Explore the realm of VHDL, a fundamental programming language in the field of Computer Science. Absorb the principles from the introduction to VHDL language, understanding its foundation, and types of variables and data. Venture into the depths of control structures, becoming au fait with loops, if statements, and case statements. Finally, grasp the practical applications with real-life examples and utilisation in computer architecture. With this comprehensive guide, you'll deepen your knowledge and skills in VHDL, enhancing your prowess in Computer Science.


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Table of contents

    Understanding VHDL Programming Language

    Often used in electronic design automation, VHDL is a powerful hardware description language that provides a means of describing, in a textual format, the structure and function of digital systems. Diving into VHDL, you'll discover a language that's versatile and explicitly built to define complex digital systems, making it an essential aspect of computer science studies.

    Introduction to Language VHDL

    Developed in the 1980s, VHDL, standing for VHSIC (Very High Speed Integrated Circuits) Hardware Description Language, was originally created for the U.S. Department of Defense. However, VHDL rapidly became the standard for representing logic circuits and systems across a myriad of fields, reaching beyond government projects.

    VHDL is a typed language, which means it uses different data types for representing different types of information.

    A VHDL program, often referred to as a 'design', is composed of interconnected 'entities'. An 'entity' is a modular component of a design, analogous to an object or a class in object-oriented programming languages, which can interact with other entities through 'ports'. The main parts of VHDL are the entity declaration, the architecture body, and the configuration declaration.
     ENTITY entity_name IS 
    PORT ( port_name : mode port_type ); 
    END entity_name; 
    ARCHITECTURE architecture_name OF entity_name IS 
    END architecture_name; 
    CONFIGURATION config_name OF entity_name IS 
    FOR architecture_name 
    END FOR; 
    END config_name; 

    Foundation of VHDL code

    Here's an example of a simple VHDL code that defines a flip flop circuit – a fundamental building block in digital circuits.

     ENTITY flip_flop IS 
    END flip_flop;
    ARCHITECTURE behavior OF flip_flop IS 
    process(D, CLK) 
      if rising_edge(CLK) then Q <= D; 
      end if; 
    END process; 
    END behavior; 
    Building on the foundation of VHDL code requires understanding the concepts of signals, variables, and ports that enable communication between entities.

    Types of Variables and Data in VHDL

    In VHDL, there are several kinds of data types. Each data type has a predefined set of operations. Here are some examples:
    • bit: Represents a binary value (0 or 1).
    • boolean: Represents a logical value (true or false).
    • integer: Represents a signed whole number.
    • real: Represents a floating-point number.

    By learning VHDL, you're empowered to create high-level system designs and abstract data models, which can then be synthesised into real hardware circuits!

    Not equal VHDL: Understanding Inequalities

    In VHDL, the inequality operator is "/=". This operator is used to compare two values. For example, if you want to check if two signals, signal1 and signal2, are not equal, you can use the following code:
     IF signal1 /= signal2 THEN 
    -- execute if not equal 
    END IF; 
    Remember to use parenthesis if needed. For example, \((A + B) /= C\) Keep exploring VHDL, and soon enough you'll be designing complex digital systems with ease and efficiency. Happy coding!

    Working with Control Structures in VHDL

    Control structures form the backbone of any programming language, and VHDL is no exception. In VHDL, you use control structures to steer the flow of execution. With them, you're able to execute blocks of code multiple times, test conditions, and make decisions. The three fundamental control structures in VHDL are the For Loop, If Statement, and Case Statement. Let's dive deep into each of these.

    Dive Deep into VHDL for loop

    In VHDL, the for loop is a powerful control structure used to execute a block of statements repetitively for a predetermined number of times. It consists of an iteration scheme and a sequence of statements. For a for loop to function, you must define an iterator, the range over which it iterates, and the set of instructions executed for each iteration.
     FOR i IN range LOOP 
    -- Statements 
    END LOOP; 
    The range can be ascending or descending, defined as follows:
     FOR i IN 0 TO 10 LOOP -- Ascending 
    FOR i IN 10 DOWNTO 0 LOOP -- Descending 

    Utilising for loop for repetitive tasks in VHDL

    The primary use of a for loop is to perform repetitive tasks, specifically when the number of repetitions is known before entering the loop. For instance, here's how you could use a for loop to initialise a 10-element array to zero:
     FOR i IN 0 TO 9 LOOP 
    array(i) <= '0'; 
    END LOOP; 
    Remember, the loop variable (in this case, i) is implicitly declared by the for loop and is read-only within the loop.

