Jan Van der Spiegel

University of Pennsylvania

Department of Electrical and Systems Engineering



VHDL Tutorial 1

1. Introduction. 1

2. Levels of representation and abstraction. 2

3. Basic Structure of a VHDL file. 3

Behavioral model 5

Concurrency. 6

Structural description. 6

4. Lexical Elements of VHDL. 10

5. Data Objects: Signals, Variables and Constants. 11

Constant 11

Variable. 12

Signal 12

6. Data types. 13

Integer types. 15

Floating-point types. 15

Physical types. 16

Array Type. 17

Record Type. 19

Signal attributes. 20

Scalar attributes. 21

Array attributes. 22

7. Operators. 22

8. Behavioral Modeling: Sequential Statements. 26

Basic Loop statement 31

While-Loop statement 32

For-Loop statement 32

9. Dataflow Modeling – Concurrent Statements. 35

10. Structural Modeling. 37

11. References. 39

            Appendix: IEEE Standard Package STD_LOGIC_1164





This tutorial gives a brief overview of the VHDL language and is mainly intended as a companion for the Digital Design Laboratory. This writing aims to give the reader a quick introduction to VHDL and to give a complete or in-depth discussion of VHDL. For a more detailed treatment, please consult any of the many good books on this topic. Several of these books are listed in the reference list.




1. Introduction


VHDL stands for VHSIC (Very High Speed Integrated Circuits) Hardware Description Language. In the mid-1980’s the U.S. Department of Defense and the IEEE sponsored the development of this hardware description language with the goal to develop very high-speed integrated circuit. It has become now one of industry’s standard languages used to describe digital systems. The other widely used hardware description language is Verilog. Both are powerful languages that allow you to describe and simulate complex digital systems.  A third HDL language is ABEL (Advanced Boolean Equation Language) which was specifically designed for Programmable Logic Devices (PLD). ABEL is less powerful than the other two languages and is less popular in industry. This tutorial deals with VHDL, as described by the IEEE standard 1076-1993.


Although these languages look similar as conventional programming languages, there are some important differences. A hardware description language is inherently parallel, i.e. commands, which correspond to logic gates, are executed (computed) in parallel, as soon as a new input arrives. A HDL program mimics the behavior of a physical, usually digital, system. It also allows incorporation of timing specifications (gate delays) as well as to describe a system as an interconnection of different components. 


2. Levels of representation and abstraction


A digital system can be represented at different levels of abstraction [1]. This keeps the description and design of complex systems manageable. Figure 1 shows different levels of abstraction.




Figure 1: Levels of abstraction: Behavioral, Structural and Physical


The highest level of abstraction is the behavioral level that describes a system in terms of what it does (or how it behaves) rather than in terms of its components and interconnection between them. A behavioral description specifies the relationship between the input and output signals. This could be a Boolean expression or a more abstract description such as the Register Transfer or Algorithmic level. As an example, let us consider a simple circuit that warns car passengers when the door is open or the seatbelt is not used whenever the car key is inserted in the ignition lock At the behavioral level this could be expressed as,


            Warning = Ignition_on AND ( Door_open  OR Seatbelt_off)


The structural level, on the other hand, describes a system as a collection of gates and components that are interconnected to perform a desired function. A structural description could be compared to a schematic of interconnected logic gates. It is a representation that is usually closer to the physical realization of a system. For the example above, the structural representation is shown in Figure 2 below.




Figure 2: Structural representation of a “buzzer” circuit.


VHDL allows one to describe a digital system at the structural or the behavioral level. The behavioral level can be further divided into two kinds of styles: Data flow and Algorithmic. The dataflow representation describes how data moves through the system. This is typically done in terms of data flow between registers (Register Transfer level). The data flow model makes use of concurrent statements that are executed in parallel as soon as data arrives at the input. On the other hand, sequential statements are executed in the sequence that they are specified. VHDL allows both concurrent and sequential signal assignments that will determine the manner in which they are executed. Examples of both representations will be given later.


3. Basic Structure of a VHDL file


A digital system in VHDL consists of a design entity that can contain other entities that are then considered components of the top-level entity. Each entity is modeled by an entity declaration and an architecture body. One can consider the entity declaration as the interface to the outside world that defines the input and output signals, while the architecture body contains the description of the entity and is composed of interconnected entities, processes and components, all operating concurrently, as schematically shown in Figure 3 below. In a typical design there will be many such entities connected together to perform the desired function.


Figure 3: A VHDL entity consisting of an interface (entity declaration) and a body (architectural description).


VHDL uses reserved keywords that cannot be used as signal names or identifiers.  Keywords and user-defined identifiers are case insensitive. Lines with comments start with two adjacent hyphens (--) and will be ignored by the compiler. VHDL also ignores line breaks and extra spaces. VHDL is a strongly typed language which implies that one has always to declare the type of every object that can have a value, such as signals, constants and variables.


a.       Entity Declaration


The entity declaration defines the NAME of the entity and lists the input and output ports. The general form is as follows,


entity NAME_OF_ENTITY is [ generic generic_declarations);]

     port (signal_names: mode type;

            signal_names: mode type;


            signal_names: mode type);



An entity always starts with the keyword entity, followed by its name and the keyword is. Next are the port declarations using the keyword port. An entity declaration always ends with the keyword end, optionally [] followed by the name of the entity.


·        The NAME_OF_ENTITY is a user-selected identifier

·        signal_names consists of a comma separated list of one or more user-selected identifiers that specify external interface signals.

·        mode: is one of the reserved words to indicate the signal direction:

o       in – indicates that the signal is an input

o       out – indicates that the signal is an output of the entity whose value can only be read by other entities that use it.

o       buffer – indicates that the signal is an output of the entity whose value can be read inside the entity’s architecture

o       inout – the signal can be an input or an output.

·        type: a built-in or user-defined signal type. Examples of types are bit, bit_vector, Boolean, character, std_logic, and std_ulogic.

o       bit – can have the value 0 and 1

o       bit_vector – is a vector of bit values (e.g. bit_vector (0 to 7)

o       std_logic, std_ulogic, std_logic_vector, std_ulogic_vector: can have 9 values to indicate the value and strength of a signal. Std_ulogic and std_logic are preferred over the bit or bit_vector types.

o       boolean – can have the value TRUE and FALSE

o       integer – can have a range of integer values

o       real – can have a range of real values

o       character – any printing character

o       time – to indicate time


·        generic: generic declarations are optional and determine the local constants used for timing and sizing (e.g. bus widths) the entity. A generic can have a default value. The syntax for a generic follows,


generic (

constant_name: type [:=value] ;

constant_name: type [:=value] ;


constant_name: type [:=value] );


For the example of Figure 2 above, the entity declaration looks as follows.



-- comments: example of the buzzer circuit of fig. 2

entity BUZZER is

     port (DOOR, IGNITION, SBELT: in std_logic;

           WARNING: out std_logic);

     end BUZZER;



The entity is called BUZZER and has three input ports, DOOR, IGNITION and SBELT and one output port, WARNING. Notice the use and placement of semicolons! The name BUZZER is an identifier. Inputs are denoted by the keyword in, and outputs by the keyword out. Since VHDL is a strongly typed language, each port has a defined type. In this case, we specified the std_logic type. This is the preferred type of digital signals. In contrast to the bit type that can only have the values ‘1’ and ‘0’, the std_logic and std_ulogic types can have nine values. This is important to describe a digital system accurately including the binary values 0 and 1, as well as the unknown value X, the uninitialized value U, “-” for don’t care, Z for high impedance, and several symbols to indicate the signal strength (e.g. L for weak 0, H for weak 1, W for weak unknown - see section on Enumerated Types). The std_logic type is defined in the std_logic_1164 package of the IEEE library. The type defines the set of values an object can have. This has the advantage that it helps with the creation of models and helps reduce errors. For instance, if one tries to assign an illegal value to an object, the compiler will flag the error.


A few other examples of entity declarations follow


Four-to-one multiplexer of which each input is an 8-bit word.


entity mux4_to_1 is

     port (I0,I1,I2,I3: in std_logic_vector(7 downto 0);

     SEL: in std_logic_vector (1 downto 0); 	

           OUT1: out std_logic­_vector(7 downto 0));

     end mux4_to_1;


An example of the entity declaration of a D flip-flop with set and reset inputs is


entity dff_sr is

     port (D,CLK,S,R: in std_logic;

           Q,Qnot: out std_logic­);

     end dff_sr;



b.      Architecture body


The architecture body specifies how the circuit operates and how it is implemented. As discussed earlier, an entity or circuit can be specified in a variety of ways, such as behavioral, structural (interconnected components), or a combination of the above.


The architecture body looks as follows,


            architecture architecture_name of NAME_OF_ENTITY is

     -- Declarations

           -- components declarations

           -- signal declarations

           -- constant declarations

           -- function declarations

           -- procedure declarations

           -- type declarations





     -- Statements




     end architecture_name;



Behavioral model

The architecture body for the example of Figure 2, described at the behavioral level, is given below,


architecture behavioral of BUZZER is



     end behavioral;



The header line of the architecture body defines the architecture name, e.g. behavioral, and associates it with the entity, BUZZER. The architecture name can be any legal identifier. The main body of the architecture starts with the keyword begin and gives the Boolean expression of the function. We will see later that a behavioral model can be described in several other ways. The “<= ” symbol represents an assignment operator and assigns the value of the expression on the right to the signal on the left. The architecture body ends with an end keyword followed by the architecture name.


