Data Object Types A VHDL object consists of

Data 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. 1

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. 2

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_name : Signal_type [: = initial_value] The Keyword SIGNAL is followed by one or more signal names. 3

• 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. 4

• 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’. 5

Example : Signal library ieee; use ieee. std_logic_1164. all; entity signal_example is port ( a : in std_logic_vector(7 downto 0); y : out std_logic ); end signal_example; 6

architecture behave_signal of signal_example is signal temp : std_logic; begin process (a) begin temp <= '1'; for i in 0 to 7 loop temp <= temp xor a(i); end loop; y <= temp; end process; end behave_signal; 7

Waveform • Waveform show one input only 8

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] 9

• 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. 10

• 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. 11

Example - Variable library ieee; use ieee. std_logic_1164. all; entity variable_example is port ( a : in std_logic_vector(7 downto 0); y : out std_logic ); end variable_example; 12

architecture behave_variable of variable_example is begin process (a) variable temp: std_logic; begin temp : = '0'; for i in 0 to 7 loop temp : = temp xor a(i); end loop; y <= temp; end process; end behave_variable; 13

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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. 15

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]; 16