarchical design by allowing the integration of smaller components (other entities) within larger designs. This is done in structural architectures, where designers can instantiate and connect sub-components to create more complex systems.

• This hierarchical approach improves the manageability and scalability of large designs, promoting a structured and organized development process.

7. Simulation and Synthesis

VHDL architectures allow for the same entity to be represented differently for simulation and synthesis:

• Behavioral simulation: A behavioral architecture can describe the system for functional verification, allowing quick and efficient testing of how the system responds to inputs.
• Synthesis-ready design: A structural architecture is usually more appropriate for synthesis, detailing the exact components and connections that should be implemented in hardware.

This distinction ensures that the design can be easily verified and then translated into real hardware.

8. Abstraction Levels

VHDL architectures support different levels of abstraction, from high-level behavioral descriptions to low-level gate or transistor-level designs. This versatility allows designers to:

• Start with a high-level design for testing and functionality verification.
• Refine it into a detailed, low-level design for synthesis and hardware implementation.

Having this flexibility across abstraction levels is critical for managing large-scale and complex hardware projects.

Example of Architectures in VHDL Programming Language

In VHDL, the architecture section defines the internal functionality of a digital component, while the entity defines its external interface. The architecture can describe behavior, structure, or a mix of both, depending on the complexity and design requirements. Let’s go through an example of how to write and understand architectures in VHDL, focusing on a simple digital design.

1. Entity Declaration

To begin, we declare the entity of a digital component. In this example, we will describe a 2-input AND gate. The entity defines the component’s inputs and outputs, i.e., its interface with the outside world.

entity AND_Gate is
    Port (
        A : in std_logic;   -- First input signal
        B : in std_logic;   -- Second input signal
        Y : out std_logic   -- Output signal
    );
end AND_Gate;

• Here, the entity AND_Gate specifies that the AND gate has:
  • Two input ports (A and B), both of type std_logic.
  • One output port (Y), also of type std_logic.

This defines the external view of the AND gate, but it does not describe what happens inside the gate.

2. Architecture Declaration (Behavioral Approach)

Next, we define the architecture. The architecture explains how the output is generated based on the inputs. In this example, we’ll start with a behavioral architecture.

Behavioral Architecture Example

A behavioral architecture describes the behavior of the component using logic expressions, processes, or signal assignments. Let’s write the architecture for the AND gate:

architecture Behavioral of AND_Gate is
begin
    -- Concurrent signal assignment
    Y <= A and B;
end Behavioral;

Explanation:

• The architecture is named Behavioral.
• The statement Y <= A and B; is a concurrent signal assignment that defines the output Y as the result of the AND operation between inputs A and B.
• This means that whenever the signals A or B change, the expression A and B is evaluated, and the result is assigned to the output Y.

This architecture defines the behavior of the AND gate in a straightforward manner.

How Behavioral Architectures Work:

• Behavioral descriptions are usually used for simulation and functional verification.
• They describe what the system does without going into detail about how the logic is implemented in hardware.

3. Architecture Declaration (Structural Approach)

Next, let’s consider a structural architecture, which describes the component’s structure in terms of smaller subcomponents. Suppose we want to build the AND gate using two NAND gates (since an AND gate can be constructed using two NAND gates). Here’s how the structural architecture would look:

Structural Architecture Example

architecture Structural of AND_Gate is
    -- Internal signal to connect the output of the first NAND gate to the second NAND gate
    signal NAND_Out : std_logic;
begin
    -- Instantiate the first NAND gate
    U1: entity work.NAND_Gate port map (
        A => A, 
        B => B, 
        Y => NAND_Out
    );
    
    -- Instantiate the second NAND gate (inverter)
    U2: entity work.NAND_Gate port map (
        A => NAND_Out, 
        B => NAND_Out, 
        Y => Y
    );
end Structural;

Explanation:

• The architecture is named Structural, indicating that it describes the internal structure of the design.
• We use two NAND gates to construct an AND gate. The first NAND gate takes inputs A and B and produces the intermediate signal NAND_Out. The second NAND gate acts as an inverter (by connecting both inputs to NAND_Out), producing the final output Y.
• The port map statement connects the inputs and outputs of the NAND gates to the external signals of the AND gate.

How Structural Architectures Work:

• Structural architectures describe how a system is composed of smaller components, like building blocks.
• This is especially useful when designing complex circuits using pre-designed modules.

4. Architecture with Mixed Design

In some designs, you may want to use both behavioral and structural elements in the same architecture. This is called a mixed-level architecture. Here’s how you can mix behavioral and structural approaches.

Mixed Architecture Example

architecture Mixed of AND_Gate is
    signal N1, N2: std_logic;  -- Internal signals
begin
    -- Behavioral description of the first NAND gate
    N1 <= not (A and B);  -- Behavioral description of NAND gate
    
    -- Structural description of an AND gate using the behavioral NAND gate
    U1: entity work.NAND_Gate port map (
        A => N1, 
        B => N1, 
        Y => Y
    );
end Mixed;

Explanation:

• In this architecture, we combine both behavioral and structural styles.
• The first NAND gate is described behaviorally with the statement N1 <= not (A and B);.
• The second NAND gate is described structurally using the port map mechanism.

This mixed approach allows designers to handle complex designs where part of the logic is defined explicitly and other parts are reused as structural components.

5. Simulation and Synthesis

• Behavioral architectures are useful for simulation, where designers want to verify the functional correctness of their design.
• Structural architectures are preferred when designing for hardware synthesis because they reflect how the design will be implemented on physical hardware.