A few other examples follow. The behavioral description of a two-input AND gate is shown below.


entity AND2 is

     port (in1, in2: in std_logic;

           out1: out std_logic);

     end AND2;


architecture behavioral_2 of AND2 is


out1 <= in1 and in2;

     end behavioral_2;


An example of a two-input XNOR gate is shown below.


entity XNOR2 is

     port (A, B: in std_logic;

           Z: out std_logic);

     end XNOR2;


architecture behavioral_xnor of XNOR2 is

     -- signal declaration (of internal signals X, Y)

     signal X, Y: std_logic;


X <= A and B;

Y <= (not A) and (not B);

Z <= X or Y;

     End behavioral_xnor;



The statements in the body of the architecture make use of logic operators. Logic operators that are allowed are: and, or, nand, nor, xor, xnor and not. In addition, other types of operators including relational, shift, arithmetic are allowed as well (see section on Operators). For more information on behavioral modeling see section on Behavioral Modeling.



It is worth pointing out that the signal assignments in the above examples are concurrent statements. This implies that the statements are executed when one or more of the signals on the right hand side change their value (i.e. an event occurs on one of the signals). For instance, when the input A changes, the internal signals X and Y change values that in turn causes the last statement to update the output Z. There may be a propagation delay associated with this change.  Digital systems are basically data-driven and an event which occurs on one signal will lead to an event on another signal, etc. The execution of the statements is determined by the flow of signal values. As a result, the order in which these statements are given does not matter (i.e., moving the statement for the output Z ahead of that for X and Y does not change the outcome). This is in contrast to conventional, software programs that execute the statements in a sequential or procedural manner.


Structural description

The circuit of Figure 2 can also be described using a structural model that specifies what gates are used and how they are interconnected. The following example illustrates it.



            architecture structural of BUZZER is

           -- Declarations

component AND2

                port (in1, in2: in std_logic;

                      out1: out std_logic);

           end component;

           component OR2

                port (in1, in2: in std_logic;

                      out1: out std_logic);

           end component;

           component NOT1

                port (in1: in std_logic;

                      out1: out std_logic);

           end component;

           -- declaration of signals used to interconnect gates

signal DOOR_NOT, SBELT_NOT, B1, B2: std_logic;


           -- Component instantiations statements

U0: NOT1 port map (DOOR, DOOR_NOT);

           U1: NOT1 port map (SBELT, SBELT_NOT);

           U2: AND2 port map (IGNITION, DOOR_NOT, B1);

U3: AND2 port map (IGNITION, SBELT_NOT, B2);

U4: OR2  port map (B1, B2, WARNING);


     end structural;



Following the header is the declarative part that gives the components (gates) that are going to be used in the description of the circuits. In our example, we use a two- input AND gate, two-input OR gate and an inverter. These gates have to be defined first, i.e. they will need an entity declaration and architecture body (as shown in the previous example). These can be stored in one of the packages one refers to in the header of the file (see Library and Packages below). The declarations for the components give the inputs (e.g. in1, in2) and the output (e.g. out1).  Next, one has to define internal nets (signal names). In our example these signals are called DOOR_NOT, SBELT_NOT, B1, B2 (see Figure 2). Notice that one always has to declare the type of the signal.


The statements after the begin keyword gives the instantiations of the components and describes how these are interconnected. A component instantiation statement creates a new level of hierarchy. Each line starts with an instance name (e.g. U0) followed by a colon and a component name and the keyword port map. This keyword defines how the components are connected. In the example above, this is done through positional association: DOOR corresponds to the input, in1 of the NOT1 gate and DOOR_NOT to the output. Similarly, for the AND2 gate where the first two signals (IGNITION and DOOR_NOT) correspond to the inputs in1 and in2, respectively, and the signal B1 to the output out1. An alternative way is to use explicit association between the ports, as shown below.


label: component-name port map (port1=>signal1, port2=> signal2,… port3=>signaln);


U0: NOT1 port map (in1 => DOOR, out1 => DOOR_NOT);

U1: NOT1 port map (in1 => SBELT, out1 => SBELT_NOT);

U2: AND2 port map (in1 => IGNITION, in2 => DOOR_NOT, out1 => B1);

U3: AND2 port map (in1 => IGNITION, in2 => SBELT_NOT, B2);

U4: OR2  port map (in1 => B1, in2 => B2, out1 => WARNING);


Notice that the order in which these statements are written has no bearing on the execution since these statements are concurrent and therefore executed in parallel. Indeed, the schematic that is described by these statements is the same independent of the order of the statements.


Structural modeling of design lends itself to hierarchical design, in which one can define components of units that are used over and over again. Once these components are defined they can be used as blocks, cells or macros in a higher level entity. This can significantly reduce the complexity of large designs. Hierarchical design approaches are always preferred over flat designs. We will illustrate the use of a hierarchical design approach for a 4-bit adder, shown in Figure 4 below. Each full adder can be described by the Boolean expressions for the sum and carry out signals,


            sum =  (A Ĺ B) Ĺ C

            carry = AB + C(A Ĺ B)




Figure 4: Schematic of a 4-bit adder consisting of full adder modules.


In the VHDL file, we have defined a component for the full adder first. We used several instantiations of the full adder to build the structure of the 4-bit adder. We have included the library and use clause as well as the entity declarations.


Four Bit Adder – Illustrating a hierarchical VHDL model

-- Example of a four bit adder

library  ieee;

use  ieee.std_logic_1164.all;

-- definition of a full adder

entity FULLADDER is

     port (a, b, c: in std_logic;

           sum, carry: out std_logic);

     end FULLADDER;

architecture fulladder_behav of FULLADDER is


sum <= (a xor b) xor c ;

carry <= (a and b) or (c and (a xor b));

     end fulladder_behav;


     -- 4-bit adder

library  ieee;

use  ieee.std_logic_1164.all;


entity FOURBITADD is

     port (a, b: in std_logic_vector(3 downto 0);

           Cin : in std_logic;

                sum: out std_logic_vector (3 downto 0);

                Cout, V: out std_logic);

     end FOURBITADD;


architecture fouradder_structure of FOURBITADD is

     signal c: std_logic_vector (4 downto 0);

component FULLADDER

           port(a, b, c: in std_logic;

sum, carry: out std_logic);

           end component;


           FA0: FULLADDER

                port map (a(0), b(0), Cin, sum(0), c(1));

           FA1: FULLADDER

                port map (a(1), b(1), C(1), sum(1), c(2));

           FA2: FULLADDER

                port map (a(2), b(2), C(2), sum(2), c(3));

           FA3: FULLADDER

                port map (a(3), b(3), C(3), sum(3), c(4));

           V <= c(3) xor c(4);

           Cout <= c(4);

end fouradder_structure;



Notice that the same input names a and b for the ports of the full adder and the 4-bit adder were used. This does not pose a problem in VHDL since they refer to different levels. However, for readability, it may be easier to use different names. We needed to define the internal signals c(4:0) to indicate the nets that connect the output carry to the input carry of the next full adder. For the first input we used the input signal Cin. For the last carry we defined c(4) as an internal signal since the last carry is needed as the input to the xor gate. We could not use the output signal Cout since VHDL does not allow the  use of outputs as internal signals! For this reason we had to define the internal carry c(4) and assign c(4) to the output carry signal Cout.


See also the section on Structural Modeling.


c.       Library and Packages: library and use keywords


A library can be considered as a place where the compiler stores information about a design project. A VHDL package is a file or module that contains declarations of commonly used objects, data type, component declarations, signal, procedures and functions that can be shared among different VHDL models.


We mentioned earlier that std_logic is defined in the package ieee.std_logic_1164 in the ieee library. In order to use the std_logic one needs to specify the library and package. This is done at the beginning of the VHDL file using the library and the use keywords as follows:


library  ieee;

use  ieee.std_logic_1164.all;


The .all extension indicates to use all of the ieee.std_logic_1164 package.


The Xilinx Foundation Express comes with several packages.


            ieee Library:



To use any of these one must include the library and use clause:

library ieee;

use ieee.std_logic_1164.all;

use ieee.std_logic_arith.all;

use ieee.std_logic_unsigned.all;


In addition, the synopsis library has the attributes package:


library SYNOPSYS;

use SYNOPSYS.attributes.all;


One can add other libraries and packages. The syntax to declare a package is as follows:


            -- Package declaration

     package name_of_package is

           package declarations

     end package name_of_package;

     -- Package body declarations
     package body name_of_package is

           package body declarations

     end package body name_of_package;


For instance, the basic functions of the AND2, OR2, NAND2, NOR2, XOR2, etc. components need to be defined before one can use them. This can be done in a package, e.g. basic_func for each of these components, as follows:


-- Package declaration

library ieee;

use ieee.std_logic_1164.all;

package basic_func is

     -- AND2 declaration

     component AND2

           generic (DELAY: time :=5ns);

           port (in1, in2: in std_logic; out1: out std_logic);

     end component;

     -- OR2 declaration

component OR2

           generic (DELAY: time :=5ns);

           port (in1, in2: in std_logic; out1: out std_logic);

     end component;

end package basic_func;


-- Package body declarations

library ieee;

use ieee.std_logic_1164.all;

package body basic_func is

     -- 2 input AND gate
     entity AND2 is

           generic (DELAY: time);

           port (in1, in2: in std_logic; out1: out std_logic);

     end AND2;

     architecture model_conc of AND2 is


                out1 <= in1 and in2 after DELAY;

     end model_conc;

-- 2 input OR gate
entity OR2 is

           generic (DELAY: time);

           port (in1, in2: in std_logic; out1: out std_logic);

     end OR2;

     architecture model_conc2 of AND2 is


                out1 <= in1 or in2 after DELAY;

     end model_conc2;

end package body basic_func;



Notice that we included a delay of 5 ns. However, it should be noticed that delay specifications are ignored by the Foundation synthesis tool. We made use of the predefined type std_logic that is declared in the package std_logic_1164. We have included the library and use clause for this package. This package needs to be compiled and placed in a library. Lets call this library my_func. To use the components of this package one has to declare it using the library and use clause:


library  ieee, my_func;

use  ieee.std_logic_1164.all, my_func.basic_func.all;


One can concatenate a series of names separated by periods to select a package. The library and use statements are connected to the subsequent entity statement. The library and use statements have to be repeated for each entity declaration.


