Short.course.introduction.to.vhdl for beginners

2
Content Discussed in this Session
 Modeling
 History
 Applications
 Design Flow
 VHDL Characteristics
 VHDL Basics/Overview
 Examples
 Summary

3
What is A Modeling Language
 A Modeling Language.
 What is Modeling.
 When do you Model?
 What do you Model
» Various Levels of Modeling & Abstraction (what does it
mean?)
– Behavioral/Architectural Level modeling
– Structural Level modeling
– Dataflow Level modeling
 Characteristics/Requirements of Models.
 What are the main advantages

4
Design Flow & Levels of Abstraction
Idea
Architectural design
Logical design
Physical design
Fabrication
New chip
Generic CAD tools
Behavioral modeling and
Simulation tool
Functional and logic minimization
logic fitting and simulation tools
Tools for partitioning,
placement, routing, etc.
CAD subproblem level
Behavioral / Architectural
Register transfer / logic
Cell / mask

5
Architectural Design
 Generally carried out by expert human engineers.
 Decisions made effect cost/performance of the design. Some
examples are
» What should be the instruction set (IS) of the Processor (P) ?
– What memory addressing modes should be supported?
– Should the IS be compatible with another in the market?
» Should instruction pipelining be employed?
– If yes, then what should be the depth?
» Should the P be provided by an on-chip cache?
– How big should the cache be?
– What should be the organization of the cache?
– Should instruction cache be separated from data cache?

6
Architectural Design
 How should the arithmetic unit be designed?
» Bit Serial or Bit Parallel?
» Bit Serial will save on hardware but lose on
performance!
 How will the Processor interface to the external
world?
» Are there any International Standards to be met?
 Notes: This level of design cannot be done by a
computer program, there are tools that can aid
the system Architect.

7
Architectural Design Tools
 This level of design cannot be done by a computer
program, there are tools that can aid the system
Architect. Example:
» Tools can be used to experiment with innovative ideas
» Tools are also available to
– tune the size of the cache, or
– Determine the depth of the pipeline, etc.

8
Data/Control Path Design
 Once the architecture is defined, it is necessary
to carry out two things:
1. Detailed logic design of individual circuit modules
2. Derive the control signals necessary to activate and
deactivate the circuit modules.
1. The First step is known as data path design.
2. The Second step is called control path design.

9
Data Path Design
 The data path of the circuit includes the various
» functional blocks (such as adders, multipliers, ALU,
etc),
» storage elements (shift register, RAM, buffers,
stacks, queues),
» hardware components to allow transfer of data among
functional blocks and storage elements.
– Data transfer is achieved using tri-state busses or a
combination of multiplexer(s)/de- multiplexer(s).

10
Control Path Design
 The control path of the circuit generates the
various control signals necessary to operate the
circuit:
» These are necessary to initialize the storage elements,
» To initiate data transfers among functional blocks and
storage elements,
» And so on.
 The control path may be implemented using
» Hardwired control (random logic), or
» Through micro-programmed logic.

11
An Example
 The Problem: It is required to design an 8-bit
adder. The two operands are stored in two 8-bit
shift registers A and B. At the end of the
addition operation, the sum must be stored in the
A register. The contents of the B register must
not be destroyed. The design must be as
economical as possible in terms of hardware.

12
Possible Solutions
 There are numerous ways to design the above
circuit,some of which are listed below.
» Use an 8-bit carry look-ahead adder.
» Use an 8-bit ripple-carry adder
» Use two 4-bit carry look-ahead adders and ripple the
carry between stages.
» Use a 1-bit adder and perform the addition serially in 8
clock cycles.

