Data Object By E. Thirumeni Department of Electronics St. Joseph’s College (Autonomous) Trichy

Data Object Object Types A VHDL object consists of one of the following: Signal, Which represents interconnection wires that connect component instantiation ports together. Variable, Which is used for local storage of temporary data, visible only inside a process. Constant, which names specific values.

Objects Objects are used to represent & store the data in the system being described in VHDL. Object contains a value of a specific type. The name given to object is called identifier. Each object has a type & class. Class indicates how the object is used in the model & what can be done with the object. Type indicates what type of data the object contains.

Every data object belongs to one of the following three classes: 1. Constant: An object of constant cla^s 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. 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. 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.

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 Signal objects are used to connect entities together to form models. Signals are the means for communication of dynamic data between entities. A signal declaration looks like this: Signal Signal_name : Signal_type [:= initial_value] The Keyword SIGNAL is followed by one or more signal names.

Each Signal name creates a new signal. Separating the signal names from the signal type is colon. Signal type specifies the data type of the information that the signal contains. The signal can contain an initial value specifier so that the signal value may initialized. Signals can be declared in entity declaration sections, architecture declarations and package declarations.

Signals in package declaration are also referred to as global signals because they can be shared among entities. Each signal has a history of values I.e. holds a list of values which include current value of signal & set of possible future values that are to appear on the signal. Computed value is assigned to signal after delay called ‘delta delay’.

Signal Declarations Here are some examples of signal declarations. signal CLOCK: BIT; signal DATA_BUS: BIT_VECTOR(0 to 7); signal GATE_DELAY: TIME := 10 ns;

Variable Variable are used for local storage in process statements and subprograms. All assignment to variable occur immediately. A variable declaration looks like this: Variable variable_name: variable_type [: value]

The keyword VARIABLE is followed by one or more variable names. Each name creates a new variable. The construct variable_type defines the data type of the variable, and an optional initial value can be specified. Variable can be declared in the process declaration and subprogram declaration sections only.

Variable are inherently more efficient because assignments happen immediately, while signals must be scheduled to occur. Variables take less memory, while signals need more information to allow for scheduling and signal attributes. Using a Signal would have required a WAIT statement to synchronize the signal assignment to the same execution iteration as the usage.

Variable Declarations Examples of variable declarations are variable CTRL_STATUS: BIT_VECTOR(10 downto 0); variable SUM: INTEGER range Oto 100 := 10; variable FOUND, DONE: BOOLEAN;

Signal vs Variables A Signal has three properties attached to it: type, value and time. while a variable has only two properties attached to it type and value. Use signals as channels of communication between concurrent statement. In non-synthesizeable models, avoid using signals to describe storage elements. Use variable instead. Signals occupy about two orders of magnitude more storage than variable during simulation. Signals also cost a performance penalty due to the simulation overhead necessary to maintain the data structures representing signals.

Constants Constant objects are names assigned to specific values of a type. Constants give the designer the ability to have a better-documented model, and a model that is easy to update. Constant declaration : Constant constant_name : type_name [:value];

Constant Declarations Examples of constant declarations are constant RISE_TIME: TIME := 10ns; constant BUS_WIDTH: INTEGER := 8: An example of another form of constant declaration is constant NO_OF_INPUTS: INTEGER; The value of the constant has not been specified in this case. Such a constant is called a deferred constant and it can appear only inside a package declaration.