One has to include the library and use clause for each entity as shown for the example of the four-bit adder above.


4. Lexical Elements of VHDL


a.       Identifiers

Identifiers are user-defined words used to name objects in VHDL models. We have seen examples of identifiers for input and output signals as well as the name of a design entity and architecture body. When choosing an identifier one needs to follow these basic rules:


·        May contain only alpha-numeric characters (A to Z, a to z, 0-9) and the underscore (_) character

·        The first character must be a letter and the last one cannot be an underscore.

·        An identifier cannot include two consecutive underscores.

·        An identifier is case insensitive (ex. And2 and AND2 or and2 refer to the same object)

·        An identifier can be of any length.


Examples of valid identifiers are: X10, x_10, My_gate1.

Some invalid identifiers are: _X10, my_gate@input, gate-input.


The above identifiers are called basic identifiers. The rules for these basic identifiers are often too restrictive to indicate signals. For example, if one wants to indicate an active low signal such as an active low RESET, one cannot call it /RESET. In order to overcome these limitations, there are a set of extended identifier rules which allow identifiers with any sequence of characters.


·        An extended identifier is enclosed by the backslash, “\”, character.

·        An extended identifier is case sensitive.

·        An extended identifier is different from reserved words (keywords) or any basic identifier (e.g. the identifier \identity\ is allowed)

·        Inside the two backslashes one can use any character in any order, except that a backslash as part of an extended identifier must be indicated by an additional backslash. As an example, to use the identifier BUS:\data, one writes: \BUS:\data\

·        Extended identifiers are allowed in the VHDL-93 version but not in VHDL-87


Some examples of legal identifiers are:


            Input,   \Input\,  \input#1\,   \Rst\\as\  


b.      Keywords (Reserved words)


Certain identifiers are used by the system as keywords for special use such as specific constructs. These keywords cannot be used as identifiers for signals or objects we define. We have seen several of these reserved words already such as in, out, or, and, port, map, end, etc. Keywords are often printed in boldface, as is done in this tutorial. For a list of all the keywords click on complete keyword list. Extended identifiers can make use of keywords since these are considered different words (e.g. the extended identifier \end\ is allowed.


c.       Numbers


The default number representation is the decimal system. VHDL allows integer literals and real literals. Integer literals consist of whole numbers without a decimal point, while real literals always include a decimal point. Exponential notation is allowed using the letter “E” or “e”. For integer literals the exponent must always be positive. Examples are:


Integer literals: 12    10    256E3   12e+6

Real literals:   1.2   256.24  3.14E-2


The number –12 is a combination of a negation operator and an integer literal.


To express a number in a base different from the base “10”, one uses the following convention: base#number#. A few examples follow.


            Base 2:   2#10010#  (representing the decimal number “18”)

            Base 16: 16#12#     

            Base 8:   8#22#


            Base 2:   2#11101#  (representing the decimal number “29”)

            Base 16: 16#1D#     

            Base 8:   8#35#


To make the readability of large numbers easier, one can insert underscores in the numbers as long as the underscore is not used at the beginning or the end.





d.      Characters, Strings and Bit Strings


To use a character literal in a VHDL code, one puts it in a single quotation mark, as shown in the examples below:


           ‘a’,  ‘B’,  ‘,’


On the other hand, a string of characters are placed in double quotation marks as shown in the following examples:


           “This is a string”, 

“To use a double quotation mark inside a string, use two double quotation marks”

“This is a “”String””.”


Any printing character can be included inside a string.


A bit-string represents a sequence of bit values. In order to indicate that this is a bit string, one places the ‘B’ in front of the string: B”1001”. One can also use strings in the hexagonal or octal base by using the X or O specifiers, respectively. Some examples are:


            Binary:  B”1100_1001”, b”1001011”

            Hexagonal: X”C9”, X”4b”

            Octal: O”311”, o”113”


Notice that in the hexadecimal system, each digit represents exactly 4 bits. As a result, the number b”1001011” is not the same as X”4b” since the former has only 7 bits while the latter represents a sequence 8 bits. For the same reason, O”113” (represents 9 bits) is not the same sequence as X”4b” (represents 8 bits).


5. 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. Up to now we have seen signals that were used as input or output ports or internal nets. 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.



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;



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.


Signals are declared outside the process using the following statement:


signal list_of_signal_names: type [ := initial value] ;


           signal SUM, CARRY: std_logic;

           signal CLOCK: bit;

     signal TRIGGER: integer :=0;

     signal DATA_BUS: bit_vector (0 to 7);

     signal VALUE: integer range 0 to 100;


Signals are updated when their signal assignment statement is executed, after a certain delay, as illustrated below,


           SUM <= (A xor B) after 2 ns;


If no delay is specified, the signal will be updated after a delta delay. One can also specify multiple waveforms using multiple events as illustrated below,


signal wavefrm : std_logic;

wavefrm <= ‘0’, ‘1’ after 5ns, ‘0’ after 10ns, ‘1’ after 20 ns;


It is important to understand the difference between variables and signals, particularly how it relates to when their value changes. A variable changes instantaneously when the variable assignment is executed. On the other hand, a signal changes a delay after the assignment expression is evaluated. If no delay is specified, the signal will change after a delta delay. This has important consequences for the updated values of variables and signals. Lets compare the two files in which a process is used to calculate the signal RESULT [7].


Example of a process using Variables


architecture VAR of EXAMPLE is

     signal TRIGGER, RESULT: integer := 0;



           variable variable1: integer :=1;

           variable variable2: integer :=2;

           variable variable3: integer :=3;


           wait on TRIGGER;

           variable1 := variable2;

           variable2 := variable1 + variable3;

           variable3 := variable2;

           RESULT <= variable1 + variable2 + variable3;

     end process;

end VAR


Example of a process using Signals


architecture SIGN of EXAMPLE is

     signal TRIGGER, RESULT: integer := 0;

signal signal1: integer :=1;

     signal signal2: integer :=2;

     signal signal3: integer :=3;




           wait on TRIGGER;

           signal1 <= signal2;

           signal2 <= signal1 + signal3;

           signal3 <= signal2;

           RESULT  <= signal1 + signal2 + signal3;

     end process;

end SIGN;



In the first case, the variables “variable1, variable2 and variable3” are computed sequentially and their values updated instantaneously after the TRIGGER signal arrives. Next, the RESULT, which is a signal, is computed using the new values of the variables and updated a time delta after TRIGGER arrives. This results in the following values (after a time TRIGGER): variable1 = 2, variable2 = 5 (=2+3), variable3= 5. Since RESULT is a signal it will be computed at the time TRIGGER and updated at the time TRIGGER + Delta. Its value will be RESULT=12.


On the other hand, in the second example, the signals will be computed at the time TRIGGER. All of these signals are computed at the same time, using the old values of signal1, 2 and 3. All the signals will be updated at Delta time after the TRIGGER has arrived. Thus the signals will have these values: signal1= 2, signal2= 4 (=1+3), signal3=2 and RESULT=6.