13
Our Choice
 Since it is specified that the cost must be
minimum, the last option seems best.
MUX A
MA
Din FA
MUX B
MB
SA
SB
Q D
S
Cout
Cin
Start
Clock
Load A
Load B
Add
MA
MB
SB
SA
RD
Rc
Read A
Read B
(a) (b)

14
Organization of a Serial Adder
Start
Clock
Load A
Load B
Add
MA
MB
SB
SA
RD
Rc
Read A
Read B
MUX A
MA
Din FA
MUX B
MB
SA
SB
Q D
S
Cout
Cin
(a) (b)

15
Data Path of Serial Adder
 Consists of two 8-bit shift regiseters
 A full adder
 A D-flipflop
 Two multiplexers
 A three bit-counter ( to count the number of
times bit-wise addition is performed)

16
Control Path of Serial Adder
 The relevant control signals are:
» SA to Shift the register A right by one bit
» SB to shift the register B right by one bit
» MA to control multiplexer A.
» MB to control multiplexer B
» RD to Reset the D flip-flop
» RC to Reset the counter
» START to commences the addition

17
Control Algorithm of Serial Adder
Forever do
While (START = 0) skip;
Reset the D flip-flop and the counter;
Set MA and MB to 0;
Repeat
Shift registers A and B right by one
counter = counter + 1;
Until counter = 8;

18
Some Observations
 Design involves trade-offs between
» Cost
» Performance
» Testability
» Power dissipation
» Fault tolerance
» Ease of design
» Ease of making changes to the design.
 Serial is cheap but slow, parallel fastest in terms of
performance but most costly
 The different ways we can think of building an 8-bit
adder constitutes what is known as design space (at a
particular level of abstraction).
» Each method of implementation is called a point in the design
space.

19
High-Level Synthesis
 A circuit specification may pose constraints on one or
more aspects of the final design.
» Example: When the spec says that the circuit must operate at
15MHz, we have a constraint on timing performance of the
circuit.
 Given a spec, the objective is to arrive at the design
which meets all the constraints, and optimizes on or more
design aspect.
 This problem is known as hardware synthesis
 Programs are available for both data path synthesis and
control path synthesis
 The automatic generation of data path and control path
is known as high-level synthesis.

20
High-Level Synthesis
 Tasks involved in HLS (the process of automatically translating
abstract behavioral models of digital systems to implementable
hardware) are scheduling and allocation
 Scheduling distributes the execution of operations throughout
time steps
 Allocation assigns hardware to operations and values.
– Allocation of hardware cells include functional unit allocation, register
allocation and bus allocation.
– Allocation determines the interconnections required.

21
Behavioral Description and its CDFG
X = W + ( S * T )
Y = ( S * T ) + ( U * V )
(a)
CDFG
W S T U V
*
+
*
+
X Y
(b)
Scheduled CDFG
W S T U V
*
+ *
+
X Y
1
2
3
(c)

22
Example 1: A CDFG and 3 Solutions
1 2
3 4 5 6
7 8
9
CSs REGs FUs
( c ) 4 4 3
( d ) 5 3 3
( e ) 5 4 2
( a ) ( b )

23
Example 1
3
1
2
1
2
1
2
4
5
6
4
5
6
3
4
7
8
3
8
5
6
9
7
7
8
9
9
( c )
( d )
( e )
1 2 3 4 5

24
Register Allocation
V3 V1 V2
+
+
V4 V5
*
+
a1 a2
a3
m1
V6
V7
1
2
3
( a )
V4 = V1 + V3
V5 = V1 + V2
V6 = V4 * V5
V7 = V2 + V6
( b )
V1
V2
V3
V4
V5
V6
V7
Lifetimes
1 2 3
( c )

25
Register Assignment & Synthesis
Register Assignment
R1 : V1 , V6
R2 : V2
R3 : V3
R4 : V4
R5 : V5, V7
( d )
R4 ( V4 ) R5 ( V5,V7 )
a2
a3
+ ( 1 ) + ( 2 ) *
a1
m1
R3 ( V3 ) R1 ( V1,V6 ) R2 ( V2 )
( e )

26
Final Output
Bus 1 Data Path
X Y W
MUX
+
S U T V
Z MUX MUX
*
(f)

27
Logic Design
 Data path and control path (derived automatically or
manually) will have components such as ALU, registers,
multiplexers, buffers, and so on.
 Further design steps depend on
» How the circuit is implemented (PCB/VLSI)
» Are all the components readily available as off-the-shelf
components/ICs
 If the circuit is implemented on PCB, then the next stage
is to select the components to minimize
cost/performance
 Following the selection procedure ICs are selected and
placed on one or more circuit boards and interconnected.
 A similar procedure is used when implementing on a VLSI
chip using pre-designed components from a module library
(called macro cells).

