VHDL Programming by Example

VHDL Programming by Example
VHDL:
Programming
by Example
Douglas L. Perry
Fourth Edition
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DOI: 10.1036/0071409548
This Book is Dedicated to
my wife Debbie and my son Brennan
Thank you for your patience and support
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CONTENTS
Foreword xiii
Preface xv
Acknowledgments
Chapter 1
Chapter 2
xviii
Introduction to VHDL
1
VHDL Terms
Describing Hardware in VHDL
Entity
Architectures
Concurrent Signal Assignment
Event Scheduling
Statement Concurrency
Structural Designs
Sequential Behavior
Process Statements
Process Declarative Region
Process Statement Part
Process Execution
Sequential Statements
Architecture Selection
Configuration Statements
Power of Configurations
2
3
3
4
5
6
6
7
8
9
9
9
10
10
11
11
12
Behavioral Modeling
15
Introduction to Behavioral Modeling
Transport Versus Inertial Delay
Inertial Delay
Transport Delay
Inertial Delay Model
Transport Delay Model
Simulation Deltas
Drivers
Driver Creation
Bad Multiple Driver Model
Generics
Block Statements
Guarded Blocks
16
20
20
21
22
23
23
27
27
28
29
31
35
vi
Contents
Chapter 3
Chapter 4
Chapter 5
Sequential Processing
39
Process Statement
Sensitivity List
Process Example
Signal Assignment Versus Variable Assignment
Incorrect Mux Example
Correct Mux Example
Sequential Statements
IF Statements
CASE Statements
LOOP Statements
NEXT Statement
EXIT Statement
ASSERT Statement
Assertion BNF
WAIT Statements
WAIT ON Signal
WAIT UNTIL Expression
WAIT FOR time_expression
Multiple WAIT Conditions
WAIT Time-Out
Sensitivity List Versus WAIT Statement
Concurrent Assignment Problem
Passive Processes
40
40
40
42
43
45
46
47
48
50
53
54
56
57
59
62
62
62
63
64
66
67
70
Data Types
73
Object Types
Signal
Variables
Constants
Data Types
Scalar Types
Composite Types
Incomplete Types
File Types
File Type Caveats
Subtypes
74
74
76
77
78
79
86
98
102
105
105
Subprograms and Packages
109
Subprograms
Function
110
110
Contents
vii
Chapter 6
Chapter 7
Conversion Functions
Resolution Functions
Procedures
Packages
Package Declaration
Deferred Constants
Subprogram Declaration
Package Body
113
119
133
135
136
136
137
138
Predefined Attributes
143
Value Kind Attributes
Value Type Attributes
Value Array Attributes
Value Block Attributes
Function Kind Attributes
Function Type Attributes
Function Array Attributes
Function Signal Attributes
Attributes ’EVENT and ’LAST_VALUE
Attribute ’LAST_EVENT
Attribute ’ACTIVE and ’LAST_ACTIVE
Signal Kind Attributes
Attribute ’DELAYED
Attribute ’STABLE
Attribute ’QUIET
Attribute ’TRANSACTION
Type Kind Attributes
Range Kind Attributes
144
144
147
149
151
151
154
156
157
158
160
160
161
164
166
168
169
170
Configurations
173
Default Configurations
Component Configurations
Lower-Level Configurations
Entity-Architecture Pair Configuration
Port Maps
Mapping Library Entities
Generics in Configurations
Generic Value Specification in Architecture
Generic Specifications in Configurations
Board-Socket-Chip Analogy
Block Configurations
Architecture Configurations
174
176
179
180
181
183
185
188
190
195
199
201
viii
Contents
Chapter 8
Chapter 9
Chapter 10
Advanced Topics
205
Overloading
Subprogram Overloading
Overloading Operators
Aliases
Qualified Expressions
User-Defined Attributes
Generate Statements
Irregular Generate Statement
TextIO
206
206
210
215
215
218
220
222
224
Synthesis
231
Register Transfer Level Description
Constraints
Timing Constraints
Clock Constraints
Attributes
Load
Drive
Arrival Time
Technology Libraries
Synthesis
Translation
Boolean Optimization
Flattening
Factoring
Mapping to Gates
232
237
238
238
239
240
240
240
241
243
243
244
245
246
247
VHDL Synthesis
251
Simple Gate — Concurrent Assignment
IF Control Flow Statements
Case Control Flow Statements
Simple Sequential Statements
Asynchronous Reset
Asynchronous Preset and Clear
More Complex Sequential Statements
Four-Bit Shifter
State Machine Example
252
253
256
257
259
261
262
264
266
Contents
ix
Chapter 11
Chapter 12
Chapter 13
High Level Design Flow
273
RTL Simulation
VHDL Synthesis
Functional Gate-Level Verification
Place and Route
Post Layout Timing Simulation
Static Timing
275
277
283
284
286
287
Top-Level System Design
289
CPU Design
Top-Level System Operation
Instructions
Sample Instruction Representation
CPU Top-Level Design
Block Copy Operation
290
290
291
292
293
299
CPU: Synthesis Description
303
ALU
Comp
Control
Reg
Regarray
Shift
Trireg
Chapter 14
Chapter 15
306
309
311
321
322
324
326
CPU: RTL Simulation
329
Testbenches
Kinds of Testbenches
Stimulus Only
Full Testbench
Simulator Specific
Hybrid Testbenches
Fast Testbench
CPU Simulation
330
331
333
337
340
342
345
349
CPU Design: Synthesis Results
357
x
Contents
Chapter 16
Chapter 17
Chapter 18
Place and Route
369
Place and Route Process
Placing and Routing the Device
Setting up a project
370
373
373
CPU: VITAL Simulation
379
VITAL Library
VITAL Simulation Process Overview
VITAL Implementation
Simple VITAL Model
VITAL Architecture
Wire Delay Section
Flip-Flop Example
SDF File
VITAL Simulation
Back-Annotated Simulation
381
382
382
383
386
386
388
392
394
397
At Speed Debugging Techniques
399
Instrumentor
Debugger
Debug CPU Design
Create Project
Specify Top-Level Parameters
Specify Project Parameters
Instrument Signals
Write Instrumented Design
Implement New Design
Start Debug
Enable Breakpoint
Trigger Position
Waveform Display
Set Watchpoint
Complex Triggers
401
401
401
402
403
403
404
405
405
406
406
408
408
409
410
Appendix A Standard Logic Package
413
Appendix B VHDL Reference Tables
435
Appendix C Reading VHDL BNF
445
Contents
xi
Appendix D VHDL93 Updates
Alias
Attribute Changes
Bit String Literal
DELAY_LENGTH Subtype
Direct Instantiation
Extended Identifiers
File Operations
Foreign Interface
Generate Statement Changes
Globally Static Assignment
Groups
Incremental Binding
Postponed Process
Pure and Impure Functions
Pulse Reject
Report Statement
Shared Variables
Shift Operators
SLL — shift left logical
SRL — shift right logical
SLA — shift left arithmetic
SRA — shift right arithmetic
ROL — rotate left
ROR — rotate right
Syntax Consistency
Unaffected
XNOR Operator
Index 469
About the Author
477
449
449
450
452
452
452
453
454
455
456
456
457
458
459
460
460
461
461
463
463
463
463
463
464
464
464
466
466
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FOREWORD
VHDL has been at the heart of electronic design productivity since initial ratification by the IEEE in 1987. For almost 15 years the electronic
design automation industry has expanded the use of VHDL from initial
concept of design documentation, to design implementation and functional verification. It can be said that VHDL fueled modern synthesis
technology and enabled the development of ASIC semiconductor companies. The editions of Doug Perry’s books have served as the authoritative
source of practical information on the use of VHDL for users of the
language around the world.
The use of VHDL has evolved and its importance increased as semiconductor devices dimensions have shrunk. Not more than 10 years ago it
was common to mix designs described with schematics and VHDL. But as
design complexity grew, the industry abandoned schematics in favor of the
hardware description language only. The successive revisions of this book
have always kept pace with the industry’s evolving use of VHDL.
The fact that VHDL is adaptable is a tribute to its architecture. The
industry has seen the use of VHDL’s package structure to allow designers, electronic design automation companies and the semiconductor industry to experiment with new language concepts to ensure good design tool
and data interoperability. When the associated data types found in the
IEEE 1164 standard were ratified, it meant that design data interoperability was possible.
All of this was facilitated by industry backing in a consortium of systems,
electronic design automation and semiconductor companies now known
as Accellera.
And when the ASIC industry needed a standard way to convey gatelevel design data and timing information in VHDL, one of Accellera’s
progenitors (VHDL International) sponsored the IEEE VHDL team to
build a companion standard. The IEEE 1076.4 VITAL (VHDL Initiative
Towards ASIC Libraries) was created and ratified as offers designers a
single language flow from concept to gate-level signoff.
In the late ’90s, the Verilog HDL and VHDL industry standards teams
collaborated on the use of a common timing data such as IEEE 1497 SDF,
set register transfer level (RTL) standards and more to improve design
xiv
Foreword
methodologies and the external connections provided to the hardware
description languages.
But from the beginning, the leadership of the VHDL community has
assured open and internationally accredited standards for the electronic
design engineering community. The legacy of this team’s work continues
to benefit the design community today as the benchmark by which one
measures openness.
The design community continues to see benefits as the electronic design
automation community continues to find new algorithms to work from
VHDL design descriptions and related standards to again push designer
productivity. And, as a new generation of designers of programmable logic
devices move to the use of hardware description languages as the basis of
their design methodology, there will be substantial growth in the number
of VHDL users.
This new generation of electronic designers, along with the current
designers of complex systems and ASICs, will find this book as invaluable as the first generation of VHDL users did with the first addition.
Updated with current use of the standard, all will benefit from the years
of use that have made the VHDL language the underpinning of successful
electronic design.
Dennis B. Brophy
Chair, Accellera
PREFACE
This is the fourth version of the book and this version now not only provides
VHDL language coverage but design methodology information as well. This
version will guide the reader through the process of creating a VHDL
design, simulating the design, synthesizing the design, placing and routing
the design, using VITAL simulation to verify the final result, and a new
technique called At-Speed debugging that provides extremely fast design
verification. The design example in this version has been updated to reflect
the new focus on the design methodology.
This book was written to help hardware design engineers learn how to
write good VHDL design descriptions. The goal is to provide enough VHDL
and design methodology information to enable a designer to quickly write
good VHDL designs and be able to verify the results. It will also attempt
to bring the designer with little or no knowledge of VHDL, to the level of
writing complex VHDL descriptions. It is not intended to show every possible construct of VHDL in every possible use, but rather to show the designer how to write concise, efficient, and correct VHDL descriptions of
hardware designs.
This book is organized into three logical sections. The first section of the
book will introduce the VHDL language, the second section walks through
a VHDL based design process including simulation, synthesis, place and
route, and VITAL simulation; and the third section walks through a design
example of a small CPU design from VHDL capture to final gate-level
implementation, and At-Speed debugging. At the back of the book are
included a number of appendices that contain useful information about the
language and examples used throughout the book.
In the first section VHDL features are introduced one or more at a time.
As each feature is introduced, one or more real examples are given to show
how the feature would be used. The first section consists of Chapters 1
through 8, and each chapter introduces a basic description capability of
VHDL. Chapter 1 discusses how VHDL design relates to schematic based
design, and introduces the basic terms of the language. Chapter 2 describes
some of the basic concepts of VHDL, including the different delay mechanisms available, how to use instance specific data, and defines VHDL drivers. Chapter 2 discusses concurrent statements while Chapter 3 introduces
the reader to VHDL sequential statements. Chapter 4 talks about the wide
xvi
Preface
range of types available for use in VHDL. Examples are given for each of
the types showing how they would be used in a real example. In Chapter
5 the concepts of subprograms and packages are introduced. The different
uses for functions are given, as well as the features available in VHDL
packages.
Chapter 6 introduces the five kinds of VHDL attributes. Each attribute
kind has examples describing how to use the specific attribute to the
designer’s best advantage. Examples are given which describe the purpose of each of the attributes.
Chapters 7 and 8 will introduce some of the more advanced VHDL
features to the reader. Chapter 7 discusses how VHDL configurations
can be used to construct and manage complex VHDL designs. Each of
the different configuration styles are discussed along with examples
showing usage. Chapter 8 introduces more of the VHDL advanced topics with discussions of overloading, user defined attributes, generate
statements, and TextIO.
The second section of the book consists of Chapters 9 through 11. Chapters 9 and 10 discuss the synthesis process and how to write synthesizable designs. These two chapters describe the basics of the synthesis
process including how to write synthesizeable VHDL, what is a technology library, what does the synthesis process look like, what are constraints and attributes, and what does the the optimization process look
like. Chapter 11 discusses the complete high level design flow from VHDL
capture through VITAL simulation.
The third section of the book walks through a description of a small
CPU design from the VHDL capture through simulation, synthesis, place
and route, and VITAL simulation. Chapter 12 describes the top level of
the CPU design from a functional point of view. In Chapter 13 the RTL
description of the CPU is presented and discussed from a synthesis point
of view. Chapter 14 begins with a discussion of VHDL testbenches and
how they are used to verify functionality. Chapter 14 finishes the discussion by describing the simulation of the CPU design. In Chapter 15 the
verified design is synthesized to a target technology. Chapter 16 takes
the synthesized design and places and routes the design to a target
device. Chapter 17 begins with a discussion of VITAL and ends with the
VITAL simulation of the placed and routed CPU design. Chapter 18 is a
new chapter that discusses the new technique of At-Speed debugging.
This chapter provides the reader with an in-depth look at how a hardware
implementation of the CPU design can help speed verification.
Finally there are three appendices at the end of the book to provide reference information. Appendix A is a listing of the IEEE 1164 STD_LOGIC
Preface
xvii
package used throughout the book. Appendix B is a set of useful tables
that condense some of the information in the rest of the book into quick
reference tables. Finally, Appendix C describes how to read the BachusNaur format(BNF) descriptions found in the VHDL Language Reference
Manual. I can only hope that you the reader will have as much fun reading this book and working with VHDL as I did in writing it.
ACKNOWLEDGMENTS
This book would not have been possible without the help of a number of
people, and I would like to express my gratitude to all of them. Rod Farrow, Cary Ussery, Alec Stanculescu, and Ken Scott answered a multitude
of questions about some of the vagaries of VHDL. Mark Beardslee and
Derek Palmer for their review of parts of the third edition. Their comments were both helpful and insightful. Paul Krol developed the chart in
Chapter 7 that describes generics. Keith Irwin helped define the style of
some of the chapters. Hoa Dinh and David Emrich for answering a lot
of questions about FPGA synthesis. Thanks to John Ott and Dennis Brophy for making the ModelSim and Leonardo Spectrum software available
during the writing and for the software on the CD. Thanks to Derek
Palmer and Robert Blake of Altera for making the MaxPlus II software
available and answering questions. Finally thanks to Endric Schubert,
Mark Beardslee, Gernot Koch, Olaf Poeppe, Matt Hall, Michael Eitelwein, Ewald Detjens, and William Vancleemput for all of their hard work
with Bridges2Silicon.
CHAPTER
1
Introduction to
VHDL
The VHSIC Hardware Description Language is an industry
standard language used to describe hardware from the
abstract to the concrete level. VHDL resulted from work
done in the ’70s and early ’80s by the U.S. Department
of Defense. Its roots are in the ADA language, as will be
seen by the overall structure of VHDL as well as other
VHDL statements.
VHDL usage has risen rapidly since its inception and
is used by literally tens of thousands of engineers around
the globe to create sophisticated electronic products. This
chapter will start the process of easing the reader into
the complexities of VHDL. VHDL is a powerful language
with numerous language constructs that are capable of
describing very complex behavior. Learning all the features
of VHDL is not a simple task. Complex features will be
introduced in a simple form and then more complex usage
will be described.
2
Chapter One
In 1986, VHDL was proposed as an IEEE standard. It went through a
number of revisions and changes until it was adopted as the IEEE 1076
standard in December 1987. The IEEE 1076-1987 standard VHDL is the
VHDL used in this book. (Appendix D contains a brief description of VHDL
1076-1993.) All the examples have been described in IEEE 1076 VHDL, and
compiled and simulated with the VHDL simulation environment from
Model Technology Inc. The synthesis examples were synthesized with the
Exemplar Logic Inc. synthesis tools.
VHDL Terms
Before we go any further, let’s define some of the terms that we use
throughout the book. These are the basic VHDL building blocks that are
used in almost every description, along with some terms that are redefined
in VHDL to mean something different to the average designer.
■ Entity. All designs are expressed in terms of entities. An entity is
the most basic building block in a design. The uppermost level of
the design is the top-level entity. If the design is hierarchical, then
the top-level description will have lower-level descriptions contained
in it. These lower-level descriptions will be lower-level entities
contained in the top-level entity description.
■ Architecture. All entities that can be simulated have an architecture description. The architecture describes the behavior of the
entity. A single entity can have multiple architectures. One architecture might be behavioral while another might be a structural
description of the design.
■ Configuration. A configuration statement is used to bind a
component instance to an entity-architecture pair. A configuration
can be considered like a parts list for a design. It describes which
behavior to use for each entity, much like a parts list describes
which part to use for each part in the design.
■ Package. A package is a collection of commonly used data types
and subprograms used in a design. Think of a package as a toolbox that contains tools used to build designs.
■ Driver. This is a source on a signal. If a signal is driven by two
sources, then when both sources are active, the signal will have
two drivers.
Introduction to VHDL
3
■ Bus. The term “bus” usually brings to mind a group of signals or
a particular method of communication used in the design of hardware. In VHDL, a bus is a special kind of signal that may have its
drivers turned off.
■ Attribute. An attribute is data that are attached to VHDL objects
or predefined data about VHDL objects. Examples are the current
drive capability of a buffer or the maximum operating temperature
of the device.
■ Generic. A generic is VHDL’s term for a parameter that passes
information to an entity. For instance, if an entity is a gate level
model with a rise and a fall delay, values for the rise and fall delays
could be passed into the entity with generics.
■ Process. A process is the basic unit of execution in VHDL. All
operations that are performed in a simulation of a VHDL description are broken into single or multiple processes.
Describing Hardware in VHDL
VHDL Descriptions consist of primary design units and secondary design
units. The primary design units are the Entity and the Package. The secondary design units are the Architecture and the Package Body. Secondary design units are always related to a primary design unit. Libraries
are collections of primary and secondary design units. A typical design
usually contains one or more libraries of design units.
Entity
A VHDL entity specifies the name of the entity, the ports of the entity,
and entity-related information. All designs are created using one or more
entities.
Let’s take a look at a simple entity example:
ENTITY mux
IS
PORT ( a, b, c, d : IN BIT;
s0, s1 : IN BIT;
x, : OUT BIT);
END mux;
4
Chapter One
The keyword ENTITY signifies that this is the start of an entity statement. In the descriptions shown throughout the book, keywords of the
language and types provided with the STANDARD package are shown in
ALL CAPITAL letters. For instance, in the preceding example, the keywords are ENTITY, IS, PORT, IN, INOUT, and so on. The standard type provided is BIT. Names of user-created objects such as mux, in the example
above, will be shown in lower case.
The name of the entity is mux. The entity has seven ports in the PORT
clause. Six ports are of mode IN and one port is of mode OUT. The four data
input ports (a, b, c, d) are of type BIT. The two multiplexer select inputs,
s0 and s1, are also of type BIT. The output port is of type BIT.
The entity describes the interface to the outside world. It specifies
the number of ports, the direction of the ports, and the type of the ports.
A lot more information can be put into the entity than is shown here,
but this gives us a foundation upon which we can build more complex
examples.
Architectures
The entity describes the interface to the VHDL model. The architecture describes the underlying functionality of the entity and contains
the statements that model the behavior of the entity. An architecture is
always related to an entity and describes the behavior of that entity. An
architecture for the counter device described earlier would look like this:
ARCHITECTURE dataflow OF mux IS
SIGNAL select : INTEGER;
BEGIN
select <= 0 WHEN s0 = ‘0’ AND s1 = ‘0’ ELSE
1 WHEN s0 = ‘1’ AND s1 = ‘0’ ELSE
2 WHEN s0 = ‘0’ AND s1 = ‘1’ ELSE
3;
x <= a
b
c
d
AFTER
AFTER
AFTER
AFTER
0.5
0.5
0.5
0.5
NS WHEN select = 0 ELSE
NS WHEN select = 1 ELSE
NS WHEN select = 2 ELSE
NS;
END dataflow;
The keyword ARCHITECTURE signifies that this statement describes an
architecture for an entity. The architecture name is dataflow. The entity
the architecture is describing is called mux.
Introduction to VHDL
5
The reason for the connection between the architecture and the entity
is that an entity can have multiple architectures describing the behavior of
the entity. For instance, one architecture could be a behavioral description,
and another could be a structural description.
The textual area between the keyword ARCHITECTURE and the keyword
BEGIN is where local signals and components are declared for later use.
In this example signal select is declared to be a local signal.
The statement area of the architecture starts with the keyword BEGIN.
All statements between the BEGIN and the END netlist statement are called
concurrent statements, because all the statements execute concurrently.
Concurrent Signal Assignment
In a typical programming language such as C or C++, each assignment
statement executes one after the other and in a specified order. The order
of execution is determined by the order of the statements in the source file.
Inside a VHDL architecture, there is no specified ordering of the assignment
statements. The order of execution is solely specified by events occurring
on signals that the assignment statements are sensitive to.
Examine the first assignment statement from architecture behave, as
shown here:
select <= 0 WHEN s0 = ‘0’ AND s1 = ‘0’ ELSE
1 WHEN s0 = ‘1’ AND s1 = ‘0’ ELSE
2 WHEN s0 = ‘0’ AND s1 = ‘1’ ELSE
3;
A signal assignment is identified by the symbol <=. Signal select will
get a numeric value assigned to it based on the values of s0 and s1. This
statement is executed whenever either signal s0 or signal s1 has an event
occur on it. An event on a signal is a change in the value of that signal. A
signal assignment statement is said to be sensitive to changes on any signals that are to the right of the <= symbol. This signal assignment statement is sensitive to s0 and s1. The other signal assignment statement in
architecture dataflow is sensitive to signal select.
Let’s take a look at how these statements actually work. Suppose that
we have a steady-state condition where both s0 and s1 have a value of 0,
and signals a, b, c, and d currently have a value of 0. Signal x will
have a 0 value because it is assigned the value of signal a whenever signals
s0 and s1 are both 0. Now assume that we cause an event on signal a that
changes its value to 1. When this happens, the first signal assignment
6
Chapter One
statement will not execute because this statement is not sensitive to
changes to signal a. This happens because signal a is not on the right
side of the operator. The second signal assignment statement will execute because it is sensitive to events on signal a. When the second signal
assignment statement executes the new value of a will be assigned to
signal x. Output port x will now change to 1.
Let’s now look at the case where signal s0 changes value. Assume that
s0 and s1 are both 0, and ports a, b, c, and d have the values 0, 1, 0,
and 1, respectively. Now let’s change the value of port s0 from 0 to 1. The
first signal assignment statement is sensitive to signal s0 and will therefore execute. When concurrent statements execute, the expression value
calculation will use the current values for all signals contained in it.
When the first statement executes, it computes the new value to be assigned to q from the current value of the signal expression on the right
side of the <= symbol. The expression value calculation uses the current
values for all signals contained in it.
With the value of s0 equal to 1 and s1 equal to 0, signal select will
receive a new value of 1. This new value of signal select will cause an
event to occur on signal select, causing the second signal assignment
statement to execute. This statement will use the new value of signal select
to assign the value of port b to port x. The new assignment will cause
port x to change from a 0 to a 1.
Event Scheduling
The assignment to signal x does not happen instantly. Each of the values
assigned to signal x contain an AFTER clause. The mechanism for delaying
the new value is called scheduling an event. By assigning port x a new
value, an event was scheduled 0.5 nanoseconds in the future that contains
the new value for signal x. When the event matures (0.5 nanoseconds in
the future), signal x receives the new value.
Statement Concurrency
The first assignment is the only statement to execute when events occur
on ports s0 or s1. The second signal assignment statement does not execute unless an event on signal select occurs or an event occurs on ports
a, b, c, d.
Introduction to VHDL
7
The two signal assignment statements in architecture behave form a
behavioral model, or architecture, for the mux entity. The dataflow architecture contains no structure. There are no components instantiated in
the architecture. There is no further hierarchy, and this architecture can
be considered a leaf node in the hierarchy of the design.
Structural Designs
Another way to write the mux design is to instantiate subcomponents that
perform smaller operations of the complete model. With a model as simple
as the 4-input multiplexer that we have been using, a simple gate level
description can be generated to show how components are described and
instantiated. The architecture shown below is a structural description of
the mux entity.
ARCHITECTURE netlist OF mux IS
COMPONENT andgate
PORT(a, b, c : IN bit; c : OUT BIT);
END COMPONENT;
COMPONENT inverter
PORT(in1 : IN BIT; x : OUT BIT);
END COMPONENT;
COMPONENT orgate
PORT(a, b, c, d : IN bit; x : OUT BIT);
END COMPONENT;
SIGNAL s0_inv, s1_inv, x1, x2, x3, x4 : BIT;
BEGIN
U1 : inverter(s0, s0_inv);
U2 : inverter(s1, s1_inv);
U3 : andgate(a, s0_inv, s1_inv, x1);
U4 : andgate(b, s0, s1_inv, x2);
U5 : andgate(c, s0_inv, s1, x3);
U6 : andgate(d, s0, s1, x4);
U7 : orgate(x2 => b, x1 => a, x4 => d, x3 => c, x => x);
END netlist;
This description uses a number of lower-level components to model the
behavior of the mux device. There is an inverter component, an andgate
component and an orgate component. Each of these components is declared
in the architecture declaration section, which is between the architecture
statement and the BEGIN keyword.
A number of local signals are used to connect each of the components
to form the architecture description. These local signals are declared using
the SIGNAL declaration.
8
Chapter One
The architecture statement area is located after the BEGIN keyword. In
this example are a number of component instantiation statements. These
statements are labeled U1-U7. Statement U1 is a component instantiation
statement that instantiates the inverter component. This statement connects port s0 to the first port of the inverter component and signal
s0_inv to the second port of the inverter component. The effect is that
port in1 of the inverter is connected to port s0 of the mux entity, and port
x of the inverter is connected to local signal s0_inv. In this statement
the ports are connected by the order they appear in the statement.
Notice component instantiation statement U7. This statement uses the
following notation:
U7 : orgate(x2 => b, x1 => a, x4 => d, x3 => c, x => x);
This statement uses named association to match the ports and signals
to each other. For instance port x2 of the orgate is connected to port b of
the entity with the first association clause. The last instantiation clause
connects port x of the orgate component to port x of the entity. The order
of the clauses is not important. Named and ordered association can be
mixed, but it is not recommended.
Sequential Behavior
There is yet another way to describe the functionality of a mux device in
VHDL. The fact that VHDL has so many possible representations for similar functionality is what makes learning the entire language a big task.
The third way to describe the functionality of the mux is to use a process
statement to describe the functionality in an algorithmic representation.
This is shown in architecture sequential, as shown in the following:
ARCHITECTURE sequential OF mux IS
(a, b, c, d, s0, s1 )
VARIABLE sel : INTEGER;
BEGIN
IF s0 = ‘0’ and s1 = ‘0’ THEN
sel := 0;
ELSIF s0 = ‘1’ and s1 = ‘0’ THEN
sel := 1;
ELSIF s0 = ‘0’ and s1 = ‘0’ THEN
sel := 2;
ELSE
sel := 3;
END IF;
CASE sel IS
Introduction to VHDL
9
WHEN 0 =>
x <= a;
WHEN 1 =>
x <= b;
WHEN 2 =>
x <= c;
WHEN OTHERS =>
x <= d;
END CASE;
END PROCESS;
END sequential;
The architecture contains only one statement, called a process statement. It starts at the line beginning with the keyword PROCESS and ends
with the line that contains END PROCESS. All the statements between
these two lines are considered part of the process statement.
Process Statements
The process statement consists of a number of parts. The first part is
called the sensitivity list; the second part is called the process declarative
part; and the third is the statement part. In the preceding example, the
list of signals in parentheses after the keyword PROCESS is called the sensitivity list. This list enumerates exactly which signals cause the process
statement to be executed. In this example, the list consists of a, b, c, d,
s0, and s1. Only events on these signals cause the process statement to
be executed.
Process Declarative Region
The process declarative part consists of the area between the end of the
sensitivity list and the keyword BEGIN. In this example, the declarative
part contains a variable declaration that declares local variable sel. This
variable is used locally to contain the value computed based on ports s0
and s1.
Process Statement Part
The statement part of the process starts at the keyword BEGIN and ends
at the END PROCESS line. All the statements enclosed by the process are
10
Chapter One
sequential statements. This means that any statements enclosed by the
process are executed one after the other in a sequential order just like a
typical programming language. Remember that the order of the statements
in the architecture did not make any difference; however, this is not true
inside the process. The order of execution is the order of the statements
in the process statement.
Process Execution
Let’s see how this works by walking through the execution of the example
in architecture sequential, line by line. To be consistent, let’s assume
that s0 changes to 0. Because s0 is in the sensitivity list for the process
statement, the process is invoked. Each statement in the process is then
executed sequentially. In this example the IF statement is executed first
followed by the CASE statment. Each check that the IF statement performs
is done sequentially starting with the first in the model.
The first check is to see if s0 is equal to a 0. This statement fails because
s0 is equal to a 1 and s1t is equal to a 0. The signal assignment statement that follows the first check will not be executed. Instead, the next
check is performed. This check succeeds and the signal assignment statements following the check for s0 = 1 and s1 = 0 are executed. This
statement is shown below.
sel := 1;
Sequential Statements
This statement will execute sequentially. Once it is executed, the next
check of the IF statement is not performed. Whenever a check succeeds,
no other checks are done. The IF statement has completed and now the CASE
statement will execute. The CASE statement will evaluate the value of sel
computed earlier by the IF statement and then execute the appropriate
statement that matches the value of sel. In this example the value of sel
is 1 therefore the following statement will be executed:
x <= b;
The value of port b will be assigned to port x and process execution will
terminate because there are no more statements in the architecture.
Introduction to VHDL
11
Architecture Selection
So far, three architectures have been described for one entity. Which architecture should be used to model the mux device? It depends on the accuracy
wanted and if structural information is required. If the model is going to
be used to drive a layout tool, then the structural architecture netlist is
probably most appropriate. If a structural model is not wanted for some
other reason, then a more efficient model can be used. Either of the other
two methods (architectures dataflow and sequential) are probably more
efficient in memory space required and speed of execution. How to choose
between these two methods may come down to a question of programming
style. Would the modeler rather write concurrent or sequential VHDL code?