6. Data types


Each data object has a type associated with it. The type defines the set of values that the object can have and the set of operations that are allowed on it. The notion of type is key to VHDL since it is a strongly typed language that requires each object to be of a certain type. In general one is not allowed to assign a value of one type to an object of another data type (e.g. assigning an integer to a bit type is not allowed). There are four classes of data types: scalar, composite, access and file types. The scalar types represent a single value and are ordered so that relational operations can be performed on them. The scalar type includes integer, real, and enumerated types of Boolean and Character. Examples of these will be given further on.


a. Data Types defined in the Standard Package


VHDL has several predefined types in the standard package as shown in the table below.  To use this package one has to include the following clause:


            library std, work;

     use std.standard.all;


Types defined in the Package Standard of the std Library


Range of values



 ‘0’, ‘1’

signal A: bit :=1;


an array with each element of type bit

signal INBUS: bit_vector(7 downto 0);



variable TEST: Boolean :=FALSE’


any legal VHDL character (see package standard); printable characters must be placed between single quotes (e.g. ‘#’)

variable VAL: character :=’$’;


read_mode, write_mode, append_mode



open_ok, status_error, name_error, mode_error



range is implementation dependent but includes at least –(231 – 1) to +(231 – 1)

constant CONST1: integer :=129;


integer starting with 0 up to the max specified in the implementation

variable VAR1: natural :=2;


integer starting from 1 up the max specified in the implementation

variable VAR2: positive :=2;


floating point number in the range of –1.0 x 1038 to +1.0x 1038 (can be implementation dependent. Not supported by the Foundation synthesis program.

variable VAR3: real :=+64.2E12;


note, warning, error, failure



array of which each element is of the type character

variable VAR4: string(1 to 12):= “@$#ABC*()_%Z”;


an integer number of which the range is implementation defined; units can be expressed in sec, ms, us, ns, ps, fs, min and hr. . Not supported by the Foundation synthesis program

variable DELAY: time :=5 ns;

* Not supported by the Foundation synthesis program


b. User-defined Types


One can introduce new types by using the type declaration, which names the type and specifies its value range. The syntax is


            type identifier is type_definition;


Here are a few examples of type definitions,


Integer types

            type small_int is range 0 to 1024;

            type my_word_length is range 31 downto 0;

            subtype data_word is my_word_length range 7 downto 0;


A subtype is a subset of a previously defined type. The last example above illustrates the use of subtypes. It defines a type called data_word that is a sybtype of my_word_length of which the range is restricted from 7 to 0. Another example of a subtype is,


            subtype int_small is  integer range -1024 to +1024;


Floating-point types


            type cmos_level is range 0.0 to 3.3;

            type pmos_level is range -5.0 to 0.0;

            type probability is range 0.0 to 1.0;

            subtype cmos_low_V is cmos_level range 0.0 to +1.8;


Note that floating point data types are not supported by the Xilinx Foundation synthesis program.


Physical types


The physical type definition includes a units identifier as follows,


            type conductance is range 0 to 2E-9



mmho = 1E-3 mho;

                umho = 1E-6 mho;

                nmho = 1E-9 mho;

                pmho = 1E-12 mho;

           end units conductance;


Here are some object declarations that use the above types,


            variable BUS_WIDTH: small_int :=24;

            signal DATA_BUS: my_word_length;

            variable VAR1: cmos_level range 0.0 to 2.5;

            constant LINE_COND: conductance:= 125 umho;


            Notice that a space must be left before the unit name.


The physical data types are not supported by the Xilinx Foundation Express synthesis program.


In order to use our own types, we need either to include the type definition inside an architecture body or to declare the type in a package. The latter can be done as follows for a package called “my_types”.


package my_types is

     type small_int is range 0 to 1024;

     type my_word_length is range 31 downto 0;

     subtype data_word is my_word_length is range 7 downto 0;

     type cmos_level is range 0.0 to 3.3;

type conductance is range 0 to 2E-9



mmho = 1E-3 mho;

                umho = 1E-6 mho;

                nmho = 1E-9 mho;

                pmho = 1E-12 mho;

           end units conductance;

end package my_types;


c.       Enumerated Types


An enumerated type consists of lists of character literals or identifiers. The enumerated type can be very handy when writing models at an abstract level. The syntax for an enumerated type is,


     type type_name is (identifier list or character literal);


Here are some examples,


type my_3values is (‘0’, ‘1’, ‘Z’);

type PC_OPER  is (load, store, add, sub, div, mult, shiftl, shiftr);

type hex_digit  is (‘0’, ‘1’, ‘2’, ‘3’, ‘4’, ‘5’, ‘6’, ‘7’, 8’, ‘9’, ‘A’, ‘B’, ‘C’, ‘D’, ‘E’, ‘F’);

type state_type is (S0, S1, S2, S3);


Examples of objects that use the above types:


            signal SIG1: my_3values;

            variable ALU_OP: pc_oper;

            variable first_digit: hex_digit :=’0’;

            signal STATE: state_type :=S2;


If one does not initialize the signal, the default initialization is the leftmost element of the list.


Enumerated types have to be defined in the architecture body or inside a package as shown in the section above.


An example of an enumerated type that has been defined in the std_logic_1164 package is the std_ulogic type, defined as follows


            type STD_ULOGIC is (

                        ‘U’,                  -- uninitialized

                        ‘X’,                  -- forcing unknown

                        ‘0’,                   -- forcing 0

                        ‘1’,                   -- forcing 1

                        ‘Z’,                  -- high impedance

                        ‘W’,                 -- weak unknown

                        ‘L’,                  --  weak 0

                        ‘H’.                  -- weak 1

                        ‘-‘);                  -- don’t care


In order to use this type one has to include the clause before each entity declaration.


            library ieee; use ieee.std_logic_1164.all;


It is possible that multiple drivers are driving a signal. In that case there could be a conflict and the output signal would be undetermined. For instance, the outputs of an AND gate and NOT gate are connected together into the output net OUT1. In order to resolve the value of the output, one can call up a resolution function. These are usually a user-written function that will resolve the signal. If the signal is of the type std_ulogic and has multiple drivers, one needs to use a resolution function. The std_logic_1164 package has such a resolution function, called RESOLVED predefined. One can then use the following declaration for signal OUT1


            signal OUT1: resolved: std_ulogic;


If there is contention, the RESOLVED function will be used to intermediate the conflict and determine the value of the signal. Alternatively, one can declare the signal directly as a std_logic type since the subtype std_logic has been defined in the std_logic_1164 package.


            signal OUT1: std_logic;


d.      Composite Types: Array  and Record


Composite data objects consist of a collection of related data elements in the form of an array or record. Before we can use such objects one has to declare the composite type first.


Array Type


An array type is declared as follows:


type array_name is array (indexing scheme) of element_type;


type MY_WORD is array (15 downto 0) of std_logic;

type YOUR_WORD is array (0 to 15) of std_logic;

type VAR is array (0 to 7) of integer;

type STD_LOGIC_1D is array (std_ulogic) of std_logic;


In the first two examples above we have defined a one-dimensional array of elements of the type std_logic indexed from 15 down to 0, and 0 up to 15, respectively.  The last example defines a one-dimensional array of the type std_logic elements that uses the type std_ulogic to define the index constraint. Thus this array looks as follows:


Index:               ‘U’  ‘X’  ‘0’  ‘1’  ‘Z’  ‘W’  ‘L’  ‘H’  ‘-‘



We can now declare objects of these data types. Some examples are given


            signal MEM_ADDR: MY_WORD;

            signal DATA_WORD: YOUR_WORD :=  B“1101100101010110”;

            constant SETTING: VAR := (2,4,6,8,10,12,14,16);


In the first example, the signal MEM_ADDR is an array of 16 bits, initialized to all ‘0’s. To access individual elements of an array we specify the index. For example, MEM_ACCR(15) accesses the left most bit of the array, while DATA_WORD(15) accesses the right most bit of the array with value ‘0’. To access a subrange, one specifies the index range, MEM_ADDR(15 downto 8) or DATA_WORD(0 to 7).


Multidimensional arrays can be declared as well by using a similar syntax as above,


type MY_MATRIX3X2 is array (1 to 3, 1 to 2) of natural;

type YOUR_MATRIX4X2 is array (1 to 4, 1 to 2) of integer;

type STD_LOGIC_2D is array (std_ulogic, std_ulogic) of std_logic;


variable DATA_ARR: MY_MATRIX :=((0,2), (1,3), (4,6), (5,7));


The variable array DATA_ARR will then be initialized to,


                0  2 

                             1  3 

                4  6 

                       5  7 


To access an element one specifies the index, e.g. DATA_ARR(3,1) returns the value 4.

The last example defines a 9x9 array or table with an index the elements of the std_ulogic type.


Sometimes it is more convenient not to specify the dimension of the array when the array type is declared. This is called an unconstrained array type. The syntax for the array declaration is,


type array_name is array (type range <>) of element_type;


Some examples are


            type MATRIX is array (integer range <>) of integer;

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

            type VECTOR2 is array (natural range <>, natural range <>) of std_logic;


The range is now specified when one declares the array object,


            variable MATRIX8: MATRIX (2 downto -8) := (3, 5, 1, 4, 7, 9, 12, 14, 20, 18);

            variable ARRAY3x2: VECTOR2 (1 to 4, 1 to 3)) := ((‘1’,’0’), (‘0’,’-‘), (1, ‘Z’));


Record Type

A second composite type is the records type. A record consists of multiple elements that may be of different types. The syntax for a record type is the following:


type name is


            identifier :subtype_indication;


            identifier :subtype_indication;

end record;


As an example,


type MY_MODULE  is


            RISE_TIME     :time;

            FALL_TIME   : time;

            SIZE                : integer range 0 to 200;

            DATA              : bit_vector (15 downto 0);

end record;


            signal A, B: MY_MODULE;


To access values or assign values to records, one can use one of the following methods:


            A.RISE_TIME <= 5ns;

A.SIZE <= 120;


            B <= A;


e.       Type Conversions


Since VHDL is a strongly typed language one cannot assign a value of one data type to a signal of a different data type. In general, it is preferred to the same data types for the signals in a design, such as std_logic (instead of a mix of std_logic and bit types). Sometimes one cannot avoid using different types. To allow assigning data between objects of different types, one needs to convert one type to the other. Fortunately there are functions available in several packages in the ieee library, such as the std_logic_1164 and the std_logic_arith packages. As an example, the std_logic_1164 package allows the following conversions:



Conversions supported by std_logic_1164 package

std_ulogic    to bit


std_logic_vector  to bit_vector


std_ulogic_vector  to bit_vector


bit  to std_ulogic


bit_vector   to   std_logic_vector


bit_vector   to   std_ulogic_vector


std_ulogic to std_logic_vector


std_logic to std_ulogic_vector



The IEEE std_logic_unsigned and the IEEE std_logic_arith packages allow additional conversions such as from an integer to std_logic_vector and vice versa.