6. Concurrency in Architectures

One key feature of architectures in VHDL is that they naturally model concurrent operations. Since hardware components operate in parallel, VHDL allows concurrent signal assignments in architectures, meaning that multiple operations can happen simultaneously.

For example, in a more complex design, different signals could be updated concurrently, which mirrors real hardware behavior.

Advantages of Architectures in VHDL Programming Language

Following are the Advantages of Architectures in VHDL Programming Language:

1. Separation of Interface and Implementation

The division between entity and architecture in VHDL promotes a clear separation between the interface and the implementation of a component. This modular design enables you to define an external interface without worrying about how to describe or structure the internal functionality.

2. Modular and Reusable Design

Architectures facilitate a modular approach to VHDL design, allowing you to modify the internal behavior of a component easily without affecting its interface. This flexibility ensures that you can reuse architectures for different implementations, reducing development time and promoting code reuse.

3. Flexibility in Design Representation

With multiple architectures for a single entity, designers can choose between behavioral, structural, or mixed approaches based on project requirements. This flexibility allows you to describe a single component at different levels of abstraction, making the design process adaptable to simulation and synthesis needs.

4. Concurrency Support

VHDL architectures allow for concurrent execution of statements, which reflects how hardware components operate in real life. Multiple operations can occur simultaneously, modeling the natural parallelism of digital circuits.

5. Ease of Simulation

Behavioral architectures are excellent for functional verification through simulation. Designers can describe how a circuit behaves logically and simulate its functionality before implementing it in hardware, ensuring correctness and reducing the risk of design errors.

6. Structural Design for Hardware Synthesis

Architectures with structural descriptions provide a clear path to hardware synthesis. By describing the design in terms of smaller components (like gates or modules), designers can ensure that the architecture matches the physical structure of the circuit when synthesized onto hardware.

7. Maintainability and Scalability

The separation between entity and architecture makes VHDL designs easier to maintain and scale. You can make changes to the internal design without affecting the external interface, allowing designers to upgrade or optimize the implementation without impacting other parts of the system.

8. Support for Mixed-Level Design

VHDL supports mixed-level design by allowing a combination of behavioral and structural descriptions within the same architecture. This flexibility enables designers to combine high-level functional descriptions with low-level structural components, balancing ease of simulation and hardware implementation.

9. Design Abstraction

By supporting different levels of abstraction (from purely behavioral to fully structural), architectures provide a way to develop complex designs progressively. Designers can start with a high-level behavioral model and gradually refine it into a structural model suitable for hardware implementation.

10. Improved Design Testing

Since architectures allow for multiple implementations of the same entity, designers can test different configurations or versions of a design without altering the entity. This facilitates thorough testing, ensuring that the best-performing architecture is used for the final implementation.

Disadvantages of Architectures in VHDL Programming Language

Following are the Disadvantages of Architectures in VHDL Programming Language:

1. Increased Complexity in Large Designs

In large projects, managing multiple architectures for various entities can introduce significant complexity. Keeping track of different architectures for different components requires rigorous documentation and careful organization, which can lead to confusion if not handled well.

2. Overhead in Design Maintenance

The separation of interface (entity) and implementation (architecture) is generally beneficial, but it can also create maintenance overhead. If changes are required in the interface, modifications must be propagated across all associated architectures, which can become cumbersome in large designs.

3. Performance Optimization Challenges

While architectures allow flexibility in describing behavior at different levels of abstraction, optimizing performance for hardware synthesis might require a significant amount of manual tweaking. The abstraction can sometimes lead to inefficiencies when the design is synthesized, requiring extra effort to optimize it for real-world hardware constraints.

4. Potential for Redundancy

When multiple architectures are used for the same entity, there is a risk of redundancy if the architectures are not carefully differentiated. Without clear guidelines or reasons for the variations, having multiple architectures can lead to confusion and duplication of efforts.

5. Learning Curve

VHDL’s strict separation between entities and architectures adds to the learning curve for beginners. Newcomers might struggle to understand the need for separating the interface from its implementation, especially if they are familiar with programming languages that don’t enforce this distinction as strictly.

6. Synthesis Limitations

While behavioral architectures are useful for simulation, not all descriptions within an architecture can be directly synthesized into hardware. This distinction can cause confusion, as not all VHDL code is synthesizable, especially at higher levels of abstraction. Designers need to be aware of what types of constructs can actually be translated into physical hardware.

7. Debugging Complexity

When issues arise, the debugging process can be more complex because problems may not always be isolated within a single architecture. Debugging in VHDL often involves checking the interaction between entities and architectures, which can be time-consuming when designs become larger or more intricate.

8. Concurrency Handling

VHDL supports concurrent execution within architectures, which accurately reflects hardware behavior. However, this can make the design harder to understand, especially for beginners or those not familiar with hardware concurrency. Tracking how different processes interact in real-time can complicate the debugging and simulation processes.

9. Tool Dependency

The ability to effectively utilize architectures in VHDL is highly dependent on the quality of the development tools (e.g., simulators and synthesizers). Inconsistent or inadequate support across different tools can lead to difficulties in testing and optimizing the architecture for hardware.

10. Risk of Over-Engineering

Due to the flexibility in having multiple architectures for a single entity, there is a tendency toward over-engineering. Designers may create multiple architectures without a clear need, leading to unnecessary complexity in the design process.