If the modeler wants to write concurrent VHDL code, then the style of
architecture dataflow is the way to go; otherwise, architecture sequential
should be chosen. Typically, modelers are more familiar with sequential coding styles, but concurrent statements are very powerful tools for
writing small efficient models.
We will also look at yet another architecture that can be written for an
entity. This is the architecture that can be used to drive a synthesis tool.
Synthesis tools convert a Register Transfer Level (RTL) VHDL description
into an optimized gate-level description. Synthesis tools can offer greatly
enhanced productivity compared to manual methods. The synthesis
process is discussed in Chapters 9, “Synthesis” and 10, “VHDL Synthesis.”
Configuration Statements
An entity can have more than one architecture, but how does the modeler
choose which architecture to use in a given simulation? The configuration
statement maps component instantiations to entities. With this powerful
statement, the modeler can pick and choose which architectures are used
to model an entity at every level in the design.
Let’s look at a configuration statement using the netlist architecture of
the rsff entity. The following is an example configuration:
CONFIGURATION muxcon1 OF mux IS
FOR netlist
FOR U1,U2 :
inverter USE ENTITY WORK.myinv(version1);
END FOR;
FOR U3,U4,U5,U6 : andgate USE ENTITY WORK.myand(version1);
END FOR;
12
Chapter One
FOR U7 : orgate USE ENTITY WORK.myor(version1);
END FOR;
END FOR;
END muxcon1;
The function of the configuration statement is to spell out exactly
which architecture to use for every component instance in the model. This
occurs in a hierarchical fashion. The highest-level entity in the design
needs to have the architecture to use specified, as well as any components
instantiated in the design.
The preceding configuration statement reads as follows: This is a configuration named muxcon1 for entity mux. Use architecture netlist as the
architecture for the topmost entity, which is mux. For the two component
instances U1 and U2 of type inverter instantiated in the netlist architecture, use entity myinv, architecture version1 from the library called
WORK. For the component instances U3-U6 of type andgate, use entity
myand, architecture version1 from library WORK. For component instance
U7 of type orgate use entity myor, architecture version1 from library
WORK. All of the entities now have architectures specified for them. Entity
mux has architecture netlist, and the other components have architectures
named version1 specified.
Power of Configurations
By compiling the entities, architectures, and the configuration specified
earlier, you can create a simulatable model. But what if you did not want
to simulate at the gate level? What if you really wanted to use architecture
BEHAVE instead? The power of the configuration is that you do not need to
recompile your complete design; you only need to recompile the new configuration. Following is an example configuration:
CONFIGURATION muxcon2 OF mux IS
FOR dataflow
END FOR;
END muxcon2;
This is a configuration named muxcon2 for entity mux. Use architecture
dataflow for the topmost entity, which is mux. By compiling this
configuration, the architecture dataflow is selected for entity mux in this
simulation.
This configuration is not necessary in standard VHDL, but gives the
designer the freedom to specify exactly which architecture will be used for
the entity. The default architecture used for the entity is the last one
compiled into the working library.
Introduction to VHDL
13
SUMMARY
In this chapter, we have had a basic introduction to VHDL and how
it can be used to model the behavior of devices and designs. The first
example showed how a simple dataflow model in VDHL is specified. The
second example showed how a larger design can be made of smaller designs
—in this case a 4-input multiplexer was modeled using AND, OR and INVERTER gates. The first example provided a structural view of VHDL.
The last example showed an algorithmic or behavioral view of the
mux. All these views of the mux successfully model the functionality of a mux
and all can be simulated with a VHDL simulator. Ultimately, however, a
designer will want to use the model to facilitate building a piece of hardware. The most common use of VHDL in actually building hardware today
is through synthesis tools. Therefore, the focus of the rest of the book is
not only on the simulation of VHDL but also on the synthesis of VHDL.
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CHAPTER
2
Behavioral
Modeling
In Chapter 1, we discussed different modeling techniques
and touched briefly on behavioral modeling. In this chapter,
we discuss behavioral modeling more thoroughly, as well
as some of the issues relating to the simulation and synthesis of VHDL models.
16
Chapter Two
Introduction to Behavioral
Modeling
The signal assignment statement is the most basic form of behavioral
modeling in VHDL. Following is an example:
a <= b;
This statement is read as follows: a gets the value of b. The effect of
this statement is that the current value of signal b is assigned to signal
a. This statement is executed whenever signal b changes value. Signal b
is in the sensitivity list of this statement. Whenever a signal in the sensitivity list of a signal assignment statement changes value, the signal
assignment statement is executed. If the result of the execution is a new
value that is different from the current value of the signal, then an event
is scheduled for the target signal. If the result of the execution is the same
value, then no event is scheduled but a transaction is still generated
(transactions are discussed in Chapter 3, “Sequential Processing”). A transaction is always generated when a model is evaluated, but only signal
value changes cause events to be scheduled.
The next example shows how to introduce a nonzero delay value for the
assignment:
a <= b after 10 ns;
This statement is read as follows: a gets the value of b when 10
nanoseconds of time have elapsed.
Both of the preceding statements are concurrent signal assignment statements. Both statements are sensitive to changes in the value of signal b.
Whenever b changes value, these statements execute and new values are
assigned to signal a.
Using a concurrent signal assignment statement, a simple AND gate
can be modeled, as follows:
ENTITY and2 IS
PORT ( a, b : IN BIT;
PORT ( c : OUT BIT );
END and2;
ARCHITECTURE and2_behav OF and2 IS
BEGIN
c <= a AND b AFTER 5 ns;
Behavioral Modeling
Figure 2-1
AND Gate Symbol.
17
A
C
B
END and2_behav;
The AND gate has two inputs a, b and one output c, as shown in Figure
2-1. The value of signal c may be assigned a new value whenever either
a or b changes value. With an AND gate, if a is a ‘0’ and b changes from a
‘1’ to a ‘0’, output c does not change. If the output does change value, then
a transaction occurs which causes an event to be scheduled on signal c;
otherwise, a transaction occurs on signal c.
The entity design unit describes the ports of the and2 gate. There are
two inputs a and b, as well as one output c. The architecture and2_behav
for entity and2 contains one concurrent signal assignment statement. This
statement is sensitive to both signal a and signal b by the fact that the
expression to calculate the value of c includes both a and b signal values.
The value of the expression a and b is calculated first, and the resulting
value from the calculation is scheduled on output c, 5 nanoseconds from
the time the calculation is completed.
The next example shows more complicated signal assignment statements and demonstrates the concept of concurrency in greater detail. In
Figure 2-2, the symbol for a four-input multiplexer is shown.
This is the behavioral model for the mux:
LIBRARY IEEE;
USE IEEE.std_logic_1164.ALL;
ENTITY mux4 IS
PORT ( i0, i1, i2, i3, a, b : IN std_logic;
PORT ( i0, i1, i2, i3, a, q : OUT std_logic);
END mux4;
ARCHITECTURE mux4 OF mux4 IS
SIGNAL sel: INTEGER;
BEGIN
WITH sel SELECT
q <= i0 AFTER 10 ns WHEN 0,
q <= i1 AFTER 10 ns WHEN 1,
18
Chapter Two
MUX4
Figure 2-2
Mux4 Symbol.
I0
I1
Q
I2
I3
A B
q <= i2 AFTER 10 ns WHEN 2,
q <= i3 AFTER 10 ns WHEN 3,
q <= ‘X’ AFTER 10 ns WHEN OTHERS;
sel <= 0 WHEN
1 WHEN
2 WHEN
3 WHEN
4 ;
END mux4;
a
a
a
a
=
=
=
=
‘0’
‘1’
‘0’
‘1’
AND
AND
AND
AND
b
b
b
b
=
=
=
=
‘0’
‘0’
‘1’
‘1’
ELSE
ELSE
ELSE
ELSE
The entity for this model has six input ports and one output port. Four
of the input ports (I0, I1, I2, I3) represent signals that will be assigned
to the output signal q. Only one of the signals will be assigned to the output signal q based on the value of the other two input signals a and b. The
truth table for the multiplexer is shown in Figure 2-3.
To implement the functionality described in the preceding, we use a
conditional signal assignment statement and a selected signal assignment.
The second statement type in this example is called a conditional signal
assignment statement. This statement assigns a value to the target signal based on conditions that are evaluated for each statement. The
statement WHEN conditions are executed one at a time in sequential order
until the conditions of a statement are met. The first statement that
matches the conditions required assigns the value to the target signal.
The target signal for this example is the local signal sel. Depending
on the values of signals a and b, the values 0 through 4 are assigned
to sel.
If more than one statement’s conditions match, the first statement that
Behavioral Modeling
Figure 2-3
Mux Functional
Table.
19
A
B
Q
0
0
I0
1
0
I1
0
1
I2
1
1
I3
matches does the assign, and the other matching statements’ values are
ignored.
The first statement is called a selected signal assignment and selects
among a number of options to assign the correct value to the target signal. The target signal in this example is the signal q.
The expression (the value of signal sel in this example) is evaluated,
and the statement that matches the value of the expression assigns the
value to the target signal. All of the possible values of the expression must
have a matching choice in the selected signal assignment (or an OTHERS
clause must exist).
Each of the input signals can be assigned to output q, depending on the
values of the two select inputs, a and b. If the values of a or b are unknown
values, then the last value, ‘X’ (unknown), is assigned to output q. In this
example, when one of the select inputs is at an unknown value, the output is set to unknown.
Looking at the model for the multiplexer, it looks like the model will
not work as written. It seems that the value of signal sel is used before
it is computed. This impression is received from the fact that the second
statement in the architecture is the statement that actually computes the
value for sel. The model does work as written, however, because of the
concept of concurrency.
The second statement is sensitive to signals a and b. Whenever either
a or b changes value, the second statement is executed, and signal sel is
updated. The first statement is sensitive to signal sel. Whenever signal
sel changes value, the first signal assignment is executed.
If this example is processed by a synthesis tool, the resulting gate
structure created resembles a 4 to 1 multiplexer. If the synthesis library
contains a 4 to 1 multiplexer primitive, that primitive may be generated
20
Chapter Two
based on the sophistication of the synthesis tool and the constraints put
on the design.
Transport Versus Inertial Delay
In VHDL, there are two types of delay that can be used for modeling
behaviors. Inertial delay is the most commonly used, while transport delay
is used where a wire delay model is required.
Inertial Delay
Inertial delay is the default in VHDL. If no delay type is specified, inertial delay is used. Inertial delay is the default because, in most cases, it
behaves similarly to the actual device.
In an inertial delay model, the output signal of the device has inertia,
which must be overcome for the signal to change value. The inertia value
is equal to the delay through the device. If there are any spikes, pulses,
and so on that have periods where a signal value is maintained for less
than the delay through the device, the output signal value does not
change. If a signal value is maintained at a particular value for longer
than the delay through the device, the inertia is overcome and the device
changes to the new state.
Figure 2-4 is an example of a very simple buffer symbol. The buffer has
a single input A and a single output B. The waveforms are shown for input
A and the output B. Signal A changes from a ‘0’ to a ‘1’ at 10 nanoseconds
and from a ‘1’ to a ‘0’ at 20 nanoseconds. This creates a pulse or spike
that is 10 nanoseconds in duration. The buffer has a 20- nanosecond delay
through the device.
The ‘0’ to ‘1’ transition on signal A causes the buffer model to be executed and schedules an event with the value ‘1’ to occur on output B at
time 30 nanoseconds. At time 20 nanoseconds, the next event on signal A
occurs. This executes the buffer model again. The buffer model predicts a
new event on output B of a 0 value at time 40 nanoseconds. The event
scheduled on output B for time 30 nanoseconds still has not occurred. The
new event predicted by the buffer model clashes with the currently
scheduled event, and the simulator preempts the event at 30 nanoseconds.
The effect of the preemption is that the spike is swallowed. The reason
for the cancellation is that, according to the inertial delay model, the first
Behavioral Modeling
21
A
B
Figure 2-4
Inertial Delay Buffer
Waveforms.
Delay = 20 ns
A
B
0
10
20
30
40
event at 30 nanoseconds did not have enough time to overcome the inertia
of the output signal.
The inertial delay model is by far the most commonly used in all currently available simulators. This is partly because, in most cases, the
inertial delay model is accurate enough for the designer’s needs. One
more reason for the widespread use of inertial delay is that it prevents
prolific propagation of spikes throughout the circuit. In most cases, this
is the behavior wanted by the designer.
Transport Delay
Transport delay is not the default in VHDL and must be specified. It represents a wire delay in which any pulse, no matter how small, is propagated
to the output signal delayed by the delay value specified. Transport delay
is especially useful for modeling delay line devices, wire delays on a PC
board, and path delays on an ASIC.
If we look at the same buffer circuit that was shown in Figure 2-4, but
replace the inertial delay waveforms with the transport delay waveforms,
we get the result shown in Figure 2-5. The same waveform is input to
signal A, but the output from signal B is quite different. With transport
delay, the spikes are not swallowed, but the events are ordered before
22
Chapter Two
A
B
Figure 2-5
Transport Delay
Buffer Waveforms.
Delay = 20 ns
A
B
0
10
20
30
40
propagation.
At time 10 nanoseconds, the buffer model is executed and schedules an
event for the output to go to a 1 value at 30 nanoseconds. At time 20
nanoseconds, the buffer model is re-invoked and predicts a new value for
the output at time 40 nanoseconds. With the transport delay algorithm,
the events are put in order. The event for time 40 nanoseconds is put in
the list of events after the event for time 30 nanoseconds. The spike is not
swallowed but is propagated intact after the delay time of the device.
Inertial Delay Model
The following model shows how to write an inertial delay model. It is
the same as any other model we have been looking at. The default delay
type is inertial; therefore, it is not necessary to specify the delay type to
be inertial:
LIBRARY IEEE;
USE IEEE.std_logic_1164.ALL;
ENTITY buf IS
PORT ( a : IN std_logic;
PORT ( b : OUT std_logic);
END buf;
Behavioral Modeling
23
ARCHITECTURE buf OF buf IS
BEGIN
b <= a AFTER 20 ns;
END buf;
Transport Delay Model
Following is an example of a transport delay model. It is similar in every
respect to the inertial delay model except for the keyword TRANSPORT in
the signal assignment statement to signal b. When this keyword exists,
the delay type used in the statement is the transport delay mechanism:
LIBRARY IEEE;
USE IEEE.std_logic_1164.ALL;
ENTITY delay_line IS
PORT ( a : IN std_logic;
PORT ( b : OUT std_logic);
END delay_line;
ARCHITECTURE delay_line OF delay_line IS
BEGIN
b <= TRANSPORT a AFTER 20 ns;
END delay_line;
Simulation Deltas
Simulation deltas are used to order some types of events during a simulation. Specifically, zero delay events must be ordered to produce consistent results. If zero delay events are not properly ordered, results can
be disparate between different simulation runs. An example of this is
shown using the circuit shown in Figure 2-6. This circuit could be part of
a clocking scheme in a complex device being modeled. It probably would
not be the entire circuit, but only a part of the circuit used to generate
the clock to the D flip-flop.
The circuit consists of an inverter, a NAND gate, and an AND gate
driving the clock input of a flip-flop component. The NAND gate and AND
gate are used to gate the clock input to the flip-flop.
Let’s examine the circuit operation, using a delta delay mechanism and
another mechanism. By examining the two delay mechanisms, we will
better understand how a delta delay orders events.
24
Chapter Two
A
Figure 2-6
Simulation Delta
Circuit.
B
Clock
E
DFF
D
CLK
Q
F
QB
D
C
To use delta delay, all of the circuit components must have zero delay
specified. The delay for all three gates is specified as zero. (Real circuits
do not exhibit such characteristics, but sometimes modeling is easier if
all of the delay is concentrated at the outputs.) Let’s examine the nondelta delay mechanism first.
When a falling edge occurs on signal A, the output of the inverter
changes in 0 time. Let’s assume that such an event occurs at time 10
nanoseconds. The output of the inverter, signal B, changes to reflect the
new input value. When signal B changes, both the AND gate and the
NAND gate are reevaluated. For this example, the clock input is assumed
to be a constant value ‘1’. If the NAND gate is evaluated first, its new
value is ‘0’.
When the AND gate evaluates, signal B is a ‘0’, and signal C is a ‘1’;
therefore, the AND gate predicts a new value of ‘0’. But what happens
if the AND gate evaluates first? The AND gate sees a ‘1’ value on signal
B, and a ‘1’ value on signal C before the NAND gate has a chance to
reevaluate. The AND gate predicts a new value of ‘1’.
Behavioral Modeling
Figure 2-7
Comparison of Two
Evaluation Mechanisms.
25
AND First
NAND First
evaluate inverter
evaluate inverter
B <= 1
B <= 1
evaluate AND (C = 1)
evaluate NAND
D <= 1
C <= 0
evaluate NAND
evaluate AND
C <= 0
D <= 0
evaluate AND
D <= 0
The NAND gate reevaluates and calculates its new value as ‘0’. The
change on the output of the NAND gate causes the AND gate to reevaluate
again. The AND gate now sees the value of B, a ‘1’ value, and the new
value of signal C, a ‘0’ value. The AND gate now predicts a ‘0’ on its
output. This process is summarized in Figure 2-7.
Both circuits arrive at the same value for signal D. However, when the
AND gate is evaluated first, a rising edge, one delta delay wide, occurs on
signal D. This rising edge can clock the flip-flop, depending on how the
flip-flop is modeled.
The point of this discussion is that without a delta synchronization
mechanism, the results of the simulation can depend on how the simulator
data structures are built. For instance, compiling the circuit the first time
might make the AND gate evaluate first, while compiling again might
make the NAND gate evaluate first—clearly not desirable results; simulation deltas prevent this behavior from occurring.
The same circuit evaluated using the VHDL delta delay mechanism
would evaluate as shown in Figure 2-8.
The evaluation of the circuit does not depend on the order of evaluation of the NAND gate or AND gate. The sequence in Figure 2-8 occurs
irrespective of the evaluation order of the AND or NAND gate.
During the first delta time point of time 10 nanoseconds, signal A receives
the value ‘0’. This causes the inverter to reevaluate with the new value.
26
Figure 2-8
Delta Delay Evaluation Mechanism.
Chapter Two
Time
10 ns
Delta
Activity
(1)
A <= 0
evaluate inverter
(2)
B <= 1
evaluate AND
evaluate NAND
(3)
D <= 1
C <= 0
evaluate AND
(4)
D <= 0
11 ns
The inverter calculates the new value for signal B, which is the value ‘1’.
This value is not propagated immediately, but is scheduled for the next
delta time point (delta 2).
The simulator then begins execution of delta time point 2. Signal B is
updated to a ‘1’ value, and the AND gate and NAND gate are reevaluated.
Both the AND gate and NAND gate now schedule their new values for
the next delta time point (delta 3).
When delta 3 occurs, signal D receives a ‘1’ value, and signal C receives
a ‘0’ value. Because signal C also drives the AND gate, the AND gate is
reevaluated and schedules its new output for delta time point 4.
To summarize, simulation deltas are an infinitesimal amount of time
used as a synchronization mechanism when 0 delay events are present.
Delta delay is used whenever 0 delay is specified, as shown in the following:
a <= b AFTER 0 ns;
Another case for using delta delay is when no delay is specified. For
example:
a <= b;
In both cases, whenever signal b changes value from an event, signal
a has a delta-delayed signal assignment to it.
An equivalent VHDL model of the circuit shown in Figure 2-6, except
for the flip-flop, is shown in the following:
Behavioral Modeling
27
ENTITY reg IS
PORT( a, clock : in bit
PORT( d : out bit);
END reg;
ARCHITECTURE test OF reg IS
SIGNAL b, c : bit;
BEGIN
b <= NOT(a); -- notice no delay
c <= NOT( clock AND b);
d <= c AND b;
END test;
Drivers
VHDL has a unique way of handling multiply driven signals. Multiply
driven signals are very useful for modeling a data bus, a bidirectional bus,
and so on. Correctly modeling these kinds of circuits in VHDL requires
the concept of signal drivers. A VHDL driver is one contributor to the
overall value of a signal.
A multiply driven signal has many drivers. The values of all of the
drivers are resolved together to create a single value for the signal.
The method of resolving all of the contributors into a single value is
through a resolution function (resolution functions are discussed in Chapter
5, “Subprograms and Packages”). A resolution function is a designerwritten function that is called whenever a driver of a signal changes value.
Driver Creation
Drivers are created by signal assignment statements. A concurrent signal
assignment inside of an architecture produces one driver for each signal assignment. Therefore, multiple signal assignments produce multiple
drivers for a signal. Consider the following architecture:
ARCHITECTURE test OF test IS
BEGIN
a <= b AFTER 10 ns;
a <= c AFTER 10 ns;
END test;
Signal a is being driven from two sources, b and c. Each concurrent
28
Chapter Two
signal assignment statement creates a driver for signal a. The first statement creates a driver that contains the value of signal b delayed by 10
nanoseconds. The second statement creates a driver that contains the
value of signal c delayed by 10 nanoseconds. How these two drivers are
resolved is left to the designer. The designers of VHDL did not want to
arbitrarily add language constraints to signal behavior. Synthesizing the
preceding example would short c and b together.
Bad Multiple Driver Model
Let’s look at a model that looks correct at first glance, but does not function
as the user intended. The model is for the 4 to 1 multiplexer discussed
earlier:
USE WORK.std_logic_1164.ALL;
ENTITY mux IS
PORT (i0, i1, i2, i3, a, b: IN std_logic;
PORT (q : OUT std_logic);
END mux;
ARCHITECTURE bad
BEGIN
q <= i0 WHEN a
q <= i1 WHEN a
q <= i2 WHEN a
q <= i3 WHEN a
END BAD;
OF mux IS
=
=
=
=
‘0’
‘1’
‘0’
‘1’
AND
AND
AND
AND
b
b
b
b
=
=
=
=
‘0’
‘0’
‘1’
‘1’
ELSE
ELSE
ELSE
ELSE
‘0’;
‘0’;
‘0’;
‘0’;
This model assigns i0 to q when a is equal to a 0 and b is equal to a 0;
i1 when a is equal to a 1 and b is equal to a 0; and so on. At first glance,
the model looks like it works. However, each assignment to signal q creates
a new driver for signal q. Four drivers to signal q are created by this model.
Each driver drives either the value of one of the i0, i1, i2, i3 inputs
or ‘0’. The value driven is dependent on inputs a and b. If a is equal to
‘0’ and b is equal to ‘0’, the first assignment statement puts the value
of i0 into one of the drivers of q. The other three assignment statements
do not have their conditions met and, therefore, are driving the value ‘0’.
Three drivers are driving the value ‘0’, and one driver is driving the value
of i0. Typical resolution functions would have a difficult time predicting
the desired output on q, which is the value of i0.
A better way to write this model is to create only one driver for signal
q, as shown in the following:
Behavioral Modeling
29
ARCHITECTURE better OF mux IS
BEGIN
q <= i0 WHEN a = ‘0’ AND b = ‘0’ ELSE
i1 WHEN a = ‘1’ AND b = ‘0’ ELSE
i2 WHEN a = ‘0’ AND b = ‘1’ ELSE
i3 WHEN a = ‘1’ AND b = ‘1’ ELSE
‘X’;
--- unknown
END better;
Generics
Generics are a general mechanism used to pass information to an instance
of an entity. The information passed to the entity can be of most types
allowed in VHDL. (Types are covered in detail later in Chapter 4, “Data
Types.”)
Why would a designer want to pass information to an entity? The
most obvious, and probably most used, information passed to an entity is
delay times for rising and falling delays of the device being modeled.
Generics can also be used to pass any user-defined data types, including
information such as load capacitance, resistance, and so on. For synthesis
parameters such as datapath widths, signal widths, and so on, can be
passed in as generics.
All of the data passed to an entity is instance-specific information. The
data values pertain to the instance being passed the data. In this way, the
designer can pass different values to different instances in the design.
The data passed to an instance is static data. After the model has been
elaborated (linked into the simulator), the data does not change during
simulation. Generics cannot be assigned information as part of a simulation run. The information contained in generics passed into a component
instance or a block can be used to alter the simulation results, but results
cannot modify the generics.
The following is an example of an entity for an AND gate that has three
generics associated with it:
ENTITY and2 IS
GENERIC(rise, fall : TIME; load : INTEGER);
PORT( a, b : IN BIT;
PORT( c : OUT BIT);
END AND2;
This entity allows the designer to pass in a value for the rise and fall
30
Chapter Two
delays, as well as the loading that the device has on its output. With this
information, the model can correctly model the AND gate in the design.
Following is the architecture for the AND gate:
ARCHITECTURE load_dependent OF and2 IS
SIGNAL internal : BIT;
BEGIN
internal <= a AND b;
c <= internal AFTER (rise + (load * 2 ns)) WHEN internal = ‘1’
ELSE internal AFTER (fall + (load * 3 ns));
END load_dependent;
The architecture declares a local signal called internal to store the
value of the expression a and b. Pre-computing values used in multiple
instances is a very efficient method for modeling.
The generics rise, fall, and load contain the values that were
passed in by the component instantiation statement. Let’s look at a
piece of a model that instantiates the components of type AND2 in another model:
LIBRARY IEEE;
USE IEEE.std_logic_1164.ALL;
ENTITY test IS
GENERIC(rise, fall : TIME; load : INTEGER);
PORT ( ina, inb, inc, ind : IN std_logic;
PORT ( out1, out2 : OUT std_logic);
END test;
ARCHITECTURE test_arch OF test IS
COMPONENT AND2
GENERIC(rise, fall : TIME; load : INTEGER);
PORT ( a, b : IN std_logic;
PORT ( c : OUT std_logic);
END COMPONENT;
BEGIN
U1: AND2 GENERIC MAP(10 ns, 12 ns, 3 )
PORT MAP (ina, inb, out1 );
U2: AND2 GENERIC MAP(9 ns, 11 ns, 5 )
PORT MAP (inc, ind, out2 );
END test_arch;
The architecture statement first declares any components that will be
used in the model. In this example, component AND2 is declared. Next, the
body of the architecture statement contains two of the component instantiation statements for components U1 and U2. Port a of component U1 is
mapped to signal ina, port b is mapped to signal inb, and port c is mapped
Behavioral Modeling
31
to out1. In the same way, component U2 is mapped to signals inc, ind,
and out2.
Generic rise of instance U1 is mapped to 10 nanoseconds, generic
fall is mapped to 12 nanoseconds, and generic load is mapped to 3. The
generics for component U2 are mapped to values 9 and 11 nanoseconds
and value 5.
Generics can also have default values that are overridden if actual
values are mapped to the generics. The next example shows two instances
of component type AND2.
In instance U1, actual values are mapped to the generics, and these
values are used in the simulation. In instance U2, no values are mapped
to the instance, and the default values are used to control the behavior of
the simulation if specified; otherwise an error occurs:
LIBRARY IEEE;
USE IEEE.std_logic_1164.ALL;
ENTITY test IS
GENERIC(rise, fall : TIME;
GENERIC(load : INTEGER);
PORT ( ina, inb, inc, ind : IN std_logic;
PORT ( out1, out2 : OUT std_logic);
END test;
ARCHITECTURE test_arch OF test IS
COMPONENT and2
GENERIC(rise, fall : TIME := 10 NS;
GENERIC(load : INTEGER := 0);
PORT ( a, b : IN std_logic;
PORT ( c : OUT std_logic);
END COMPONENT;
BEGIN
U1: and2 GENERIC MAP(10 ns, 12 ns, 3 )
PORT MAP (ina, inb, out1 );
U2: and2 PORT MAP (inc, ind, out2 );
END test_arch;
As we have seen, generics have many uses. The uses of generics are
limited only by the creativity of the model writer.
Block Statements
Blocks are a partitioning mechanism within VHDL that allow the designer
32
Chapter Two
to logically group areas of the model. The analogy with a typical Schematic
Entry system is a schematic sheet. In a typical Schematic Entry system,
a level or a portion of the design can be represented by a number of
schematic sheets. The reason for partitioning the design may relate to
C design standards about how many components are allowed on a sheet,
or it may be a logical grouping that the designer finds more understandable.
The same analogy holds true for block statements. The statement area
in an architecture can be broken into a number of separate logical areas.
For instance, if you are designing a CPU, one block might be an ALU,
another a register bank, and another a shifter.
Each block represents a self-contained area of the model. Each block
can declare local signals, types, constants, and so on. Any object that can
be declared in the architecture declaration section can be declared in the
block declaration section. Following is an example:
LIBRARY IEEE;
USE IEEE.std_logic_1164.ALL;
PACKAGE bit32 IS
TYPE tw32 IS ARRAY(31 DOWNTO 0) OF std_logic;
END bit32;
LIBRARY IEEE;
USE IEEE.std_logic_1164.ALL;
USE WORK.bit32.ALL;
ENTITY cpu IS
PORT( clk, interrupt : IN std_logic;
PORT( addr : OUT tw32; data : INOUT tw32 );
END cpu;
ARCHITECTURE cpu_blk OF cpu IS
SIGNAL ibus, dbus : tw32;
BEGIN
ALU : BLOCK
SIGNAL qbus : tw32;
BEGIN
-- alu behavior statements
END BLOCK ALU;
REG8 : BLOCK
SIGNAL zbus : tw32;
BEGIN
REG1: BLOCK
SIGNAL qbus : tw32;
BEGIN
-- reg1 behavioral statements
END BLOCK REG1;
-- more REG8 statements
Behavioral Modeling
33
END BLOCK REG8;
END cpu_blk;
Entity cpu is the outermost entity declaration of this model. (This is
not a complete model, only a subset.) Entity cpu declares four ports that
are used as the model interface. Ports clk and interrupt are input ports,
addr is an output port, and data is an inout port. All of these ports are
visible to any block declared in an architecture for this entity. The input
ports can be read from and the output ports can be assigned values.
Signals ibus and dbus are local signals declared in architecture
cpu_blk. These signals are local to architecture cpu_blk and cannot be
referenced outside of the architecture. However, any block inside of the
architecture can reference these signals. Any lower-level block can reference signals from a level above, but upper-level blocks cannot reference
lower-level local signals.
Signal qbus is declared in the block declaration section of block ALU.
This signal is local to block ALU and cannot be referenced outside of the
block. All of the statements inside of block ALU can reference qbus, but
statements outside of block ALU cannot use qbus.
In exactly the same fashion, signal zbus is local to block REG8. Block
REG1 inside of block REG8 has access to signal zbus, and all of the other
statements in block REG8 also have access to signal zbus.
In the declaration section for block REG1, another signal called qbus is
declared. This signal has the same name as the signal qbus declared in
block ALU. Doesn’t this cause a problem? To the compiler, these two signals
are separate, and this is a legal, although confusing, use of the language.
The two signals are declared in two separate declarative regions and are
valid only in those regions; therefore, they are considered to be two separate signals with the same name. Each qbus can be referenced only in
the block that has the declaration of the signal, except as a fully qualified
name, discussed later in this section.