An example follows.


entity QUAD_NAND2 is

     port (A, B: in bit_vector(3 downto 0);

           out4: out std_logic_vector (3 downto 0));      

     end QUAD_NAND2;


architecture behavioral_2 of QUAD_NAND2 is


out4 <= to_StdLogicVector(A and B);

     end behavioral_2;



The expression “A and B” which is of the type bit_vector has to be converted to the type std_logic_vector to be of the same type as the output signal out4.


The syntax of a type conversion is as follows:


            type_name (expression);


In order for the conversion to be legal, the expression must return a type that can be converted into the type type_name. Here are the conditions that must be fulfilled for the conversion to be possible.


·        Type conversions between integer types or between similar array types are possible

·        Conversion between array types is possible if they have the same length and if they have identical element types or convertible element types.

·        Enumerated types cannot be converted.



f.        Attributes


VHDL supports 5 types of attributes. Predefined attributes are always applied to a prefix such as a signal name, variable name or a type. Attributes are used to return various types of information about a signal, variable or type. Attributes consist of a quote mark (‘) followed by the name of the attribute.


Signal attributes


The following table gives several signal attributes.





returns the Boolean value True if an event on the signal occurred, otherwise gives a False


returns the Boolean value True there has been a transaction (assignment) on the signal, otherwise gives a False


returns a signal of the type “bit” that toggles (0 to 1 or 1 to 0) every time there is a transaction on the signal.


returns the time interval since the last event on the signal


returns the time interval since the last transaction on the signal


gives the value of the signal before the last event occurred on the signal


gives a signal that is the delayed version (by time T) of the original one. [T is optional, default T=0]


returns a Boolean value, True, if no event has occurred on the signal during the interval T, otherwise returns a False. [T is optional, default T=0]


returns a Boolean value, True, if no transaction has occurred on the signal during the interval T, otherwise returns a False. [T is optional, default T=0]


An example of an attribute is


if (CLOCK’event and CLOCK=’1’) then


This expression checks for the arrival of a positive clock edge.  To find out how much time has passed since the last clock edge, one can use the following attribute:




Scalar attributes


Several attributes of a scalar type, scalar-type, are supported.  The following table shows some of these attributes.





returns the first or leftmost value of scalar-type in its defined range


returns the last or rightmost value of scalar-type in its defined range


returns the lowest value of scalar-type in its defined range


returns the greatest value of scalar-type in its defined range


True if T is an ascending range, otherwise False


returns the value in T that is represented by s (s stands for string value).


Here are a few examples.


type conductance is range 1E-6 to 1E3

            units mho;

            end units conductance;

type my_index is range 3 to 15;

type my_levels is (low, high, dontcare, highZ);


conductance’right                     returns: 1E3

conductance’high                                              1E3

conductance’low                                              1E-6

my_index’left                                                    3

my_index’value(5)                                            “5”

my_levels’left                                                    low

my_levels’low                                                   low

my_levels’high                                      highZ

my_levels’value(dontcare)                                 “dontcare”


Array attributes


By using array attributes one can return an index value corresponding to the array range.


The following attributes are supported.












left-most element index

right-most index

upper bound

lower bound

the number of elements


reverse range

a Boolean value TRUE if index is an ascending range, otherwise FALSE          


The number N between parentheses refers to the dimension. For a one-dimensional array, one can omit the number N as shown in the examples below. Lets assume the following arrays, declared as follows:


            type MYARR8x4 is array (8 downto 1, 0 to 3) of boolean;

            type MYARR1 is array (-2 to 4)  of integer;


MYARR1’left              returns:             -2

MYARR1’right                                                4

MYARR1’high                                     4

MYARR1’reverse_range                                  4 downto to -2


            MYARR8x4’left(1)                                          8

MYARR8x4’left(2)                                          0

MYARR8x4’right(2)                                        3

MYARR8x4’high(1)                                         8

MYARR8x4’low(1)                                         1

MYARR8x4’ascending(1)                                False


7. Operators


VHDL supports different classes of operators that operate on signals, variables and constants. The different classes of operators are summarized below.









1. Logical operators







2. Relational operators







3. Shift operators







4.Addition operators







5. Unary operators







6. Multiplying op.







7. Miscellaneous op.








The order of precedence is the highest for the operators of class 7, followed by class 6 with the lowest precedence for class 1. Unless parentheses are used, the operators with the highest precedence are applied first. Operators of the same class have the same precedence and are applied from left to right in an expression. As an example, consider the following std_ulogic_vectors, X (=’010’), Y(=’10’), and Z (‘10101’). The expression


            not X & Y xor Z rol 1 


is equivalent to  ((not X) & Y) xor (Z rol 1)  = ((101) & 10) xor (01011)  =(10110) xor (01011) = 11101. The xor is executed on a bit-per-bit basis.





a.       Logic operators


The logic operators (and, or, nand, nor, xor and xnor)  are defined for the “bit”, “boolean”, “std_logic” and “std_ulogic” types and their vectors. They are used to define Boolean logic expression or to perform bit-per-bit operations on arrays of bits.  They give a result of the same type as the operand (Bit or Boolean). These operators can be applied to signals, variables and constants.


Notice that the nand and nor operators are not associative. One should use parentheses in a sequence of nand or nor operators to prevent a syntax error:


X nand Y nand Z will give a syntax error and should be written as (X nand Y) nand Z.


b.      Relational operators


The relational operators test the relative values of two scalar types and give as result a Boolean output of “TRUE” or “FALSE”.




Operand Types

Result Type



any type




any type



Smaller than

scalar or discrete array types



Smaller than or equal

scalar or discrete array types



Greater than

scalar or discrete array types



Greater than or equal

scalar or discrete array types



Notice that symbol of the operator “<=” (smaller or equal to) is the same one as the assignment operator used to assign a value to a signal or variable. In the following examples the first “<=” symbol is the assignment operator. Some examples of relational operations are:


            variable STS               : Boolean;

constant A                  : integer :=24;

            constant B_COUNT   : integer :=32;

            constant C                  : integer :=14;

            STS <= (A < B_COUNT) ;  -- will assign the value “TRUE” to STS

            STS <=  ((A >= B_COUNT) or (A > C));    -- will result in “TRUE”

            STS <=  (std_logic (‘1’, ‘0’, ‘1’) < std_logic(‘0’, ‘1’,’1’));--makes STS “FALSE”


            type new_std_logic is (‘0’, ‘1’, ‘Z’, ‘-‘);

            variable A1: new_std_logic :=’1’;

            variable A2: new_std_logic :=’Z’;

            STS <=  (A1 < A2); will result in “TRUE” since ‘1’ occurs to the left of ‘Z’.


For discrete array types, the comparison is done on an element-per-element basis, starting from the left towards the right, as illustrated by the last two examples. 


c.       Shift operators


These operators perform a bit-wise shift or rotate operation on a one-dimensional array of elements of the type bit (or std_logic) or Boolean.




Operand Type

Result Type


Shift left logical (fill right vacated bits with the 0)

Left: Any one-dimensional array type with elements of type bit or Boolean; Right: integer

Same as left type


Shift right logical (fill left vacated bits with 0)

 same as above

Same as left type


Shift left arithmetic (fill right vacated bits with rightmost bit)

same as above

Same as left type


Shift right arithmetic (fill left vacated bits with leftmost bit)

same as above

Same as left type


Rotate left (circular)

same as above

Same as left type


Rotate right (circular)

same as above

Same as left type


The operand is on the left of the operator and the number (integer) of shifts is on the right side of the operator. As an example,


            variable NUM1          :bit_vector := “10010110”;

            NUM1 srl 2;


will result in the number “00100101”.


When a negative integer is given, the opposite action occurs, i.e. a shift to the left will be a shift to the right. As an example


NUM1 srl –2 would be equivalent to NUM1 sll 2 and give the result “01011000”.


Other examples of shift operations are for the bit_vector A = “101001”


variable A: bit_vector :=”101001”;


A sll 2   results  in      “100100”

A srl 2   results in                  “001010”

A sla 2   results in                  “100111”

A sra 2   results in                 “111010”

A rol 2   results in                  “100110”

A ror 2   results in                 “011010”



d.      Addition operators


The addition operators are used to perform arithmetic operation (addition and subtraction) on operands of any numeric type. The concatenation (&) operator is used to concatenate two vectors together to make a longer one. In order to use these operators one has to specify the ieee.std_logic_unsigned.all or std_logic_arith package package in addition to the ieee.std_logic_1164 package.




Left Operand Type

Right Operand Type

Result Type



Numeric type

Same as left operand

Same type



Numeric type

Same as left operand

Same type



Array or element type

Same as left operand

Same array type


An example of concatenation is the grouping of signals into a single bus [4].


signal MYBUS                        :std_logic_vector (15 downto 0);

signal STATUS                       :std_logic_vector (2 downto 0);

signal RW, CS1, CS2             :std_logic;

signal MDATA                                  :std_logic_vector ( 0 to 9);



Other examples are


MYARRAY (15 downto 0) <= “1111_1111” & MDATA (2 to 9);

NEWWORD <= “VHDL” & “93”;


The first example results in filling up the first 8 leftmost bits of MYARRAY with 1’s and the rest with the 8 rightmost bits of MDATA. The last example results in an array of characters “VHDL93”.


e.       Unary operators


The unary operators “+” and “-“ are used to specify the sign of a numeric type.




