Which Data Type in VHDL Is Defined by the User?


Larry Thompson

Which Data Type in VHDL Is Defined by the User?

VHDL (VHSIC Hardware Description Language) is a powerful language used for designing and describing digital systems. It provides various data types to represent different kinds of values, such as integers, booleans, and characters. However, VHDL also allows users to define their own custom data types.

Why Define Custom Data Types?

Defining custom data types in VHDL can be beneficial for several reasons. Firstly, it allows you to create meaningful names for your data, making the code more readable and understandable. Secondly, it enables you to encapsulate complex structures into a single user-defined type, simplifying the design process.

Let’s take a look at some examples of user-defined data types in VHDL:

  • Enumeration Types

    An enumeration type is a user-defined type that represents a set of named values. It is useful when you have a discrete set of options or states. For example:

    type DayOfWeek is (Monday, Tuesday, Wednesday, Thursday, Friday, Saturday, Sunday);

    In this example, we define an enumeration type called “DayOfWeek” with seven possible values representing each day of the week.

  • Record Types

    A record type is a user-defined composite type that can contain multiple elements of different data types. It is useful when you need to group related variables together.

    For example:

    type Person is record
      Name: string(1 to 50);
      Age: integer range 0 to 150;
      Height: real;
    end record;

    In this example, we define a record type called “Person” with three elements representing a person’s name, age, and height.

  • Array Types

    An array type is a user-defined type that allows you to group multiple elements of the same data type into a single object. It is useful when you need to store and manipulate collections of data. For example:

    type Matrix is array (natural range <>, natural range <>) of integer;

    In this example, we define an array type called “Matrix” that can hold a two-dimensional matrix of integers.

Using User-Defined Data Types

Once you have defined your custom data types, you can use them in your VHDL code just like any other built-in data types. You can declare variables with these types, create objects of these types, and perform operations on them.

For example:

signal today: DayOfWeek; -- Variable declaration using enumeration type
variable john: Person; -- Object creation using record type
variable m: Matrix(0 to 2, 0 to 2); -- Object creation using array type

In this example, we declare a signal called “today” of the enumeration type “DayOfWeek,” create a variable called “john” of the record type “Person,” and create a variable called “m” of the array type “Matrix.”


User-defined data types in VHDL provide flexibility and abstraction, allowing designers to create meaningful abstractions for their designs. Whether it is an enumeration type, record type, or array type, custom data types enhance code readability and simplify the design process. By utilizing user-defined data types effectively, VHDL designers can create more organized and maintainable code.

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