Another interesting case is shown here:
BLK1 : BLOCK
SIGNAL qbus : tw32;
BEGIN
BLK2 : BLOCK
SIGNAL qbus : tw32;
BEGIN
-- blk2 statements
END BLOCK BLK2;
-- blk1 statements
34
Chapter Two
END BLOCK BLK1;
In this example, signal qbus is declared in two blocks. The interesting
feature of this model is that one of the blocks is contained in the other. It
would seem that BLK2 has access to two signals called qbus —the first from
the local declaration of qbus in the declaration section of BLK2 and the
second from the declaration section of BLK1. BLK1 is also the parent block
of BLK2. However, BLK2 sees only the qbus signal from the declaration in
BLK2. The qbus signal from BLK1 has been overridden by a declaration of the
same name in BLK2.
The qbus signal from BLK1 can be seen inside of BLK2, if the name of
signal qbus is qualified with the block name. For instance, in this example,
to reference signal qbus from BLK1, use BLK1.qbus.
In general, this can be a very confusing method of modeling. The
problem stems from the fact that you are never quite sure which qbus is
being referenced at a given time without fully analyzing all of the declarations carefully.
As mentioned earlier, blocks are self-contained regions of the model.
But blocks are unique because a block can contain ports and generics.
This allows the designer to remap signals and generics external to the
block to signals and generics inside the block. But why, as designers,
would we want to do that?
The capability of ports and generics on blocks allows the designer to
reuse blocks written for another purpose in a new design. For instance,
let’s assume that you are upgrading a CPU design and need extra functionality in the ALU section. Let’s also assume that another designer has
a new ALU model that performs the functionality needed. The only trouble with the new ALU model is that the interface port names and generic
names are different than the names that exist in the design being upgraded.
With the port and generic mapping capability within blocks, this is no
problem. Map the signal names and the generic parameters in the design
being upgraded to ports and generics created for the new ALU block.
Following is an example illustrating this:
PACKAGE math IS
TYPE tw32 IS ARRAY(31 DOWNTO 0) OF std_logic;
FUNCTION tw_add(a, b : tw32) RETURN tw32;
FUNCTION tw_sub(a, b : tw32) RETURN tw32;
END math;
Behavioral Modeling
35
USE WORK.math.ALL;
LIBRARY IEEE;
USE IEEE.std_logic_1164.ALL;
ENTITY cpu IS
PORT( clk, interrupt : IN std_logic;
PORT( addr : OUT tw32; cont : IN INTEGER;
PORT( data : INOUT tw32 );
END cpu;
ARCHITECTURE cpu_blk OF cpu IS
SIGNAL ibus, dbus : tw32;
BEGIN
ALU : BLOCK
PORT( abus, bbus : IN tw32;
PORT( d_out : OUT tw32;
PORT( ctbus : IN INTEGER);
PORT MAP ( abus => ibus, bbus => dbus, d_out => data,
PORT MAP ( ctbus => cont);
SIGNAL qbus : tw32;
BEGIN
d_out <= tw_add(abus, bbus)
WHEN ctbus = 0 ELSE
d_out <= tw_sub(abus, bbus)
WHEN ctbus = 1 ELSE
d_out <= abus;
END BLOCK ALU;
END cpu_blk;
Basically, this is the same model shown earlier except for the port and
port map statements in the ALU block declaration section. The port statement declares the number of ports used for the block, the direction of the
ports, and the type of the ports. The port map statement maps the new
ports with signals or ports that exist outside of the block. Port abus is
mapped to architecture CPU_BLK local signal ibus; port bbus is mapped to
dbus. Ports d_out and ctbus are mapped to external ports of the entity.
Mapping implies a connection between the port and the external signal
such that, whenever there is a change in value on the signal connected to
a port, the port value changes to the new value. If a change occurs in the
signal ibus, the new value of ibus is passed into the ALU block and port
abus obtains the new value. The same is true for all ports.
Guarded Blocks
Block statements have another interesting behavior known as guarded
blocks. A guarded block contains a guard expression that can enable and
disable drivers inside the block. The guard expression is a boolean expression: when true, drivers contained in the block are enabled, and when
false, the drivers are disabled. Let’s look at the following example to show
36
Chapter Two
some more of the details:
LIBRARY IEEE;
USE IEEE.std_logic_1164.ALL;
ENTITY latch IS
PORT( d, clk : IN std_logic;
q, qb : OUT std_logic);
END latch;
ARCHITECTURE latch_guard OF latch IS
BEGIN
G1 : BLOCK( clk = ‘1’)
BEGIN
q <= GUARDED d AFTER 5 ns;
qb <= GUARDED NOT(d) AFTER 7 ns;
END BLOCK G1;
END latch_guard;
This model illustrates how a latch model could be written using a
guarded block. This is a very simple-minded model; however, more complex
and more accurate models will be shown later. The entity declares the four
ports needed for the latch, and the architecture has only one statement in
it. The statement is a guarded block statement. A guarded block statement
looks like a typical block statement, except for the guard expression after
the keyword BLOCK. The guard expression in this example is (clk = ‘1’).
This is a boolean expression that returns TRUE when clk is equal to a ‘1’
value and FALSE when clk is equal to any other value.
When the guard expression is true, all of the drivers of guarded signal
assignment statements are enabled, or turned on. When the guard
expression is false, all of the drivers of guarded signal assignment statements are disabled, or turned off. There are two guarded signal assignment
statements in this model: One is the statement that assigns a value to q
and the other is the statement that assigns a value to qb. A guarded signal
assignment statement is recognized by the keyword GUARDED between the
<= and the expression part of the statement.
When port clk of the entity has the value ‘1’, the guard expression is
true, and the value of input d is scheduled on the q output after 5 nanoseconds, and the NOT value of d is scheduled on the qb output after 7
nanoseconds. When port clk has the value ‘0’ or any other legal value
of the type, outputs q and qb turn off and the output value of the signal
is determined by the default value assigned by the resolution function.
When clk is not equal to ‘1’, the drivers created by the signal assignments
for q and qb in this architecture are effectively turned off. The drivers do
not contribute to the overall value of the signal.
Signal assignments can be guarded by using the keyword GUARDED. A
Behavioral Modeling
37
new signal is implicitly declared in the block whenever a block has a guard
expression. This signal is called GUARD. Its value is the value of the guard
expression. This signal can be used to trigger other processes to occur.
Blocks are useful for partitioning the design into smaller, more manageable units. They allow the designer the flexibility to create large
designs from smaller building blocks and provide a convenient method of
controlling the drivers on a signal.
SUMMARY
In the first chapter, concepts of structurally building models were discussed.
This chapter is the first of many that discusses behavioral modeling. In this
chapter, we discussed:
■ How signal assignments are the most basic form of behavioral
modeling
■ Signal assignment statements can be selected or conditional
■ Signal assignment statements can contain delays
■ VHDL contains inertial delay and transport delay
■ Simulation delta time points are used to order events in time
■ Drivers on a signal are created by signal assignment statements
■ Generics are used to pass data to entities
■ Block statements allow grouping within an entity
■ Guarded block statements allow the capability of turning off
drivers within a block
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CHAPTER
3
Sequential
Processing
In Chapter 2, we examined behavioral modeling using
concurrent statements. We discussed concurrent signal
assignment statements, as well as block statements and
component instantiation. In this chapter, we focus on
sequential statements. Sequential statements are statements that execute serially one after the other. Most
programming languages, such as C and C++, support this
type of behavior. In fact, VHDL has borrowed the syntax
for its sequential statements from ADA.
40
Chapter Three
Process Statement
In an architecture for an entity, all statements are concurrent. So where
do sequential statements exist in VHDL? There is a statement called
the process statement that contains only sequential statements. The
process statement is itself a concurrent statement. A process statement
can exist in an architecture and define regions in the architecture
where all statements are sequential.
A process statement has a declaration section and a statement part. In
the declaration section, types, variables, constants, subprograms, and so on
can be declared. The statement part contains only sequential statements.
Sequential statements consist of CASE statements, IF THEN ELSE statements, LOOP statements, and so on. We examine these statements later in
this chapter. First, let’s look at how a process statement is structured.
Sensitivity List
The process statement can have an explicit sensitivity list. This list defines
the signals that cause the statements inside the process statement to
execute whenever one or more elements of the list change value. The sensitivity list is a list of the signals that will cause the process to execute.
The process has to have an explicit sensitivity list or, as we discuss later,
a WAIT statement.
As of this writing, synthesis tools have a difficult time with sensitivity
lists that are not fully specified. Synthesis tools think of process statements as either describing sequential logic or combinational logic. If a
process contains a partial sensitivity list, one that does not contain every
input signal used in the process, there is no way to map that functionality
to either sequential or combinational logic.
Process Example
Let’s look at an example of a process statement in an architecture to see
how the process statement fits into the big picture, and discuss some more
details of how it works. Following is a model of a two-input NAND gate:
LIBRARY IEEE;
USE IEEE.std_logic_1164.ALL;
ENTITY nand2 IS
Sequential Processing
41
PORT( a, b : IN std_logic;
PORT( c : OUT std_logic);
END nand2;
ARCHITECTURE nand2 OF nand2 IS
BEGIN
PROCESS( a, b )
VARIABLE temp : std_logic;
BEGIN
temp := NOT (a and b);
IF (temp = ‘1’) THEN
c <= temp AFTER 6 ns;
ELSIF (temp = ‘0’) THEN
c <= temp AFTER 5 ns;
ELSE
c <= temp AFTER 6 ns;
END IF;
END PROCESS;
END nand2;
This example shows how to write a model for a simple two-input NAND
gate using a process statement. The USE statement declares a VHDL package that provides the necessary information to allow modeling this NAND
gate with 9 state logic. (This package is described in Appendix A, “Standard Logic Package.”) We discuss packages later in Chapter 5, “Subprograms and Packages.” The USE statement was included so that the model
could be simulated with a VHDL simulator without any modifications.
The entity declares three ports for the nand2 gate. Ports a and b are the
inputs to the nand2 gate and port c is the output. The name of the architecture is the same name as the entity name. This is legal and can save
some of the headaches of trying to generate unique names.
The architecture contains only one statement, a concurrent process
statement. The process declarative part starts at the keyword PROCESS
and ends at the keyword BEGIN. The process statement part starts at the
keyword BEGIN and ends at the keywords END PROCESS. The process declaration section declares a local variable named temp. The process statement part has two sequential statements in it; a variable assignment
statement:
temp := NOT (a AND b);
and an IF THEN ELSE statement:
IF (temp = ‘1’) THEN
42
Chapter Three
c <= temp
ELSIF (temp
c <= temp
ELSE
c <= temp
END IF;
AFTER 6 ns;
= ‘0’) THEN
AFTER 5 ns;
AFTER 6 ns;
The process contains an explicit sensitivity list with two signals contained in it:
PROCESS( a, b )
The process is sensitive to signals a and b. In this example, a and b are
input ports to the model. Input ports create signals that can be used as
inputs; output ports create signals that can be used as outputs; and inout
ports create signals that can be used as both. Whenever port a or b has a
change in value, the statements inside of the process are executed. Each
statement is executed in serial order starting with the statement at the
top of the process statement and working down to the bottom. After all of
the statements have been executed once, the process waits for another
change in a signal or port in its sensitivity list.
The process declarative part declares one variable called temp. Its type
is std_logic. This type is explained in Appendix A, “Standard Logic
Package,” as it is used throughout the book. For now, assume that the type
defines a signal that is a single bit and can assume the values 0, 1, and
X. Variable temp is used as temporary storage in this model to save the precomputed value of the expression (a and b). The value of this expression is
precomputed for efficiency.
Signal Assignment Versus
Variable Assignment
The first statement inside of the process statement is a variable assignment that assigns a value to variable temp. In the previous chapter, we
discussed how signals received values that were scheduled either after
an amount of time or after a delta delay. A variable assignment happens
immediately when the statement is executed. For instance, in this
model, the first statement has to assign a value to variable temp for the
second statement to use. Variable assignment has no delay; it happens
immediately.
Sequential Processing
43
MUX4
Figure 3-1
Four Input Mux Symbol and Function.
I0
I1
Q
I2
I3
A B
A
B
Q
0
0
I0
1
0
I1
0
1
I2
1
1
I3
Let’s look at two examples that illustrate this point more clearly. Both
examples are models of a 4 to 1 multiplexer device. The symbol and truth
table for this device are shown in Figure 3-1. One of the four input signals
is propagated to the output depending on the values of inputs A and B.
The first model for the multiplexer is an incorrect model, and the second
is a corrected version of the model.
Incorrect Mux Example
The incorrect model of the multiplexer has a flaw in it that causes the
model to produce incorrect results. This is shown by the following model:
LIBRARY IEEE;
USE IEEE.std_logic_1164.ALL;
ENTITY mux IS
PORT (i0, i1, i2, i3, a, b : IN std_logic;
44
Chapter Three
q : OUT std_logic);
END mux;
ARCHITECTURE wrong of mux IS
SIGNAL muxval : INTEGER;
BEGIN
PROCESS ( i0, i1, i2, i3, a, b )
BEGIN
muxval <= 0;
IF (a = ‘1’) THEN
muxval <= muxval + 1;
END IF;
IF (b = ‘1’) THEN
muxval <= muxval + 2;
END IF;
CASE muxval IS
WHEN 0 =>
q <= I0 AFTER
WHEN 1 =>
q <= I1 AFTER
WHEN 2 =>
q <= I2 AFTER
WHEN 3 =>
q <= I3 AFTER
WHEN OTHERS =>
NULL;
END CASE;
END PROCESS;
END wrong;
10 ns;
10 ns;
10 ns;
10 ns;
Whenever one of the input signals in the process sensitivity list changes
value, the sequential statements in the process are executed. The process
statement in the first example contains four sequential statements. The first
statement initializes the local signal muxval to a known value (0). The subsequent statements add values to the local signal depending on the value
of the a and b input signals. Finally, the case statement chooses an input
to propagate to the output based on the value of signal muxval. This model
has a significant flaw, however. The first statement:
muxval <= 0;
causes the value 0 to be scheduled as an event for signal muxval. In fact,
the value 0 is scheduled in an event for the next simulation delta because
no delay was specified. When the second statement:
IF (a = ‘1’) THEN
muxval <= muxval + 1;
END IF;
Sequential Processing
45
is executed, the value of signal muxval is whatever was last propagated
to it. The new value scheduled from the first statement has not propagated yet. In fact, when multiple assignments to a signal occur within the
same process statement, the last assigned value is the value propagated.
The signal muxval has a garbage value when entering the process. Its
value is not changed until the process has completed execution of all
sequential statements contained in the process. In fact, if signal b is a ‘1’
value, then whatever garbage value the signal had when entering the
process will have the value 2 added to it.
A better way to implement this example is shown in the next example.
The only difference between the next model and the previous one is the
declaration of muxval and the assignments to muxval. In the previous
model, muxval was a signal, and signal assignment statements were used
to assign values to it. In the next example, muxval is a variable, and
variable assignments are used to assign to it.
Correct Mux Example
In this example, the incorrect model is rewritten to reflect a solution to
the problems with the last model:
LIBRARY IEEE;
USE IEEE.std_logic_1164ALL;
ENTITY mux IS
PORT (i0, i1, i2, i3, a, b : IN std_logic;
PORT (q : OUT std_logic);
END mux;
ARCHITECTURE better OF mux IS
BEGIN
PROCESS ( i0, i1, i2, i3, a, b )
VARIABLE muxval : INTEGER;
BEGIN
muxval := 0;
IF (a = ‘1’) THEN
muxval := muxval + 1;
END IF;
IF (b = ‘1’) THEN
muxval := muxval + 2;
END IF;
CASE muxval IS
WHEN 0 =>
q <= I0 AFTER 10 ns;
WHEN 1 =>
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Chapter Three
q <= I1 AFTER 10 ns;
WHEN 2 =>
q <= I2 AFTER 10 ns;
WHEN 3 =>
q <= I3 AFTER 10 ns;
WHEN OTHERS =>
NULL;
END CASE;
END PROCESS;
END better;
This simple coding difference makes a tremendous operational difference.
When the first statement:
muxval := 0;
is executed, the value 0 is placed in variable muxval immediately. The
value is not scheduled because muxval, in this example, is a variable, not
a signal. Variables represent local storage as opposed to signals, which
represent circuit interconnect. The local storage is updated immediately,
and the new value can be used later in the model for further computations.
Because muxval is initialized to 0 immediately, the next two statements
in the process use 0 as the initial value and add appropriate numbers,
depending on the values of signals a and b. These assignments are also
immediate, and therefore when the CASE statement executes, variable
muxval contains the correct value. From this value, the correct input signal
can be propagated to the output.
Sequential Statements
Sequential statements exist inside the boundaries of a process statement
as well as in subprograms. In this chapter, we are most concerned with
sequential statements inside process statements. In Chapter 5, we discuss
subprograms and the statements contained within them.
The sequential statements that we discuss are:
■ IF
■ CASE
■ LOOP
■ EXIT
■ ASSERT
■ WAIT
Sequential Processing
47
IF Statements
In Appendix A of the VHDL Language Reference Manual, all VHDL constructs are described using a variant of the Bachus-Naur format (BNF)
that is used to describe typical programming languages. If you are not
familiar with BNF, Appendix C gives a cursory description. Becoming
familiar with the BNF will help you better understand how to construct
complex VHDL statements.
The BNF description of the IF statement looks like this:
if_statement ::=
IF condition THEN
sequence_of_statements
{ELSIF condition THEN
sequence_of_statements}
[ELSE
sequence_of_statements]
END IF;
From the BNF description, we can conclude that the IF statement
starts with the keyword IF and ends with the keywords END IF spelled
out as two separate words. There are also two optional clauses: the ELSIF
clause and the ELSE clause. The ELSIF clause is repeatable—more than
one ELSIF clause is allowed; but the ELSE clause is optional, and only
one is allowed. The condition construct in all cases is a boolean expression. This is an expression that evaluates to either true or false. Whenever a condition evaluates to a true value, the sequence of statements
following is executed. If no condition is true, then the sequence of statements for the ELSE clause is executed, if one exists. Let’s analyze a few
examples to get a better understanding of how the BNF relates to the
VHDL code.
The first example shows how to write a simple IF statement:
IF (x < 10) THEN
a := b;
END IF;
The IF statement starts with the keyword IF. Next is the condition
(x < 10), followed by the keyword THEN. The condition is true when the
value of x is less than 10; otherwise it is false. When the condition is true,
the statements between the THEN and END IF are executed. In this example, the assignment statement (a := b) is executed whenever x is less than
10. What happens if x is greater than or equal to 10? In this example, there
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Chapter Three
is no ELSE clause, so no statements are executed in the IF statement. Instead, control is transferred to the statement after the END IF.
Let’s look at another example where the ELSE clause is useful:
IF (day = sunday) THEN
weekend := TRUE;
ELSIF (day = saturday) THEN
weekend := TRUE;
ELSE
weekday := TRUE;
END IF;
In this example, there are two variables— weekend and weekday —that
are set depending on the value of a signal called day. Variable weekend is
set to TRUE whenever day is equal to saturday or sunday. Otherwise, variable weekday is set to TRUE. The execution of the IF statement starts by
checking to see if variable day is equal to sunday. If this is true, then the
next statement is executed and control is transferred to the statement
following END IF. Otherwise, control is transferred to the ELSIF statement
part, and day is checked for saturday. If variable day is equal to saturday,
then the next statement is executed and control is again transferred to the
statement following the END IF statement. Finally, if day is not equal to
sunday or saturday, then the ELSE statement part is executed.
The IF statement can have multiple ELSIF statement parts, but only
one ELSE statement part. More than one sequential statement can exist
between each statement part.
CASE Statements
The CASE statement is used whenever a single expression value can be
used to select between a number of actions. Following is the BNF for the
CASE statement:
case_statement ::=
CASE expression IS
case_statement_alternative
{case_statement_alternative}
END CASE;
case_statement_alternative ::=
WHEN choices =>
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49
sequence_of_statements
sequence_of_statements ::=
{sequential_statement}
choices ::=
choice{| choice}
choice ::=
SIMPLE_expression|
discrete_range|
ELEMENT_simple_name|
OTHERS
A CASE statement consists of the keyword CASE followed by an expression
and the keyword IS. The expression either returns a value that matches one
of the CHOICES in a WHEN statement part, or matches an OTHERS clause. If the
expression matches the CHOICE part of a WHEN choices => clause, the
sequence_of_statements following is executed. After these statements are
executed, control is transferred to the statement following the END CASE
clause.
Either the CHOICES clause must enumerate every possible value of
the type returned by the expression, or the last choice must contain an
OTHERS clause.
Let’s look at some examples to reinforce what the BNF states:
CASE instruction IS
WHEN load_accum =>
accum <= data;
WHEN store_accum =>
data_out <= accum;
WHEN load|store =>
process_IO(addr);
WHEN OTHERS =>
process_error(instruction);
END CASE;
The CASE statement executes the proper statement depending on the
value of input instruction. If the value of instruction is one of the choices
listed in the WHEN clauses, then the statement following the WHEN clause
is executed. Otherwise, the statement following the OTHERS clause is executed. In this example, when the value of instruction is load_accum, the
first assignment statement is executed. If the value of instruction is load
or store, the process_IO procedure is called.
If the value of instruction is outside the range of the choices given, then
the OTHERS clause matches the expression and the statement following the
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Chapter Three
OTHERS clause is executed. It is an error if an OTHERS clause does not ex-
ist, and the choices given do not cover every possible value of the expression
type.
In the next example, a more complex type is returned by the expression.
(Types are discussed in Chapter 4, “Data Types.”) The CASE statement
uses this type to select among the choices of the statement:
TYPE vectype IS ARRAY(0 TO 1) OF BIT;
VARIABLE bit_vec : vectype;
.
.
CASE bit_vec IS
WHEN “00” =>
RETURN 0;
WHEN “01” =>
RETURN 1;
WHEN “10” =>
RETURN 2;
WHEN “11” =>
RETURN 3;
END CASE;
This example shows one way to convert an array of bits into an integer.
When both bits of variable bit_vec contain ‘0’ values, the first choice
“00” matches and the value 0 is returned. When both bits are ‘1’ values,
the value 3, or “11”, is returned. This CASE statement does not need an
OTHERS clause because all possible values of variable bit_vec are enumerated by the choices.
LOOP Statements
The LOOP statement is used whenever an operation needs to be repeated.
LOOP statements are used when powerful iteration capability is needed to
implement a model. Following is the BNF for the LOOP statement:
loop_statement ::=
[LOOP_label : ] [iteration_scheme] LOOP
sequence_of_statements
END LOOP[LOOP_label];
iteration_scheme ::=
WHILE condition | FOR LOOP_parameter_specification
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51
LOOP_parameter_specification ::=
identifier IN discrete_range
The LOOP statement has an optional label, which can be used to
identify the LOOP statement. The LOOP statement has an optional
iteration_scheme that determines which kind of LOOP statement is being
used. The iteration_scheme includes two types of LOOP statements: a
WHILE
condition LOOP statement and a “FOR identifier IN
discrete_range” statement. The FOR loop loops as many times as specified
in the discrete_range, unless the loop is exited. The WHILE condition
LOOP statement loops as long as the condition expression is TRUE.
Let’s look at a couple of examples to see how these statements work:
WHILE (day = weekday) LOOP
day := get_next_day(day);
END LOOP;
This example uses the WHILE condition form of the LOOP statement.
The condition is checked each time before the loop is executed. If the condition is TRUE, the LOOP statements are executed. Control is then transferred
back to the beginning of the loop. The condition is checked again. If TRUE,
the loop is executed again; if not, statement execution continues on the
statement following the END LOOP clause.
The other version of the LOOP statement is the FOR loop:
FOR i IN 1 to 10 LOOP
i_squared(i) := i * i;
END LOOP;
This loop executes 10 times whenever execution begins. Its function is
to calculate the squares from 1 to 10 and insert them into the i_squared
signal array. The index variable i starts at the leftmost value of the range
and is incremented until the rightmost value of the range.
In some languages, the loop index (in this example, i) can be assigned
a value inside the loop to change its value. VHDL does not allow any
assignment to the loop index. This also precludes the loop index existing
as the return value of a function, or as an out or inout parameter of a
procedure.
Another interesting point about FOR LOOP statements is that the index
value i is locally declared by the FOR statement. The variable i does not
need to be declared explicitly in the process, function, or procedure. By
virtue of the FOR LOOP statement, the loop index is declared locally. If
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Chapter Three
another variable of the same name exists in the process, function, or
procedure, then these two variables are treated as separate variables
and are accessed by context. Let’s look at an example to illustrate this
point:
PROCESS(i)
BEGIN
x <= i + 1; -- x is a signal
FOR i IN 1 to a/2 LOOP
q(i) := a; -- q is a variable
END LOOP;
END PROCESS;
Whenever the value of the signal i in the process sensitivity list
changes value, the process will be invoked. The first statement schedules
the value i + 1 on the signal x. Next, the FOR loop is executed. The index
value i is not the same object as the signal i that was used to calculate
the new value for signal x. These are separate objects that are each
accessed by context. Inside the FOR loop, when a reference is made to i,
the local index is retrieved. But outside the FOR loop, when a reference is
made to i, the value of the signal i in the sensitivity list of the process
is retrieved.
The values used to specify the range in the FOR loop need not be specific
integer values, as has been shown in the examples. The range can
be any discrete range. A discrete_range can be expressed as a
subtype_indication or a range statement. Let’s look at a few more examples of how FOR loops can be constructed with ranges:
PROCESS(clk)
TYPE day_of_week IS (sun, mon, tue, wed, thur, fri,
sat);
BEGIN
FOR i IN day_of_week LOOP
IF i = sat THEN
son <= mow_lawn;
ELSIF i = sun THEN
church <= family;
ELSE
dad <= go_to_work;
END IF;
END LOOP;
END PROCESS;
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53
In this example, the range is specified by the type. By specifying the
type as the range, the compiler determines that the leftmost value is sun,
and the rightmost value is sat. The range then is determined as from
sun to sat.
If an ascending range is desired, use the to clause. The downto clause
can be used to create a descending range. Here is an example:
PROCESS(x, y)
BEGIN
FOR i IN x downto y LOOP
q(i) := w(i);
END LOOP;
END PROCESS;
When different values for x and y are passed in, different ranges of the
array w are copied to the same place in array q.
NEXT Statement
There are cases when it is necessary to stop executing the statements in
the loop for this iteration and go to the next iteration. VHDL includes a
construct that accomplishes this. The NEXT statement allows the designer
to stop processing this iteration and skip to the successor. When the NEXT
statement is executed, processing of the model stops at the current point
and is transferred to the beginning of the LOOP statement. Execution begins
with the first statement in the loop, but the loop variable is incremented
to the next iteration value. If the iteration limit has been reached, processing stops. If not, execution continues.
Following is an example showing this behavior:
PROCESS(A, B)
CONSTANT max_limit : INTEGER := 255;
BEGIN
FOR i IN 0 TO max_limit LOOP
IF (done(i) = TRUE) THEN
NEXT;
ELSE
done(i) := TRUE;
END IF;
q(i) <= a(i) AND b(i);
END LOOP;
END PROCESS;
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Chapter Three
The process statement contains one LOOP statement. This LOOP statement logically “and”s the bits of arrays a and b and puts the results in
array q. This behavior continues whenever the flag in array done is not
true. If the done flag is already set for this value of index i, then the NEXT
statement is executed. Execution continues with the first statement of the
loop, and index i has the value i + 1. If the value of the done array is
not true, then the NEXT statement is not executed, and execution continues
with the statement contained in the ELSE clause for the IF statement.
The NEXT statement allows the designer the ability to stop execution of
this iteration and go on to the next iteration. There are other cases when
the need exists to stop execution of a loop completely. This capability is
provided with the EXIT statement.
EXIT Statement
During the execution of a LOOP statement, it may be necessary to jump
out of the loop. This can occur because a significant error has occurred
during the execution of the model or all of the processing has finished
early. The VHDL EXIT statement allows the designer to exit or jump out
of a LOOP statement currently in execution. The EXIT statement causes
execution to halt at the location of the EXIT statement. Execution continues at the statement following the LOOP statement.
Here is an example illustrating this point:
PROCESS(a)
variable int_a : integer;
BEGIN
int_a := a;
FOR i IN 0 TO max_limit LOOP
IF (int_a <= 0) THEN -- less than or
EXIT;
-- equal to
ELSE
int_a := int_a -1;
q(i) <= 3.1416 / REAL(int_a * i); -- signal
END IF;
-- assign
END LOOP;
y <= q;
END PROCESS;
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55
Inside this process statement, the value of int_a is always assumed to
be a positive value greater than 0. If the value of int_a is negative or zero,
then an error condition results and the calculation should not be completed. If the value of int_a is less than or equal to 0, then the IF statement is true and the EXIT statement is executed. The loop is immediately
terminated, and the next statement executed is the assignment statement
to y after the LOOP statement.
If this were a complete example, the designer would also want to alert
the user of the model that a significant error had occurred. A method to
accomplish this function would be with an ASSERT statement, which is discussed later in this chapter.
The EXIT statement has three basic types of operations. The first involves
an EXIT statement without a loop label, or a WHEN condition. If these
conditions are true, then the EXIT statement behaves as follows.
The EXIT statement only exits from the most current LOOP statement
encountered. If an EXIT statement is inside a LOOP statement that is
nested inside another LOOP statement, the EXIT statement only exits the
inner LOOP statement. Execution still remains in the outer LOOP statement. The exit statement only exits from the most recent LOOP statement.
This case is shown in the previous example.
If the EXIT statement has an optional loop label, then the EXIT statement, when encountered, completes the execution of the loop specified by
the loop label. Therefore, the next statement executed is the one following
the END LOOP of the labeled loop. Here is an example:
PROCESS(a)
BEGIN
first_loop: FOR i IN 0 TO 100 LOOP
second_loop:FOR j IN 1 TO 10 LOOP
. . . . . .
EXIT second_loop; -- exits the second loop only
. . . . . .
EXIT first_loop; -- exits the first loop and second
EXIT first_loop; -- loop
END LOOP;
END LOOP;
END PROCESS;
The first EXIT statement only exits the innermost loop because it completes execution of the loop labeled second_loop. The last EXIT statement
completes execution of the loop labeled first_loop, which exits from the
first loop and the second loop.
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Chapter Three
If the EXIT statement has an optional WHEN condition, then the EXIT
statement only exits the loop if the condition specified is true. The next
statement executed depends on whether the EXIT statement has a loop
label specified or not. If a loop label is specified, the next statement executed
is contained in the LOOP statement specified by the loop label. If no loop
label is present, the next statement executed is in the next outer loop. Here
is an example of an EXIT statement with a WHEN condition:
EXIT first_loop WHEN (i < 10);
This statement completes the execution of the loop labeled first_loop
when the expression i < 10 is true.