Operand Type

Result Type



Any numeric type

Same type



Any numeric type

Same type


f.        Multiplying operators


The multiplying operators are used to perform mathematical functions on numeric types (integer or floating point).





Left Operand Type

Right Operand Type

Result Type





Any integer or floating point

Same type

Same type

Any physical type

Integer or real type

Same as left

Any integer or real type

Any physical type

Same as right



Any integer or floating point

Any integer or floating point

Same type

Any physical type

Any integer or real t ype

Same as left

Any physical type

Same type




Any integer type

Same type



Any integer type

Same type


The multiplication operator is also defined when one of the operands is a physical type and the other an integer or real type.


The remainder (rem) and modulus (mod) are defined as follows:


            A rem B = A –(A/B)*B                        (in which A/B in an integer)

            A mod B = A – B * N              (in which N is an integer)


The result of the rem operator has the sign of its first operand while the result of the mod operators has the sign of the second operand.


Some examples of these operators are given below.


11 rem 4                      results in 3

(-11) rem 4                   results in -3

9 mod 4                        results in 1

7 mod (-4)                    results in –1  (7 – 4*2 = -1).


g.       Miscellaneous operators


These are the absolute value and exponentation operators that can be applied to numeric types. The logical negation (not) results in the inverse polarity but the same type.




Left Operand Type

Right Operand Type

Result Type



Integer type

Integer type

Same as left

Floating point

Integer type

Same as left


Absolute value

Any numeric type

Same type


Logical negation

Any bit or Boolean type

Same type


Delays or timing information

Packages (list standard, 1164 packages).


8. Behavioral Modeling: Sequential Statements


As discussed earlier, VHDL provides means to represent digital circuits at different levels of representation of abstraction, such as the behavioral and structural modeling. In this section we will discuss different constructs for describing the behavior of components and circuits in terms of sequential statements. The basis for sequential modeling is the process construct. As you will see, the process construct allows us to model complex digital systems, in particular sequential circuits.


a. Process


A process statement is the main construct in behavioral modeling that allows you to use sequential statements to describe the behavior of a system over time. The syntax for a process statement is


            [process_label:] process [ (sensitivity_list) ] [is]

                        [ process_declarations]


                        list of sequential statements such as:

                                    signal assignments

                                    variable assignments 

                                    case statement

                                    exit statement

                                    if statement

                                    loop statement

                                    next statement           

                                    null statement

                                    procedure call

                                    wait statement

            end process [process_label];


An example of a positive edge-triggered D flip-flop with asynchronous clear input follows.


library ieee;

use ieee.std_logic_1164.all;

entity DFF_CLEAR is

     port (CLK, CLEAR, D : in std_logic;

           Q : out std_logic);



architecture BEHAV_DFF of DFF_CLEAR is




           if (CLEAR = ‘1’) then

                Q <= ‘0’;

           elsif (CLK’event and CLK = ‘1’) then

                Q <= D;

           end if;

     end process;



A process is declared within an architecture and is a concurrent statement. However, the statements inside a process are executed sequentially. Like other concurrent statements, a process reads and writes signals and values of the interface (input and output) ports to communicate with the rest of the architecture. One can thus make assignments to signals that are defined externally (e.g. interface ports) to the process, such as the Q output of the flip-flop in the above example. The expression CLK’event and CLK = ‘1’ checks for a positive clock edge (clock event AND clock high).


The sensitivity list is a set of signals to which the process is sensitive. Any change in the value of the signals in the sensitivity list will cause immediate execution of the process. If the sensitivity list is not specified, one has to include a wait statement to make sure that the process will halt. Notice that one cannot include both a sensitivity list and a wait statement. Variables and constants that are used inside a process have to be defined in the process_declarations part before the keyword begin. The keyword begin signals the start of the computational part of the process.  The statements are sequentially executed, similarly as a conventional software program. It should be noted that variable assignments inside a process are executed immediately and denoted by the “:=” operator. This is in contrast to signal assignments denoted by “<=” and which changes occur after a delay. As a result, changes made to variables will be available immediately to all subsequent statements within the same process. For an example that illustrates the difference between signal and variable assignments see the section on Data Types (difference between signals and variables).


The previous example of the D flip-flop illustrates how to describe a sequential circuit with the process statement. Although the process is mainly used to describe sequential circuits, one can also describe combinational circuits with the process construct. The following example illustrates this for a Full Adder, composed of two Half Adders. This example also illustrates how one process can generate signals that will trigger other processes when events on the signals in its sensitivity list occur [3]. We can write the Boolean expression of a Half Adder and Full Adder as follows:


            S_ha = (AĹB)             and C_ha = AB


            For the Full Adder:


Sum = (AĹB)ĹCin = S_ha ĹCin

            Cout = (AĹB)Cin + AB = S_ha.Cin + C_ha


Figure 5 illustrates how the Full Adder has been modeled.




Figure 5: Full Adder composed of two Half Adders, modeled with two processes P1 and P2.


library ieee;

use ieee.std_logic_1164.all;

entity FULL_ADDER is

     port (A, B, Cin : in std_logic;

           Sum, Cout : out std_logic);



architecture BEHAV_FA of FULL_ADDER is

signal int1, int2, int3: std_logic;


-- Process P1 that defines the first half adder

P1: process (A, B)


           int1<= A xor B;

           int2<= A and B;

     end process;

-- Process P2 that defines the second half adder and the OR -- gate

P2: process (int1, int2, Cin)


          Sum <= int1 xor Cin;

           int3 <= int1 and Cin;

           Cout <= int2 or int3;

     end process;



Of course, one could simplify the behavioral model significantly by using a single process.


b. If Statements


The if statement executes a sequence of statements whose sequence depends on one or more conditions. The syntax is as follows:


            if condition then

           sequential statements

        [elsif condition then

           sequential statements ]


           sequential statements ]

     end if;


Each condition is a Boolean expression. The if statement is performed by checking each condition in the order they are presented until a “true” is found. Nesting of if statements is allowed. An example of an if statement was given earlier for a D Flip-flop with asynchronous clear input. The if statement can be used to describe combinational circuits as well. The following example illustrates this for a 4-to-1 multiplexer with inputs A, B, C and D, and select signals S0 and S1. This statement must be inside a process construct. We will see that other constructs, such as the Conditional Signal Assignment (“When-else”) or “Select” construct may be more convenient for these type of combinational circuits.


entity MUX_4_1a is

   port (S1, S0, A, B, C, D: in std_logic;

           Z: out std_logic);

   end MUX_4_1a;

architecture behav_MUX41a of MUX_4_1a is


   P1: process (S1, S0, A, B, C, D)


     if (( not S1 and not S0 )=’1’) then

           Z <= A;

     elsif (( not S1 and S0) = ‘1’) then


     elsif ((S1 and not S0) =’1’) then

           Z <=C;



end if;

   end process P1;

end behav_MUX41a;


A slightly different way of modeling the same multiplexer is shown below,


if S1=’0’ and  S0=’0’ then

     Z <= A;

elsif S1=’0’ and  S0=’1’ then

     Z <= B;

elsif S1=’1’ and  S0=’0’ then

           Z <= C;

elsif S1=’1’ and  S0=’1’ then

           Z <= D;

     end if;


If statements are often used to implement state diagrams. For an example of a Mealy machine see Example Mealy Machine later on.


c.       Case statements


The case statement executes one of several sequences of statements, based on the value of a single expression. The syntax is as follows,


            case expression is

           when choices =>

                sequential statements

           when choices =>

                sequential statements

                -- branches are allowed

           [ when others => sequential statements ]

     end case;


The expression must evaluate to an integer, an enumerated type of a one-dimensional array, such as a bit_vector. The case statement evaluates the expression and compares the value to each of the choices. The when clause corresponding to the matching choice will have its statements executed. The following rules must be adhered to:



An example of a case statement using an enumerated type follows. It gives an output D=1 when the signal GRADES has a value between 51 and 60, C=1 for grades between 61 and 70, the when others covers all the other grades and result in an F=1.


library ieee;

use ieee.std_logic_1164.all;

entity GRD_201 is

     port(VALUE: in integer range 0 to 100;

           A, B, C, D: out bit);

end GRD_201;

architecture behav_grd of GRD_201 is


     process (VALUE)

       A <= ’0’;

       B <= ’0’;

       C <= ’0’;

       D <= ’0’;

       F <= ’0’;


           case VALUE is

            when 51 to 60 =>

                D <= ’1’;

            when 61 to 70 | 71 to 75 =>

                C <= ’1’;

            when 76 to 85 =>

                B <= ’1’;

             when 86 to 100 =>

                A <= ’1’;

             when others  =>

F <= ‘1’;

           end case;

     end process;

end behav_grd;


We used the vertical bar ( | ) which is equivalent to the “or” operator, to illustrate how to express a range of values. This is a useful operator to indicate ranges that are not adjacent (e.g. 0 to 4 | 6 to 10). 