Historical Glimpses/Background of HDLs
28
 1960's - 1980's AHPL, CDL, DDL in
classroom.
» Number of languages mushrooms to over 200
languages, all of them either proprietary or academic
 1983 VHSIC Program initiates definition of
VHDL
 1987 VHDL Standard (IEEE 1076) approved
 1990 Verilog dominates the marketplace.
» VHDL gaining acceptance as the second language due
to standards effort and DoD mandated use.

 VHDL = VHSIC Hardware Description Language
29
VHDL- Time Line
Very High Speed Integrated Circuits
First Publication
(Base-Line)
Start Of
1981
1985
VHDL Development
1987
Publication Of Revised
1993
First Publication Of
VHDL Standard
VHDL Standard

30
Background, continued
 1992 IEEE 1164 (3, 4, 9-valued logic
standard adopted)
 1993 VHDL re-balloted
» minor changes make it more user-friendly.
 1994 Widespread acceptance of VHDL.
» CAE tools operate at speeds comparable to
Verilog. Mentor, Cadence, Viewlogic, Synopsys
all provide full VHDL compilation/simulation and
synthesize with subsets of VHDL.

31
Applications of HDLs
 HDLs exist to satisfy a variety of purposes and
are used in a number of different ways.
 Therefore,
» there are features of VHDL (and any other design
language) that will be useless in some applications, and
confusing in other applications.
» Some descriptive features of VHDL may even lead to bad
synthesis.
 A major aspect of this course will be understanding
how VHDL should be used to permit synthesis tools
to produce good implementations of designs.

32
Objective of VHDL
 The main objective of VHDL was to provide
a unified notation to describe Electronic
Systems (digital hardware) at various levels
of abstractions.
 Abstraction, what does it mean
 Levels of Abstractions (gate level, upwards)

33
Objectives (re-visited)
 The models (or VHDL descriptions) are both human/machine
readable.
» Like all HDLs, VHDL also comes with a simulator (to verify the
correctness of models)
 Also, one single hardware description is used for
» simulation and verification,
» synthesis (translation to hardware), and
» testing.
 To support the communication of design data (no more black
boxes)
 To aid the maintenance, modification, and procurement of
hardware.

34
Purposes Served by VHDL
To understand why VHDL looks the way it does, it will
help to examine the variety of purposes it serves
» Design Specification
» Design Documentation
» Design Verification
» Product Test Generation
» Hardware Synthesis
 Language must seek to support these!

35
Design Specification
 Definition of functional interfaces
» concurrent ==> structure (space)
» sequential ==> behavior (time)
 Definition of design functions (behavior)
» by control flow (procedural)
» by data flow (concurrent)
 Project partitioning
» in space (structural)
» in time (behavioral)

36
Design Documentation
 Language standardization
» to improve quality and efficiency of communication,
broaden audience
 Interface description for users
» often as components of next level higher system:
physical, structural, and "pin" functions
 Express usage constraints
» e.g. disallowed input timings, combinations, output loads
 Express functional behavior of the design:
» internal states, data structures (e.g. format used in a
floating point unit)
» algorithms implemented
 Show communication protocols between design
entities

37
Design Verification - SIMULATION
 The Usual Method
» Highly developed tools
» Inherently behavioral (structural simulators consist of ordered calls
to primitive procedures to model corresponding primitive
structures)
» Limited by the effectiveness of the design test program developed
 Requires
» Language semantics to be executable

38
Design Verification - Formal Methods
 Require
» common language for specification of design goals and
description of implementation to meet those goals
» formal (mathematically rigorous) language definition to permit
logical transformation of descriptions to prove equivalence.
Such mathematical languages inherently are declarative
» language that can describe both time and structure
 Correctness by construction (silicon
compilation) more nearly realized
» than automatic verification systems

39
Product Test Generation
Requires
» constrained sets of values, especially for inputs
» behavioral descriptions of sequential designs
» machine analyzable design specifications
UUT
Stimuli
Generation
Response
control
model tb_model

