VHDL Data Objects

DATA OBJECTS

A data object holds a value of a specified type, created by using object declaration.

Every data object belongs to one of the following three classes:

An object declaration is used to declare an object, its type, and its class, and optionally assign it a value.

1.   Constant

An object of constant class can hold a single value of a given type. This value is assigned to the object before simulation starts and the value cannot be changed during the course of the simulation.

         Eg.,  constant RISE_TIME: TIME := 10ns;

constant BUS_WIDTH: INTEGER := 8:

The first declaration declares the object RISE_TIME that can hold a value of type TIME, with the value assigned at the start of simulation is 10 ns.

The second constant declaration declares a constant BUS_WIDTH of type INTEGER with a value of 8.

The value of the constant has not been specified, called as a deferred constant and it can appear only inside a package declaration. The complete constant declaration with the associated value must appear in the corresponding package body.

constant NO_OF_INPUTS: INTEGER;

2.   Variable

An object of variable class can also hold a single value of a given type. However in this case, different values can be assigned to the object at different times using a variable assignment statement.

Eg.,

variable COUNT: INTEGER;

This results in the creation of a data object called COUNT which can hold integer values and the object COUNT is also declared to be of variable class.

variable CTRL_STATUS: BIT_VECTOR(10 downto 0);

variable SUM: INTEGER range 0 to 100 := 10;

variable FOUND, DONE: BOOLEAN;

The first declaration specifies a variable object CTRL_STATUS as an array of 11 elements, with each array element of type BIT.

In the second declaration, an explicit initial value has been assigned to the variable SUM, with a an initial value of 10 at the start of simulation.

If no initial value is specified for a variable object, a default value is used as an initial value. This default value is T'LEFT, where T is the object type and LEFT is a predefined attribute of a type that gives the leftmost value in the set of values belonging to type T.

In the third declaration, the initial values assigned to FOUND and DONE at start of simulation is FALSE. The initial value for all the array elements of CTRL_STATUS is '0'.

3.   Signal

An object belonging to the signal class has a past history of values, a current value, and a set of future values. Future values can be assigned to a signal object using a signal assignment statement.

The Signal objects can be regarded as wires in a circuit while variable and constant objects are analogous to their counterparts in a high-level programming language like C or Pascal.

Signal objects are typically used to model wires and flip-flops while variable and constant objects are typically used to model the behavior of the circuit.

signal CLOCK: BIT;

signal DATA_BUS: BIT_VECTOR(0 to 7);

signal GATE_DELAY: TIME := 10 ns;

The first signal declaration declares the signal object CLOCK of type BIT and gives it an initial value of '0'.

The Second Signal Declaration declares the signal object DATA_BUS of type BIT_VECTOR with 1-dimensional array of 8 bits with an intial value of “00000000”.

The third signal declaration declares a signal object GATE_DELAY of type TIME that has an initial value of 10 ns.

Other Ways to Declare Objects

The object declarations that are not created using object declarations are declared as

1.   ports of an entity. All ports are signal objects.

2.   generics of an entity which are constant objects.

3.   formal parameters of functions and procedures. Function parameters are constant objects or signal objects while procedure parameters can belong to any object class.

4.   a file declared by a file declaration.

There are two other types of objects that are implicitly declared, these are the indices of a for. . . loop statement and the generate statement

Eg., an implicit declaration for the loop index in a for. . . loop statement

for COUNT in 1 to 10 loop

SUM := SUM + COUNT;

end loop;

In this for . . . loop statement, object COUNT has an implicit declaration of type INTEGER with range I to 10. The object COUNT is created when the loop is first entered and ceases to exist after the loop is exited.