VHDL (VHSIC Hardware Description Language) is becoming increasingly popular as a way to capture complex digital electronic circuits for both simulation and synthesis. Digital circuits captured using VHDL can be easily simulated, are more likely to be synthesizable into multiple target technologies, and can be archived for later modification and reuse.

2. Agenda
• Introduction
• VHDL Invariants
• Basic structures in VHDL
• Data objects
• VHDL operators
• Sequential statements
• Concurrent statements
• Reference
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Reading Assignment

3. Introduction
V
H
D
L
Hardware
Description
Language
V
H
S
I
C
High
Integrated
Circuit
Very
• Designed to develop a very high speed
integrated circuit.
Speed
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4. Introduction…
• It is an industry standard language used to
describe hardware from the abstract to
concrete level.
• It is human and machine readable
• It supports both behavioral and structural
description of a system
• It has a standard package
• It support both synchronous and asynchronous
timing model
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5. VHDL Invariants
• Qualities of VHDL:
• It is not case sensitive
• Example:
• It is not sensitive to white space (spaces and tabs)
• Example:
• Comments in VHDL begin with “--“ (two
consecutive dashes)
• Example:
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Dout <= A and B; doUt <= a AND b;
nQ <= In_a or In_b; nQ <= in_a OR in_b;
-- This next section of code is used to blah-blah

6. VHDL Invariants…
• Qualities of VHDL…
• VHDL is relatively lax on its requirement for using
parenthesis
• Example:
• every VHDL statement is terminated with a
semicolon
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if x = ‘0’ and y = ‘0’ or z = ‘1’ then
-- statements;
End if;
if ( ((x = ‘0’) and (y = ‘0’)) or (z = ‘1’) ) then
-- statements;
End if;

7. VHDL Invariants…
• Qualities of VHDL…
• the VHDL language contains if, case, and loop
statements
• Note:
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• Every if statement has a corresponding then
component
• Each if statement is terminated with an “end if”
• If you need to use an “else if” construct, the VHDL
version is “elsif”
• Each case statement is terminated with an “end
case”
• Each loop statement has a corresponding “end
loop“ statement

8. VHDL Invariants…
• Qualities of VHDL…
• Identifiers:
• the name given to discern various items in VHDL
• Note:
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• Identifiers should be self-commenting
• Identifiers can be as long as you want (contain many
characters).
• Identifiers can only contain some combination of
letters (A-Z and a-z), digits (0-9), and the underscore
character (‘_’)
• Identifiers must start with an alphabetic character
• Identifiers must not end with an underscore and must
never have two consecutive underscores

9. VHDL Invariants…
• Qualities of VHDL…
• Identifiers…
• Example:
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10. Basic structures in VHDL
• Basic building blocks of a VHDL description
can be classified into five groups:
i. Entity
ii. Architecture
iii. Package
iv. Configuration
v. Library
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11. Basic structures in VHDL…
i. Entity:
• provides a method to abstract the functionality of a
circuit description to a higher level (i.e. does not
provide information about how a component is
implemented).
• Syntax:
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entity entity_name is
port (
port_name : mode data_type;
port_name : mode data_type;
port_name : mode data_type
);
end entity_name;
Mode:
specifies the
direction of a
port signal.
Data_type:
specifies the
data type of
a port,

12. Basic structures in VHDL…
i. Entity…
• Port modes:
• In : indicates that the signal is an input
• Out: indicates that the signal is an output of the entity
whose value can only be read by other entities that use it
• Inout: the signal can be an input and output
• Buffer: indicates that the signal is an output of the
entity whose value can be read inside the entities
architecture
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13. Basic structures in VHDL…
i. Entity…
• Types:
• A built-in or user-defined signal type.
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Boolean: An enumeration type with two values, false and true
Bit: An enumeration type with two values, ‘0’ and ‘1’.
Character: any printing character
Integer: Represents positive and negative numbers. Range is
specified from -2,147,483,647 to +2,147,483,647 .
Natural: Subtype of integers used for representing natural (non-
negative) numbers.
Positive: Subtype of integers used for representing positive (non-
negative, nonzero) numbers.

14. Basic structures in VHDL…
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i. Entity…
• Types…
Bit_vector: Represents an array of BIT values.
String: An array of CHARACTERs. A STRING value is
enclosed in double quotation marks.
REAL: Represents real numbers. Range is -1.0E+38 to
+1.0E+38.
Physical type Time: Represents a time value used for
simulation.
Std_logic, Std_ulogic, Std_logic_vector, Std_logic_uvector:
can have nine values to indicate the value and strength of a
signal.