40
Hardware Synthesis
 Stepwise refinement of design
» by architectural decomposition (either structural or
behavioral)
 Transformations from behavioral models to
corresponding structural and physical models,
» e.g. PLA generators, standard cells for shift registers,
adders, etc.
 Relating scaling parameters with expressions
 Enforcement of design constraints
 Register transfer level allocation,
» dataflow optimizations
» expression transformations for optimization

VHSIC Motivations for Creating/Using VHDL
41
 Standardization of Documentation
» improved communication of requirements
between military, contractors, and
subcontractors
 System Design Time and Cost
» reduced ambiguity in specification of design
interfaces and design functions
» reusability of existing designs

VHSIC Motivations for Creating/Using VHDL
42
 Open-system CAE Tools
» can change CAE system without losing use of existing designs
» elimination of language translators
 Improved Integration of Multi-vendor Designs
» shared design databases become possible
» standard cells, behavioral models
 Improved Understanding of Design Science
» top-down, middle-out, bottom-up

43
VHDL Basics/Overview
 VHDL an acronym : V (of VHSIC, stands for Very
High Speed Integrated Circuit) Hardware Description
Language.
 Names coined by DOD (department of defense, the
first institution to understand the benefits of design
language based documentation, modeling, and
simulation of electronic devices)
 VHDL simulators took hold in early ‘90s.
 PLA/FPGA designers started using them on larger
designs.
 Note: The word “synthesis’’ was not mentioned as one
of the reasons for creating VHDL (as it was primarily
intended for documentation, and modeling/simulation)

44
How a System Communicates
 How a System Communicates with its environment?
» Whatever function a system performs, it must get some
input from its environment and output some data.
» In other words the system must communicate with its
environment.
» The communication part of a system specification is called
the interface (without which the system would be useless).
 The system’s interface is described in VHDL by its
entity.
» This is the basic design unit for any system
» It is impossible to have a VHDL system without entity

45
Body of a System
 To accomplish the desired functionality, data
must undergo some transformation inside the
system.
 This is handled by the system’s inner part of
body (also called the architecture).
 The functionality can be simple as turning
some switch on/off, or as complicated as an
autopilot of a jetliner.
 However, in each case a system can be
considered as a composition of a body
(architecture) and an interface (entity)

46
External Support
 Some systems may derive additional features through
supportive elements.
» Example: A plug-in card that speed graphic in PC
 While such an add-on card can be considered a part
of the system, for the sake of clarity let’s consider it
as a third element of systems’s design (called package
in VHDL).
 All 3 elements of a system design
» Interface (entity)
» Body (architecture)
» Add-ons (package)
will be discussed in this session.

47
Entity
 A good practice to start any design is with
the analysis of its environment, entity is the
main part of any spec.
 In VHDL, the name of the system is the same
as the name of its entity.
 Entity comprises two parts:
» parameters of the system as seen from outside
such as bus-width of a processor or max clock
frequency
» connections which are transferring information to
and from the system (system’s inputs and outputs)

48
Illustration of Entity
Din1 Din2 Din3 Din4 Din5 Din6 Din7 Din8
CLK
8-bit register
fmax = 50MHz
Dout1 Dout2 Dout3 Dout4 Dout5 Dout6 Dout7 Dout8
entity Eight_bit_register is
parameters
connections
CLK one-bit input
end entity Eight_bit_register

49
Parameters/Connections
 All parameters are declared as generics and are
passed on to the body of the system
 Connections, which carry data to and from the
system, are called ports. They form the second part
of the entity.
 Note: The importance of entity is so great that the
architecture in VHDL is specified as the architecture
of entity
» Example
entity My-Digi-System is
..
..
..
end entity My-Digi-System;

50
Entity
 Examples
entity ALU is
Port ( A,B : in bit_vector(7 downto 0); -- comment 1
Mod1, Mod2 : inout bit_vector(3 downto 0); -- comment 2
C : out bit_vector(7 downto 0) -- comment 3
);
end entity ALU;
-- name = Mod
-- mode = bidirectional
-- type = 4-bit wide bus
-- note,
 port declarations are separated by semicolon,
 no semicolon after the last port declaration.
 Double hyphen for comments.
 Multiple ports of the same mode and type can be declared in one
statement.
 Properly documenting ports is as good as a good entity description
 Also you will see generics (and their applications) in subsequent
lectures.