The EXIT statement provides a quick and easy method of exiting a
LOOP statement when all processing is finished or an error or warning
condition occurs.
ASSERT Statement
The ASSERT statement is a very useful statement for reporting textual
strings to the designer. The ASSERT statement checks the value of a
boolean expression for true or false. If the value is true, the statement
does nothing. If the value is false, the ASSERT statement outputs a userspecified text string to the standard output to the terminal.
The designer can also specify a severity level with which to output the
text string. The four levels are, in increasing level of severity, note, warning, error, and failure. The severity level gives the designer the ability to
classify messages into proper categories.
The note category is useful for relaying information to the user about
what is currently happening in the model. For instance, if the model had
a giant loop that took a long time to execute, an assertion of severity level
note could be used to notify the designer when the loop was 10 percent
complete, 20 percent complete, 30 percent complete, and so on.
Assertions of category warning can be used to alert the designer of conditions that, although not catastrophic, can cause erroneous behavior
later. For instance, if a model expected a signal to be at a known value while
some process was executing, but the signal was at a different value, it may
not be an error as in the EXIT statement example, but a warning to the
user that results may not be as expected.
Sequential Processing
57
Assertions of severity level error are used to alert the designer of conditions that will cause the model to work incorrectly, or not work at all. If
the result of a calculation was supposed to return a positive value, but instead returned a negative value, depending on the operation, this could
be considered an error.
Assertions of severity level failure are used to alert the designer of conditions within the model that can have disastrous effects. An example of
such a condition was discussed in the EXIT statement section. Division
by 0 is an example of an operation that could cause a failure in the
model. Another is addressing beyond the end of an array. In both cases,
the severity level failure can let the designer know that the model is
behaving incorrectly.
The severity level is a good method for classifying assertions into informational messages to the designer that can describe conditions during
execution of the model.
The ASSERT statement is currently ignored by synthesis tools. Because
the ASSERT statement is used mainly for exception handling while writing a model, no hardware is built.
Assertion BNF
Following is the BNF description for the ASSERT statement:
assert_statement ::=
ASSERT condition
[REPORT expression]
[SEVERITY expression];
The keyword ASSERT is followed by a boolean-valued expression called
a condition. The condition determines whether the text expression specified
by the REPORT clause is output or not. If false, the text expression is output;
if true, the text expression is not output.
There are two optional clauses in the ASSERT statement. The first is the
REPORT clause. The REPORT clause gives the designer the ability to specify
the value of a text expression to output. The second is the SEVERITY clause.
The SEVERITY clause allows the designer to specify the severity level of
the ASSERT statement. If the REPORT clause is not specified, the default
value for the ASSERT statement is assertion violation. If the SEVERITY
clause is not specified, the default value is error.
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Chapter Three
Let’s look at a practical example of an ASSERT statement to illustrate
how it works. The example performs a data setup check between two
signals that control a D flip-flop. Most flip-flops require the din (data)
input to be at a stable value a certain amount of time before a clock edge
appears. This time is called the setup time and guarantees that the din
value will be clocked into the flip-flop if the setup time is met. This is
shown in the following model. The assertion example issues an error
message to the designer if the setup time is violated (assertion is false):
PROCESS(clk, din)
VARIABLE last_d_change : TIME := 0 ns;
VARIABLE last_d_value
: std_logic := ‘X’;
VARIABLE last_clk_value : std_logic := ‘X’;
BEGIN
IF (last_d_value /= din) THEN — /= is
last_d_change := NOW;
— not equal
last_d_value := din;
END IF;
IF (last_clk_value /= clk) THEN
last_clk_value := clk;
IF
(clk = ‘1’) THEN
ASSERT (NOW - last_d_change >= 20 ns)
REPORT “setup violation”
SEVERITY WARNING;
END IF;
END IF;
END PROCESS;
The process makes use of three local variables to record the time and
last value of signal din as well as the value of the clk signal. By storing
the last value of clk and din, we can determine if the signal has changed
value or not. By recording the last time that din changed, we can measure
from the current time to the last din transition to see if the setup time
has been violated or not. (An easier method using attributes is shown in
Chapter 5, “Subprograms and Packages.”)
Whenever din or clk changes, the process is invoked. The first step in
the process is to see if the din signal has changed. If it has, the time of
the transition is recorded using the predefined function NOW. This function
returns the current simulation time. Also, the latest value of din is stored
for future checking.
The next step is to see if signal clk has made a transition. If the
last_clk_value variable is not equal to the current value of clk, then
we know that a transition has occurred. If signal clk is a ‘1’ value,
Sequential Processing
59
then we know that a rising edge has occurred. Whenever a rising edge
occurs on signal clk, we need to check the setup time for a violation. If
the last transition on signal d was less than 20 nanoseconds ago, then
the expression:
(NOW - last_D_change)
returns a value that is less than 20 nanoseconds. The ASSERT statement
triggers and reports the assertion message setup violation as a warning
to the designer. If the last transition on signal d occurred more than 20
nanoseconds in the past, then the expression returns a value larger than
20 nanoseconds and the ASSERT statement does not write out the message.
Remember, the ASSERT statement writes out the message when the assert
condition is false.
The message reported to the user has, at a minimum, the user string
and the error classification. Some simulators also include the time of the
assertion report as well as the line number in the file of the assertion.
The ASSERT statement used in this example was a sequential ASSERT
statement, because it was included inside a PROCESS statement. A concurrent version of the ASSERT statement also exists. It has exactly the
same format as the sequential ASSERT statement and only exists outside
a PROCESS statement or subprogram.
The concurrent ASSERT statement executes whenever any signals that
exist inside of the condition expression have an event upon them. This is
as opposed to the sequential ASSERT statement in which execution occurs
when the sequential ASSERT statement is reached inside the PROCESS
statement or subprogram.
WAIT Statements
The WAIT statement gives the designer the ability to suspend the sequential execution of a process or subprogram. The conditions for resuming execution of the suspended process or subprogram can be specified by the
following three different means:
■ WAIT ON signal changes
■ WAIT UNTIL an expression is true
■ WAIT FOR a specific amount of time
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Chapter Three
WAIT statements can be used for a number of different purposes. The
most common use today is for specifying clock inputs to synthesis tools.
The WAIT statement specifies the clock for a process statement that is read
by synthesis tools to create sequential logic such as registers and flip-flops.
Other uses are to delay process execution for an amount of time or to
modify the sensitivity list of the process dynamically.
Let’s take a look at a process statement with an embedded WAIT statement that is used to generate sequential logic:
PROCESS
BEGIN
WAIT UNTIL clock = ‘1’ AND clock’EVENT;
q <= d;
END PROCESS;
This process is used to generate a flip-flop that clocks the value of d into
q when the clock input has a rising edge. The attribute ‘EVENT attached to
input clock is true whenever the clock input has had an event during the
current delta timepoint. (‘EVENT is discussed in great detail in Chapter 5.)
The combination of looking for a ‘1’ value and a change on clock creates
the necessary functionality to look for a rising edge on input clock. The
effect is that the process is held at the WAIT statement until the clock has
a rising edge. Then the current value of d is assigned to q.
Reading this description into a synthesis tool creates a D flip-flop
without a set or reset input. A synchronous reset can be created by
the following:
PROCESS
BEGIN
WAIT UNTIL clock = ‘1’ AND clock’EVENT;
IF (reset = ‘1’) THEN
q <= ‘0’;
ELSE
q <= d;
END IF;
END PROCESS;
When the clock occurs, the reset signal is tested first. If it is active, then
the reset value (‘0’) is assigned to q; otherwise, the d input is assigned.
Finally, an asynchronous reset can be added as follows:
PROCESS
BEGIN
IF (reset = ‘1’) THEN
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61
q <= ‘0’;
ELSIF clock’EVENT AND clock = ‘1’ THEN
q <= d;
END IF;
WAIT ON reset, clock;
END PROCESS;
This process statement contains a WAIT ON statement that causes the
process to halt execution until an event occurs on either reset or clock.
The IF statement is then executed and, if reset is active, the flip-flop is
asynchronously reset; otherwise, the clock is checked for a rising edge
with which to transfer the d input to the q output of the flip-flop.
A WAIT statement can also be used to control the signals a process or subprogram is sensitive to at any point in the execution. Here is an example:
PROCESS
BEGIN
WAIT ON a; -- 1.
.
.
WAIT ON b; -- 2.
.
.
END PROCESS;
Execution of the statements in the PROCESS statement proceeds until
point 1 in the VHDL fragment shown in the preceding. The WAIT statement causes the process to halt execution at that point. The process does
not continue execution until an event occurs on signal a. The process is
therefore sensitive to changes in signal a at this point in the execution.
When an event occurs on signal a, execution starts again at the statement
directly after the WAIT statement at point 1. Execution proceeds until the
WAIT statement at point 2 is encountered. Once again, execution is halted,
and the process is now sensitive to events on signal b. Therefore, by
adding in two WAIT statements, we can alter the process sensitivity list
dynamically.
Next, let’s discuss the three different options available to the WAIT
statement:
■ WAIT ON signal [,signal]
■ WAIT UNTIL boolean_expression
■ WAIT FOR time_expression
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Chapter Three
WAIT ON Signal
We have already seen an example of the first type in the previous process
example. The WAIT ON signal clause specifies a list of one or more signals
that the WAIT statement will wait for events upon. If any signal in the
signal list has an event occur on it, execution continues with the statement
following the WAIT statement. Here is an example:
WAIT ON a, b;
When an event occurs on either a or b, the process resumes with the
statement following the WAIT statement.
WAIT UNTIL Expression
The WAIT UNTIL boolean_expression clause suspends execution of the
process until the expression returns a value of true. This statement effectively creates an implicit sensitivity list of the signals used in the expression. When any of the signals in the expression have events occur upon
them, the expression is evaluated. The expression must return a boolean
type or the compiler complains. When the expression returns a true
value, execution continues with the statement following the WAIT statement. Otherwise, the process continues to be suspended. For example:
WAIT UNTIL (( x * 10 ) < 100 );
In this example, as long as the value of signal x is greater than or equal
to 10, the WAIT statement suspends the process or subprogram. When the
value of x is less than 10, execution continues with the statement following
the WAIT statement.
WAIT FOR time_expression
The WAIT FOR time_expression clause suspends execution of the
process for the time specified by the time expression. After the time
specified in the time expression has elapsed, execution continues on the
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63
statement following the WAIT statement. A couple of examples are
shown here:
WAIT FOR 10 ns;
WAIT FOR ( a * ( b + c ));
In the first example, the time expression is a simple constant value.
The WAIT statement suspends execution for 10 nanoseconds. After 10
nanoseconds has elapsed, execution continues with the statement following
the WAIT statement.
In the second example, the time expression is an expression that first
must be evaluated to return a time value. After this value is calculated,
the WAIT statement uses this value as the time value to wait for.
Multiple WAIT Conditions
The WAIT statement examples we have examined so far have shown the different options of the WAIT statement used separately. The different options
can be used together. A single statement can include an ON signal, UNTIL
expression, and FOR time_expression clauses. Following is an example:
WAIT ON nmi,interrupt UNTIL ((nmi = TRUE) or
(interrupt = TRUE)) FOR 5 usec;
This statement waits for an event on signals nmi and interrupt and
continues only if interrupt or nmi is true at the time of the event, or until
5 microseconds of time has elapsed. Only when one or more of these
conditions are true does execution continue.
When using a statement such as this:
WAIT UNTIL (interrupt = TRUE) OR ( old_clk = ‘1’);
be sure to have at least one of the values in the expression contain a signal.
This is necessary to ensure that the WAIT statement does not wait forever.
If both interrupt and old_clk are variables, the WAIT statement does not
reevaluate when these two variables change value. (In fact, the variables
cannot change value because they are declared in the suspended process.)
Only signals have events on them, and only signals can cause a WAIT
statement or concurrent signal assignment to reevaluate.
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Chapter Three
WAIT Time-Out
There are instances while designing a model when you are not sure that a
condition will be met. To prevent the WAIT statement from waiting forever, add a time-out clause. The time-out clause allows execution to
proceed whether or not the condition has been met. Be careful, though,
because this method can cause erroneous behavior unless properly handled.
The following example shows this problem:
ARCHITECTURE wait_example of wait_example IS
SIGNAL sendB, sendA : std_logic;
BEGIN
sendA <= ‘0’;
A : PROCESS
BEGIN
WAIT UNTIL sendB = ‘1’;
sendA <= ‘1’ AFTER 10 ns;
WAIT UNTIL sendB = ‘0’;
sendA <= ‘0’ AFTER 10 ns;
END PROCESS A;
B : PROCESS
BEGIN
WAIT UNTIL sendA = ‘0’;
sendB <= ‘0’ AFTER 10 ns;
WAIT UNTIL sendA = ‘1’;
sendB <= ‘1’ AFTER 10 ns;
END PROCESS B;
END wait_example;
This architecture has two processes that communicate through two
signals, sendA and sendB. This example does not do anything real but is
a simple illustration of how WAIT statements can wait forever, a condition
commonly referred to as deadlock.
During simulator initialization, all processes are executed exactly once.
This allows the processes to always start at a known execution point at the
start of simulation. In this example, the process labeled A executes at
startup and stops at the following line:
WAIT UNTIL sendB = 1;
The process labeled B also executes at startup. Execution starts at the
first line of the process and continues until this line:
Sequential Processing
65
WAIT UNTIL sendA = 1;
Execution stops at the first WAIT statement of the process even though
the expression sendA = 0 is satisfied by the first signal assignment of
signal sendA. This is because the WAIT statement needs an event to occur
on signal sendA to cause the expression to be evaluated. Both processes
are now waiting on each other. Neither process can continue because they
are both waiting for a signal set by the other process. If a time-out interval is inserted on each WAIT statement, execution can be allowed to continue. There is one catch to this last statement. Execution continues when
the condition is not met. An ASSERT statement can be added to check for
continuation of the process without the condition being met. The following
example shows the architecture wait_example rewritten to include timeout clauses:
ARCHITECTURE wait_timeout OF wait_example IS
SIGNAL sendA, sendB : std_logic;
BEGIN
A : PROCESS
BEGIN
WAIT UNTIL (sendB = ‘1’) FOR 1 us;
ASSERT (sendB = ‘1’)
REPORT “sendB timed out at ‘1’”
SEVERITY ERROR;
sendA <= ‘1’ AFTER 10 ns;
WAIT UNTIL (sendB = ‘0’) FOR 1 us;
ASSERT (sendB = ‘0’)
REPORT “sendB timed out at ‘0’”
SEVERITY ERROR;
sendA <= ‘0’ AFTER 10 ns;
END PROCESS A;
B : PROCESS
BEGIN
WAIT UNTIL (sendA = ‘0’) FOR 1 us;
ASSERT (sendA = ‘0’)
REPORT “sendA timed out at ‘0’”
SEVERITY ERROR;
sendB <= ‘0’ AFTER 10 ns;
WAIT UNTIL (sendA = ‘1’) FOR 1 us;
ASSERT (sendA = ‘1’)
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Chapter Three
REPORT “sendA timed out at ‘1’”
SEVERITY ERROR;
sendB <= ‘1’ AFTER 10 ns;
end PROCESS B;
END wait_timeout;
Each of the WAIT statements now has a time-out expression specified as
1 usec. However, if the time out does happen, the ASSERT statement reports
an error that the WAIT statement in question has timed out.
Sensitivity List Versus WAIT Statement
A process with a sensitivity list is an implicit WAIT ON the signals in the
sensitivity list. This is shown by the following example:
PROCESS (clk)
VARIABLE last_clk : std_logic := ‘X’;
BEGIN
IF (clk /= last_clk ) AND (clk = ‘1’) THEN
q <= din AFTER 25 ns;
END IF;
last_clk := clk;
END PROCESS;
This example can be rewritten using a WAIT statement:
PROCESS
VARIABLE last_clk : std_logic := ‘X’;
BEGIN
IF (clk /= last_clk ) AND (clk = ‘1’) THEN
q <= din AFTER 25 ns;
END IF;
last_clk := clk;
WAIT ON clk;
END PROCESS;
The WAIT statement at the end of the process is equivalent to the sensitivity list at the beginning of the process. But why is the WAIT statement
at the end of the process and not at the beginning? During initialization
of the simulator, all processes are executed once. To mimic the behavior
of the sensitivity list, the WAIT statement must be at the end of the process
to allow the PROCESS statement to execute once.
Sequential Processing
67
Concurrent Assignment Problem
One of the problems that most designers using sequential signal assignment
statements encounter is that the value assigned in the last statement
does not appear immediately. This can cause erroneous behavior in the
model if the designer is depending on the new value. An example of this
problem is shown here:
LIBRARY IEEE;
USE IEEE.std_logic_1164ALL;
ENTITY mux IS
PORT (I0, I1, I2, I3, A, B : IN std_logic;
PORT (Q : OUT std_logic);
END mux;
ARCHITECTURE mux_behave OF mux IS
SIGNAL sel : INTEGER RANGE 0 TO 3;
BEGIN
B : PROCESS(A, B, I0, I1, I2, I3)
BEGIN
sel <= 0;
IF (A = ‘1’) THEN sel <= sel + 1; END IF;
IF (B = ‘1’) THEN sel <= sel + 2; END IF;
CASE sel IS
WHEN 0 =>
Q <= I0;
WHEN 1 =>
Q <= I1;
WHEN 2 =>
Q <= I2;
WHEN 3 =>
Q <= I3;
END CASE;
END PROCESS;
END mux_behave;
This model is for a 4 to 1 multiplexer. Depending on the values of A and
B, one of the four inputs (I0 to I3) is transferred to output Q.
The architecture starts processing by initializing internal signal sel to
the value 0. Then, based on the values of A and B, the values 1 or 2 are
added to sel to select the correct input. Finally, a CASE statement selected
by the value of sel transfers the value of the input to output Q.
This architecture does not work as presently implemented. The value
of signal sel will never be initialized by the first line in the architecture:
sel <= 0;
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Chapter Three
This statement inside of a process statement schedules an event for
signal sel on the next delta time point, with the value 0. However, processing continues in the process statement with the next sequential statement. The value of sel remains at whatever value it had at the entry to
the process. Only when the process has completed is this current delta
finished and the next delta time point started. Only then is the new value
of sel reflected. By this time, however, the rest of the process has already
been processed using the wrong value of sel.
There are two ways to fix this problem. The first is to insert WAIT statements after each sequential signal assignment statement as shown here:
ARCHITECTURE mux_fix1 OF mux IS
SIGNAL sel : INTEGER RANGE 0 TO 3;
BEGIN
PROCESS
BEGIN
sel <= 0;
WAIT FOR 0 ns; -- or wait on sel
IF (a = ‘1’) THEN sel <= sel + 1; END IF;
WAIT for 0 ns;
IF (b = ‘1’) THEN sel <= sel + 2; END IF;
WAIT FOR 0 ns;
CASE sel IS
WHEN 0 =>
Q <= I0;
WHEN 1 =>
Q <= I1;
WHEN 2 =>
Q <= I2;
WHEN 3 =>
Q <= I3;
END CASE;
WAIT ON A, B, I0, I1, I2, I3;
END PROCESS;
END mux_fix1;
The WAIT statements after each signal assignment cause the process to
wait for one delta time point before continuing with the execution. By
waiting for one delta time point, the new value has a chance to propagate.
Therefore, when execution continues after the WAIT statement, signal sel
has the new value.
One consequence of the WAIT statements, however, is that the process can
no longer have a sensitivity list. A process with WAIT statements contained
within it or within a subprogram called from within the process cannot
Sequential Processing
69
have a sensitivity list. A sensitivity list implies that execution starts from
the beginning of the procedure, while a WAIT statement allows suspending
a process at a particular point. The two are mutually exclusive.
Because the process can no longer have a sensitivity list, a WAIT statement has been added to the end of the process that exactly imitates the
behavior of the sensitivity list. This is the following statement:
WAIT ON A, B, I0, I1, I2, I3;
The WAIT statement proceeds whenever any of the signals on the right
side of the keyword ON have an event upon them.
This method of solving the sequential signal assignment problem causes
the process to work, but a better solution is to use an internal variable instead of the internal signal, as shown here:
ARCHITECTURE mux_fix2 OF mux IS
BEGIN
PROCESS(A, B, I0, I1, I2, I3)
VARIABLE sel : INTEGER RANGE 0 TO 3;
BEGIN
sel := 0;
IF (A = ‘1’) THEN sel := sel + 1; END IF;
IF (B = ‘1’) THEN sel := sel + 2; END IF;
CASE sel IS
WHEN 0 =>
Q <= I0;
WHEN 1 =>
Q <= I1;
WHEN 2 =>
Q <= I2;
WHEN 3 =>
Q <= I3;
END CASE;
END PROCESS;
END mux_fix2;
The signal sel from the preceding example has been converted from
an internal signal to an internal variable. This was accomplished by
moving the declaration from the architecture declaration section to the
process declaration section. Variables can only be declared in the process
or subprogram declaration section.
Also, the signal assignments to sel have been changed to variable
assignment statements. Now, when the first assignment to sel is executed, the value is updated immediately. Each successive assignment is
also executed immediately so that the correct value of sel is available in
each statement of the process.
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Chapter Three
Passive Processes
Passive processes are processes that exist in the entity statement part of an
entity. They are different from a normal process in that no signal assignment is allowed. These processes are used to do all sorts of checking
functions. For instance, one good use of a passive process is to check the
data setup time on a flip-flop.
The advantage of the passive process over the example discussed in the
ASSERT statement section is that, because the passive process exists in
the entity, it can be applied to any architecture of the entity. Take a look
at the following example:
LIBRARY IEEE;
USE IEEE.std_logic_1164ALL;
ENTITY dff IS
PORT( CLK, din : IN std_logic;
PORT( Q, QB : OUT std_logic);
BEGIN
PROCESS(CLK, din)
VARIABLE last_d_change : TIME := 0 ns;
VARIABLE last_clk, last_d_value : std_logic := ‘X’;
BEGIN
IF (din /= last_d_value) THEN
last_d_change := now;
last_d_value := din;
END IF;
IF (CLK /= last_clk) THEN
IF (CLK = ‘1’) THEN
ASSERT(now - last_d_change >= 15 ns)
REPORT “setup error”
SEVERITY ERROR;
END IF;
last_clk := CLK;
END IF;
END PROCESS;
END dff;
ARCHITECTURE behave OF dff IS
BEGIN
.
.
.
.
END behave;
ARCHITECTURE struct OF dff IS
BEGIN
.
Sequential Processing
71
.
.
.
END struct;
ARCHITECTURE switch OF dff IS
BEGIN
.
.
.
.
END switch;
This example shows the entity for a D flip-flop with a passive process
included in the entity that performs a data setup check with respect to the
clock. This setup check function was described in detail in the ASSERT statement description. What this example shows is that, when the setup check
function is contained in the entity statement part, each of the architectures
for the entity have the data setup check performed automatically. Without this functionality, each of the architectures would have to have the
setup check code included. This introduces more code to maintain and can
introduce inconsistencies between architectures.
The only restriction on these processes, as mentioned earlier, is that
no signal assignment is allowed in a passive process. In the preceding example, a process statement was used to illustrate a passive process. A passive process can also exist as a concurrent statement that does not do any
signal assignment. Examples of such statements are concurrent ASSERT
statements and concurrent subprogram invocations. An example of two
concurrent ASSERT statements as passive processes are shown here:
ENTITY adder IS
PORT( A, B : IN INTEGER;
PORT( X : OUT INTEGER);
BEGIN
ASSERT (A < 256)
REPORT “A out of range”
SEVERITY ERROR;
ASSERT (B < 256)
REPORT “B out of range”
SEVERITY ERROR;
END adder;
The first ASSERT statement checks to make sure that input A is not out
of range, and the second assertion checks that input B is not out of the
range of the adder. Each of these statements acts as an individual process
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Chapter Three
that is sensitive to the signal in its expression. For instance, the first assertion is sensitive to signal A because that signal is contained in its expression.
SUMMARY
In this chapter, we discussed the following:
■ How process statements are concurrent statements that delineate
areas of sequential statements.
■ How process statements can be used to control when a process is
activated.
■ How signal assignments are scheduled and variable assignments
happen immediately within a process statement.
■ How IF, CASE, and LOOP statements can be used to control the flow
of execution within a model.
■ How ASSERTION statements can be used to check for error conditions or report information to the user.
■ The three forms of the WAIT statement. How WAIT UNTIL is used
for specifying clocks for synthesis, and how WAIT ON can be used to
modify the sensitivity list.
■ How passive processes can be used to perform error checking and
other tasks across a number of architectures by existing in an
ENTITY statement.
The next chapter focuses on all of the different data types of VHDL that
can be used in models.
CHAPTER
4
Data Types
In this chapter, we examine the object types used in
VHDL. The types allowed in VHDL consist of everything
from scalar numeric types to composite arrays and
records to file types. The first step in looking at the varied VHDL types is to review the VHDL objects that can
attain the varied types. Then we use examples to show
how many types of descriptions can be made easier to
read by using the power of enumerated and composite
data types.
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Chapter Four
Object Types
A VHDL object consists of one of the following:
■ Signal, which represents interconnection wires that connect component instantiation ports together.
■ Variable, which is used for local storage of temporary data, visible
only inside a process.
■ Constant, which names specific values.
Signal
Signal objects are used to connect entities together to form models. Signals
are the means for communication of dynamic data between entities. A
signal declaration looks like this:
SIGNAL signal_name : signal_type [:= initial_value];
The keyword SIGNAL is followed by one or more signal names. Each
signal name creates a new signal. Separating the signal names from the
signal type is a colon. The signal type specifies the data type of the information that the signal contains. Finally, the signal can contain an initial
value specifier so that the signal value may be initialized.
Signals can be declared in entity declaration sections, architecture
declarations, and package declarations. Signals in package declarations
are also referred to as global signals because they can be shared among
entities.
Following is an example of signal declarations:
LIBRARY IEEE;
USE IEEE.std_logic_1164.ALL;
PACKAGE sigdecl IS
TYPE bus_type IS ARRAY(0 to 7) OF std_logic;
SIGNAL vcc
: std_logic := ‘1’;
SIGNAL ground : std_logic := ‘0’;
FUNCTION magic_function( a : IN bus_type) RETURN
bus_type;
END sigdecl;
USE WORK.sigdecl.ALL;
LIBRARY IEEE;
Data Types
75
USE IEEE.std_logic_1164.ALL;
ENTITY board_design is
PORT( data_in : IN bus_type;
PORT( data_out : OUT bus_type);
SIGNAL sys_clk : std_logic := ‘1’;
END board_design;
ARCHITECTURE data_flow OF board_design IS
SIGNAL int_bus : bus_type;
CONSTANT disconnect_value : bus_type
:= (‘X’, ‘X’, ‘X’, ‘X’, ‘X’, ‘X’, ‘X’, ‘X’);
BEGIN
int_bus <= data_in WHEN sys_clk = ‘1’
ELSE int_bus;
data_out <= magic_function(int_bus) WHEN sys_clk = ‘0’
ELSE disconnect_value;
sys_clk <= NOT(sys_clk) after 50 ns;
END data_flow;
Signals vcc and ground are declared in package sigdecl. Because
these signals are declared in a package, they can be referenced by more
than one entity and are therefore global signals. For an entity to reference these signals, the entity needs to use package sigdecl. To use the
package requires a VHDL USE clause, as shown here:
USE work.sigdecl.vcc;
USE work.sigdecl.ground;
Or:
USE work.sigdecl.ALL;
In the first example, the objects are included in the entity by specific
reference. In the second example, the entire package is included in the entity. In the second example, problems may arise because more than what
is absolutely necessary is included. If more than one object of the same
name results because of the USE clause, none of the objects is visible, and a
compile operation that references the object fails.
SIGNALS GLOBAL TO ENTITIES Inside the entity declaration
section for entity board_design is a signal called sys_clk. This signal can
be referenced in entity board_design and any architecture for entity
board_design. In this example, there is only one architecture, data_flow,
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Chapter Four
for board_design. The signal sys_clk can therefore be assigned to and
read from in entity board_design and architecture data_flow.
ARCHITECTURE
LOCAL
SIGNALS
Inside
of
architecture
data_flow is a signal declaration for signal int_bus. Signal int_bus is of
type bus_type, a type defined in package sigdecl. The sigdecl package is
used in entity board; therefore, the type bus_type is available in architecture data_flow. Because the signal is declared in the architecture declaration section, the signal can only be referenced in architecture data_flow
or in any process statements in the architecture.
Variables
Variables are used for local storage in process statements and subprograms.
(Subprograms are discussed in Chapter 6, “Predefined Attributes.”) As
opposed to signals, which have their values scheduled in the future, all
assignments to variables occur immediately. A variable declaration looks
like this:
VARIABLE variable_name {,variable_name} : variable_type[:=
value];
The keyword VARIABLE is followed by one or more variable names. Each
name creates a new variable. The construct variable_type defines the
data type of the variable, and an optional initial value can be specified.
Variables can be declared in the process declaration and subprogram
declaration sections only. An example using two variables is shown here:
LIBRARY IEEE;
USE IEEE.std_logic_1164.ALL;
ENTITY and5 IS
PORT ( a, b, c, d, e : IN std_logic;
PORT ( q : OUT std_logic);
END and5;
ARCHITECTURE and5 OF and5 IS
BEGIN
PROCESS(a, b, c, d, e)
VARIABLE state : std_logic;
VARIABLE delay : time;
BEGIN
state := a AND b AND c AND d AND e;
IF state = ‘1’ THEN
Data Types
77
delay := 4.5 ns;
ELSIF state = ‘0’ THEN
delay := 3 ns;
ELSE
delay := 4 ns;
END IF;
q <= state AFTER delay;
END PROCESS;
END and5;
This example is the architecture for a five-input AND gate. There are two
variable declarations in the process declaration section: one for variable
state and one for variable delay. Variable state is used as a temporary storage area to hold the value of the AND function of the inputs. Temporary-storage value delay is used to hold the delay value that will be
used when scheduling the output value. Both of these values cannot be static data because their values depend on the values of inputs a, b, c, d, and
e. Signals could have been used to store the data, but there are several reasons why a signal was not used:
■ Variables are inherently more efficient because assignments happen immediately, while signals must be scheduled to occur.
■ Variables take less memory, while signals need more information
to allow for scheduling and signal attributes.
■ Using a signal would have required a WAIT statement to synchronize
the signal assignment to the same execution iteration as the usage.
When any of the input signals a, b, c, d, or e change, the process is invoked. Variable state is assigned the AND of all of the inputs. Next, based
on the value of variable state, variable delay is assigned a delay value.
Based on the delay value assigned to variable delay, output signal q will
have the value of variable state assigned to it.