    Importance of VHDL if statement

    The if statement in VHDL offers a means of conditionally executing statements, thereby enabling decision-making in your code. It's one of the most powerful control structures in VHDL, and its versatility makes it indispensable. The structure is as follows:
     IF condition THEN 
    -- Statements if condition is true 
    ELSIF another_condition THEN 
    -- Statements if another_condition is true 
    -- Statements if no conditions are true 
    END IF; 

    Creating conditions with VHDL if statement

    The conditions in the if statement can compare variables, constants, or expressions with relational operators, such as <, >, \(=\), \( \neq \), \( \leq \), \( \geq \). Here’s an example of using an if statement to compare two signals, signal1, and signal2:
     IF signal1 > signal2 THEN 
    -- execute if signal1 is greater than signal2 
    END IF; 

    The VHDL case statement Explained

    The case statement in VHDL allows multi-way branching, meaning that it lets you select one among several available options. Unlike an if statement, which tests for multiple conditions sequentially, a case statement categorizes conditions and applies different rules to each category using an expression.
     CASE expression IS 
    WHEN value1 => 
    -- Statements for value1 
    WHEN value2 => 
    -- Statements for value2 
    -- Statements for values not included above 
    END CASE; 

    Implementing decision-making using VHDL case statement

    The strength of the case statement lies in its ability to efficiently handle decision-making based on different cases of expression value. It’s especially helpful when working with enumerated types, like state machines.

    For instance, consider a traffic light system with the states 'stop', 'go', and 'wait'. This can be expressed with a case statement as follows:

     CASE state IS 
    WHEN 'stop' => 
    -- execute stop state logic 
    WHEN 'go' => 
    -- execute go state logic 
    WHEN 'wait' => 
    -- execute wait state logic 
    END CASE; 
    Remember, the case-statement requires a WHEN OTHERS clause by default, although there are exceptions based on the exhaustive nature of your conditional logic. Using these control structures wisely and adequately, you will be programming like a VHDL expert.

    VHDL Examples and Applications

    VHDL, thanks to its capacity to describe and simulate complex digital systems, serves a myriad of applications across the digital design world. You'll often find it in use in a diversity of sectors, from commercial industries to government and military projects. Some practical examples of VHDL's potent applications are digital circuits, arithmetic circuits, combinational and sequential circuits, like adders, multiplexers, encoders, or decoders.

    How 2 bit comparator VHDL with select code works

    A 2 bit comparator is a common application of VHDL. A 2 bit comparator compares two 2 bit numbers, A and B. The result provides three possibilities, which can be represented as bits - A is greater than B, A is equal to B, or A is less than B. The working model of a 2 bit comparator involves a series of if-then-else statements. They assure a comparison on a bit level. Here is a simplified VHDL code, which illustrates a 2 bit comparator using select code:
    ENTITY Comparator IS 
       PORT ( A, B : IN integer range 0 to 3; 
              L, G, E : OUT std_logic 
    END Comparator; 
    ARCHITECTURE Behavior OF Comparator IS 
          L <= '1' when 0, 
               '0' when others; 
          G <= '0' when 0, 
               '1' when others; 
          E <= '1' when B, 
               '0' when others; 
    END Behavior; 
    In this code, 'A' and 'B' are the input ports representing two 2 bit numbers. 'L', 'G', and 'E' are the output ports, indicating Less than, Greater than, and Equal situations, respectively. The WITH SELECT block acts as a decoding logic to present the output based on input 'A'.

    Real-life example of 2 bit comparator VHDL application

    A typical real-world application of a 2 bit comparator is in digital systems that require constant comparison of binary values. For instance, in a CPU, a comparator is used for making conditional jumps depending upon the comparison between two operands. With the comparator sending signals indicating whether a value is less, equal, or greater than the other, the CPU decides the next course of execution.