15. Basic structures in VHDL…
i. Entity…
• Types…
• The nine std_logic types are:
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Type STD_LOGIC is
( ‘U’ --Uninitialized
‘X’ --Forcing unknown
‘0’ --Forcing low
‘1’ --Forcing high
‘Z’ --High impedance
‘W’ --Weak unknown
‘L’ --Weak low
‘H’ --Weak high
‘_’ --Don’t care
) ;

16. Basic structures in VHDL…
i. Entity…
• Example: 4 X 1 Multiplexer
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entity mux4_8 is
port (
a_data : in std_logic_vector(0 to 7);
b_data : in std_logic_vector(0 to 7);
c_data : in std_logic_vector(0 to 7);
d_data : in std_logic_vector(0 to 7);
sel1,sel0 : in std_logic;
a_st_1, a_st_2 : out std_logic_vector(0 to 7));
end mux4_8;

17. Basic structures in VHDL…
ii. Architecture:
• Specifies how the circuit operates and how it is
implemented (i.e. it describes what the circuit
actually does)
• Syntax:
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Architecture architecture_name of entity_name is
architecture_declarative_part
begin
--statements
End architecture_name;
- Components
- Signals
- Constant
- Function
- Procedure
- Type etc.

18. Basic structures in VHDL…
ii. Architecture…
• It defines the relationships between the inputs and
the outputs of a design entity which may be
expressed in terms of :
A) Behavioral style
B) Data flow style
C) Structural style
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19. Basic structures in VHDL…
ii. Architecture…
A) Behavioral architecture
• specifies what a particular system does in a program
like description using processes, but provides no details
as to how a design is to be implemented.
• Example:
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architecture behavior of FULL_ADDER is
Begin
process (A, B, CIN)
Begin
if (A=‘0’ and B=‘0’ and CIN=‘0’) then
SUM <= ‘0’;
COUT <=‘0’;
Name of
the Entity
Name of
the Port

20. Basic structures in VHDL…
ii. Architecture…
A) Behavioral architecture…
• Example:
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elsif (A=‘0’ and B=‘0’ and CIN=‘1’) or
(A=‘0’ and B=‘1’ and CIN=‘0’) or
(A=‘1’ and B=‘0’ and CIN=‘1’) then
SUM <= ‘1’;COUT <=‘0’;
elsif (A=‘0’ and B=‘1’ and CIN=‘1’) or
(A=‘1’ and B=‘0’ and CIN=‘1’) or
(A=‘1’ and B=‘1’ and CIN=‘0’) then
SUM <= ‘0’;
COUT <=‘1’;
elsif (A=‘1’ and B=‘1’ and CIN=‘1’) then
SUM <= ‘1’;COUT <=‘1’;
end if ;
end process ;
end behavior ; Signal assignment

21. Basic structures in VHDL…
ii. Architecture…
B) Data flow architecture
• specifies a system as a concurrent representation of the
flow of control and movement of data.
• Example:
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architecture DATAFLOW of
FULL_ADDER is signal S : BIT ;
Begin
S <= A xor B ;
SUM <= S xor CIN after 10ns ;
COUT <= (A and B) or (S and CIN) after
5ns ;
end DATAFLOW ;

22. Basic structures in VHDL…
ii. Architecture…
C) Structural architecture
• defines the structural
implementation using
component declarations and
component instantiations.
• Example:
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architecture STRUCTURE of
FULL_ADDER is
component HALF_ADDER
port ( L1, L2 : in BIT ;
CARRY, SUM : out BIT ) ;
end component ;
component OR_GATE
port ( L1, L2 : in BIT ;
O : out BIT ) ;
end component ;
signal N1, N2, N3 : BIT ;
Begin
HA1 : HALF_ADDER
port map (A, B, N1, N2) ;
HA2 : HALF_ADDER
port map (N2, CIN, N3, SUM) ;
OR1 : OR_GATE
port map (N1, N3, COUT) ;
end STRUCTURE ;

23. Basic structures in VHDL…
iii. Package
• The primary purpose of a package is to collect
elements that can be shared among two or more
design units.
• Syntax:
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Package package_name is
package_declarative_item
End package_name;
package body package_name is
package_declarative_item
End package_name];