51
Entity
FULL ADDER
A
B
C
SUM
CARRY
Examples
entity FULLADDER is
- -(After a double minus sign (-) the rest of
-- the line is treated as a comment)
- -
- -Interface description of FULLADDER
port ( A, B, C: in bit;
SUM, CARRY: out bit);
end FULLADDER;

52
Entity
-- Example Data Flipflop;
entity DFF is
-- parameter: width of the data
generic (width: integer);
--input and output signals
port ( CLK, NR: in bit;
D: in bit_vector(1 to width);
Q: out bit_vector(1 to width));
end DFF;

Mixing S/B in Architecture Hierarchical Design
53
S
S B
S B/S B
B B B B B
S: structural description
B: behavioral description
B/S: mixed description

54
Types of Architecture Descriptions
 Each system can be either in terms of its
functionality (behavior) or structure, which requires
different kinds of information about the system.
 Usually the synthesis tool works with both of them.
» First the expected functionality has to be specified and
formalized;
» And then it has to be transformed into structural equivalent
» The structural equivalent is more suitable for synthesis tool
 Parts of this translation can be done automatically
 However a fully functional (behavioral) synthesis tools
is not yet available.
 This is because the behavioral spec is only a
description of outputs’ response to inputs’ changes.

Relationship between Entity & Architecture
55
Example
entity My-Digi-System is
(
...
…
);
end entity My-Digi-System;
architecture MY-Arch of My-Digi-System is
...
end architecture MY-Arch;

56
VHDL
PACKAGE
DECLARATION
PACKAGE
BODY
(often used
functions,
constants,
components, …. )
ENTITY
(interface description)
ARCHITECTURE
(functionality)
CONFIGURATION
(connection entity  architecture)

57
Structural Description
 Does not cope with functionality of the system, but
instead specifies
» what components should be used
» and how they should be connected to achieve the expected
results
 Structural design is much easier to synthesis than
behavioral because
» it refers to concrete physical components
 However, creating a structural system specification is
more difficult because it requires an experienced
designer to do it most effectively.

58
Structure/Behavior
STRUCTURE BEHAVIOR
SYSTEM
CHIP
REGISTER
GATE
CIRCUIT
SILICON
SCHEMETIC
Ideal
Design tools
HDL Description
Boolean
Equations

59
Structural Description
» Example:
architecture STRUCTURAL of FULLADDER is
signal S1, C1, C2 : bit;
component HA
port (I1, I2 : in bit; S, C : out bit);
end component;
component XOR
port (I1, I2 : in bit; X : out bit);
end component;
begin
INST_HA1 : HA
port map (I1 => B, I2 => C, S => S1, C => C1);
INST_HA2 : HA
port map (I1 => A, I2 => S1, S => SUM, C => C2);
INST_XOR : XOR
port map (I1 => C2, I2 => C1, X => CARRY);
end STRUCTURAL;

60
Structure of Full Adder
I1 S
HA
I2 C
I1 S
HA
I2 C I1
XOR
I2 x
A
B
C
SUM
CARRY
S1
C1
C2

61
Concurrent Behavioral Description
architecture CONCURRENT of FULLADDER is
begin
SUM <= A xor B xor C after 5 ns;
CARRY <= (A and B) or (B and C) or (A and C)
after 3 ns;
end CONCURRENT;

62
Concurrent Behavioral Description
T T +  T + 2
A
SUM
PROD1
PROD2
PROD3
CARRY

63
Basic VHDL Constructs
 To specify architectures, or describe
behavior, we need some basic VHDL
constructs, and some basic VHDL types.
 Every piece of info inside a digital system is
stored in the form of bits and bit vectors.
 Instead of long bit-vectors, hex characters
may be used.
 Complex but regular data structures can be
represented by arrays.