    Working with VHDL code: A step-by-step guide and examples

    Understanding and writing VHDL code, particularly for a beginner, can seem daunting. A structured, step-by-step approach makes the task easier. Here's a simplified guide to get you started: 1. Declare the entity: Every VHDL design begins with declaring an entity. That includes defining its name and declaring its ports and types.
     ENTITY entity_name IS 
    PORT ( port_name : mode port_type ); 
    END entity_name; 

    If we are creating an AND gate, an example is:

     ENTITY AND_Gate IS 
    PORT ( A, B : IN STD_LOGIC; 
           Z : OUT STD_LOGIC); 
    END AND_Gate; 
    2. Establish the architecture: Next, define the architecture, which contains the processing logic for the entity.
     ARCHITECTURE architecture_name OF entity_name IS 
    -- logic 
    END architecture_name; 

    An example of logic for an AND gate is:

     ARCHITECTURE Dataflow OF AND_Gate IS 
    Z <= A AND B; 
    END Dataflow; 
    3. Testing environment: Finally, for verification and testing, set up a testbench to mimic the expected inputs and see if the entity behaves as intended.

    Using VHDL in computer organisation and architecture

    Computer Organisation and Architecture is one field where VHDL plays a crucial role. VHDL allows for the description of structures in step with how they are manufactured or organised. You can easily design CPUs, ALUs and Memory Units using VHDL, allowing each design choice to reflect the desired aspect of organisational efficiency. In most cases, you'd use VHDL to synthesise hardware components at the register transfer level. For example, VHDL is routinely used to design Arithmetic Logic Units (ALUs). An ALU is a complex digital circuit that iterates a set of operations including arithmetic and logic operations. The sequence of an ALU design could include defining the data signals, creating the ALU entity, creating the ALU's architecture body, and finally, the simulation or translation of the design. Often, in the field of Computer Architecture, VHDL is used to create simulations of CPUs and memory subsystems. Simulations at this level allow you to see how hardware components interact, enabling the fine-tuning of your designs. So whether you're aiming to optimise the throughput of a CPU or developing efficient memory access strategies, VHDL continues to be an effective and critical design tool in Computer Organisation and Architecture.

    VHDL - Key takeaways

    • VHDL is a hardware description language used in electronic design automation that describes the structure and function of digital systems in a textual format.
    • VHDL language uses types of data for representing different types of information and is composed of interconnected 'entities', each being a modular component of the design. Base structure of VHDL includes parts: entity declaration, architecture body, and configuration declaration.
    • In VHDL programming, the key control structures include the For Loop, If Statement, and Case Statement. The For Loop is useful for executing a block of statements repetitively, If Statement allows for conditionally executing statements, and Case Statement enables multi-way branching.
    • VHDL statements include constructs such as inequality, represented by "/=", and the VHDL case statement which is used for implementing decision-making based on different cases of expression values.
    • Applications of VHDL include digital circuits, arithmetic circuits and other digital design frameworks including 2 bit comparator VHDL with select code and development of complex CPUs, ALUs and Memory Units in the field of Computer Organisation and Architecture.
    Frequently Asked Questions about VHDL
    What is the main function of VHDL in computer science?
    VHDL (VHSIC Hardware Description Language) is primarily used in computer science for describing the behaviour and structure of electronic systems, primarily digital ones. It plays a crucial role in FPGA design, ASIC design, and digital logic design, used for coding, simulation and synthesis in digital electronics.
    What are the key components to consider while learning VHDL?
    The key components to consider while learning VHDL are understanding the syntax and semantics, learning to use data types and operators, mastering the Sequential and Concurrent Statements, and the implementation of design units like entities, architectures and packages.
    How can I simulate a circuit using VHDL?
    To simulate a circuit using VHDL, you first write a test bench code to apply test cases to your VHDL design. Then, use an HDL simulator such as ModelSim to load the VHDL files, and run the testbench. This process applies stimulus to the circuit and shows you the output responses.
    What is the difference between VHDL and other hardware description languages?
    VHDL is a strongly-typed, concurrent hardware description language that describes hardware structure and behaviour. Unlike other hardware description languages like Verilog, VHDL is less forgiving of programming mistakes, making for robust designs, but also a steep learning curve. VHDL also emphasises concurrency, where multiple operations are assumed to happen simultaneously.
    What are the common uses of VHDL in digital system design?
    VHDL is commonly used in digital system design for the precise description and simulation of electronic systems, particularly digital circuits. It's also utilised for FPGA programming, hardware modelling, and generating test benches for simulations.

    Test your knowledge with multiple choice flashcards

    What does VHDL stand for and when was it developed?

    How are different types of information represented in VHDL?

    What is the structure of a VHDL program?


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