Data Objects: Signals, Variables and Constants

Data Objects: Signals, Variables and Constants

A data object is created by an object declaration and has a value and type associated with it. An object can be a Constant, Variable, Signal or a File.

Signals can be considered wires in a schematic that can have a current value and future values, and that are a function of the signal assignment statements. On the other hand, Variables and Constants are used to model the behavior of a circuit and are used in processes, procedures and functions, similarly as they would be in a programming language. Following is a brief discussion of each class of objects.

Constant

A constant can have a single value of a given type and cannot be changed during the simulation. A constant is declared as follows,

constant list_of_name_of_constant: type [ := initial value] ;

where the initial value is optional. Constants can be declared at the start of an architecture and can then be used anywhere within the architecture. Constants declared within a process can only be used inside that specific process.

constant RISE_FALL_TME: time := 2 ns;

constant DELAY1: time := 4 ns;

constant RISE_TIME, FALL_TIME: time:= 1 ns;

constant DATA_BUS: integer:= 16;

Variable

A variable can have a single value, as with a constant, but a variable can be updated using a variable assignment statement. The variable is updated without any delay as soon as the statement is executed. Variables must be declared inside a process (and are local to the process). The variable declaration is as follows:

variable list_of_variable_names: type [ := initial value] ;

A few examples follow:

variable CNTR_BIT: bit :=0;

variable VAR1: boolean :=FALSE;

variable SUM: integer range 0 to 256 :=16;

variable STS_BIT: bit_vector (7 downto 0);

The variable SUM, in the example above, is an integer that has a range from 0 to 256 with initial value of 16 at the start of the simulation. The fourth example defines a bit vector or 8 elements: STS_BIT(7), STS_BIT(6),… STS_BIT(0).

A variable can be updated using a variable assignment statement such as

Variable_name := expression;

As soon as the expression is executed, the variable is updated without any delay.