64
An Example of Behavioral Description
entity LOW_HIGH is
port (A,B C: integer;
MI, MA: out integer);
end LOW_HIGH;
architecture BEHAV of LOW_HIGH is
begin
L: process
variable LOW: integer := 0;
begin
wait on A, B, C;
if A < B then LOW := A;
else LOW :=
B;
end if;
if C < LOW then LOW := C;
end if;
MI <= LOW after 1 ns;
end process;
H:process
variable HIGH: integer := 0;
begin
wait on A, B, C;
if A > B then HIGH := A;
else HIGH := B;
end if;
if C > HIGH then HIGH := C;
end if;
MA <= HIGH after 1 ns;
end process;
end BEHAV;


65
Quick Overview: Types
 Scalar type is a generic name referring to all types
whose objects have a single value at any time instant.
 All values of scalar type are ordered and the values
are specified either as falling within a range or
explicitly enumerated. Examples:
» Boolean (T/F),
» Character (all characters defined by ISO 8859-1,
» Integer (all integers within a specified range),
» Real (floating point numbers with a specified range),
» Bit (enumeration type specified by ‘0’ or ‘1’).
 We can also have user defined enumeration types,
such as
» type FSMStates is (Idle, Fetch, Decode, Execute);
 There are also other, such as physical types, user
defined records, arrays, etc. (You will see this in a
later lecture).

Quick Overview: Expressions/Operators
66
 Input signals must always be transformed or
processed in some way to generate the desired output
signals.
» Outputs <- transformations(inputs)
 Transformations are performed by expressions
(formulae that consist of operators with an
appropriate number of operands)
 Any specification of a systems behavior can be seen
as an ordered set of expressions on the inputs
assigned to the outputs.
 The basic element of each expression is an operator
(which is assigned a specific type) and requires one or
more operands.
 It is illegal to use an operator on operands of non
supported type (except via operator overloading).

67
Quick Overview: Other Operators
 Logical Operators (and, or, nand, nor, xor, xnor, not, these
are defined for types Bit, Boolean, and Bit_Vector)
 Numerical Operators (+, -, *, /, mod, rem, **, abs)
 Relational Operators (=, /=, <, >, <=, >=) that return T/F
 Shift Operators (sll, srl, sla, sra, rol, ror) restricted to
arrays whose elements are of the type Bit or Boolean
 Catenation operator (denoted by ‘&’)
 Assignment operator (assigning expressions to signals
» x<=y<=z; (y less or equal to z, to a Boolean signal x)
» a<= b or c; (logical operation, correct for Boolean, bit and
bit_vector)
» k<= ‘1’; (assignment of a single bit or character, note apostrophes)
» m<=“0101”; (assignment of bit vector or string)
» n<= m & k; (target signal must be declared as 5-bits, why?).

68
Quick Overview: Other Operators
 Other things you will see later include
» Delays
– How an assignment can be delayed?
– What is propagation/inertial delay?
– What is transport delay?
» How to declare constants
– Use of constants and constants versus generics

69
One Entity Many Descriptions
Entity
Architecture
A
Architecture
B
Architecture
C
Architecture
D

70
One Entity Many Descriptions
 Since different types of architectures can
perform the same function, a system (an
entity) can be specified with different
architectures. For example:
» A number of 80xx51 processors produced by
different vendors can be used in place of each
other as long as they have the same interface.
» This means that one entity can be assigned
multiple architectures.
 The reverse is not true, since architectures
may NOT have different interfaces.

71
The Concept of Package
 What do you do when you face an unknown concept or
word like “subclavian”? Most probably you would turn
for some help, either from a person or a dictionary!
 If you are planning to use a dictionary, you could say
that you are going to use a library unit (i.e., a book)
 Moreover, if you live far away from any library, the
book may be delivered to you by mail as a package.
 Use these packages (external sources of description)
when something is undefined in the standard
language.

72
More on the Concept of Package
 A packages is used as a collection of often used
» datatypes,
» components,
» functions, and so on.
 Once these object are declared and defined in a
package, they can be used by different VHDL design
units.
 In particular, the definition of global information and
important shared parameters in complex designs or
within a project team is recommended to be done in
packages.
 It is possible to split a package into a declaration part
and the so-called body.
» Advantage is that after changing definitions in the package
body only this part has to be recompiled and the rest of the
design can be left untouched. Therefore, a lot of time
consumed by compiling can be saved.