Constants
Constant objects are names assigned to specific values of a type. Constants
give the designer the ability to have a better-documented model, and a
model that is easy to update. For instance, if a model requires a fixed
value in a number of instances, a constant should be used. By using a
constant, the designer can change the value of the constant and recompile,
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Chapter Four
and all of the instances of the constant value are updated to reflect the
new value of the constant.
A constant also provides a better-documented model by providing more
meaning to the value being described. For instance, instead of using the
value 3.1414 directly in the model, the designer should create a constant
as in the following:
CONSTANT PI: REAL := 3.1414;
Even though the value is not going to change, the model becomes more
readable.
A constant declaration looks like this:
CONSTANT constant_name {,constant_name} : type_name[:=
value];
The value specification is optional, because VHDL also supports deferred
constants. These are constants declared in a package declaration whose
value is specified in a package body.
A constant has the same scoping rules as signals. A constant declared
in a package can be global if the package is used by a number of entities. A
constant in an entity declaration section can be referenced by any architecture of that entity. A constant in an architecture can be used by any
statement inside the architecture, including a process statement. A constant
declared in a process declaration can be used only in a process.
Data Types
All of the objects we have been discussing until now—the signal, the
variable, and the constant—can be declared using a type specification to
specify the characteristics of the object. VHDL contains a wide range of
types that can be used to create simple or complex objects.
To define a new type, you must create a type declaration. A type declaration defines the name of the type and the range of the type. Type
declarations are allowed in package declaration sections, entity declaration sections, architecture declaration sections, subprogram declaration
sections, and process declaration sections.
A type declaration looks like this:
TYPE type_name IS type_mark;
Data Types
79
A type_mark construct encompasses a wide range of methods for specifying a type. It can be anything from an enumeration of all of the values
of a type to a complex record structure. In the next few sections, type
marks are examined. All of the scoping rules that were defined for signals
and variables apply to type declarations also.
Figure 4-1 is a diagram showing the types available in VHDL. The four
broad categories are scalar types, composite types, access types, and file
types. Scalar types include all of the simple types such as integer and real.
Composite types include arrays and records. Access types are the equivalent of pointers in typical programming languages. Finally, file types give
the designer the ability to declare file objects with designer-defined file
types.
Scalar Types
Scalar types describe objects that can hold, at most, one value at a time.
The type itself can contain multiple values, but an object that is declared
Types
Figure 4-1
VHDL Data Types
Diagram.
File
Composite
Access
Record
Array
Scalar
Physical
Enumerated
Real
Integer
80
Chapter Four
to be a scalar type can hold, at most, one of the scalar values at any point
in time. Referencing the name of the object references the entire object.
Scalar types encompass these four classes of types:
■ Integer types
■ Real types
■ Enumerated types
■ Physical types
INTEGER TYPES are exactly like mathematical integers. All of the normal predefined mathematical functions like add, subtract, multiply, and divide apply to integer types. The VHDL LRM does not specify a maximum
range for integers, but does specify the minimum range: from -2,147,483,647
to 12,147,483,647. The minimum range is specified by the Standard
package contained in the Standard Library.
The Standard package defines all of the predefined VHDL types provided with the language. The Standard Library is used to hold any packages
or entities provided as standard with the language.
It may seem strange to some designers who are familiar with two’s
complement representations that the integer range is specified from
2,147,483,647 to 2,147,483,647 when two’s complement integer representations usually allow one smaller negative number, 2,147,483,648. The
language defines the integer range to be symmetric around 0.
Following are some examples of integer values:
ARCHITECTURE test OF test IS
BEGIN
PROCESS(X)
VARIABLE a : INTEGER;
VARIABLE b : int_type;
BEGIN
a := 1;
--Ok
1
a := -1;
--Ok
2
a := 1.0; --error 3
END PROCESS;
END test;
The first two statements (1 and 2) show examples of a positive integer
assignment and a negative integer assignment. Line 3 shows a noninteger assignment to an integer variable. This line causes the compiler
to issue an error message. Any numeric value with a decimal point is considered a real number value. Because VHDL is a strongly typed language,
Data Types
81
for the assignment to take place, either the base types must match or a
type-casting operation must be performed.
REAL TYPES Real types are used to declare objects that emulate
mathematical real numbers. They can be used to represent numbers out
of the range of integer values as well as fractional values. The minimum
range of real numbers is also specified by the Standard package in the
Standard Library, and is from 1.0E38 to 1.0E38. These numbers
are represented by the following notation:
or -number.number[E or -number]
Following are a few examples of some real numbers:
ARCHITECTURE test OF test IS
SIGNAL a : REAL;
BEGIN
a <= 1.0;
--Ok 1
a <= 1;
--error 2
a <= -1.0E10;
--Ok
3
a <= 1.5E-20;
--Ok
4
a <= 5.3 ns;
--error 5
END test;
Line 1 shows how to assign a real number to a signal of type REAL. All
real numbers have a decimal point to distinguish them from integer values.
Line 2 is an example of an assignment that does not work. Signal a is of
type REAL, and a real value must be assigned to signal a. The value 1 is
of type INTEGER, so a type mismatch is generated by this line.
Line 3 shows a very large negative number. The numeric characters to
the left of the character E represent the mantissa of the real number,
while the numeric value to the right represents the exponent.
Line 4 shows how to create a very small number. In this example, the
exponent is negative so the number is very small.
Line 5 shows how a type TIME cannot be assigned to a real signal. Even
though the numeric part of the value looks like a real number, because of
the units after the value, the value is considered to be of type TIME.
ENUMERATED TYPES An enumerated type is a very powerful tool
for abstract modeling. A designer can use an enumerated type to represent exactly the values required for a specific operation. All of the values
of an enumerated type are user-defined. These values can be identifiers
or single-character literals. An identifier is like a name. Examples are x,
abc, and black. Character literals are single characters enclosed in quotes,
such as ‘X’, ‘1’, and ‘0’.
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Chapter Four
A typical enumerated type for a four-state simulation value system looks
like this:
TYPE fourval IS ( ‘X’, ‘0’, ‘1’, ‘Z’ );
This type contains four character literal values that each represent
a unique state in the four-state value system. The values represent the
following conditions:
■ ‘X’ —An unknown value
■ ‘0’ —A logical 0 or false value
■ ‘1’ —A logical 1 or true value
■ ‘Z’ —A tristate or open collector value
Character literals are needed for values ‘1’ and ‘0’ to separate these
values from the integer values 1 and 0. It would be an error to use the values 1 and 0 in an enumerated type, because these are integer values. The
characters X and Z do not need quotes around them because they do not
represent any other type, but the quotes were used for uniformity.
Another example of an enumerated type is shown here:
TYPE color IS ( red, yellow, blue, green, orange );
In this example, the type values are very abstract—that is, not representing physical values that a signal might attain. The type values in type
color are also all identifiers. Each identifier represents a unique value of
the type; therefore, all identifiers of the type must be unique.
Each identifier in the type has a specific position in the type, determined
by the order in which the identifier appears in the type. The first identifier
has a position number of 0, the next a position number of 1, and so on.
(Chapter 5, “Subprograms and Packages” includes some examples using
position numbers of a type.)
A typical use for an enumerated type would be representing all of the
instructions for a microprocessor as an enumerated type. For instance, an
enumerated type for a very simple microprocessor could look like this:
TYPE instruction IS ( add, sub, lda, ldb, sta, stb, outa,
xfr );
The model that uses this type might look like this:
PACKAGE instr IS
TYPE instruction IS ( add, sub, lda, ldb, sta, stb,
outa, xfr );
Data Types
83
END instr;
USE WORK.instr.ALL;
ENTITY mp IS
PORT (instr : IN instruction;
PORT (addr : IN INTEGER;
PORT (data : INOUT INTEGER);
END mp;
ARCHITECTURE mp OF mp IS
BEGIN
PROCESS(instr)
TYPE regtype IS ARRAY(0 TO 255) OF INTEGER;
VARIABLE a, b : INTEGER;
VARIABLE reg : regtype;
BEGIN
--select instruction to
CASE instr is
--execute
WHEN lda =>
a := data;
--load a accumulator
WHEN ldb =>
b := data;
--load b accumulator
WHEN add =>
a := a 1 b;
--add accumulators
WHEN sub =>
a := a -b;
--subtract accumulators
WHEN sta =>
reg(addr) := a; --put a accum in reg array
WHEN stb =>
reg(addr) := b; --put b accum in reg array
WHEN outa =>
data <= a;
WHEN xfr =>
a := b;
--output a accum
--transfer b to a
END CASE;
END PROCESS;
END mp;
The model receives an instruction stream (instr), an address stream
(addr), and a data stream (data). Based on the value of the enumerated
value of instr, the appropriate instruction is executed. A CASE statement
is used to select the instruction to execute. The statement is executed and
the process then waits for the next instruction.
Another common example using enumerated types is a state machine.
State machines are commonly used in designing the control logic for ASIC
84
Chapter Four
or FPGA devices. They represent a very easy and understandable method
for specifying a sequence of actions over time, based on input signal values.
ENTITY traffic_light IS
PORT(sensor : IN std_logic;
PORT(clock : IN std_logic;
PORT(red_light : OUT std_logic;
PORT(green_light : OUT std_logic;
PORT(yellow_light : OUT std_logic);
END traffic_light;
ARCHITECTURE simple OF traffic_light IS
TYPE t_state is (red, green, yellow);
Signal present_state, next_state : t_state;
BEGIN
PROCESS(present_state, sensor)
BEGIN
CASE present_state IS
WHEN green =>
next_state <= yellow;
red_light <= ‘0’;
green_light <= ‘1’;
yellow_light <= ‘0’;
WHEN red =>
red_light <= ‘1’;
green_light <= ‘0’;
yellow_light <= ‘0’;
IF (sensor = ‘1’) THEN
next_state <= green;
ELSE
next_state <= red;
END IF;
WHEN yellow =>
red_light <= ‘0’;
green_light <= ‘0’;
yellow_light <= ‘1’;
next_state <= red;
END CASE;
END PROCESS;
PROCESS
BEGIN
WAIT UNTIL clock’EVENT and clock = ‘1’;
present_state <= next_state;
END PROCESS;
END simple;
The state machine is described by two processes: the first calculates the
next state logic, and the second latches the next state into the current
state. Notice how the enumerated type makes the model much more
readable because the state names represent the color of the light that is
currently being displayed.
Data Types
85
PHYSICAL TYPES Physical types are used to represent physical
quantities such as distance, current, time, and so on. A physical type provides for a base unit, and successive units are then defined in terms of this
unit. The smallest unit representable is one base unit; the largest is determined by the range specified in the physical type declaration. An example
of a physical type for the physical quantity current is shown here:
TYPE current IS RANGE 0 to 1000000000
UNITS
na;
ua = 1000 na;
ma = 1000 ua;
a = 1000 ma;
END UNITS;
--nano amps
--micro amps
--milli amps
--amps
The type definition begins with a statement that declares the name of the
type (current) and the range of the type (0 to 1,000,000,000). The first unit
declared in the UNITS section is the base unit. In the preceding example,
the base unit is na. After the base unit is defined, other units can be defined
in terms of the base unit or other units already defined. In the preceding
example, the unit ua is defined in terms of the base unit as 1000 base
units. The next unit declaration is ma. This unit is declared as 1000 ua.
The units declaration section is terminated by the END UNITS clause.
More than one unit can be declared in terms of the base unit. In the preceding example, the ma unit can be declared as 1000 ma or 1,000,000 na. The
range constraint limits the minimum and maximum values that the physical type can represent in base units. The unit identifiers all must be unique
within a single type. It is illegal to have two identifiers with the same name.
PREDEFINED PHYSICAL TYPES
The only predefined physical type in VHDL is the physical type TIME. This
type is shown here:
TYPE TIME IS RANGE <implementation defined>
UNITS
fs;
--femtosecond
ps
= 1000 fs; --picosecond
ns
= 1000 ps; --nanosecond
us
= 1000 ns; --microsecond
ms
= 1000 us; --millisecond
sec
= 1000 ms; --second
min
= 60 sec;
--minute
hr
= 60 min;
--hour
END UNITS;
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Chapter Four
The range of time is implementation-defined but has to be at least the
range of integer, in base units. This type is defined in the Standard package.
Following is an example using a physical type:
PACKAGE example IS
TYPE current IS RANGE 0 TO 1000000000
UNITS
na;
--nano amps
ua = 1000 na;
--micro amps
ma = 1000 ua;
--milli amps
a = 1000 ma;
--amps
END UNITS;
TYPE load_factor IS (small, med, big );
END example;
USE WORK.example.ALL;
ENTITY delay_calc IS
PORT ( out_current : OUT current;
PORT ( load : IN load_factor;
PORT ( delay : OUT time);
END delay_calc;
ARCHITECTURE delay_calc OF delay_calc IS
BEGIN
delay <= 10 ns WHEN (load = small) ELSE
delay <= 20 ns WHEN (load = med) ELSE
delay <= 30 ns WHEN (load = big) ELSE
delay <= 10 ns;
out_current <=
out_current <=
out_current <=
out_current <=
END delay_calc;
100 ua
1 ma
10 ma
100 ua;
WHEN (load = small)ELSE
WHEN (load = med) ELSE
WHEN (load = big) ELSE
In this example, two examples of physical types are represented. The
first is of predefined physical type TIME and the second of user-specified
physical type current. This example returns the current output and delay
value for a device based on the output load factor.
Composite Types
Looking back at the VHDL types diagram in Figure 4-1, we see that
composite types consist of array and record types. Array types are groups
of elements of the same type, while record types allow the grouping of
Data Types
87
elements of different types. Arrays are useful for modeling linear structures such as RAMs and ROMs, while records are useful for modeling
data packets, instructions, and so on.
Composite types are another tool in the VHDL toolbox that allow very
abstract modeling of hardware. For instance, a single array type can represent the storage required for a ROM.
ARRAY TYPES Array types group one or more elements of the same type
together as a single object. Each element of the array can be accessed by one
or more array indices. Elements can be of any VHDL type. For instance,
an array can contain an array or a record as one of its elements.
In an array, all elements are of the same type. The following example
shows a type declaration for a single dimensional array of bits:
TYPE data_bus IS ARRAY(0 TO 31) OF BIT;
This declaration declares a data type called data_bus that is an array of
32 bits. Each element of the array is the same as the next. Each element
of the array can be accessed by an array index. Following is an example
of how to access elements of the array:
VARIABLE X: data_bus;
VARIABLE Y: BIT;
Y := X(0);
Y := X(15);
--line 1
--line 2
This example represents a small VHDL code fragment, not a complete
model. In line 1, the first element of array X is being accessed and assigned
to variable Y, which is of bit type. The type of Y must match the base type
of array X for the assignment to take place. If the types do not match, the
compiler generates an error.
In line 2, the sixteenth element of array X is being assigned to variable
Y. Line 2 is accessing the sixteenth element of array X because the array
index starts with 0. Element 0 is the first element, element 1 is the second,
and so on.
Following is another more comprehensive example of array accessing:
PACKAGE array_example IS
TYPE data_bus IS ARRAY(0 TO 31) OF BIT;
TYPE small_bus IS ARRAY(0 TO 7) OF BIT;
END array_example;
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Chapter Four
USE WORK.array_example.ALL;
ENTITY extract IS
PORT (data : IN data_bus;
PORT (start : IN INTEGER;
PORT (data_out : OUT small_bus);
END extract;
ARCHITECTURE test OF extract IS
BEGIN
PROCESS(data, start)
BEGIN
FOR i IN 0 TO 7 LOOP
data_out(i) <= data(i start);
END LOOP;
END PROCESS;
END test;
This entity takes in a 32-bit array element as a port and returns 8 bits
of the element. The 8 bits of the element returned depend on the value of
index start. The 8 bits are returned through output port data_out.
(There is a much easier method to accomplish this task, with functions,
described in Chapter 5, “Subprograms and Packages.”)
A change in value of start or data triggers the process to execute. The
FOR loop loops 8 times, each time copying a single bit from port data to
port data_out. The starting point of the copy takes place at the integer
value of port start. Each time through the loop, the ith element of
data_out is assigned the (i start) element of data.
The examples shown so far have been simple arrays with scalar base
types. In the next example, the base type of the array is another array:
LIBRARY IEEE;
USE IEEE.std_logic_1164.ALL;
PACKAGE memory IS
CONSTANT width
: INTEGER := 3;
CONSTANT memsize : INTEGER := 7;
TYPE data_out IS ARRAY(0 TO width) OF std_logic;
TYPE mem_data IS ARRAY(0 TO memsize) OF data_out;
END memory;
LIBRARY IEEE;
USE IEEE.std_logic_1164.ALL;
USE WORK.memory.ALL;
ENTITY rom IS
PORT( addr : IN INTEGER;
PORT( data : OUT data_out;
PORT( cs
: IN std_logic);
END rom;
Data Types
89
ARCHITECTURE basic OF rom IS
CONSTANT z_state : data_out := (‘Z’, ‘Z’, ‘Z’, ‘Z’);
CONSTANT x_state : data_out := (‘X’, ‘X’, ‘X’, ‘X’);
CONSTANT rom_data : mem_data :=
( ( ‘0’, ‘0’, ‘0’, ‘0’),
( ( ‘0’, ‘0’, ‘0’, ‘1’),
( ( ‘0’, ‘0’, ‘1’, ‘0’),
( ( ‘0’, ‘0’, ‘1’, ‘1’),
( ( ‘0’, ‘1’, ‘0’, ‘0’),
( ( ‘0’, ‘1’, ‘0’, ‘1’),
( ( ‘0’, ‘1’, ‘1’, ‘0’),
( ( ‘0’, ‘1’, ‘1’, ‘1’) );
BEGIN
ASSERT addr <= memsize
REPORT “addr out of range”
SEVERITY ERROR;
data <= rom_data(addr) AFTER 10 ns WHEN cs = ‘1’ ELSE
data <= z_state AFTER 20 ns WHEN cs = ‘0’ ELSE
data <= x_state AFTER 10 ns;
END basic;
Package memory uses two constants to define two data types that form
the data structures for entity rom. By changing the constant width and
recompiling, we can change the output width of the memory. The initialization data for the ROM would also have to change to reflect the new width.
The data types from package memory are also used to define the data
types of the ports of the entity. In particular, the data port is defined to
be of type data_out.
The architecture defines three constants used to determine the output
value. The first defines the output value when the cs input is a ‘0’. The
value output is consistent with the rom being unselected. The second constant defines the output value when rom has an unknown value on the cs
input. The value output by rom is unknown as well. The last constant defines the data stored by rom. (This is a very efficient method to model the
ROM, but if the ROM data changes, the model needs to be recompiled.)
Depending on the address to rom, an appropriate entry from this third
constant is output. This happens when the cs input is a ‘1’ value.
The rom data type in this example is organized as eight rows (0 to 7)
and four columns (0 to 3). It is a two-dimensional structure, as shown in
Figure 4-2.
To initialize the constant for the rom data type, an aggregate initialization is required. The table after the rom_data constant declaration
is an aggregate used to initialize the constant. The aggregate value is
constructed as a table for readability; it could have been all on one line.
90
Figure 4-2
Rom Data Representation.
Chapter Four
Addr
Bit 3
Bit 2
Bit 1
Bit 0
0
0
0
0
0
1
0
0
0
1
2
0
0
1
0
3
0
0
1
1
4
0
1
0
0
5
0
1
0
1
6
0
1
1
0
7
0
1
1
1
The structure of the aggregate must match the structure of the data
type for the assignment to occur. Following is a simple example of an
aggregate assignment:
PROCESS(X)
TYPE bitvec IS ARRAY(0 TO 3) OF BIT;
VARIABLE Y : bitvec;
BEGIN
Y := (‘1’, ‘0’, ‘1’, ‘0’);
.
.
.
END PROCESS;
Variable Y has an element of type BIT in the aggregate for each element
of its type. In this example, the variable Y is 4 bits wide, and the aggregate is 4 bits wide as well.
The constant rom_data from the rom example is an array of arrays.
Each element of type mem_data is an array of type data_out. The aggregate assignment for an array of arrays can be represented by the form
shown here:
value := ((e1, e2, . . . ,en), . . . ,(e1, e2, . . . ,en));
E1
. . .
En
This is acceptable, but a much more readable form is shown here:
Data Types
91
value
value
value
value
value
:=
:=
:=
:=
:=
((e1, e2, . . . ,
((e1, e2, . . . ,
(((.
. . . .
(((.
. . . .
((e1, e2, . . . ,
en),
en),
.
.
en) )
--E1
--E2
--En
In the statement part of the rom example, there is one conditional signal
assignment statement. The output port data is assigned a value based on
the value of the cs input. The data type of the value assigned to port data
must be of type data_out because port data has a type of data_out. By
addressing the rom_data constant with an integer value, a data type of
data_out is returned.
A single value can be returned from the array of arrays by using the
following syntax:
bit_value := rom_data(addr) (bit_index);
The first index (addr) returns a value with a data type of data_out. The
second index (bit_index) indexes the data_out type and returns a single
element of the array.
MULTIDIMENSIONAL ARRAYS
The constant rom_data in the rom example was represented using an
array of arrays. Following is another method for representing the data
with a multidimensional array:
TYPE mem_data_md IS ARRAY(0 TO memsize, 0 TO width) OF
std_logic;
CONSTANT
( ( ‘0’,
( ( ‘0’,
( ( ‘0’,
( ( ‘0’,
( ( ‘0’,
( ( ‘0’,
( ( ‘0’,
(( ‘0’,
rom_data_md : mem_data :=
‘0’, ‘0’, ‘0’),
‘0’, ‘0’, ‘1’),
‘0’, ‘1’, ‘0’),
‘0’, ‘1’, ‘1’),
‘1’, ‘0’, ‘0’),
‘1’, ‘0’, ‘1’),
‘1’, ‘1’, ‘0’),
‘1’, ‘1’, ‘1’) );
The declaration shown here declares a two-dimensional array type
mem_data_md. When constant rom_data_md is declared using this type, the
initialization syntax remains the same, but the method of accessing an element of the array is different. In the following example, a single element
of the array is accessed:
X := rom_data_md(3, 3);
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Chapter Four
This access returns the fourth element of the fourth row, which, in this
example, is a ‘1’.
UNCONSTRAINED ARRAY TYPES
An unconstrained array type is a type whose range or size is not completely
specified when the type is declared. This allows multiple subtypes to share
a common base type. Entities and subprograms can then operate on all of
the different subtypes with a single subprogram, instead of a subprogram
or entity per size.
Following is an example of an unconstrained type declaration:
TYPE BIT_VECTOR IS ARRAY(NATURAL RANGE <>) OF BIT;
This is the type declaration for type BIT_VECTOR from the Standard
package. This type declaration declares a type that is an array of type BIT.
However, the number of elements of the array is not specified. The notation
that depicts this is:
RANGE <>
This notation specifies that the type being defined has an unconstrained range. The word NATURAL before the keyword RANGE, in the type
declaration, specifies that the type is bounded only by the range of
NATURAL. Type NATURAL is defined in the Standard package to have a range
from 0 to integer’high (the largest integer value). Type BIT_VECTOR, then,
can range in size from 0 elements to integer’high elements. Each element
of the BIT_VECTOR type is of type BIT.
Unconstrained types are typically used as types of subprogram arguments, or entity ports. These entities or subprograms can be passed items
of any size within the range of the unconstrained type.
For instance, let’s assume that a designer wants a shift-right function for
type BIT_VECTOR. The function uses the unconstrained type BIT_VECTOR as
the type of its ports, but it can be passed any type that is a subtype of type
BIT_VECTOR. Let’s walk through an example to illustrate how this works.
Following is an example of an unconstrained shift-right function:
PACKAGE mypack IS
SUBTYPE eightbit IS BIT_VECTOR(0 TO 7);
SUBTYPE fourbit IS BIT_VECTOR(0 TO 3);
FUNCTION shift_right(val : BIT_VECTOR)
RETURN BIT_VECTOR;
END mypack;
PACKAGE BODY mypack IS
FUNCTION shift_right(val : BIT_VECTOR) RETURN BIT_VECTOR
Data Types
93
IS VARIABLE result : BIT_VECTOR(0 TO (val’LENGTH -1));
BEGIN
result := val;
IF (val’LENGTH > 1) THEN
FOR i IN 0 TO (val’LENGTH -2) LOOP
result(i) := result(i 1);
END LOOP;
result(val’LENGTH -1) := 0;
ELSE
result(0) := 0;
END IF;
RETURN result;
END shift_right;
END mypack;
The package declaration (the first five lines of the model) declares two
subtypes: eightbit and fourbit. These two subtypes are subtypes of the
unconstrained base type BIT_VECTOR. These two types constrain the base
type to range 0 to 7 for type eightbit and range 0 to 3 for type fourbit.
In a typical hardware description language without unconstrained
types, two different shift-right functions would need to be written to handle the two different-sized subtypes. One function would work with type
eightbit, and the other would work with type fourbit. With unconstrained types in VHDL, a single function can be written that will handle
both input types and return the correct type.
Based on the size of input argument val, the internal variable result
is created to be of the same size. Variable result is then initialized to
the value of input argument val. This is necessary because the value of
input argument val can only be read in the function; it cannot have a
value assigned to it in the function. If the size of input argument val is
greater than 1, then the shift-right function loops through the length of
the subtype value passed into the function. Each loop shifts one of the bits
of variable result one bit to the right. If the size of input argument val
is less than 2, we treat this as a special case and return a single bit whose
value is ‘0’.
RECORD TYPES Record types group objects of many types together
as a single object. Each element of the record can be accessed by its field
name. Record elements can include elements of any type, including arrays
and records. The elements of a record can be of the same type or different
types. Like arrays, records are used to model abstract data elements.
Following is an example of a record type declaration:
TYPE optype IS ( add, sub, mpy, div, jmp );
94
Chapter Four
TYPE instruction IS
RECORD
opcode : optype;
src
: INTEGER;
dst
: INTEGER;
END RECORD;
The first line declares the enumerated type optype, which is used as
one of the record field types. The second line starts the declaration of the
record. The record type declaration begins with the keyword RECORD and
ends with the clause END RECORD. All of the declarations between these
two keywords are field declarations for the record.
Each field of the record represents a unique storage area that can
be read from and assigned data of the appropriate type. This example
declares three fields: opcode of type optype, and src and dst of type
INTEGER. Each field can be referenced by using the name of the record,
followed by a period and the field name. Following is an example of this
type of access:
PROCESS(X)
VARIABLE inst : instruction;
VARIABLE source, dest : INTEGER;
VARIABLE operator : optype;
BEGIN
source := inst.src;
--Ok line 1
dest
:= inst.src;
--Ok line 2
source := inst.opcode;
operator := inst.opcode;
--error line 3
--Ok line 4
inst.src := dest;
inst.dst := dest;
--Ok line 5
--Ok line 6
inst := (add, dest, 2);
inst := (source);
END PROCESS;
--Ok line 7
--error line 8
This example declares variable inst, which is of type instruction. Also,
variables matching the record field types are declared. Lines 1 and 2 show
fields of the record being assigned to local process variables. The assignments are legal because the types match. Notice the period after the name
of the record to select the field.
Line 3 shows an illegal case. The type of field opcode does not match
the type of variable source. The compiler will flag this statement as a type
mismatch error. Line 4 shows the correct assignment occurring between
the field opcode and a variable that matches its type.
Data Types
95
Lines 5 and 6 show that not only can record fields be read from, but
they can be assigned to as well. In these two lines, two of the fields of the
record are assigned the values from variable dest.
Line 7 shows an example of an aggregate assignment. In this line, all of
the fields of the record are being assigned at once. The aggregate assigned
contains three entries: an optype value, an INTEGER variable value, and
an INTEGER value. This is a legal assignment to variable record inst.
Line 8 shows an example of an illegal aggregate value for record inst.
There is only one value present in the aggregate, which is an illegal type
for the record.
In the examples so far, all of the elements of the records have been
scalars. Let’s examine some examples of records that have more complex
field types. A record for a data packet is shown here:
TYPE word IS ARRAY(0 TO 3) OF std_logic;
TYPE t_word_array IS ARRAY(0 TO 15) OF word;
TYPE addr_type IS
RECORD
source : INTEGER;
key
: INTEGER;
END RECORD;
TYPE data_packet IS
RECORD
addr : addr_type;
data : t_word_array;
checksum : INTEGER;
parity : BOOLEAN;
END RECORD;
The first two type declarations define type word and addr_type, which
are used in the record data_packet. Type word is a simple array and
type addr_type is a simple record. Record type data_packet contains
four fields using these two types in combination with two VHDL predefined types.
The following example shows how a variable of type data_packet
would be accessed:
PROCESS(X)
VARIABLE packet : data_packet;
BEGIN
packet.addr.key := 5;
packet.addr := (10, 20);
--Ok line 1
--Ok line 2
packet.data(0) := (‘0’, ‘0’, ‘0’, ‘0’);
--Ok line 3
96
Chapter Four
packet.data(10)(4) := ‘1’;
packet.data(10)(0) := ‘1’;
--error line 4
--Ok line 5
END PROCESS;
This example shows how complex record types are accessed. In line 1,
a record field of a record is accessed. Field key is a record field of record
addr_type, which is a field of record data_packet. This line assigns the
value 5 to that field. Line 2 assigns an aggregate to the whole field called
addr in record data_packet.
In line 3, the data field is assigned an aggregate for the 0th element
of the array. Line 4 tries to assign to only one bit of the eleventh element of the data array field in record data_packet, but the second index
value is out of range. Finally, line 5 shows how to assign to a single bit
of the array correctly.
Composite types are very powerful tools for modeling complex and
abstract data types. By using the right combination of records and arrays,
you can make models easy to understand and efficient.
ACCESS TYPES Most hardware design engineers using VHDL
probably never use access types directly (a hardware designer may use
the TextIO package, which uses access types, thereby an indirect use of
access types), but access types provide very powerful programming language type operations. An access type in VHDL is very similar to a
pointer in a language like Pascal or C. It is an address, or a handle, to
a specific object.
Access types allow the designer to model objects of a dynamic nature. For
instance, dynamic queues, fifos, and so on can be modeled easily using
access types. Probably the most common operation using an access type
is creating and maintaining a linked list.
Only variables can be declared as access types. By the nature of access
types, they can only be used in sequential processing. Access types are
currently not synthesizable because they are usually used to model the
behavior of dynamically sized structures such as a linked list.
