Process: Basic Functional Unit in VHDL

Piyush Gupta

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The design and modeling of digital systems sometimes employ the potent language VHDL (Very High-Speed Integrated Circuit Hardware Description Language). It offers a methodical, systematic way of expressing how electronic circuits behave and are built. The process, which acts as the core functional unit for modeling concurrent behavior, is one of the essential building blocks in VHDL.

For successfully developing and implementing digital systems using VHDL, understanding processes is crucial. Processes give you the ability to concurrently and event-driven define the activity of your circuits. You may record the intricate relationships and interactions between various signals and system components by using processes.

We will delve into the concept of processes in VHDL and examine their function in modeling digital systems in this blog post. We’ll look at a process’ syntax and structure, how they’re used to define both combinational and sequential logic, and how they’re used to distinguish between concurrent and sequential execution within processes.

What is a Process in VHDL?

A. Definition of a process in VHDL:

A process in VHDL is a key component for modeling and describing the behavior of digital systems. It is an illustration of a concurrent control structure that specifies a series of sequential statements that are carried out in response to specific occurrences. These occurrences may include adjustments to the input signals or the fulfillment of particular time requirements.

B. Role of processes in modeling digital systems:

In order to fully capture the concurrent nature of digital systems, processes are essential. They let designers express complicated behaviors and interactions between various components of a design by enabling the description of both combinational and sequential logic. Designers can produce modular, reusable parts that can be combined to form larger systems by applying procedures.

C. How processes are used to describe concurrent behavior:

Multiple statements may run concurrently in VHDL because it is a concurrent language by nature. In VHDL, processes offer a means to specify the timing and sequence of these concurrent operations. Sensitivity lists enable processes to be launched in response to changes in predefined signals or events, providing fine-grained control over the execution flow.

D. Comparison with sequential behavior in VHDL:

While VHDL processes provide a means to describe concurrent behavior, they can also be used to model sequential logic. Sequential processes represent a sequence of statements that are executed one after another, simulating the behavior of flip-flops, registers, and other sequential elements in a digital system. By combining concurrent and sequential processes, designers can accurately represent the behavior of complex systems that exhibit both concurrent and sequential aspects.

Understanding the concept of processes in VHDL is essential for effectively designing and simulating digital systems. In the next section, we will explore the syntax and structure of a process, providing a foundation for implementing behaviors using this fundamental VHDL construct

Syntax and Structure of a Process

A basic building component for defining the behavior of digital systems in VHDL is called a process. It enables you to specify concurrent behavior, which enables many processes to run simultaneously and communicate with one another via signals. In this section, we’ll look at the syntax and organization of a process in VHDL.

A. Process Declaration and Begin/End Keywords:

In VHDL, a process is declared using the “process” keyword followed by a sensitivity list. The sensitivity list specifies the signals that the process is sensitive to, meaning that the process will be triggered whenever a change occurs on any of the listed signals. The process body is enclosed between the “begin” and “end” keywords.


process (clk, rst)
   — Process statements go here
end process;

B. Sensitivity List and Signal Events:

The sensitivity list plays a crucial role in determining when a process is executed. It consists of a comma-separated list of signals enclosed in parentheses. Whenever a change, known as a signal event, occurs on any of the signals in the sensitivity list, the process is triggered and its statements are executed.


process (a, b)
   — Process statements go here
end process;

C. Sequential Statements within a Process:

Inside a process, you can write a sequence of sequential statements that define the behavior of the digital system. These statements are executed sequentially within the process body. Common sequential statements include variable assignments, conditional statements (if-else), loops, and procedure calls.


process (a, b)
   if (a = ‘1’) then
      c <= b;
      c <= ‘0’;
   end if;
end process;

D. Importance of Process Sensitivity and Event Triggering:

The sensitivity list and event-triggering mechanism are crucial for the proper simulation and synthesis of VHDL designs. By accurately specifying the signals in the sensitivity list, you ensure that the process is triggered only when necessary, conserving simulation resources. Additionally, precise sensitivity lists help the synthesis tool infer the correct hardware structure for the described behavior.

Behavioral Modeling with Processes

Behavioral modeling is a crucial aspect of VHDL that allows designers to describe the functionality of digital systems. Processes play a vital role in this modeling approach by enabling the representation of both combinational and sequential logic.

A. Describing Combinational Logic using Processes:

Combinational logic circuits are those in which the outputs depend solely on the current values of the inputs, without any memory or feedback. Processes in VHDL provide an effective way to model such circuits.

To describe combinational logic using processes, you can follow these steps:

  • Declare a process using the process keyword, specifying the input and output signals.
  • Inside the process, use the beginning and end keywords to enclose the sequential statements.
  • Use signal assignments (<=) or conditional statements (if-then-else) to define the relationship between the input and output signals.
  • Ensure that all necessary signals are included in the sensitivity list to trigger the process when their values change.

For example, let’s consider a 2-input AND gate modeled using a process in VHDL:

architecture Behavioral of AndGate is
   process (input1, input2)
      output <= input1 and input2;
   end process;
end Behavioral;

B. Describing Sequential Logic using Processes:

Sequential logic circuits have memory elements, such as flip-flops or registers, where the outputs depend not only on the current inputs but also on past inputs and internal states. Processes in VHDL are well-suited for modeling such sequential behavior.