73
Using Packages in VHDL
 A similar situation to the one described in the
previous slide happens with VHDL specs.
 You may occasionally need to use something not
defined in the standard VHDL libraries.
 Packages allow you to define items that are outside
the VHDL standards.
 The only restriction in using packages is that they
need to be declared in advance, usually at the
beginning of an entity.
 There are two clauses, which serve this purpose:
» Library and
» Use.

74
Using Packages in VHDL
entity SomeSyst is
….
end entity SomeSyst;
architecture FirstARC of SomeSyst is
…
Logarithm
…
end architecture FirstArch;

75
Non Standard Concept
Call a library NewConceptLib; “use concept Logarithm
which is defined in package Arithm stored in library
NewConceptLib” (read it always from the end).
Library NewConceptLib;
Use NewConceptLib.Arithm.Logarithm
entity SomeSyst is
…
end entity SomeSyst;
architecture FirstArch of SomeSyst is
Logarithm
end architecture FirstArch;

76
Predefined Packages
 VHDL is a robust design environment and has some
well defined packages. The most popular packages
defined by the IEEE are:
» STANDARD – contains basic declarations and definitions of
language constructs and it is included in all VHDL
specifications by default;
» TEXTIO – contains declarations of basic operations on texts;
» STD_LOGIC_1164 – this package is not a part of the VHDL
standard but is a standard on its own; it contains the most
often used language extensions.
 Apart from the three IEEE packages, each VHDL—
tool vendor adds (and encourages to use) his/her own
package.
 In such cases the library usually bears the vendor’s
name.

77
Predefined Packages--Standard
 STANDARD
» The package STANDARD is usually integrated directly in
the simulation or synthesis program
» therefore, it does not exist as a VHDL description.
» It contains all basic types: boolean, bit, bit_vector,
character, integer, and the like.
» Additional logical, comparison and arithmetic operators
are defined for these types within the package.
» The package STANDARD is a part of the STD library.
» Thus, it does not have to be explicitly included by the use
statement

78
Predefined Packages--TEXTIO
 TEXTIO
» The package TEXTIO contains procedures and functions
which are needed to read from and write to text files.
» This package is also a part of the library STD.
» It is not included in every VHDL description by default.
» Therefore, if required, it has to be included by the
statement use STD.TEXTIO.all;.

Predefined Packages—STD_LOGIC_1164
79
 STD_LOGIC_1164
» The STD_LOGIC_1164 package has been developed and
standardized by the IEEE.
» It introduces a special type called std_ulogic which has nine
different logic values (shown below).
» The reason for this enhancement is that the type bit is not
suitable for the precise modeling of digital circuits due to the
missing values, such as un-initialized or high impedance.
» Declaration:
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"

80
More VHDL Examples
 VHDL is NOT CaSe-SeNsItIvE , Thus:
Begin = begin = beGiN
 Semicolon “ ; ” Terminates Declarations or
Statements.
 Line Feeds and Carriage Returns are not
Significant in VHDL.
 Lets see some views (or descriptions) of a certain
design (ones count)!

81
Ones Count Interface Specification
entity ONES_CNT is
port ( A : in BIT_VECTOR(2 downto 0);
C : out BIT_VECTOR(1 downto 0));
-- Function Documentation of ONES_CNT
-- (Truth Table Form)
-- ---------------------------------------------------
-- This is a COMMENT
--
-- A2 A1 A0 C1 C0
-- 0 0 0 0 0
-- 0 0 1 0 1
-- 0 1 0 0 1
-- 0 1 1 1 0
-- 1 0 0 0 1
-- 1 0 1 1 0
-- 1 1 0 1 0
-- 1 1 1 1 1
-- --
end ONES_CNT;

82
Ones Count Architectural/Behavioral
architecture Algorithmic of ONES_CNT is
begin
Process(A) -- Sensitivity List Contains only Vector A
Variable num: INTEGER range 0 to 3;
begin
num :=0;
For i in 0 to 2
Loop
IF A(i) = '1' then
num := num+1;
end if;
end Loop;
-- Transfer "num" Variable Value to a SIGNAL
CASE num is
WHEN 0 => C <= "00";
WHEN 1 => C <= "01";
WHEN 2 => C <= "10";
WHEN 3 => C <= "11";
end CASE;
--
end process;
end Algorithmic;