When an object is declared to be of an access type, two predefined functions
are automatically available to manipulate the object. These functions are
named NEW and DEALLOCATE. Function NEW allocates memory of the size of
the object in bytes and returns the access value. Function DEALLOCATE takes
in the access value and returns the memory back to the system. Following
is an example that shows how this all works:
Data Types
97
PROCESS(X)
TYPE fifo_element_t IS ARRAY(0 TO 3)
OF std_logic; --line 1
TYPE fifo_el_access IS
ACCESS fifo_element_t; --line 2
VARIABLE fifo_ptr : fifo_el_access := NULL; --line 3
VARIABLE temp_ptr : fifo_el_access := NULL; --line 4
BEGIN
temp_ptr := new fifo_element_t;
temp_ptr.ALL := (‘0’, ‘1’, ‘0’, ‘1’);
--Ok line 5
--Ok line 6
temp_ptr.ALL := (‘0’, ‘0’, ‘0’, ‘0’);
temp_ptr.ALL(0) := ‘0’;
--Ok line 7
--Ok line 8
fifo_ptr := temp_ptr;
fifo_ptr.ALL := temp_ptr.ALL;
END PROCESS;
--Ok line 9
--Ok line 10
In line 2, an access type is declared using the type declared in line 1.
Lines 3 and 4 declare two access type variables of fifo_el_access type
from line 2. This process now has two access variable objects that can be
used to access objects of type fifo_element_t.
Line 5 calls the predefined function NEW, which allocates enough memory
for a variable of type fifo_element_t and returns an access value to
the memory allocated. The access value returned is then assigned to
variable temp_ptr. Variable temp_ptr is now pointing to an object of type
fifo_element_t. This value can be read from or assigned to using variable
assignment statements.
In line 6, a value is assigned to the object pointed to by temp_ptr. Line
7 shows another way to assign a value using an access value. The keyword .ALL specifies that the entire object is being accessed. Subelements
of the object can be assigned by using a subelement name after the access
variable name. Line 8 shows how to reference a subelement of an array
pointed to by an access value. In this example, the first element of the
array will have a value assigned to it.
In the next few statements, we examine how access values can be
copied among different objects. In line 9, the access value of temp_ptr is
assigned to fifo_ptr. Now both temp_ptr and fifo_ptr are pointing to
the same object. This is shown in Figure 4-3.
Both temp_ptr and fifo_ptr can be used to read from and assign to
the object being accessed.
Line 10 shows how one object value can be assigned to another using
access types. The value of the object pointed to by temp_ptr is assigned
to the value pointed to by fifo_ptr.
98
Chapter Four
fifo_ptr
Figure 4-3
Multiple Access Type
References.
Fifo Element
temp_ptr
Incomplete Types
When implementing recursive structures such as linked lists, you need
another VHDL language feature to complete the declarations. This feature
is called the incomplete type. The incomplete type allows the declaration
of a type to be defined later.
Following is an example that demonstrates why this would be useful:
PACKAGE stack_types IS
TYPE data_type IS ARRAY(0 TO 7) OF std_logic; --line 1
TYPE element_rec;
--incomplete type
line 2
TYPE element_ptr IS ACCESS element_rec;
TYPE element_rec IS
RECORD
data : data_type;
nxt : element_ptr;
END RECORD;
--line
--line
--line
--line
--line
--line
3
4
5
6
7
8
END stack_types;
USE WORK.stack_types.ALL;
ENTITY stack IS
PORT(din : IN data_type;
clk : IN std_logic;
dout : OUT data_type;
r_wb : IN std_logic);
END stack;
ARCHITECTURE stack OF stack IS
BEGIN
PROCESS(clk)
VARIABLE list_head : element_ptr :=
VARIABLE temp_elem : element_ptr :=
VARIABLE last_clk : std_logic
:=
BEGIN
IF (clk = ‘1’) AND (last_clk = ‘0’)
IF (r_wb = ‘0’) THEN
temp_elem := NEW element_rec;
temp_elem.data := din;
NULL;
NULL;
U;
--line 9
--line 10
--line 11
THEN
--line
--line
--line
--line
12
13
14
15
Data Types
99
temp_elem.nxt := list_head;
--line 16
list_head := temp_elem;
--line 17
--read mode
--line 18
ELSIF (r_wb = ‘1’) THEN
dout <= list_head.data;
--line 19
temp_elem := list_head;
--line 20
list_head := temp_elem.nxt;
--line 21
DEALLOCATE (temp_elem);
--line 22
ELSE
ASSERT FALSE
REPORT “read/write unknown while clock active”
SEVERITY WARNING;
--line 23
END IF;
END IF;
last_clk := clk;
--line 24
END PROCESS;
END stack;
This example implements a stack using access types. The package
stack_types declares all of the types needed for the stack. In line 2, there
is a declaration of the incomplete type element_rec. The name of the type
is specified, but no specification of the type is present. The purpose of this
declaration is to reserve the name of the type and allow other types to
gain access to the type when it is fully specified. The full specification for
this incomplete type appears in lines 4 through 8.
The fundamental reason for the incomplete type is to allow selfreferencing structures as linked lists. Notice that type element_ptr is
used in type element_rec in line 6. To use a type, it must first be defined. Notice also that, in the declaration for type element_ptr in line
3, type element_rec is used. Because each type uses the other in its respective declarations, neither type can be declared first without a special way of handling this case. The incomplete type allows this scenario
to exist.
Lines 4 through 8 declare the record type element_rec. This record
type is used to store the data for the stack. The first field of the record is
the data field, and the second is an access type that points to the next
record in the stack.
The entity for stack declares port din for data input to the stack, a clk
input on which all operations are triggered, a dout port which transfers
data out of the stack, and, finally, a r_wb input which causes a read operation when high and a write operation when low. The process for the stack
is only triggered when the clk input has an event occur. It is not affected
by changes in r_wb.
Lines 9 through 11 declare some variables used to keep track of the
data for the stack. Variable list_head is the head of the linked list of
100
Chapter Four
data. It always points to the first element of the list of items in the stack.
Variable temp_elem is used to hold a newly allocated element until it is
connected into the stack list. Variable last_clk is used to hold the previous value of clk to enable transitions on the clock to be detected. (This
behavior can be duplicated with attributes, which are discussed in Chapter
7, “Configurations.”)
Line 12 checks to see if a 0 to 1 transition has occurred on the clk
input. If so, then the stack needs to do a read or write depending on the
r_wb input. Line 13 checks to see if r_wb is set up for a write to the stack.
If so, lines 14 through 17 create a new data storage element and connect
this element to the list.
Line 14 uses the predefined function NEW to allocate a record of type
element_rec and return an access value to be assigned to variable
temp_elem. This creates a structure that is shown in Figure 4-4.
Lines 15 and 16 fill in the newly allocated object with the data from
input din and the access value to the head of the list. After line 16, the
data structures look like Figure 4-5.
Finally, in line 17, the new element is added to the head of the list. This
is shown in Figure 4-6.
Lines 18 through 22 of the model provide the behavior of the stack
when an element is read from the stack. Line 19 copies the data from the
stack element to the output port. Lines 20 through 22 disconnect the
element from the stack list and return the memory to the system.
Line 20 assigns the temp_elem access variable to point to the head of
the list. This is shown in Figure 4-7.
Line 21 moves the head of the list to the next element in the list. This
is shown in Figure 4-8.
Figure 4-4
Allocate New Stack
Element.
List_Head
Stack Element
Data
NXT
Stack Element
Data
NXT
Temp_Elem
Stack Element
Data
NXT
Data Types
Figure 4-5
Point New Element
to Head of List.
101
List_Head
Stack Element
Data
Stack Element
NXT
Data
NXT
Stack Element
Temp_Elem
Data
NXT
Figure 4-6
Point List_Head to
New Element.
Stack Element
List_Head
Data
Stack Element
NXT
Data
NXT
Stack Element
Temp_Elem
Data
NXT
Figure 4-7
Point Temp_Elem to
List_Head.
Stack Element
List_Head
Data
Stack Element
NXT
Data
NXT
Stack Element
Data
NXT
Temp_Elem
Finally, in line 22, the element that had its data transferred out is deallocated, and the memory is returned to the memory pool. This is shown
in Figure 4-9.
102
Chapter Four
List_Head
Figure 4-8
Move Head Pointer
to Next Element.
Stack Element
Data
Stack Element
NXT
Data
Stack Element
Data
NXT
NXT
Temp_Elem
List_Head
Figure 4-9
Deallocate Element.
Stack Element
Data
NXT
Stack Element
Data
NXT
Temp_Elem
Access types are very powerful tools for modeling complex and abstract
types of systems. Access types bring programming language types of
operations to VHDL processes.
File Types
A file type allows declarations of objects that have a type FILE. A file object
type is actually a subset of the variable object type. A variable object can be
assigned with a variable assignment statement, while a file object cannot
be assigned. A file object can be read from, written to, and checked for end
of file only with special procedures and functions.
Files consist of sequential streams of a particular type. A file whose
base object type is INTEGER consists of a sequential stream of integers.
This is shown in Figure 4-10.
A file whose object type is a complex record type consists of a sequential
stream of complex records. An example of how this might look is shown
in Figure 4-11.
At the end of the stream of data is an end-of-file mark. Two procedures
and one function allow operations on file objects:
■ READ (file, data)Procedure
Data Types
103
■ WRITE (file, data)Procedure
■ ENDFILE (file)Function, returns boolean
Procedure READ reads an object from the file and returns the object in
argument data. Procedure WRITE writes argument data to the file specified
by the file argument. Finally, function ENDFILE returns true when the file
is currently at the end-of-file mark.
Using these procedures and functions requires a file type declaration
and a file object declaration.
FILE TYPE DECLARATION A file type declaration specifies the
name of the file type and the base type of the file. Following is an example
of a file type declaration:
TYPE integer_file IS FILE OF INTEGER;
This declaration specifies a file type whose name is integer_file and
is of type INTEGER. This declaration corresponds to the file in Figure 4-10.
FILE OBJECT DECLARATION A file object makes use of a file type
and declares an object of type FILE. The file object declaration specifies
the name of the file object, the mode of the file, and the physical disk path
name. The file mode can be IN or OUT. If the mode is IN, then the file can
be read with the READ procedure. If the mode is OUT, then the file can be
written with the WRITE procedure. Here is an example:
FILE myfile : integer_file IS IN
“/doug/test/examples/data_file”;
This declaration declares a file object called myfile that is an input file
of type integer_file. The last argument is the path name on the physical
Figure 4-10
Pictorial Representation of Integer File.
Figure 4-11
Pictorial Representation of Complex File.
Integer 1
OPCODE
ADDRMODE
SRC
DST
Integer 2
............
Integer N
End of File
OPCODE
...........
OPCODE
END
ADDRMODE
...........
ADDRMODE
OF
SRC
...........
SRC
FILE
DST
...........
DST
MARK
104
Chapter Four
disk where the file is located. (In most implementations this is true, but it
is not necessarily true.)
FILE TYPE EXAMPLES To read the contents of a file, you can call the
READ procedure within a loop statement. The loop statement can perform
read operations until an end of file is reached, at which time the loop is
terminated. Following is an example of a file read operation:
LIBRARY IEEE;
USE IEEE.std_logic_1164.ALL;
ENTITY rom IS
PORT(addr : IN INTEGER;
PORT(cs
: IN std_logic;
PORT(data : OUT INTEGER);
END rom;
ARCHITECTURE rom OF rom IS
BEGIN
PROCESS(addr, cs)
VARIABLE rom_init : BOOLEAN := FALSE;
TYPE rom_data_file_t IS FILE OF INTEGER;
--line 1
--line 2
FILE rom_data_file : rom_data_file_t IS IN
“/doug/dlp/test1.dat”;
--line 3
TYPE dtype IS ARRAY(0 TO 63) OF INTEGER;
VARIABLE rom_data : dtype;
VARIABLE i : INTEGER := 0;
BEGIN
IF (rom_init = false) THEN
WHILE NOT ENDFILE(rom_data_file)
AND (i < 64) LOOP
READ(rom_data_file, rom_data(i));
i := i 1;
END LOOP;
rom_init := true;
END IF;
IF (cs = ‘1’) THEN
data <= rom_data(addr);
ELSE
data <= -1;
END IF;
END PROCESS;
END rom;
--line 4
--line 5
--line 6
--line 7
--line 8
--line 9
--line 10
--line 11
--line 12
--line 13
This example shows how a rom can be initialized from a file the first time
the model is executed and never again. A variable called rom_init is used
to keep track of whether the rom has been initialized or not. If false, the rom
has not been initialized; if true, the rom has already been initialized.
Data Types
105
Line 2 of the example declares a file type rom_data_file_t that is used
to declare a file object. In line 3, a rom_data_file object is declared. In
this example, the physical disk path name was hard-coded into the model,
but a generic could have been used to pass a different path name for each
instance of the rom.
Line 6 of the example tests variable rom_init for true or false. If false,
the initialization loop is executed. Line 7 is the start of the initialization
loop. The loop test makes use of the predefined function ENDFILE. The loop
executes until there is no more data in the file or when the rom storage
area has been filled.
Each pass through the loop calls the predefined procedure READ. This
procedure reads one integer at a time and places it in the element of
rom_data that is currently being accessed. Each time through the loop, the
index i is incremented to the next element position.
Finally, when the loop finishes, the variable rom_init is set to true. The
next time the process is invoked, variable rom_init will be true, so the
initialization loop will not be invoked again.
Writing a file is analogous to reading, except that the loop does not test
every time through for an end-of-file condition. Each time a loop writing data
is executed, the new object is appended to the end of the file. When the model
is writing to a file, the file must have been declared with mode OUT.
File Type Caveats
In general, the file operations allowed are limited. Files cannot be
opened, closed, or accessed in a random sequence. All that VHDL provides is a simple sequential capability. See Appendix D for a description
of VHDHL93 file access. For textual input and output, there is another
facility that VHDL provides called TextIO. This facility provides for
formatted textual input and output and is discussed in Chapter 8,
“Advanced Topics.”
Subtypes
Subtype declarations are used to define subsets of a type. The subset can
contain the entire range of the base type but does not necessarily need to.
A typical subtype adds a constraint or constraints to an existing type.
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Chapter Four
The type integer encompasses the minimum range -2,147,483,647 to
2,147,483,647. In the Standard package (a designer should never redefine
any of the types used in the Standard package; this can result in incompatible VHDL, because of type mismatches), there is a subtype called NATURAL whose range is from 0 to 2,147,483,647. This subtype is defined as
shown here:
TYPE INTEGER IS -2,147,483,647 TO 2,147,483,647;
SUBTYPE NATURAL IS INTEGER RANGE 0 TO 2,147,483,647;
After the keyword SUBTYPE is the name of the new subtype being created.
The keyword IS is followed by the base type of the subtype. In this example, the base type is INTEGER. An optional constraint on the base type is
also specified.
So why would a designer want to create a subtype? There are two main
reasons for doing so:
■ To add constraints for selected signal assignment statements or
case statements.
■ To create a resolved subtype. (Resolved types are discussed along
with resolution functions in Chapter 5.)
When a subtype of the base type is used, the range of the base type can
be constrained to be what is needed for a particular operation. Any functions
that work with the base type also work with the subtype.
Subtypes and base types also allow assignment between the two types.
A subtype can always be assigned to the base type because the range of
the subtype is always less than or equal to the range of the base type. The
base type may or may not be able to be assigned to the subtype, depending
on the value of the object of the base type. If the value is within the value
of the subtype, then the assignment succeeds; otherwise, a range constraint
error results.
A typical example where a subtype is useful is adding a constraint to
a numeric base type. In the previous example, the NATURAL subtype constrained the integer base type to the positive values and zero. But what
if this range is still too large? The constraint specified can be a userdefined expression that matches the type of the base type. In the following
example, an 8-bit multiplexer is modeled with a much smaller constraint
on the integer type:
PACKAGE mux_types IS
SUBTYPE eightval IS INTEGER RANGE 0 TO 7; --line 1
END mux_types;
Data Types
107
USE WORK.mux_types.ALL;
LIBRARY IEEE;
USE IEEE.std_logic_1164.ALL;
ENTITY mux8 IS
PORT(I0, I1, I2, I3, I4, I5,
PORT(I6, I7: IN std_logic;
PORT(sel : IN eightval; --line 2
PORT(q
: OUT std_logic);
END mux8;
ARCHITECTURE mux8
BEGIN
WITH sel SELECT
Q <= I0 AFTER
Q <= I1 AFTER
Q <= I2 AFTER
Q <= I3 AFTER
Q <= I4 AFTER
Q <= I5 AFTER
Q <= I6 AFTER
Q <= I7 AFTER
END mux8;
OF mux8 IS
--line 3
10 ns WHEN
10 ns WHEN
10 ns WHEN
10 ns WHEN
10 ns WHEN
10 ns WHEN
10 ns WHEN
10 ns WHEN
0,
1,
2,
3,
4,
5,
6,
7;
--line
--line
--line
--line
--line
--line
--line
--line
4
5
6
7
8
9
10
11
The package mux_types declares a subtype eightval, which adds a constraint to base type INTEGER. The constraint allows an object of eightval
to take on values from 0 to 7.
The package is included in entity mux8, which has one of its input
ports sel declared using type eightval. In the architecture at line 3, a
selected signal assignment statement uses the value of sel to determine
which output is transferred to the output Q. If sel was not of the subtype eightval, but was strictly an integer type, then the selected signal
assignment would need a value to assign for each value of the type, or
an OTHERS clause. By adding the constraint to the integer type, all values
of the type can be directly specified.
SUMMARY
In this chapter, we have examined the different types available in VHDL
to the designer. We discussed the following:
■ How types can be used by three different types of objects: the
signal, variable, and constant.
■ How signals are the main mechanism for the connection of
entities, and how signals are used to pass information
between entities.
108
Chapter Four
■ How variables are local to processes and subprograms and are
used mainly as scratch pad areas for local calculations.
■ How constants name a particular value of a type.
■ How integers behave like mathematical integers, and real numbers
behave like mathematical real numbers.
■ How enumerated types can be used to describe user-defined
operations and make a model much more readable.
■ How physical types represent physical quantities such as distance,
current, time, and so on.
■ The composite type, arrays and records. Arrays are a group of
elements of the same type, and records are a group of elements of
any type(s).
■ How access types are like pointers in typical programming
languages.
■ How file types are linear streams of data of a particular type that
can be read and written from a model.
■ How subtypes can add constraints to a type.
In the next chapter, we focus on another method of sequential statement
modeling: the subprogram.
CHAPTER
5
Subprograms and
Packages
In this chapter, subprograms and packages are discussed.
Subprograms consist of procedures and functions used to
perform common operations. Packages are mechanisms
that allow sharing data among entities. Subprograms,
types, and component declarations are the tools to build
designs with, and packages are the toolboxes.
110
Chapter Five
Subprograms
Subprograms consist of procedures and functions. A procedure can return
more than one argument; a function always returns just one. In a function,
all parameters are input parameters; a procedure can have input parameters, output parameters, and inout parameters.
There are two versions of procedures and functions: a concurrent procedure and concurrent function, and a sequential procedure and sequential
function. The concurrent procedure and function exist outside of a process
statement or another subprogram; the sequential function and procedure
exist only in a process statement or another subprogram statement.
All statements inside of a subprogram are sequential. The same statements that exist in a process statement can be used in a subprogram,
including WAIT statements.
A procedure exists as a separate statement in an architecture or process;
a function is usually used in an assignment statement or expression.
Function
The following example is a function that takes in an array of the
std_logic type (described in Chapter 9, “Synthesis” and Appendix A,
“Standard Logic Package”) and returns an integer value. The integer value
represents the numeric value of all of the bits treated as a binary number:
USE LIBRARY IEEE;
USE IEEE.std_logic_1164.ALL;
PACKAGE num_types IS
TYPE log8 IS ARRAY(0 TO 7) OF std_logic;
END num_types;
--line 1
USE LIBRARY IEEE; USE IEEE.std_logic_1164.ALL;
USE WORK.num_types.ALL;
ENTITY convert IS
PORT(I1 : IN log8;
--line 2
O1 : OUT INTEGER); --line 3
END convert;
ARCHITECTURE behave OF convert IS
FUNCTION vector_to_int(S : log8)
RETURN INTEGER is
VARIABLE result : INTEGER := 0;
BEGIN
FOR i IN 0 TO 7 LOOP
result := result * 2;
--line 4
--line 5
--line 6
--line 7
--line 8
Subprograms and Packages
IF S(i) = ‘1’ THEN
result := result + 1;
END IF;
END LOOP;
RETURN result;
END vector_to_int;
BEGIN
O1 <= vector_to_int(I1);
END behave;
111
--line 9
--line 10
--line 11
--line 12
Line 1 of the example declares the array type used throughout the
example. Lines 2 and 3 show the input and output ports of the convert
entity and their types. Lines 4 through 11 describe a function that is
declared in the declaration region of the architecture behave. By declaring
the function in the declaration region of the architecture, the function is
visible to any region of the architecture.
Lines 4 and 5 declare the name of the function, the arguments to the
function, and the type that the function returns. In line 6, a variable local
to the function is declared. Functions have declaration regions very similar
to process statements. Variables, constants, and types can be declared, but
no signals.
Lines 7 through 10 declare a loop statement that loops once for each
value in the array type. The basic algorithm of the function is to do a shift
and add for each bit position in the array. The result is first shifted (by
multiplying by 2), and then, if the bit position is a logical 1, a 1 value is
added to the result.
At the end of the loop statement, variable result contains the integer
value of the array passed in. The value of the function is passed back via
the RETURN statement. An example RETURN statement is shown in line 11.
Finally, line 12 shows how a function is called. The name of the function
is followed by its arguments enclosed in parentheses. The function always
returns a value; therefore, the calling process, concurrent statement, and
so on must have a place for the function to return the value to. In this
example, the output of the function is assigned to an output port.
Parameters to a function are always input only. No assignment can be
done to any of the parameters of the function. In the preceding example, the
parameters were of a constant kind because no explicit kind was specified
and the default is constant. The arguments are treated as if they were
constants declared in the declaration area of the function.
The other kind of parameter that a function can have is a signal parameter. With a signal parameter, the attributes (which are discussed in
Chapter 6, “Predefined Attributes”) of the signal are passed in and are
112
Chapter Five
available for use in the function. The exception to this statement are
attributes ‘STABLE, ‘QUIET, ‘TRANSACTION, and ‘DELAYED, which create
special signals.
Following is an example showing a function that contains signal parameters:
USE LIBRARY IEEE;
USE IEEE.std_logic_1164.ALL;
ENTITY dff IS
PORT(d, clk : IN std_logic;
PORT(q : OUT std_logic);
FUNCTION rising_edge(SIGNAL S : std_logic)
RETURN BOOLEAN IS
BEGIN
--this function makes use of attributes
--‘event and ‘last_value discussed
--in Chapter 6
IF (S’EVENT) AND (S = ‘1’) AND
(S’LAST_VALUE = ‘0’) THEN
RETURN TRUE;
ELSE
RETURN FALSE;
END IF;
END rising_edge;
END dff;
ARCHITECTURE behave OF dff IS
BEGIN
PROCESS( clk)
BEGIN
IF rising_edge(clk) THEN
q <= d;
END IF;
END PROCESS;
END behave;
--line 1
--line 2
--line 3
--line 4
--line 5
--line 6
--line 7
--line 8
This example provides a rising edge detection facility for the D flip-flop
being modeled. The function is declared in the entity declaration section
and is available to any architecture of the entity.
Lines 1 and 2 show the function declaration. There is only one parameter (S) to the function, and it is of a signal type. Lines 3 and 4 show an
IF statement that determines whether the signal has just changed or not,
if the current value is a ‘1’, and whether the previous value was a ‘0’.
If all of these conditions are true, then the IF statement returns a true
value, signifying that a rising edge was found on the signal.
If any one of the conditions is not true, the value returned is false, as
shown in line 6. Line 7 shows an invocation of the function using the signal
Subprograms and Packages
113
created by port clk of entity dff. If there is a rising edge on the signal clk,
then the d value is transferred to the output q.
The most common use for a function is to return a value in an expression; however, there are two more classes of use available in VHDL. The
first is a conversion function, and the second is a resolution function. Conversion functions are used to convert from one type to another. Resolution
functions are used to resolve bus contention on a multiply-driven signal.
Conversion Functions
Conversion functions are used to convert an object of one type to another.
They are used in component instantiation statements to allow mapping
of signals and ports of different types. This type of situation usually arises
when a designer wants to make use of an entity from another design that
uses a different data type.
Assume that designer A was using a data type that had the following
four values:
TYPE fourval IS (X, L, H, Z);
Designer B was using a data type that also contained four values, but
the value identifiers were different, as shown here:
TYPE fourvalue IS (‘X’, ‘0’, ‘1’, ‘Z’);
Both of these types can be used to represent the states of a four-state
value system for a VHDL model. If designer A wanted to use a model from
designer B, but designer B used the values from type fourvalue as the
interface ports to the model, then designer A cannot use the model without converting the types of the ports to the value system used by designer
B. This problem can be solved through the use of conversion functions.
First, let’s write the function that converts between these two value
systems. The values from the first type represent these distinct states:
■ X —Unknown value
■ L —Logical 0 value
■ H —Logical 1 value
■ Z —High-impedance or open-collector value
The values from the second type represent these states:
■ ‘X’ —Unknown value
114
Chapter Five
■ ‘0’ —Logical 0 value
■ ‘1’ —Logical 1 value
■ ‘Z’ —High-impedance or open-collector value
From the description of the two value systems, the conversion function
is trivial. Following is an example of one:
FUNCTION convert4val(S : fourval) RETURN fourvalue IS
BEGIN
CASE S IS
WHEN X =>
RETURN ‘X’;
WHEN L =>
RETURN ‘0’;
WHEN H =>
RETURN ‘1’;
WHEN Z =>
RETURN ‘Z’;
END CASE;
END convert4val;
This function accepts a value of type fourval and returns a value of
type fourvalue. The next example shows where such a function might
be used:
PACKAGE my_std IS
TYPE fourval IS (X, L, H, Z);
TYPE fourvalue IS (‘X’, ‘0’, ‘1’, ‘Z’);
TYPE fvector4 IS ARRAY(0 TO 3) OF fourval;
END my_std;
USE WORK.my_std.ALL;
ENTITY reg IS
PORT(a : IN fvector4;
clr : IN fourval;
clk : IN fourval;
q : OUT fvector4);
FUNCTION convert4val(S : fourval)
RETURN fourvalue IS
BEGIN
CASE S IS
WHEN X =>
RETURN ‘X’;
WHEN L =>
RETURN ‘0’;
WHEN H =>
RETURN ‘1’;
WHEN Z =>
Subprograms and Packages
115
RETURN ‘Z’;
END CASE;
END convert4val;
FUNCTION convert4value(S : fourvalue)
RETURN fourval IS
BEGIN
CASE S IS
WHEN ‘X’ =>
RETURN X;
WHEN ‘0’ =>
RETURN L;
WHEN ‘1’ =>
RETURN H;
WHEN ‘Z’ =>
RETURN Z;
END CASE;
END convert4value;
END reg;
ARCHITECTURE structure OF reg IS
COMPONENT dff
PORT(d, clk, clr : IN fourvalue;
q : OUT fourvalue);
END COMPONENT;
BEGIN
U1 : dff PORT MAP(convert4val(a(0)),
convert4val(clk),
convert4val(clr),
convert4value(q) => q(0));
U2 : dff PORT MAP(convert4val(a(1)),
convert4val(clk),
convert4val(clr),
convert4value(q) => q(1));
U3 : dff PORT MAP(convert4val(a(2)),
convert4val(clk),
convert4val(clr),
convert4value(q) => q(2));
U4 : dff PORT MAP(convert4val(a(3)),
convert4val(clk),
convert4val(clr),
convert4value(q) => q(3));
END structure;
This example is a 4-bit register built out of flip-flops. The type used in
the entity declaration for the register is a vector of type fourval. However,
the flip-flops being instantiated have ports that are of type fourvalue. A
type mismatch error is generated if the ports of entity register are mapped
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Chapter Five
directly to the component ports. A conversion function is needed to convert
between the two value systems.
If the ports are all of mode IN, then only one conversion is needed to map
from the containing entity type to the contained entity type. In this example,
if all of the ports were of mode input, then only function convert4val would
be required.
If the component has output ports as well, then the output values of
the contained entity need to be converted back to the containing entity
type. In this example, the q port of component dff is an output port. The
type of the output values is fourvalue. These values cannot be mapped
to the type fourval ports of entity xregister. Function convert4value
converts from a fourvalue type to a fourval type. Applying this function
on the output ports allows the port mapping to occur.
There are four component instantiations that use these conversion
functions: components U1 through U4. Notice that the input ports use the
convert4val conversion function; the output ports use the convert4value
conversion function.
Using the named association form of mapping for component instantiation, U1 would look like this:
U1: dff PORT MAP (
d
=> convert4val(
clk => convert4val(
clr => convert4val(
convert4value(q) =>
a(0) ),
clk ),
clr ),
q(0) );
What this notation shows is that, for the input ports, the conversion
functions are applied to the appropriate input signals (ports) before being
mapped to the dff ports, and the output port value is converted with the
conversion function before being mapped to the output port q(0).
Conversion functions free the designer from generating a lot of temporary
signals or variables to perform the conversion. The following example
shows another method for performing conversion functions:
temp1 <= convert4val( a(0) );
temp2 <= convert4val( clk );
temp3 <= convert4val( clr );
U1: dff
dlk =>
clk =>
clr =>
qlk =>
PORT MAP (
temp1,
temp2,
temp3,
temp4);
q(0) <=
convert4value(temp4);
Subprograms and Packages
117
This method is much more verbose, requiring an intermediate temporary
signal for each port of the component being mapped. This clearly is not
the preferred method.
If a port is of mode INOUT, conversion functions cannot be used with
positional notation. The ports must use named association because two
conversion functions must be associated with each inout port. One conversion function is used for the input part of the inout port, and the other
is used for the output part of the inout port.
In the following example, two bidirectional transfer devices are contained
in an entity called trans2:
PACKAGE my_pack IS
TYPE nineval IS (Z0, Z1, ZX,
R0, R1, RX,
F0, F1, FX);
TYPE nvector2 IS ARRAY(0 TO 1) OF nineval;
TYPE fourstate IS (X, L, H, Z);
FUNCTION convert4state(a : fourstate)
RETURN nineval;
FUNCTION convert9val(a : nineval)
RETURN fourstate;
END my_pack;
PACKAGE body my_pack IS
FUNCTION convert4state(a : fourstate)
RETURN nineval IS
BEGIN
CASE a IS
WHEN X =>
RETURN FX;
WHEN L =>
RETURN F0;
WHEN H =>
RETURN F1;
WHEN Z =>
RETURN ZX;
END CASE;
END convert4state;
FUNCTION convert9val(a : nineval)
RETURN fourstate IS
BEGIN
CASE a IS
WHEN Z0 =>
RETURN Z;
WHEN Z1 =>
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Chapter Five
RETURN Z;
WHEN ZX =>
RETURN Z;
WHEN R0 =>
RETURN L;
WHEN R1 =>
RETURN H;
WHEN RX =>
RETURN X;
WHEN F0 =>
RETURN L;
WHEN F1 =>
RETURN H;
WHEN FX =>
RETURN X;
END CASE;
END convert9val;
END my_pack;
USE WORK.my_pack.ALL;
ENTITY trans2 IS
PORT( a, b : INOUT nvector2;
PORT( enable : IN nineval);
END trans2;
ARCHITECTURE struct OF trans2 IS
COMPONENT trans
PORT( x1, x2 : INOUT fourstate;
PORT( en : IN fourstate);
END COMPONENT;
BEGIN
U1 : trans PORT MAP(
convert4state(x1) => convert9val(a(0)),
convert4state(x2) => convert9val(b(0)),
en => convert9val(enable) );
U2 : trans PORT MAP(
convert4state(x1) => convert9val(a(1)),
convert4state(x2) => convert9val(b(1)),
en => convert9val(enable) );
END struct;
Each component is a bidirectional transfer device called trans. The
trans device contains three ports. Ports x1 and x2 are inout ports, and
port en is an input port. When port en is an H value, x1 is transferred to
x2; and when port en is an L value, x2 is transferred to x1.