To describe sequential logic using processes, you can follow these steps:

  • Declare a process with the necessary signals and variables.
  • Inside the process, use the beginning and end keywords to enclose the sequential statements.
  • Utilize variables to store internal states or intermediate values.
  • Use signal assignments (<=) or conditional statements (if-then-else) to define the relationship between the input, output signals, and internal states.
  • Include all relevant signals in the sensitivity list to trigger the process when their values change.

Let’s take an example of a D flip-flop modeled using a process in VHDL:

architecture Behavioral of DFlipFlop is
   signal d, clk, q, qn: std_logic;
   process (clk)
      if rising_edge(clk) then
         q <= d;
         qn <= not d;
      end if;
   end process;
end Behavioral;

In this example, the process triggers when a rising edge is detected on the clock signal (clk). It updates the output signals (q and qn) based on the value of the input signal (d).

By using processes, designers can effectively capture the behavior of various digital circuits, including more complex sequential elements like registers and state machines. Understanding how to use processes for behavioral modeling is essential for implementing accurate and efficient digital systems in VHDL.

Concurrent vs. Sequential Execution in Processes

In VHDL, processes can be executed concurrently or sequentially, depending on their sensitivity list and the events that trigger them. Understanding the difference between concurrent and sequential execution is crucial for designing reliable and efficient digital systems.

A. Concurrent Execution in VHDL:

Concurrent execution refers to the simultaneous execution of multiple processes or statements. In VHDL, concurrent execution allows multiple processes to run concurrently, with each process potentially influencing the others through shared signals.

Concurrent execution offers the advantage of modeling systems with parallel behavior, where different components or modules can operate independently and concurrently. This can lead to more efficient designs and better use of hardware resources.

B. Sequential Execution in VHDL:

Sequential execution refers to the ordered execution of processes or statements, one after the other, following a specific control flow. In VHDL, sequential execution is used to model behaviors that require a specific order of operations or have dependencies between different stages or states.

Sequential execution is commonly employed in modeling sequential logic, where the outputs depend on the current and past inputs or internal states. By controlling the order of execution, sequential processes can accurately model the flow of data and timing within a system.

C. Handling Concurrent Events and Event Triggering:

In VHDL, processes are triggered by specific events or changes in the signals included in their sensitivity list. Understanding how events are handled during concurrent execution is essential for the proper modeling of digital systems.

  1. Concurrent Event Handling:
  • When an event occurs on a signal in the sensitivity list of a process, the process is triggered and its statements are executed.
  • Concurrent processes with overlapping sensitivity lists can execute concurrently, potentially leading to race conditions or unexpected behavior.
  • To avoid race conditions, proper care must be taken to ensure that concurrent processes do not access or modify the same signals simultaneously.
  1. Event Triggering and Process Sensitivity:
  • The sensitivity list of a process determines which events can trigger the process.
  • Adding all the necessary signals to the sensitivity list is crucial to ensure that the process is triggered when the expected events occur.
  • Missing signals in the sensitivity list may result in incorrect or incomplete behavior of the design.

Understanding the distinction between concurrent and sequential execution in VHDL is vital for designing accurate and efficient digital systems. Proper handling of concurrent events and thoughtful selection of process sensitivity lists are essential to avoid race conditions and ensure the desired behavior of the system.

Process Attributes and Configurations

In VHDL, process attributes and configurations provide additional flexibility and customization options when working with processes. Understanding process attributes and how to configure processes can enhance the design and improve the overall functionality of digital systems.

A. Introduction to Process Attributes:

Process attributes in VHDL provide metadata or additional information about processes, allowing designers to access and utilize specific characteristics of processes during simulation or synthesis.

B. Commonly used process attributes include:

  1. ‘EVENT: This attribute returns the current event causing the process to execute, providing information about the event triggering the process.
  2. ‘ACTIVE: This attribute indicates whether the process is currently active or idle.
  3. ‘STABLE: This attribute checks if a signal’s value has remained unchanged since the last time the process was triggered.
  4. ‘LAST_EVENT: This attribute returns the last event causing the process to execute.

Process attributes enable dynamic behavior based on the context and can be utilized to optimize or control the execution of processes.

C. Configuration Options for Processes:

Configuration statements in VHDL allow customization and fine-tuning of processes to suit specific design requirements.

  1. Process Configuration:
  • Processes can be configured using the configuration block to override default settings or customize their behavior.
  • Configuration blocks specify different process attributes, such as sensitivity lists, delay values, or resolution functions.
  • By configuring processes, designers can modify the process behavior without modifying the process itself, promoting reusability and flexibility.
  1. Conditional Compilation and Configuration:
  • Conditional compilation directives, such as ifdef, ifndef, and elsif, can be used to conditionally include or exclude specific process configurations based on predefined conditions.
  • Conditional compilation allows designers to adapt processes based on different design scenarios or target platforms.

Process attributes and configurations enable designers to fine-tune the behavior of processes and adapt them to specific design requirements. They provide flexibility and control over process execution, enhancing the overall functionality and efficiency of digital systems.


Processes are the fundamental building blocks in VHDL that enable the modeling of digital systems. They play a crucial role in capturing both combinational and sequential logic, allowing designers to accurately describe the behavior of complex circuits. Understanding processes, their syntax, and execution are essential for creating effective VHDL models. By utilizing process attributes and configurations, designers can customize the behavior of processes and enhance their functionality. Mastering processes empowers designers to create robust and efficient digital systems. Embrace the power of processes in VHDL to unleash your creativity and design innovative solutions.


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