83
Ones Count Data_Flow
• C1 = A1 A0 + A2 A0 + A2 A1
• C0 = A2 A1’ A0’ + A2’ A1’ A0 + A2 A1 A0 + A2’ A1 A0’
architecture Two_Level of ONES_CNT is
begin
C(1) <=(A(1) and A(0)) or (A(2) and A(0))or (A(2) and A(1));
--
C(0) <= (A(2) and not A(1) and not A(0))
or (not A(2) and not A(1) and A(0))
or (A(2) and A(1) and A(0))
or (not A(2) and A(1) and not A(0));
end Two_Level;

Architectural/Behavioral (Data Flow) Using Functions
84
architecture Macro of ONES_CNT is
begin
C(1) <= MAJ3(A);
C(0) <= OPAR3(A);
end Macro ;
Functions OPAR3 and MAJ3 Must Have
Been Declared and Defined Previously

85
Architectural Body (another view)
architecture Truth_Table of ONES_CNT is
begin
--
Process(A) -- Sensitivity List Contains only Vector A
Variable num: BIT_VECTOR(2 downto 0);
begin
num :=A;
CASE num is
WHEN "000" => C <= "00";
WHEN "001" => C <= "01";
WHEN "010" => C <= "01";
WHEN "011" => C <= "10";
WHEN "100" => C <= "01";
WHEN "101" => C <= "10";
WHEN "110" => C <= "10";
WHEN "111" => C <= "11";
end CASE;
end process;
end Truth_Table;

86
 Modeling Adder(s) as a function
 Describing Latches
 Modeling Flip-flops (DFF, edge triggered)
 Describing FSMs
 Structural Descriptions and Components
Instantiations
 Etc.
Other Examples

87
Adder as a Function
function "+" (A, B: bit_vector (3 downto 0)) return bit_vector is
variable SUM: bit_vector (3 downto 0);
variable CARRY: bit;
begin
CARRY := '0';
for I in 0 to 3 loop
SUM(I) := A(I) xor B(I) xor CARRY;
CARRY := ((A(I) and B(I)) or (A(I) and CARRY) or (B(I) and CARRY));
end loop;
return SUM;
end;

88
DFF Edge Triggered
entity DFLOP is -- D-Type FF
port (CLK, D: in std_logic; Q: out std_logic)
end DFLOP;
architecture BEHAV of DFLOP is
begin
process (CLK)
begin
if (CLK = '1') and -- CLK = 1
(CLK'event) and -- and a new event
(CLK'last_value = '0')
-- and previous value was 0 (because of `X`...)
then Q <= D; -- rising edge
end if;
end process;

89
Summary & Discussion
 Superset of ADA
 Concurrent modeling (Blocks for example)
 Generating of instances
 Use of packages, libraries
 Configurations, generics
 Grammar and BNF
 Simulation (and what they will do in the lab)

90
Summary
 OBJECTIVES: Provide a Standard Medium For
» Design Modeling
» Design Documentation
 VHDL is an IEEE Standard (Language Standard)
» IEEE VHDL-87
» IEEE VHDL-93
» VHDL-93 is Upward Compatible with the VHDL-87 (Minor
Differences May Require Code Modification)
 Requires Simulation and Synthesis Tools
 By The End of 1995, Most Simulation Tools Have
Incorporated Support for the VHDL-93
 VHDL Is Also Used For Design Synthesis

91
Summary
 Modular, Hierarchical, Allows Design Description
(TOP - DOWN, BOTTOM – UP), Portable, etc.
 Can Describe the Same Design Entity using More than
one View (Domain):
» The Behavioral View (e.g. as an algorithm, Register-Transfer
(Data Flow), Input-Output Relations, etc)
» The Structural View.
 This Allows Investigation of Design Alternatives of
the Same Entity
 It Also Allows Delayed Detailed Implementations.
 Can Model Systems at Various Levels of Abstraction
(System, chip RTL, Logic (Gate))
 VHDL Can be Made to Simulate Timing At Reasonable
Accuracy.

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