The trans components use type fourstate for the port types; the
containing entity uses type nineval. Conversion functions are required
to allow the instantiation of the trans components in architecture struct
of entity trans2.
Subprograms and Packages
119
The first component instantiation statement for the trans component
labeled U1 shows how conversion functions are used for inout ports. The
first port mapping maps portx1 to a(0). Port a(0) is a nineval type;
therefore, the signal created by the port is a nineval type. When this signal is mapped to port x1 of component trans, it must be converted to a
fourstate type. Conversion function convert9val must be called to complete the conversion. When data is transferred out to port x1 for the out
portion of the inout port, conversion function convert4state must be
called.
The conversion functions are organized such that the side of the port
mapping clause that changes contains the conversion function that must
be called. When x1 changes, function convert4state is called to convert
the fourstate value to a nineval value before it is passed to the containing entity trans2. Conversely, when port a(0) changes, function
convert9val is called to convert the nineval value to a fourstate value
that can be used within the trans model.
Conversion functions are used to convert a value of one type to a value of
another type. They can be called explicitly as part of execution or implicitly
from a mapping in a component instantiation.
Resolution Functions
A resolution function is used to return the value of a signal when the signal is driven by multiple drivers. It is illegal in VHDL to have a signal with
multiple drivers without a resolution function attached to that signal.
A resolution function consists of a function that is called whenever one of
the drivers for the signal has an event occur on it. The resolution function
is executed and returns a single value from all of the driver values; this
value is the new value of the signal.
In typical simulators, resolution functions are built in, or fixed. With
VHDL, the designer has the ability to define any type of resolution function
desired, wired-or, wired-and, average signal value, and so on.
A resolution function has a single-argument input and returns a single
value. The single-input argument consists of an unconstrained array of
driver values for the signal that the resolution function is attached to. If
the signal has two drivers, the unconstrained array is two elements long;
if the signal has three drivers, the unconstrained array is three elements
long. The resolution function examines the values of all of the drivers and
returns a single value called the resolved value of the signal.
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Chapter Five
Let’s examine a resolution function for the type fourval that was used
in the conversion function examples. The type declaration for fourval is
shown here:
TYPE fourval IS (X, L, H, Z);
Four distinct values are declared that represent all of the possible
values that the signal can obtain. The value L represents a logical 0, the
value H represents a logical 1, the value Z represents a high-impedance
or open-collector condition, and, finally, the value X represents an unknown
condition in which the value can represent an L or an H, but we’re not sure
which. This condition can occur when two drivers are driving a signal, one
driver driving with an H, and the other driving with an L.
Listed by order of strength, with the weakest at the top, the values are
as follows:
■ Z —Weakest, H, L, or X can override
■ H,L —Medium strength, only X can override
■ X —Strong, no override
Using this information, a truth table for two inputs can be developed,
as shown in Figure 5-1.
This truth table is for two input values. It can be expanded to more
inputs by successively applying it to two values at a time. This can be done
because the table is commutative and associative. An L and a Z, or a Z and
an L, gives the same results. An (L, Z) with H gives the same results as an
(H, Z) with an L. These principles are very important, because the order of
driver values within the input argument to the resolution function is nondeterministic from the designer’s point of view. Any dependence on order
can cause nondeterministic results from the resolution function.
Figure 5-1
Four State Truth
Table.
Z
L
H
X
Z
Z
L
H
X
L
L
L
X
X
H
H
X
H
X
X
X
X
X
X
Subprograms and Packages
121
Using all of this information, a designer can write a resolution function
for this type. The resolution function maintains the highest strength seen
so far and compares this value with new values a single element at a time,
until all values have been exhausted. This algorithm returns the higheststrength value.
Following is an example of such a resolution function:
PACKAGE fourpack IS
TYPE fourval IS (X, L, H, Z);
TYPE fourval_vector IS ARRAY (natural RANGE <> ) OF
fourval;
FUNCTION resolve( s: fourval_vector) RETURN fourval;
END fourpack;
PACKAGE BODY fourpack IS
FUNCTION resolve( s: fourval_vector) RETURN fourval IS
VARIABLE result : fourval := Z;
BEGIN
FOR i IN s’RANGE LOOP
CASE result IS
WHEN Z =>
CASE s(i) IS
WHEN H =>
result := H;
WHEN L =>
result := L;
WHEN X =>
result := X;
WHEN OTHERS =>
NULL;
END CASE;
WHEN L =>
CASE s(i) IS
WHEN H =>
result := X;
WHEN X =>
result := X;
WHEN OTHERS =>
NULL;
END CASE;
WHEN H =>
CASE s(i) IS
WHEN L =>
result := X;
WHEN X =>
result := X;
WHEN OTHERS =>
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Chapter Five
NULL;
END CASE;
WHEN X =>
result := X;
END CASE;
END LOOP;
RETURN result;
END resolve;
END fourpack;
The input argument is an unconstrained array of the driver-base
type, fourval. The resolution function examines all of the values of the
drivers passed in argument s one at a time and returns a single value
of fourval type to be scheduled as the signal value.
Variable result is initialized to a Z value to take care of the case of zero
drivers for the signal. In this case, the loop is never executed, and the
result value returned is the initialization value. It is also a good idea to
initialize the result value to the weakest value of the value system to allow
overwriting by stronger values.
If a nonzero number of drivers exists for the signal being resolved, then
the loop is executed once for each driver value passed in argument s. Each
driver value is compared with the current value stored in variable result.
If the new value is stronger according to the rules outlined earlier, then
the current result is updated with the new value.
Let’s look at some example driver values to see how this works. Assuming
that argument s contained the driver values shown in Figure 5-2, what
would the result be?
Initial
Figure 5-2
Four State Resolution
with Two Values.
Value
Driver Values
Z
Z
H
Z
H
Resultant Value
Subprograms and Packages
123
Because there are two drivers, the loop is executed twice. The first time
through, the loop variable result contains the initial value Z. The first
driver value is also a Z value. Value Z compared with value Z produces a
resulting value Z.
The next iteration through the loop retrieves the next driver value,
which is H. The value H compared with value Z returns value H. The
function therefore returns the value H as the resolved value of the signal.
Another case is shown in Figure 5-3. In this example, there are three
drivers, and the resolution function executes the loop three times. In the
first iteration of the loop, the initial value of result (Z) is compared with
the first driver value (H). The value H is assigned to result. In the next
iteration, result (H) is compared with the second driver (Z). The value H
remains in result because the value Z is weaker. Finally, the last iteration result (H) is compared with the last driver value (L). Because these
values are of the same strength, the value X is assigned to result. The
value X is returned from the function as the resolved value for the signal.
NINE-VALUE RESOLUTION FUNCTION Some simulators use
more complex types to represent the value of a signal. For instance, what
might a resolution function look like for a nine-value system, typical of
most workstation-based simulators in use currently? Following are the
nine values in the value system:
Figure 5-3
Four State Resolution
with Three Values.
Initial
Value
Driver Values
Z
H
Z
L
H
H
X
Resultant Value
124
Chapter Five
Z0, Z1, ZX, R0, R1, RX, F0, F1, FX
weakest-----------------------------strongest
The system consists of three strengths and three logic values. The three
strengths represent the following:
■ Z —High impedance strength, few hundred k of resistance
■ R —Resistive, few k of resistance
■ F —Forcing, few ohms of resistance
The three logic levels are represented as follows:
■ 0 —Logical 0 or false
■ 1 —Logical 1 or true
■ X —Logical unknown
The nine states are described as follows:
■ Z0 —High-impedance 0
■ Z1 —High-impedance 1
■ ZX —High-impedance unknown
■ R0 —Resistive 0
■ R1 —Resistive 1
■ RX —Resistive unknown
■ F0 —Forcing 0
■ F1 —Forcing 1
■ FX —Forcing unknown
A few simple rules can be used to define how the resolution function
should work:
—Strongest strength always wins.
—If strengths are the same and values are different, return same
strength but X value.
Following are the type declarations needed for the value system:
PACKAGE ninepack IS
TYPE strength IS (Z, R, F);
TYPE nineval IS ( Z0, Z1, ZX,
TYPE nineval IS ( R0, R1, RX,
TYPE nineval IS ( F0, F1, FX );
TYPE ninevalvec IS ARRAY(natural RANGE <>) OF nineval;
Subprograms and Packages
125
TYPE ninevaltab IS ARRAY(nineval’LOW TO
nineval’HIGH) OF nineval;
TYPE strengthtab IS ARRAY(strength’LOW TO
strength’HIGH) OF nineval;
FUNCTION resolve9( s: ninevalvec) RETURN nineval;
END ninepack;
The package body contains the resolution function (package bodies are
discussed near the end of this chapter).
PACKAGE BODY ninepack IS
FUNCTION resolve9( s: ninevalvec) RETURN nineval IS
VARIABLE result: nineval;
CONSTANT get_strength : ninevaltab :=
(Z,
--Z0
Z,
--Z1
Z,
--ZX
R,
--R0
R,
--R1
R,
--RX
F,
--F0
F,
--F1
F);
--FX
CONSTANT x_tab : strengthtab :=
(ZX,
--Z
RX,
--R
FX); --F
BEGIN
IF s’LENGTH = 0 THEN RETURN ZX; END IF;
result := s(0);
FOR i IN s’RANGE LOOP
IF get_strength(result) < get_strength(s(i)) THEN
result := s(i);
ELSIF get_strength(result) = get_strength(s(i)) THEN
IF result /= s(i) THEN
result := x_tab(get_strength(result));
END IF;
END IF;
END LOOP;
RETURN result;
END resolve9;
END ninepack;
The package ninepack declares a number of types used in this example,
including some array types to make the resolution function easier to
126
Chapter Five
implement. The basic algorithm of the function is the same as the fourval
resolution function; however, the operations with nine values are a little
more complex. Function resolve9 still does a pairwise comparison of the
input values to determine the resultant value. With a nine-value system,
the comparison operation is more complicated, and therefore some constant
arrays were declared to make the job easier.
The constant get_strength returns the driving strength of the driver
value. The constant x_tab returns the appropriate unknown nine-state
value, given the strength of the input. These constants could have been
implemented as IF statements or CASE statements, but constant arrays
are much more efficient.
In the nine-value system, there are three values at the lowest strength
level, so the variable result has to be initialized more carefully to predict
correct results. If there are no drivers, the range attribute of argument s
returns 0, and the default value (ZX) is returned.
Let’s look at a few examples of driver-input arguments and see what
the resolution function predicts. An example of two drivers is shown in
Figure 5-4.
This example contains two driver values, Z1 and R0. Variable result is
initialized to the first driver value, and the loop executes as many times
as there are drivers. The first time through the loop, result equals Z1 and
the first driver equals Z1. Variable result remains at Z1 because the
values are equal. The next time through the loop, variable result contains Z1, and the second driver contains R0. The constant get_strength
returns strength R. The constant get_strength for variable result returns
strength Z. Strength R is lexically greater than strength Z. This is because
value R has a higher position number than Z, because R is listed after Z
in the type declaration for type strength. The fact that the new driver has
Initial
Figure 5-4
Nine State Resolution
with Two Values.
Value
Driver Values
Z1
Z1
R0
Z1
R0
Resultant Value
Subprograms and Packages
127
a stronger strength value than variable result causes variable result to
be updated with the stronger value, R0.
Another example shows how the constant x_tab is used to predict the
correct value for conflicting inputs. The driver values are shown in the
array in Figure 5-5.
In this example, variable result is initialized to F0. The first iteration
of the loop does nothing because the first driver and the resultinitialization value are the same value. The next iteration starts with
variable result containing the value F0, and the next driver value as R0.
Because the value in variable result is greater in strength than the value
of the new driver, no action is implemented, except to advance the loop to
the next driver.
The last driver contains the value F1. The strength of the value contained
in variable result and the new driver value are the same. Therefore, the
IF statement checking this condition is executed and succeeds. The next
IF statement checks to see if the logical values are the same for both variable result and the new driver. Variable result contains an F0, and the
new driver value contains an F1. The values are not the same, and the
x_tab table is used to return the correct unknown value for the strength
of the driver values. The x_tab table returns the value FX, which is returned
as the resolved value.
A more efficient method to implement the loop would be to skip the
first iteration where the first driver is compared to itself, because the
value in variable result is initialized to the first driver value. It is left
as an exercise to the reader to write this new loop iteration mechanism.
Figure 5-5
Nine State Resolution
with Three Values.
Initial
Value
Driver Values
F0
F0
R0
F1
F0
F0
FX
Resultant Value
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Chapter Five
Although VHDL simulators can support any type of resolution that can
be legally written in the language, synthesis tools can only support a
subset. The reason stems from the fact that the synthesis tools must build
actual hardware from the VHDL description. If the Resolution Function
maps into a common hardware behavior such as wired-or or wired-and,
then most synthesis tools allow the user the ability to tag the resolution
function appropriately. For instance, a Resolution Function that performs
a wired-or function is tagged with an attribute that tells the synthesis
tools to connect the outputs together.
COMPOSITE TYPE RESOLUTION For simple signal values such as
the nineval and fourval types, it is easy to see how to create the resolution function. But for signals of composite types, it is not so obvious. How
can one value of a composite type be stronger than another?
The answer is that one value must be designated as weaker than all of
the other values. Then the principle is the same as any other type being
resolved. In the fourval type, the value Z was considered the weakest
state, and any of the other values could overwrite this value. In the
nineval type, all values with a strength of Z could be overridden by values with a strength of R or F, and all values with strength R could be overridden by strength F.
To resolve a composite type, designate one value of the composite type
as unusable except to indicate that the signal is not currently being driven.
The resolution function checks how many drivers have this value and how
many drivers have a driving value. If only one driving value exists, then
the resolution function can return this value as the resolved value. If more
than one driving value is present, then an error condition probably exists
and the resolution function can announce the error.
A typical application for a composite type resolution function is shown
in Figure 5-6.
Signal XBUS can be driven from a number of sources, but hopefully only
one at a time. The resolution function must determine how many drivers
are trying to drive XBUS and return the correct value for the signal.
Following is the type declarations and resolution function for a composite type used in such a circuit:
PACKAGE composite_res IS
TYPE xtype IS
RECORD
addr : INTEGER;
data : INTEGER;
Subprograms and Packages
129
MEMORY
Figure 5-6
Block Diagram of
Computer.
CPU
XBUS
IO_PORT
DISK_CONTROL
END RECORD;
TYPE xtypevector IS ARRAY( natural RANGE <>) OF xtype;
CONSTANT notdriven : xtype := (-1,-1);
FUNCTION cresolve( t : xtypevector) RETURN xtype;
END composite_res;
PACKAGE BODY composite_res IS
FUNCTION cresolve( t : xtypevector) RETURN xtype IS
VARIABLE result : xtype := notdriven;
VARIABLE drive_count : INTEGER := 0;
BEGIN
IF t’LENGTH = 0 THEN RETURN notdriven;
END IF;
FOR i IN t’RANGE LOOP
IF t(i) /= notdriven THEN
drive_count := drive_count + 1;
IF drive_count = 1 THEN
result := t(i);
ELSE
result := notdriven;
ASSERT FALSE
REPORT “multiple drivers detected”
SEVERITY ERROR;
END IF;
END IF;
END LOOP;
RETURN result;
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Chapter Five
END cresolve;
END composite_res;
Type xtype declares the record type for signal xbus. Type xtypevector
is an unconstrained array type of xtype values used for the resolution
function input argument t. Constant notdriven declares the value of the
record that is used to signify that a signal driver is not driving. Negative
number values were used to represent the notdriven state because, in this
example, only positive values are used in the addr and data fields. But
what happens if all of the values must be used for a particular type? The
easiest solution is probably to declare a new type which is a record, containing the original type as one field of the record, and a new field which
is a boolean that determines whether the driver is driving or not driving.
In this example, resolution function cresolve first checks to make certain
that at least one driver value is passed in argument t (drivers can be turned
off using guarded signal assignment). If at least one driver is driving, the
loop statement loops through all driver values, looking for driving values.
If a driving value is detected, and it is the first, then this value is assumed
to be the output resolved value, until proven otherwise. If only one driving
value occurs, that value is returned as the resolved value.
If a second driving value appears, the output is set to the nondriven
value, signifying that the outcome is uncertain, and the ASSERT statement
writes out an error message to that effect.
In this example, the negative numbers of the integer type were not
used except to indicate whether the signal was driving or not. We reserved
one value to indicate this condition. Another value could be reserved to
indicate the multiple-driven case such that when multiple drivers are
detected on the signal, this value would be returned as the resolved value.
An example might look like this:
CONSTANT multiple_drive : xtype := (-2,-2);
This constant provides the capability of distinguishing between a nondriven signal and a multiple-driven signal.
RESOLVED SIGNALS So far we have discussed how to write resolution functions that can resolve signals of multiple drivers, but we have not
discussed how all of the appropriate declarations are structured to accomplish this.
Resolved signals are created using one of two methods. The first is
to create a resolved subtype and declare a signal using this type. The
second is to declare a signal specifying a resolution function as part of
the signal declaration.
Subprograms and Packages
131
Let’s discuss the resolved subtype method first. To create a resolved subtype, the designer declares the base type, then declares the subtype specifying the resolution function to use for this type. An example looks like this:
TYPE fourval IS (X, L, H, Z);
-- won’t compile
SUBTYPE resfour IS resolve fourval; -- as is
The first declaration declares the enumerated type fourval. The second
declaration is used to declare a subtype named resfour, which uses a
resolution function named resolve to resolve the base type fourval. This
syntax does not compile as is because the function resolve is not visible.
To declare a resolved subtype requires a very specific combination of
statements, in a very specific ordering.
Following is a correct example of the resolved type:
PACKAGE fourpack IS
TYPE fourval IS (X, L, H, Z); -- line 1
TYPE fourvalvector IS ARRAY(natural RANGE <>)
OF fourval;
-- line 2
FUNCTION resolve( s: fourvalvector) RETURN fourval;
-- line 3
SUBTYPE resfour IS resolve fourval; -- line 4
END fourpack;
The statement in line 2 declares an unconstrained array of the base
type that is used to contain the driver values passed to the resolution
function. The statement in line 3 declares the definition of the resolution
function resolve so that the subtype declaration can make use of it. The
body of the resolution function is implemented in the package body. Finally,
the statement in line 4 declares the resolved subtype using the base type
and the resolution function declaration.
The order of the statements is important, because each statement
declares something that is used in the next statement. If the unconstrained array declaration is left out, the resolution function could not be
declared, and if the resolution function was not declared, the subtype
could not be declared.
The second method of obtaining a resolved signal is to specify the resolution function in the signal declaration. In the following example, a signal
is declared using the resolution function resolve:
PACKAGE fourpack IS
TYPE fourval IS (X, L, H, Z);
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Chapter Five
TYPE fourvalvector IS ARRAY(natural RANGE <>) OF fourval;
FUNCTION resolve( s: fourvalvector) RETURN fourval;
SUBTYPE resfour IS resolve fourval;
END fourpack;
USE WORK.fourpack.ALL;
ENTITY mux2 IS
PORT( i1, i2, a : IN fourval;
q : OUT fourval);
END mux2;
ARCHITECTURE different OF mux2 IS
COMPONENT and2
PORT( a, b : IN fourval;
c : OUT fourval);
END COMPONENT;
COMPONENT inv
PORT( a : IN fourval;
b : OUT fourval);
END COMPONENT;
SIGNAL nota : fourval;
-- resolved signal
SIGNAL intq : resolve fourval := X;
BEGIN
U1: inv PORT MAP(a, nota);
U2: and2 PORT MAP(i1, a, intq);
U3: and2 PORT MAP(i2, nota, intq);
q <= intq;
END different;
The package fourpack declares all of the appropriate types and function
declarations so that the resolution function resolve is visible in the entity.
In the architecture declaration section, signal intq is declared of type
fourval, using the resolution function resolve. This signal is also given
an initial value of X.
Signal intq is required to have a resolution function because it is the
output signal for components U2 and U3. Each component provides a driver
to signal intq. Resolution function resolve is used to determine the end
result of the two driver values. Signal nota is not required to have a resolution function because it only has one driver, component U1.
Subprograms and Packages
133
Procedures
In the earlier section describing functions, we discussed how functions can
have a number of input parameters and always return one value. In contrast, procedures can have any number of in, out, and inout parameters. A
procedure call is considered a statement of its own; a function usually exists as part of an expression. The most usual case of using a procedure is
when more than one value is returned.
Procedures have basically the same syntax and rules as functions. A
procedure declaration begins with the keyword PROCEDURE, followed by
the procedure name, and then an argument list. The main difference between a function and a procedure is that the procedure argument list
most likely has a direction associated with each parameter; the function
argument list does not. In a procedure, some of the arguments can be mode
IN, OUT, or INOUT; in a function, all arguments are of mode IN by default
and can only be of mode IN.
A typical example where a procedure is very useful is during the conversion from an array of a multivalued type to an integer. A procedure
showing an example of how to accomplish this is shown here:
USE LIBRARY IEEE;
USE IEEE.std_logic_1164.ALL;
PROCEDURE vector_to_int (z : IN std_logic_vector;
x_flag : OUT BOOLEAN; q : INOUT INTEGER) IS
BEGIN
q := 0;
x_flag := false;
FOR i IN z’RANGE LOOP
q := q * 2;
IF z(i) = ‘1’ THEN
q := q + 1;
ELSIF z(i) /= F0 THEN
x_flag := TRUE;
END IF;
END LOOP;
END vector_to_int;
The behavior of this procedure is to convert the input argument z from
an array of a type to an integer. However, if the input array has unknown
values contained in it, an integer value cannot be generated from the array. When this condition occurs, output argument x_flag is set to true,
indicating that the output integer value is unknown. A procedure was
required to implement this behavior because more than one output value
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results from the procedure. Let’s examine what the result from the procedure is from the input array value shown here:
‘0’ ‘0’ ‘1’ ‘1’
The first step for the procedure is to initialize the output values to
known conditions, in case a zero length input argument is passed in.
Output argument x_flag is initialized to false and stays false until
proven otherwise.
The loop statement loops through the input vector z and progressively
adds each value of the vector until all values have been added. If the value
is a ‘1’, then it is added to the result. If the value is a ‘0’, then no addition is done. If any other value is found in the vector, the x_flag result is
set true, indicating that an unknown condition was found on one of the
inputs. (Notice that parameter q is defined as an inout parameter. This is
needed because the value is read in the procedure.)
PROCEDURE WITH INOUT PARAMETERS The examples we have
discussed so far have dealt mostly with in and out parameters, but procedures can have inout parameters also. The next example shows a procedure
that has an inout argument that is a record type. The record contains an
array of eight integers, along with a field used to hold the average of
all of the integers. The procedure calculates the average of the integer
values, writes the average in the average field of the record, and returns
the updated record:
PACKAGE intpack IS
TYPE bus_stat_vec IS ARRAY(0 to 7) OF INTEGER;
TYPE bus_stat_t IS
RECORD
bus_val: bus_stat_vec;
average_val : INTEGER;
END RECORD;
PROCEDURE bus_average( x : inout bus_stat_t );
END intpack;
PACKAGE BODY intpack IS
PROCEDURE bus_average( x : inout bus_stat_t ) IS
VARIABLE total : INTEGER := 0;
BEGIN
FOR i IN 0 TO 7 LOOP
total := total + x.bus_val(i);
END LOOP;
x.average_val := total / 8;
Subprograms and Packages
135
END bus_average;
END intpack;
A process calling the procedure might look as shown below:
PROCESS( mem_update )
VARIABLE bus_statistics : bus_stat_t;
BEGIN
bus_statistics.bus_val :=
(50, 40, 30, 35, 45, 55, 65, 85 );
bus_average(bus_statistics);
average <= bus_statistics.average_val;
END PROCESS;
The variable assignment to bus_statistics.bus_val fills in the appropriate bus utilization values to be used for the calculation. The next line is the
call to the bus_average procedure, which performs the averaging calculation.
Initially, the argument to the bus_average procedure is an input value, but
after the procedure has finished, the argument becomes an output value that
can be used inside the calling process. The output value from the procedure
is assigned to an output signal in the last line of the process.
SIDE EFFECTS Procedures have an interesting problem that is not
shared by their function counterparts. Procedures can cause side effects to
occur. A side effect is the result of changing the value of an object inside a
procedure when that object was not an argument to the procedure. For instance, a signal of an architecture can be assigned a value from within a
procedure, without that signal being an argument passed into the procedure. For instance, if two signals are not declared in the argument list of
a procedure, but are assigned from within a procedure called from the
current procedure, any assignments to these signals are side effects.
This is not a recommended method for writing a model. The debugging
and maintenance of a model of this type can be very difficult. This feature
was presented so the reader would understand the behavior if such a
model were examined.
Packages
The primary purpose of a package is to encapsulate elements that can be
shared (globally) among two or more design units. A package is a common
storage area used to hold data to be shared among a number of entities.
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Declaring data inside of a package allows the data to be referenced by
other entities; thus, the data can be shared.
A package consists of two parts: a package declaration section and a
package body. The package declaration defines the interface for the package,
much the same way that the entity defines the interface for a model. The
package body specifies the actual behavior of the package in the same
method that the architecture statement does for a model.
Package Declaration
The package declaration section can contain the following declarations:
■ Subprogram declaration
■ Type, subtype declaration
■ Constant, deferred constant declaration
■ Signal declaration creates a global signal
■ File declaration
■ Alias declaration
■ Component declaration
■ Attribute declaration, a user-defined attribute (Chapter 8,
“Advanced Topics”)
■ Attribute specification
■ Disconnection specification
■ Use clause
All of the items declared in the package declaration section are visible
to any design unit that uses the package with a USE clause. The interface to
a package consists of any subprograms or deferred constants declared in
the package declaration. The subprogram and deferred constant declarations must have a corresponding subprogram body and deferred constant
value in the package body or an error results.
Deferred Constants
Deferred constants are constants that have their name and type declared
in the package declaration section but have the actual value specified in
the package body section. Following is an example of a deferred constant
in the package declaration:
Subprograms and Packages
137
PACKAGE tpack IS
CONSTANT timing_mode : t_mode;
END tpack;
This example shows a deferred constant called timing_mode being
defined as type t_mode. The actual value of the constant is specified when
the package body for package tpack is compiled. This feature allows late
binding of the value of a constant so that the value of the constant can be
specified at the last possible moment and can be changed easily. Any design
unit that uses a deferred constant from the package declaration need not
be recompiled if the value of the constant is changed in the package body.
Only the package body needs to be recompiled.
Subprogram Declaration
The other item that forms the interface to the package is the subprogram
declaration. A subprogram declaration allows the designer to specify the
interface to a subprogram separately from the subprogram body. This
functionality allows any designers using the subprogram to start or continue
with the design, while the specification of the internals of the subprograms
are detailed. It also gives the designer of the subprogram bodies freedom
to change the internal workings of the subprograms, without affecting any
designs that use the subprograms. Following is an example of a subprogram declaration:
PACKAGE cluspack IS
TYPE nineval IS (Z0, Z1, ZX,
TYPE nineval IS (R0, R1, RX,
TYPE nineval IS (F0, F1, FX );
TYPE t_cluster IS ARRAY(0 to 15) OF nineval;
TYPE t_clus_vec IS ARRAY(natural range <>) OF t_cluster;
FUNCTION resolve_cluster( s: t_clus_vec )
RETURN t_cluster;
SUBTYPE t_wclus IS resolve_cluster t_cluster;
CONSTANT undriven : t_wclus;
END cluspack;
The subprogram declaration for resolve_cluster specifies the name
of the subprogram, any arguments to the subprogram, their types and
modes, and the return type if the subprogram is a function. This declaration can be used to compile any models that intend to use it, without the
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actual subprogram body specified yet. The subprogram body must exist
before the simulator is built, during elaboration.
Package Body
The main purpose of the package body is to define the values for deferred
constants and specify the subprogram bodies for any subprogram declarations from the package declaration. However, the package body can also
contain the following declarations:
■ Subprogram declaration
■ Subprogram body
■ Type, subtype declaration
■ Constant declaration, which fills in the value for the deferred constant
■ File declaration
■ Alias declaration
■ Use clause
All of the declarations in the package body, except for the constant
declaration that is specifying the value of a deferred constant and the subprogram body declaration, are local to the package body.
Let’s examine a package body for the package declaration that was
discussed in the last section:
PACKAGE BODY cluspack
CONSTANT undriven :
(ZX, ZX, ZX,
ZX, ZX, ZX,
ZX, ZX, ZX,
ZX, ZX, ZX,
IS
t_wclus :=
ZX,
ZX,
ZX,
ZX);
FUNCTION resolve_cluster ( s: t_clus_vec )
return t_cluster IS
VARIABLE result : t_cluster;
VARIABLE drive_count : INTEGER;
BEGIN
IF s’LENGTH = 0 THEN RETURN undriven;
END IF;
FOR i in s’RANGE LOOP
IF s(i) /= undriven THEN
drive_count := drive_count + 1;
IF drive_count = 1 THEN
Subprograms and Packages
139
result := s(i);
ELSE
result := undriven;
ASSERT FALSE
REPORT “multiple drivers detected”
SEVERITY ERROR;
END IF;
END IF;
END LOOP;
RETURN result;
END resolve_cluster;
END cluspack;
The package body statement is very similar to the package declaration,
except for the keyword BODY after package. The contents of the two design
units are very different, however. This package body example contains
only two items: the deferred constant value for deferred constant undriven
and the subprogram body for subprogram resolve_cluster. Notice how
the deferred constant value specification matches the deferred constant
declaration in the package declaration, and the subprogram body matches
the subprogram declaration in the package declaration. The subprogram
body must match the subprogram declaration exactly in the number of
parameters, the type of parameters, and the return type.