24. Basic structures in VHDL…
iii. Package…
• Example:
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Package my_pkg is
constant delay : time := 10ns ;
end my_pkg ;
Variable
assignment

25. Basic structures in VHDL…
iv. Configuration
• used to provide fast substitutions of component
instances of a structural design.
• Syntax:
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Configuration configuration_name of entity_name is
{configuration_declarative_part}
For block_specification
{use_clause}
{configuration_item}
End for;

26. Basic structures in VHDL…
iv. Configuration…
• Example:
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configuration FADD_CONFIG of FULL_ADDER is
for STRUCTURE
for HA1, HA2 : HALF_ADDER use entity
burcin.HALF_ADDER(STRUCTURE) ;
for OR1 : OR_GATE use entity burcin.OR_GATE ;
end for ;
end FADD_CONFIG ;
Name of
the Entity
Name of the
architecture Library

27. Basic structures in VHDL…
v. Library
• Used as a place where the compiler stores
information about a design project.
• It contains the following library units:
• Packages
• Entities
• Architectures
• Configurations
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28. Basic structures in VHDL…
V. Library
• Used as a place where the compiler stores
information about a design project.
• Example:
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Library ieee;
Use ieee.std_logic_1164.all
Use ieee.std_logic_arith.all
Use ieee.std_logic_unsigned.all
Indicates to use
all of the
ieee.std_logic_u
nsigned package
package

29. Data objects
• An object which holds both a name (associated
identifier) and a specific type
• Types:
1. Constants
• Have a single value of a given type and can not be
changed during the simulation.
2. Variables
• Can have a single value, but it can be updated using a
variable assignment statement.
3. signals
• can be interpreted as wires or busses in an actual circuit.
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30. Data objects…
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31. VHDL operators
• Used to operate a signal, variable, and constant.
• Operators in VHDL are grouped into seven
different types: logical, relational, shift, addition,
unary, multiplying, and “others”
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32. 12/22/2019 Introduction to VHDL 32

33. Reference
1. Sjoholm, S. and Lindh, L., 1997. VHDL for Designers. Prentice Hall PTR.
2. Skahill, K., 1996. VHDL for programmable logic. Addison-Wesley
Longman Publishing Co., Inc..
3. Holland, B., Vacas, M., Aggarwal, V., DeVille, R., Troxel, I. and George,
A.D., 2005, September. Survey of C-based application mapping tools for
reconfigurable computing. In Proceedings of the 8th International
Conference on Military and Aerospace Programmable Logic Devices
(MAPLD 2005), Washington, DC, USA (September 2005).
4. Sagahyroon, A.A., 2000. From AHPL to VHDL: A course in hardware
description languages. IEEE Transactions on Education, 43(4), pp.449-
454.
5. Brown, S., 2010. Fundamentals of digital logic design with VHDL.
6. Navabi, Z., 1997. VHDL: Analysis and modeling of digital systems.
McGraw-Hill, Inc..
7. Camposano, R. and Tabet, R.M., 1988. Design representation for the synthesis of
behavioral VHDL models. IBM Thomas J. Watson Research Division.
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34. Reference…
8. Camposano, R. and Tabet, R.M., 1988. Design representation for the synthesis of
behavioral VHDL models. IBM Thomas J. Watson Research Division.
9. Ferrandi, F., Fummi, F. and Sciuto, D., 1998, October. Implicit test generation for
behavioral VHDL models. In Test Conference, 1998. Proceedings.,
International (pp. 587-596). IEEE.
10. Micheli, G.D., 1994. Synthesis and optimization of digital circuits. McGraw-Hill
Higher Education.
11. Navabi, Z., 1997. VHDL: Analysis and modeling of digital systems. McGraw-Hill,
Inc..
12. Armstrong, J.R., 1989. Chip-level modeling with VHDL. Englewood Cliffs:
Prentice Hall.
13. Sieh, V., Tschache, O. and Balbach, F., 1997, June. VERIFY: Evaluation of
reliability using VHDL-models with embedded fault descriptions. In Fault-
Tolerant Computing, 1997. FTCS-27. Digest of Papers., Twenty-Seventh Annual
International Symposium on (pp. 32-36). IEEE.
14. Navabi, Z., 1997. VHDL: Analysis and modeling of digital systems. McGraw-Hill,
Inc..
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