Another example using the case construct is a 4-to-1 MUX.


entity MUX_4_1 is

   port ( SEL: in std_logic_vector(2 downto 1);

           A, B, C, D: in std_logic;

           Z: out std_logic);

   end MUX_4_1;

architecture behav_MUX41 of MUX_4_1 is


   PR_MUX: process (SEL, A, B, C, D)


     case SEL is

           when “00” => Z <= A; 

           when “01” => Z <= B;      

           when “10” => Z <= C;                  

           when “11” => Z <= D;

           when others => Z <= ‘X’;

     end case;

   end process PR_MUX;

end behav_MUX41;


The “when others” covers the cases when SEL=”0X”, “0Z”, “XZ”, “UX”, etc. It should be noted that these combinational circuits can be expressed in other ways, using concurrent statements such as the “With – Select” construct. Since the case statement is a sequential statement, one can have nested case statements.


d. Loop statements


A loop statement is used to repeatedly execute a sequence of sequential statements. The syntax for a loop is as follows:


            [ loop_label :]iteration_scheme loop

           sequential statements

           [next  [label] [when condition];

           [exit  [label] [when condition];

     end loop [loop_label];


Labels are optional but are useful when writing nested loops. The next and exit statement are sequential statements that can only be used inside a loop.



There are three types of iteration schemes:


·        basic loop

·        while … loop

·        for … loop


Basic Loop statement


This loop has no iteration scheme. It will be executed continuously until it encounters an exit or next statement.


[ loop_label :] loop

           sequential statements

           [next  [label] [when condition];

           [exit  [label] [when condition];

     end loop [ loop_label];


The basic loop (as well as the while-loop) must have at least one wait statement. As an example, lets consider a 5-bit counter that counts from 0 to 31. When it reaches 31, it will start over from 0. A wait statement has been included so that the loop will execute every time the clock changes from ‘0’ to ‘1’.


Example of a basic loop to implement a counter that counts from 0 to 31

entity COUNT31 is

   port ( CLK: in std_logic;

           COUNT: out integer);

   end COUNT31;

architecture behav_COUNT of COUNT31 is


   P_COUNT: process

     variable intern_value: integer :=0;


     COUNT <= intern_value;


        wait until CLK=’1’;

   intern_value:=(intern_value + 1) mod 32;

        COUNT <= intern_value;

     end loop;

end process P_COUNT;

end behav_COUNT;



We defined a variable intern_value inside the process because output ports cannot be read inside the process.


While-Loop statement


The while … loop evaluates a Boolean iteration condition. When the condition is TRUE, the loop repeats, otherwise the loop is skipped and the execution will halt. The syntax for the while…loop is as follows,


[ loop_label :] while condition loop

           sequential statements

           [next  [label] [when condition];

           [exit  [label] [when condition];

     end loop[ loop_label ];


The condition of the loop is tested before each iteration, including the first iteration. If it is false, the loop is terminated.


For-Loop statement


The for-loop uses an integer iteration scheme that determines the number of iterations. The syntax is as follows,


[ loop_label :] for identifier in range loop

           sequential statements

           [next  [label] [when condition];

           [exit  [label] [when condition];

     end loop[ loop_label ];



e.       Next and Exit Statement


The next statement skips execution to the next iteration of a loop statement and proceeds with the next iteration. The syntax is


     next [label] [when  condition];


The when keyword is optional and will execute the next statement when its condition evaluates to the Boolean value TRUE.


The exit statement skips the rest of the statements, terminating the loop entirely, and continues with the next statement after the exited loop. The syntax is as follows:


     exit [label] [when  condition];


The when keyword is optional and will execute the next statement when its condition evaluates to the Boolean value TRUE.


Notice that the difference between the next and exit statement, is that the exit statement terminates the loop.


f.        Wait statement


The wait statement will halt a process until an event occurs. There are several forms of the wait statement,


wait until condition;

            wait for time expression;

            wait on signal;



The Xilinx Foundation Express has implemented only the first form of the wait statement. The syntax is as follows,


            wait until signal = value;

            wait until signal’event and signal = value;

            wait until not signal’stable and signal = value;


The condition in the “wait until” statement must be TRUE for the process to resume. A few examples follow.


            wait until CLK=’1’;

wait until CLK=’0’;

     wait until CLK’event and CLK=’1’;

wait until not CLK’stable and CLK=’1’;


For the first example the process will wait until a positive-going clock edge occurs, while for the second example, the process will wait until a negative-going clock edge arrives. The last two examples are equivalent to the first one (positive-edge or 0-1 transitions). The hardware implementation for these three statements will be identical.


It should be noted that a process that contains a wait statement can not have a sensitivity list. If a process uses one or more wait statements, the Foundation Express synthesizer will use sequential logic. The results of the computations are stored in flip-flops.


g.       Null statement


 The null statement states that no action will occur. The syntax is as follows,




It can be useful in a case statement where all choices must be covered, even if some of them can be ignored. As an example, consider a control signal CNTL in the range 0 to 31. When the value of CNTL is 3 or 15, the signals A and B will be xor-ed, otherwise nothing will occur.


entity EX_WAIT is

   port ( CNTL: in integer range 0 to 31;

           A, B: in std_logic_vector(7 downto 0);

           Z: out std_logic_vector(7 downto 0) );

   end EX_WAIT;

architecture arch_wait of EX_WAIT is


   P_WAIT: process (CNTL)


     Z <=A;

     case CNTL is

           when 3 | 15 =>

Z <= A xor B;

                when others =>


     end case;

end process P_WAIT;

end arch_wait;


h.       Example of a Mealy Machine

The sequence following detector recognizes the input bit sequence X: "1011". The machine will keep checking for the proper bit sequence and does not reset to the initial state after it recognizes the string. In case we are implementing a Mealy machine, the output is associated with the transitions as indicated on the following state diagram (Figure 6).

Figure 6: Sequence detector (1011), realized as a Mealy Machine.


The VHDL file is given below.


VHDL file for a sequence detector (1011) implemented as a Mealy Machine

library ieee;

use ieee.std_logic_1164.all;


entity myvhdl is

    port (CLK, RST, X: in STD_LOGIC;

              Z: out STD_LOGIC);



architecture myvhdl_arch of myvhdl is

-- SYMBOLIC ENCODED state machine: Sreg0

type Sreg0_type is (S1, S2, S3, S4);

signal Sreg0: Sreg0_type;


--concurrent signal assignments

Sreg0_machine: process (CLK)


if CLK'event and CLK = '1' then

    if RST='1' then

        Sreg0 <= S1;


    case Sreg0 is

        when S1 =>

            if X='0' then

                Sreg0 <= S1;

            elsif X='1' then

                Sreg0 <= S2;

            end if;

        when S2 =>

            if X='1' then

                Sreg0 <= S2;

            elsif X='0' then

                Sreg0 <= S3;

            end if;

        when S3 =>

            if X='1' then

                Sreg0 <= S4;

            elsif X='0' then

                Sreg0 <= S1;

            end if;

        when S4 =>

            if X='0' then

                Sreg0 <= S3;

            elsif X='1' then

                Sreg0 <= S2;

            end if;

        when others =>


    end case;

    end if;

end if;

end process;

-- signal assignment statements for combinatorial outputs


Z <= '0' when (Sreg0 = S1 and X='0') else

        '0' when (Sreg0 = S1 and X='1') else

        '0' when (Sreg0 = S2 and X='1') else

        '0' when (Sreg0 = S2 and X='0') else

        '0' when (Sreg0 = S3 and X='1') else

        '0' when (Sreg0 = S3 and X='0') else

        '0' when (Sreg0 = S4 and X='0') else

        '1' when (Sreg0 = S4 and X='1') else


end myvhdl_arch;



9. Dataflow Modeling – Concurrent Statements


Behavioral modeling can be done with sequential statements using the process construct or with concurrent statements. The first method was described in the previous section and is useful to describe complex digital systems. In this section, we will use concurrent statements to describe behavior. This method is usually called dataflow modeling. The dataflow modeling describes a circuit in terms of its function and the flow of data through the circuit. This is different from the structural modeling that describes a circuit in terms of the interconnection of components.


Concurrent signal assignments are event triggered and executed as soon as an event on one of the signals occurs. In the remainder of the section we will describe several concurrent constructs for use in dataflow modeling.


a.       Simple Concurrent signal assignments.


We have discussed several concurrent examples earlier in the tutorial. In this section we will review the different types of concurrent signal assignments.


A simple concurrent signal assignment is given in the following examples,


Sum <= (A xor B) xor Cin;

Carry <= (A and B);

Z <= (not X) or Y after 2 ns;


The syntax is as follows:


Target_signal <= expression;


in which the value of the expression transferred to the target_signal. As soon as an event occurs on one of the signals, the expression will be evaluated. The type of the target_signal has to be the same as the type of the value of the expression.


Another example is given below of a 4-bit adder circuit. Notice that we specified the package: IEEE.std_logic_unsigned in order to be able to use the “+” (addition) operator.



Example of a Four bit Adder using concurrent/behavioral modeling

library ieee;

use IEEE.std_logic_1164.all;

use IEEE.std_logic_unsigned.all;


entity ADD4 is

    port (

        A: in STD_LOGIC_VECTOR (3 downto 0);

        B: in STD_LOGIC_VECTOR (3 downto 0);

        CIN: in STD_LOGIC;

        SUM: out STD_LOGIC_VECTOR (3 downto 0);

        COUT: out STD_LOGIC


end ADD4;


architecture ADD4_concurnt of ADD4 is


-- define internal SUM signal including the carry

signal SUMINT: STD_LOGIC_VECTOR(4 downto 0);



  -- <<enter your statements here>>


  SUMINT <=  ('0' & A) + ('0' & B) + ("0000" & CIN);

  COUT <= SUMINT(4);

  SUM <= SUMINT(3 downto 0);

end ADD4_concurnt;




b.      Conditional Signal assignments


The syntax for the conditional signal assignment is as follows:


Target_signal <= expression when Boolean_condition else

 expression when Boolean_condition else




The target signal will receive the value of the first expression whose Boolean condition is TRUE. If no condition is found to be TRUE, the target signal will receive the value of the final expression. If more than one condition is true, the value of the first condition that is TRUE will be assigned.


An example of a 4-to-1 multiplexer using conditional signal assignments is shown below.


entity MUX_4_1_Conc is

   port (S1, S0, A, B, C, D: in std_logic;

           Z: out std_logic);

   end MUX_4_1_Conc;

architecture concurr_MUX41 of MUX_4_1_Conc is


                        Z <= A when S1=’0’ and S0=’0’ else

B when S1=’0’ and S0=’1’ else

C when S1=’1’ and S0=’0’ else


            end concurr_MUX41;


The conditional signal assignment will be re-evaluated as soon as any of the signals in the conditions or expression change. The when-else construct is useful to express logic function in the form of a truth table. An example of the same multiplexer as above is given below in a more compact form.


entity MUX_4_1_funcTab is

   port (A, B, C, D: in std_logic;

         SEL: in std_logic_vector (1 downto  0);

           Z: out std_logic);

   end MUX_4_1_ funcTab;

architecture concurr_MUX41 of MUX_4_1_ funcTab is


                        Z <= A when SEL = ”00” else

B when SEL = ”01” else

C when SEL = “10” else


            end concurr_MUX41;



Notice that this construct is simpler than the If-then-else construct using the process statement or the case statement. An alternative way to define the multiplexer is the case construct inside a process statement, as discussed earlier.


c.       Selected Signal assignments


The selected signal assignment is similar to the conditional one described above. The syntax is as follows,


            with choice_expression select

           target_name <= expression when choices,

           target_name <= expression when choices,


                        target_name <= expression when choices;


The target is a signal that will receive the value of an expression whose choice includes the value of the choice_expression. The expression selected is the first with a matching choice. The choice can be a static expression (e.g. 5) or a range expression (e.g. 4 to 9). The following rules must be followed for the choices:



An example of a 4-to-1 multiplexer is given below.


entity MUX_4_1_Conc2 is

   port (A, B, C, D: in std_logic;

           SEL: in std_logic_vector(1 downto 0);

           Z: out std_logic);

   end MUX_4_1_Conc2;

architecture concurr_MUX41b of MUX_4_1_Conc2 is


     with SEL select

                Z <= A when “00”,

B when “01”,

C when “10”,

D when “11”;

            end concurr_MUX41b;


The equivalent process statement would make use of the case construct. Similarly to the when-else construct, the selected signal assignment is useful to express a function as a truth table, as illustrated above.


The choices can express a single value, a range or combined choices as shown below.



           target <= value1 when “000”,

                                                value2 when “001” | “011”  | “101” ,

                     value3 when others;


In the above example, all eight choices are covered and only once.  The others choice must the last one used.


Notice that the Xilinx Foundation Express does not allow a vector as choice_expression such as std_logic_vector’(A,B,C).


As an example, lets consider a full adder with inputs A, B and C and outputs sum and cout,


entity FullAdd_Conc is

   port (A, B, C: in std_logic;

           sum, cout: out std_logic);

   end FullAdd_Conc;

architecture FullAdd_Conc of FullAdd_Conc is

     --define internal signal: vector INS of the input signals

     signal INS: std_logic_vector (2 downto 0);



--define the components of vector INS of the input signals

INS(2) <= A;

INS(1) <= B;

INS(0) <= C;


with INS select

           (sum, cout) <=  std_logic_vector’(“00”) when “000”,

std_logic_vector’(“10”) when “001”, 

std_logic_vector’(“10”) when “010”, 

std_logic_vector’(“01”) when “011”, 

std_logic_vector’(“10”) when “100”, 

std_logic_vector’(“01”) when “101”, 

std_logic_vector’(“01”) when “110”, 

std_logic_vector’(“11”) when “111”,

std_logic_vector’(“11”) when others;


end FullAdd_Conc; ]


Notice: In the example above we had to define an internal vector INS(A,B,C) of the input signals to use as part of the with-select-when statement.  This was done because the Xilinx Foundation does not support the construct std_logic_vector’(A,B,C).



10. Structural Modeling


Structural modeling was described briefly in the section Structural Modeling in “Basic Structure of a VHDL file”. A structural way of modeling describes a circuit in terms of components and its interconnection. Each component is supposed to be defined earlier (e.g. in  package) and can be described as structural, a behavioral or dataflow model. At the lowest hierarchy each component is described as a behavioral model, using the basic logic operators defined in VHDL. In general structural modeling is very good to describe complex digital systems, though a set of components in a hierarchical fashion.


A structural description can best be compared to a schematic block diagram that can be described by the components and the interconnections. VHDL provides a formal way to do this by


·        Declare a list of components being used

·        Declare signals which define the nets that interconnect components

·        Label multiple instances of the same component so that each instance is uniquely defined.


The components and signals are declared within the architecture body,


architecture architecture_name of NAME_OF_ENTITY is

     -- Declarations

           component declarations

           signal declarations


     -- Statements

           component instantiation and connections   


     end architecture_name;


a. Component declaration


Before components can be instantiated they need to be declared in the architecture declaration section or in the package declaration. The component declaration consists of the component name and the interface (ports). The syntax is as follows:


     component component_name [is]

[port (port_signal_names: mode type;

             port_signal_names: mode type;


                port_signal_names: mode type);]

     end component [component_name];


The component name refers to either the name of an entity defined in a library or an entity explicitly defined in the VHDL file (see example of the four bit adder).


The list of interface ports gives the name, mode and type of each port, similarly as is done in the entity declaration.


A few examples of component declaration follow:


component OR2

           port (in1, in2: in std_logic;

                 out1: out std_logic);

     end component;


     component PROC

           port (CLK, RST, RW, STP: in std_logic;

                 ADDRBUS: out std_logic_vector (31 downto 0);

                 DATA: inout integer range 0 to 1024);


component FULLADDER

     port(a, b, c: in std_logic;

sum, carry: out std_logic);

     end component;


As mentioned earlier, the component declaration has to be done either in the architecture body or in the package declaration. If the component is declared in a package, one does not have to declare it again in the architecture body as long as one uses the library and use clause.


b. Component Instantiation and interconnections


The component instantiation statement references a component that can be



The syntax for the components instantiation is as follows,


instance_name : component name  

      port map (port1=>signal1, port2=> signal2,… port3=>signaln);


The instance name or label can be any legal identifier and is the name of this particular instance. The component name is the name of the component declared earlier using the component declaration statement. The port name is the name of the port and signal is the name of the signal to which the specific port is connected. The above port map associates the ports to the signals through named association. An alternative method is the positional association shown below,


      port map (signal1,  signal2,…signaln);


in which the first port in the component declaration corresponds to the first signal, the second port to the second signal, etc. The signal position must be in the same order as the declared component’s ports. One can mix named and positional associations as long as one puts all positional associations before the named ones. The following examples illustrates this,


component NAND2

                port (in1, in2: in std_logic;

                      out1: out std_logic);

           end component;

                        signal int1, int2, int3: std_logic;

                        architecture struct of EXAMPLE is

                                    U1: NAND2 port map (A,B,int1);

                                    U2: NAND2 port map (in2=>C, in2=>D, out1=>int2);

                                    U3: NAND3 port map (in1=>int1, int2, Z);



Another example is the Buzzer circuit of Figure 2.



11. References


  1. D. Gajski and R. Khun, “Introduction: New VLSI Tools,” IEEE Computer, Vol. 16, No. 12, pp. 11-14, Dec. 1983.
  2. M. Mano and C. Kime, “Logic and Computer Design Fundamentals,” 2nd Edition, Prentice Hall, Upper Saddle River, 2001.
  3. S. Yalamanchili, “VHDL Starter’s Guide,” Prentice Hall, Upper Saddle River, 1998.
  4. J. Bhasker, “VHDL Primer,” 3rd Edition, Prentice Hall, Upper Saddle River, 1998.
  5. P. J. Ashenden, “The Student’s Guide to VHDL,” Morgan Kaufmann Publishers, Inc, San Francisco, 1998.
  6. A. Dewey, “Analysis and Design of Digital Systems,” PWS Publishing Company, New York, 1997.
  7. C. H. Roth, “Digital System Design using VHDL”, PWS Publishing Company, New York, 1998.
  8. D. Pellerin and D. Taylor, “VHDL Made Easy!”, ,” Prentice Hall, Upper Saddle River, 1997.
  9. VHDL Reference Guide, Xilinx, Inc., 1999 (available on line: http://toolbox.xilinx.com/docsan/  (select Foundation Series)


Copyright 2001; Created by Jan Van der Spiegel, Sept. 28, 2001; Updated August 6, 2006

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