A package body can also contain local declarations that are used only
within the package body to create other subprogram bodies, or deferred
constant values. These declarations are not visible outside of the package
body but can be very useful within the package body. Following is an
example of a complete package making use of this feature:
USE LIBRARY IEEE;
USE IEEE.std_logic_1164.ALL;
PACKAGE math IS
TYPE st16 IS ARRAY(0 TO 15) OF std_logic;
FUNCTION add(a, b: IN st16) RETURN st16;
FUNCTION sub(a, b: IN st16) RETURN st16;
END math;
PACKAGE BODY math IS
FUNCTION vect_to_int(S : st16) RETURN INTEGER IS
VARIABLE result : INTEGER := 0;
BEGIN
FOR i IN 0 TO 7 LOOP
result := result * 2;
IF S(i) = ‘1’ THEN
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Chapter Five
result := result + 1;
END IF;
END LOOP;
RETURN result;
END vect_to_int;
FUNCTION int_to_st16(s : INTEGER) RETURN st16 IS
VARIABLE result : st16;
VARIABLE digit : INTEGER := 2**15;
VARIABLE local : INTEGER;
BEGIN
local : = s;
FOR i IN 15 DOWNTO 0 LOOP
IF local/digit >>= 1 THEN
result(i) := ‘1’;
local := local - digit;
ELSE
result(i) := ‘0’;
END IF;
digit := digit/2;
END LOOP;
RETURN result;
END int_to_st16;
FUNCTION add(a, b: IN st16) RETURN st16 IS
VARIABLE result : INTEGER;
BEGIN
result := vect_to_int(a) + vect_to_int(b);
RETURN int_to_st16(result);
END add;
FUNCTION sub(a, b: IN st16) RETURN st16 IS
VARIABLE result : INTEGER;
BEGIN
result := vect_to_int(a) - vect_to_int(b);
RETURN int_to_st16(result);
END sub;
END math;
The package declaration declares a type st16 and two functions, add
and sub, that work with this type. The package body has function bodies
for function declarations add and sub and also includes two functions that
are only used in the package body. These functions are int_to_st16 and
vect_to_int. These functions are not visible outside of the package body.
To make these functions visible, a function declaration would need to be
added to the package declaration, for each function.
Subprograms and Packages
141
Functions vect_to_int and int_to_st16 must be declared ahead of
function add to compile correctly. All functions must be declared before
they are used to compile correctly.
SUMMARY
In this chapter, we discussed the different kinds of subprograms and some
of the uses for them. Specifically, we covered the following:
■ How subprograms consist of functions and procedures. Functions
have only input parameters and a single return value; procedures
can have any number of in, out, and inout parameters.
■ How functions can be used as conversion functions to convert from
one type to another.
■ How functions can be used as resolution functions to calculate the
proper value on a multiple-driven network.
■ How procedures are considered statements; functions are usually
part of an expression. Procedures can exist alone; functions are
usually called as part of a statement.
■ How packages are used to encapsulate information that is to be
shared among multiple design units.
■ How packages consist of a package declaration in which all of the
type, subprogram, and other declarations exist and a package body
in which subprogram bodies and deferred constants exist.
In the next chapter, we discuss how attributes can make some descriptions easier to read and more compact.
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CHAPTER
6
Predefined
Attributes
This chapter discusses VHDL predefined attributes and
the way that concise readable models can be written
using attributes. Predefined attributes are data that can
be obtained from blocks, signals, and types or subtypes.
The data obtained falls into one of the following categories
shown:
■ Value kind—A simple value is returned.
■ Function kind—A function call is performed to return
a value.
■ Signal kind—A new signal is created whose value is
derived from another signal.
■ Type kind—A type mark is returned.
■ Range kind—A range value is returned.
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Chapter Six
Predefined attributes have a number of very important applications.
Attributes can be used to detect clock edges, perform timing checks in
concert with ASSERT statements, return range information about unconstrained types, and much more. All of these applications are examined in
this chapter. First, we discuss each of the predefined attribute kinds and
the ways that these attributes can be applied to modeling.
Value Kind Attributes
Value attributes are used to return a particular value about an array of a
type, a block, or a type in general. Value attributes can be used to return
the length of an array or the lowest bound of a type. Value attributes can
be further broken down into three subclasses:
■ Value type attributes, which return the bounds of a type
■ Value array attributes, which return the length of an array
■ Value block attributes, which return block information
Value Type Attributes
Value type attributes are used to return the bounds of a type. For instance,
a type defined as shown in the following would have a low bound of 0 and
a high bound of 7:
TYPE state IS (0 TO 7);
There are four predefined attributes in the value type attribute category:
■ T’LEFT, which returns the left bound of a type or subtype
■ T’RIGHT, which returns the right bound of a type or subtype
■ T’HIGH, which returns the upper bound of a type or subtype
■ T’LOW, which returns the lower bound of a type or subtype
Attributes are specified by the character ’ and then the attribute name.
The object preceding the ’ is the object that the attribute is attached to.
The capital T in the preceding descriptions means that the object that the
attribute is attached to is a type. The ’ character is pronounced “tick”
among VHDL hackers. Therefore, the first attribute in the preceding list
is specified “T tick left.”
Predefined Attributes
145
The left bound of a type or subtype is the leftmost entry of the range
constraint. The right bound is the rightmost entry of the type or subtype.
In the following example, the left bound is -32,767, and the right bound
is 32,767:
TYPE smallint IS -32767 TO 32767;
The upper bound of a type or subtype is the bound with the largest
value, and the lower bound is the bound with the lowest value. In the preceding example, for the type smallint, the upper bound is 32,767, and the
lower bound is -32,767.
To use one of these value attributes, the type mark name is followed
by the attribute desired. For example, following is the syntax to return the
left bound of a type:
PROCESS(x)
SUBTYPE smallreal IS REAL RANGE -1.0E6 TO 1.0E6;
VARIABLE q : real;
BEGIN
q := smallreal’LEFT;
-- use of ’left returns
-- -1.0E6
END test;
In this example, variable q is assigned the left bound of type smallreal.
Variable q must have the same type as the bounds of the type for the
assignment to occur. (The assignment could also occur if variable q was cast
into the appropriate type.) After the assignment has occurred, variable q
contains -1.0E6, which is the left bound of type smallreal.
In the next example, all of the attributes are used to show what happens
when a DOWNTO range is used for a type:
PROCESS(a)
TYPE bit_range IS ARRAY(31 DOWNTO 0) OF BIT;
VARIABLE left_range, right_range, uprange, lowrange :
integer;
BEGIN
left_range := bit_range’LEFT;
-- returns 31
right_range := bit_range’RIGHT;
-- returns 0
uprange
:= bit_range’HIGH;
-- returns 31
lowrange
:= bit_range’LOW;
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Chapter Six
-- returns 0
END PROCESS;
This example shows how the different attributes can be used to return
information about a type. When ranges of a type are defined using (a TO
b) where b > a, the ’LEFT attribute will always equal the ’LOW attribute;
but when a range specification using (b DOWNTO a) where b > a is used,
the ’HIGH and ’LOW can be used to determine the upper and lower bounds
of the type.
Value type attributes are not restricted to numeric types. These attributes
can also be used with any scalar type. Following is an example using
enumerated types:
ARCHITECTURE b OF a IS
TYPE color IS (blue, cyan, green, yellow, red, magenta);
SUBTYPE reverse_color IS color RANGE red DOWNTO green;
SIGNAL color1, color2, color3,
color4, color5, color6,
color7, color8 : color;
BEGIN
color1 <= color’LEFT;
color2 <= color’RIGHT;
-- returns blue
-- returns magenta
color3 <= color’HIGH;
color4 <= color’LOW;
-- returns magenta
-- returns blue
color5 <= reverse_color’LEFT;
-- returns red
color6 <= reverse_color’RIGHT;
-- returns green
color7 <= reverse_color’HIGH;
-- returns red
color8 <= reverse_color’LOW;
-- returns green
END b;
This example illustrates how value type attributes can be used with
enumerated types to return information about the type. Signals color1
and color2 are assigned blue and magenta, respectively, the left and right
bounds of the type. It is easy to see how these values are obtained by
examining the declaration of the type. The left bound of the type is blue
and the right bound is magenta. What is returned for the ’HIGH and ’LOW
attributes of an enumerated type? The answer relates to the position
numbers of the type. For an integer and real type, the position numbers
Predefined Attributes
147
of a value are equal to the value itself; but for an enumerated type, the
position numbers of a value are determined by the declaration of the type.
Values declared earlier have lower position numbers than values declared
later. Value blue from the preceding example has a position number of 0,
because it is the first value of the type. Value cyan has a position number
1, green has 2, and so on. From these position numbers, the high and low
bounds of the type can be found.
Signals color5 through color8 are assigned attributes of the type
reverse_color. This type has a DOWNTO range specification. Attributes
’HIGH and ’RIGHT do not return the same value because the range is
reversed. Value red has a higher position number than value green, and
therefore a DOWNTO is needed for the range specification.
Value Array Attributes
There is only one value array attribute: ’LENGTH. Given an array type, this
attribute returns the total length of the array range specified. This
attribute works with array ranges of any scalar type and with multidimensional arrays of scalar-type ranges. Following is a simple example:
PROCESS(a)
TYPE bit4 IS ARRAY(0 TO 3) of BIT;
TYPE bit_strange IS ARRAY(10 TO 20) OF BIT;
VARIABLE len1, len2 : INTEGER;
BEGIN
len1 := bit4’LENGTH;
-- returns 4
len2 := bit_strange’LENGTH; -- returns 11
END PROCESS;
The assignment to len1 assigns the value of the number of elements
in array type bit4. The assignment to len2 assigns the value of the number of elements of type bit_strange.
This attribute also works with enumerated-type ranges, as shown by
the following example:
PACKAGE p_4val IS
TYPE t_4val IS (’x’, ’0’, ’1’, ’z’);
TYPE t_4valX1 IS ARRAY(t_4val’LOW TO t_4val’HIGH) OF
t_4val;
TYPE t_4valX2 IS ARRAY(t_4val’LOW TO t_4val’HIGH) OF
t_4valX1;
TYPE t_4valmd IS ARRAY(t_4val’LOW TO t_4val’HIGH,
t_4val’LOW TO t_4val’HIGH) OF t_4val;
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Chapter Six
CONSTANT andsd : t_4valX2 :=
((’x’,
-- xx
’0’,
-- x0
’x’,
-- x1
(Notice this is an
’x’),
-- xz
array of arrays.)
(’0’,
-- 0x
’0’,
-- 00
’0’,
-- 01
’0’),
-- 0z
(’x’,
-- 1x
’0’,
-- 10
’1’,
-- 11
’x’),
-- 1z
(’x’,
-- zx
’0’,
-- z0
’x’,
-- z1
’x’)); -- zz
CONSTANT andmd
((’x’,
-’0’,
-’x’,
-’x’),
-(’0’,
-’0’,
-’0’,
-’0’),
-(’x’,
-’0’,
-’1’,
-’x’),
-(’x’,
-’0’,
-’x’,
-’x’)); -END p_4val;
: t_4valmd :=
xx
x0
x1
xz
(Notice this example
0x
is a multidimensional
00
array.)
01
0z
1x
10
11
1z
zx
z0
z1
zz
The two composite type constants, andsd and andmd, provide a lookup
table for an AND function of type t_4val. The first constant andsd uses an
array of array values, while the second constant andmd uses a multidimensional array to store the values. The initialization of both constants
is specified by the same syntax. If the ’LENGTH attribute is applied to these
types as shown in the following, the results shown in the VHDL comments
are obtained:
PROCESS(a)
VARIABLE len1, len2, len3, len4 : INTEGER;
BEGIN
len1 := t_4valX1’LENGTH;
-- returns 4
len2 := t_4valX2’LENGTH;
-- returns 4
Predefined Attributes
len3 := t_4valmd’LENGTH(1);
len4 := t_4valmd’LENGTH(2);
END PROCESS;
149
-- returns 4
-- returns 4
Type t_4valX1 is a four-element array of type t_4val. The range of
the array is specified using the predefined attributes ’LOW and ’HIGH of the
t_4val type. Assigning the length of type t_4valX1 to len1 returns
the value 4, the number of elements in array type t_4valX1. The assignment to len2 also returns the value 4, because the range of type t_valX2
is from ’LOW to ’HIGH of element type t_4valX1.
The assignments to len3 and len4 make use of a multidimensional
array type t_4valmd. Because a multidimensional array has more than
one range, an argument is used to specify a particular range. The range
defaults to the first range, if none is specified. In the type t_4valmd
example, the designer can pick the first or second range, because there
are only two to choose from. To pick a range, the argument passed to the
attribute specifies the number of the range starting at 1. An argument
value of 1 picks the first range, an argument value of 2 picks the second
range, and so on.
The assignment to len3 in the previous example passed in the value 1
to pick the first range. The first range is from t_4val’LOW to t_4val’HIGH,
or four entries. The second range is exactly the same as the first; therefore, both assignments return 4 as the length of the array.
If the argument to ’LENGTH is not specified, it defaults to 1. This was the
case in the first examples of ’LENGTH, when no argument was specified.
There was only one range, so the correct range was selected.
Value Block Attributes
There are two attributes that form the set of attributes that work with
blocks and architectures. Attributes ’STRUCTURE and ’BEHAVIOR return
information about how a block in a design is modeled. Attribute ’BEHAVIOR
returns true if the block specified by the block label, or architecture
specified by the architecture name, contains no component instantiation
statements. Attribute ’STRUCTURE returns true if the block or architecture contains only component instantiation statements and/or passive
processes.
The following two examples illustrate how these attributes work. The
first example contains only structural VHDL:
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Chapter Six
LIBRARY IEEE;
USE IEEE.std_logic_1164.ALL;
ENTITY shifter IS
PORT( clk, left : IN std_logic;
right : OUT std_logic);
END shifter;
ARCHITECTURE structural OF shifter IS
COMPONENT dff
PORT( d, clk : IN std_logic;
q : OUT std_logic);
END COMPONENT;
SIGNAL i1, i2, i3: std_logic;
BEGIN
u1: dff PORT MAP(d => left, clk => clk, q => i1);
u2: dff PORT MAP(d => i1, clk => clk, q => i2);
u3: dff PORT MAP(d => i2, clk => clk, q => i3);
u4: dff PORT MAP(d => i3, clk => clk, q => right);
checktime: PROCESS(clk)
VARIABLE last_time : time := time’left;
BEGIN
ASSERT (NOW - last_time = 20 ns)
REPORT “spike on clock”
SEVERITY WARNING;
last_time := now;
END PROCESS checktime;
END structural;
The preceding example is a shift register modeled using four dff components connected in series. A passive process statement exists in the
architecture for entity shifter, used to detect spikes on the clk input.
The following example shows the results of the attributes for the architecture structural:
structural’BEHAVIOR: returns false
structural’STRUCTURE: returns true
The passive process checktime has no effect on the fact that the
architecture is structural. If the process contained signal assignment
statements, then the process would no longer be considered passive, and
attribute ’STRUCTURE would also return false.
Predefined Attributes
151
For any block or architecture that does not contain any component
instantiation statements, attribute ’BEHAVIOR is true, and attribute
’STRUCTURE is false. For blocks or architectures that mix structure and
behavior, both attributes return false.
Function Kind Attributes
Function attributes return information to the designer about types,
arrays, and signals. When a function kind attribute is used in an expression, a function call occurs that uses the value of the input argument to
return a value. The value returned can be a position number of an enumerated value, an indication of whether a signal has changed this delta,
or one of the bounds of an array.
Function attributes can be subdivided into three general classifications:
■ Function type attributes, which return type values
■ Function array attributes, which return array bounds
■ Function signal attributes, which return signal history information
Function Type Attributes
Function type attributes return particular information about a type.
Given the position number of a value within a type, the value can be
returned. Also values to the left or right of an input value of a particular
type can be returned.
Function type attributes are one of the following:
■ ’POS (value), which returns position number of value passed in
■ ’VAL (value), which returns value from position number passed in
■ ’SUCC (value), which returns next value in type after input value
■ ’PRED (value), which returns previous value in type before input
value
■ ’LEFTOF (value), which returns value immediately to the left of the
input value
■ ’RIGHTOF (value), which returns value immediately to the right of
the input value
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Chapter Six
A typical use of a function type attribute is to convert from an enumerated or physical type to an integer type. Following is an example of
conversion from a physical type to an integer type:
PACKAGE ohms_law IS
TYPE current IS RANGE 0 TO 1000000
UNITS
ua;
-- micro amps
ma = 1000 ua;
-- milli amps
a = 1000 ma;
-- amps
END UNITS;
TYPE voltage IS RANGE 0 TO 1000000
UNITS
uv;
-- micro volts
mv = 1000 uv;
-- milli volts
v = 1000 mv;
-- volts
END UNITS;
TYPE resistance IS RANGE 0 TO 100000000
UNITS
ohm;
-- ohms
Kohm = 1000 ohm; -- kilo ohms
Mohm = 1000 Kohm;-- mega ohms
END UNITS;
END ohms_law;
use work.ohms_law.all;
ENTITY calc_resistance IS
PORT( i : IN current; e : IN voltage;
r : OUT resistance);
END calc_resistance;
ARCHITECTURE behave OF calc_resistance IS
BEGIN
ohm_proc: PROCESS( i, e )
VARIABLE convi, conve, int_r : integer;
BEGIN
convi := current’POS(i); -- current in ua
conve := voltage’POS(e); -- voltage in uv
-- resistance in ohms
int_r := conve / convi;
r <= resistance’VAL(int_r);
-----
another way to write this example
is shown below
r <=resistance’VAL(current’POS(i)
/ voltage’POS(e));
Predefined Attributes
153
END PROCESS;
END behave;
Package ohms_law declares three physical types used in this example.
Types current, voltage, and resistance are used to show how physical
types can be converted to type INTEGER and back to a physical type.
Whenever ports i or e have an event occur on them, process ohm_proc
is invoked and calculates a new value of resistance (r) from the current (i)
and the voltage (e). Variables conve, convi, and int_r were not necessary
in this example but were added for ease of understanding. The commentedout assignment to output r shows an example where the internal variables
are not needed.
The first statement of the process assigns the position number of the
input value to variable convi. If the input value is 10 ua, then 10 is
assigned to variable convi.
The second statement assigns the position number of the value of
input e to variable conve. The base unit of type voltage is uv (microvolts);
therefore, the position number of any voltage value is determined based
on how many uv the input value is equal to.
The last line in the process converts the resistance value calculated
from the previous line to the appropriate ohms value in type resistance.
The ’VAL attribute is used to convert a position number to a physical type
value of type resistance.
The preceding example illustrates how ’POS and ’VAL work, but not
’SUCC, ’PRED, ’RIGHTOF, and ’LEFTOF. Following is a very simple example
using these attributes:
PACKAGE p_color IS
TYPE color IS ( red, yellow, green, blue, purple,
orange );
SUBTYPE reverse_color is color RANGE orange downto red ;
END p_color;
Assuming the preceding types, the following results are obtained:
■ color’SUCC (blue) returns purple.
■ color’PRED (green) returns yellow.
■ reverse_color’SUCC (blue) returns green.
■ reverse_color’PRED (green) returns blue.
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Chapter Six
■ color’RIGHTOF (blue) returns purple.
■ color’LEFTOF (green) returns yellow.
■ reverse_color’RIGHTOF (blue) returns green.
■ reverse_color’LEFTOF (green) returns blue.
For ascending ranges, the following is true:
’SUCC(x) = ’RIGHTOF(x);
’PRED(x) = ’LEFTOF(x);
For descending ranges, the opposite is true:
’SUCC(x) = ’LEFTOF(x);
’PRED(x) = ’RIGHTOF(x);
What happens if the value passed to ’SUCC, ’PRED, and so on is at the
limit of the type? For instance, for type color, what is the value of the
expression shown below:
y := red;
x := color’PRED(y);
The second expression causes a runtime error to be reported, because
a range constraint has been violated.
Function Array Attributes
Function array attributes return the bounds of array types. An operation
that requires accessing every location of an array can use these attributes
to find the bounds of the array.
The four kinds of function array attributes are:
■ array’LEFT (n), which returns the left bound of index range n
■ array’RIGHT (n), which returns the right bound of index range n
■ array’HIGH (n), which returns the upper bound of index range n
■ array’LOW (n), which returns the lower bound of index range n
These attributes are exactly like the value type attributes that were
discussed earlier, except that these attributes work with arrays.
For ascending ranges, the following is true:
array’LEFT = array’LOW
array’RIGHT = array’HIGH
Predefined Attributes
155
For descending ranges, the opposite is true:
array’LEFT = array’HIGH
array’RIGHT = array’LOW
Following is an example where these attributes are very useful:
PACKAGE p_ram IS
TYPE t_ram_data IS ARRAY(0 TO 511) OF INTEGER;
CONSTANT x_val : INTEGER := -1;
CONSTANT z_val : INTEGER := -2;
END p_ram;
USE WORK.p_ram.ALL;
LIBRARY IEEE; USE IEEE.std_logic_1164.ALL;
ENTITY ram IS
PORT( data_in : IN INTEGER;
PORT( addr : IN INTEGER;
PORT( data : OUT INTEGER;
PORT( cs : IN std_logic;
PORT( r_wb: in std_logic);
END ram;
ARCHITECTURE behave_ram OF ram IS
BEGIN
main_proc: PROCESS( cs, addr, r_wb )
VARIABLE ram_data : t_ram_data;
VARIABLE ram_init : boolean := false;
BEGIN
IF NOT(ram_init) THEN
FOR i IN ram_data’LOW TO ram_data’HIGH LOOP
ram_data(i) := 0;
END LOOP;
ram_init := TRUE;
END IF;
IF (cs = ’X’) OR (r_wb = ’X’)THEN
data <= x_val;
ELSIF ( cs = ’0’ ) THEN
data <=z_val;
ELSIF (r_wb = ’1’) THEN
IF (addr = x_val) OR (addr = z_val) THEN
data <=x_val;
ELSE
data <= ram_data(addr);
END IF;
ELSE
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Chapter Six
IF (addr = x_val) OR (addr = z_val) THEN
ASSERT FALSE
REPORT “ writing to unknown address”
SEVERITY ERROR;
data <= x_val;
ELSE
ram_data(addr) :=data_in;
data <= ram_data(addr);
END IF;
END IF;
END PROCESS;
END behave_ram;
This example implements an integer-based RAM device. There are
512 integer locations in the RAM, which is controlled by two control
lines. The first is cs (chip select), and the second is r_wb (read/write bar).
The model contains an IF statement that initializes the contents of the
RAM to a known value. A boolean variable (ram_init) is declared to
keep track of whether the RAM has been initialized or not. If this variable is false, the RAM has not yet been initialized. If true, initialization
has been performed.
The first time the process is executed, variable ram_init is false, and
the IF statement is executed. Inside the IF statement is a loop statement
that loops through every location of the RAM and sets the location to a
known value. This process is necessary because the starting value of type
INTEGER is the value integer’LEFT, or -2,147,483,647. Notice the use of
function array attributes ’LOW and ’HIGH to control the range of the initialization loop.
After the loop has been executed and all RAM locations have been
initialized, the ram_init variable is set to true. Setting the variable
ram_init to true prevents the initialization loop from executing again.
The rest of the model implements the read and write functions based
on the values of addr, data_in, r_wb, and cs. This model performs a lot of
error checking for unknown values on input ports. The model tries to intelligently handle these unknown input values.
Function Signal Attributes
Function signal attributes are used to return information about the behav-ior
of signals. These attributes can be used to report whether a signal
has just changed value, how much time has passed since the last event
Predefined Attributes
157
transition, or what the previous value of the signal was. There are five attributes that fall into this category. Following is a brief description of each:
■ S’EVENT, which returns true if an event occurred during the current delta; otherwise, returns false
■ S’ACTIVE, which returns true if a transaction occurred during the
current delta; otherwise, returns false
■ S’LAST_EVENT, which returns time elapsed since the previous
event transition of signal
■ S’LAST_VALUE, which returns previous value of S before the last
event
■ S’LAST_ACTIVE, which returns time elapsed since the previous
transaction of signal
Attributes ’EVENT and ’LAST_VALUE
Attribute ’EVENT is very useful for determining clock edges. By checking
if a signal is at a particular value, and if the signal has just changed, it
can be deduced that an edge has occurred on the signal. Following is an
example of a rising edge detector:
LIBRARY IEEE;
USE IEEE.std_logic_1164.ALL;
ENTITY dff IS
PORT( d, clk : IN std_logic;
PORT( q : OUT std_logic);
END dff;
ARCHITECTURE dff OF dff IS
BEGIN
PROCESS(clk)
BEGIN
IF ( clk = ’1’) AND ( clk’EVENT ) THEN
q <= d;
END IF;
END PROCESS;
END dff;
This example shows a very simple dff model. The clk input is used to
transfer the d input to the q output, on a rising edge of the clk. To detect
the rising edge of the clk input, this model makes use of the ’EVENT
attribute. If the value of the clk input is a ’1’, and the value has just
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Chapter Six
changed, then a rising edge must have occurred. (When a synthesis tool
is applied to the preceding example, a flip-flop results.)
What the preceding example ignores is the fact that an ’X’ value to a
’1’ value also looks like a rising edge when it is not. The next example
shows how to correct this problem using the ’LAST_VALUE attribute. The
IF statement from the preceding example is rewritten here:
IF ( clk = ’1’ ) AND ( clk’EVENT )
and ( clk’LAST_VALUE = ’0’) THEN
q <= d;
END IF;
In this example, one more check is made to make certain that the last
value of the clk input was a ’0’ before the new event occurred.
In both examples, the ’EVENT attribute was not really needed, because
the process statement had only clk as its sensitivity list. The only way
that the process statement could be executed would be because of an
event on signal clk. This is a true statement, but it is a good modeling
practice to check for the event anyway. Some time in the future, the model
may be modified to include an asynchronous preset or clear, and these
signals will be added to the sensitivity list for the process statement. Now,
when an event occurs on any of the inputs, the process is invoked. Using
the ’EVENT attribute, the process can determine which input caused the
process to be invoked.
Attribute ’LAST_EVENT
Attribute ’LAST_EVENT returns the time since the previous event occurred
on the signal. This attribute is very useful for implementing timing
checks, such as setup checks, hold checks, and pulse width checks. An
example of a setup time and a hold time are shown in Figure 6-1.
The rising edge of signal clk is the reference edge to which all checks
are performed. A setup time check guarantees that the data input does
not change during the setup time, and the hold time check guarantees
that the data input does not change during the time equal to the hold time
after the reference edge. This ensures correct operation of the device.
Following is an example of the setup time check using the ’LAST_EVENT
attribute:
LIBRARY IEEE;
USE IEEE.std_logic_1164.ALL;
ENTITY dff IS
Predefined Attributes
Figure 6-1
Setup and Hold Time
Waveform Description.
159
DATA
CLK
Setup
Time
Hold
Time
Reference
Clock Edge
GENERIC ( setup_time, hold_time : TIME );
PORT( d, clk : IN std_logic;
q : OUT std_logic);
BEGIN
setup_check : PROCESS ( clk )
BEGIN
IF ( clk = ’1’ ) and ( clk’EVENT ) THEN
ASSERT ( d’LAST_EVENT >= setup_time )
REPORT “setup violation”
SEVERITY ERROR;
END IF;
END PROCESS setup_check;
END dff;
ARCHITECTURE dff_behave OF dff IS
BEGIN
dff_process : PROCESS ( clk )
BEGIN
IF ( clk = ’1’ ) AND ( clk’EVENT ) THEN
q <= d;
END IF;
END PROCESS dff_process;
END dff_behave;
The setup_check procedure is contained in a passive process in the entity
for the dff model. The check could have been included in the architecture
for the dff model, but having the check in the entity allows the timing
check to be shared among any architecture of the entity.
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Chapter Six
The passive process executes for each event on signal clk. When the
clk input has a rising edge, the ASSERT statement is executed and performs the check for a setup violation.
The ASSERT statement checks to see that input d has not had an event
during the setup time passed in by the generic setup_time. Attribute
d’LAST_EVENT returns the time since the most recent event on signal d. If
the time returned is less than the setup time, the assertion fails and
reports a violation.
Attribute ’ACTIVE and ’LAST_ACTIVE
Attributes ’ACTIVE and ’LAST_ACTIVE trigger on transactions of the signal
attached to AND events. A transaction on a signal occurs when a model
in or inout port has an event occur that triggers the execution of the
model. The model is executed, but the result of the execution produces
the same output values. For instance, if an AND gate has a ’1’ value on
one input and a ’0’ on the other, the output value is ’0’. If the input
with a ’1’ value changes to a ’0’ value, the output remains ’0’; no event
is generated, but a transaction will have been generated on the output of
the AND gate.
Attribute ’ACTIVE returns true when a transaction or event occurs on
a signal, and attribute ’LAST_ACTIVE returns the time since a previous
transaction or event occurred on the signal it is attached to. Both of these
attributes are counterparts for attributes ’EVENT and ’LAST_EVENT, which
provide the same behavior for events.
Signal Kind Attributes
Signal kind attributes are used to create special signals, based on other signals. These special signals return information to the designer about the
signal that the attribute is attached to. The information returned is very
similar to some of the functionality provided by some of the function
attributes. The difference is that these special signals can be used anywhere that a normal signal can be used, including sensitivity lists.
Signal attributes return information such as whether a signal has been
stable for a specified amount of time, when a transaction has occurred on
a signal, and a delayed version of the signal can be created.
Predefined Attributes
161
One restriction on the use of these attributes is that they cannot be
used within a subprogram. A compiler error message results if a signal
kind attribute is used within a subprogram.
There are four attributes in the signal kind category:
■ s’DELAYED [(time)], which creates a signal of the same type as
the reference signal that follows the reference signal, delayed by
the time of the optional time expression
■ s’STABLE [(time)], which creates a boolean signal that is true
whenever the reference signal has had no events for the time
specified by the optional time expression
■ s’QUIET [(time)], which creates a boolean signal that is true
whenever the reference signal has had no transactions or events
for the time specified by the optional time expression
■ s’TRANSACTION, which creates a signal of type BIT that toggles its
value for every transaction or event that occurs on s
Attribute ’DELAYED
Attribute ’DELAYED creates a delayed version of the signal that it is attached
to. The same functionality can be obtained using a transport-delayed signal assignment. The difference between a transport-delay assignment and
the ’DELAYED attribute is that the designer has to do more bookkeeping
with the transport signal assignment method. With a transport signal assignment, a new signal must be declared.
Let’s look at one use for the ’DELAYED attribute. One method for modeling ASIC devices is to place path-related delays on the input pins of the
ASIC library part. An example of this method is shown in Figure 6-2.
Typically, before the layout process, educated guesses are made for the
delays of each input. After layout, the real delay values are back-annotated
to the model, and the simulation is run again with the real delays. One
method to provide for back annotation of the delay values is to use generic
values specified in the configuration for the device. (Configurations are discussed in Chapter 7, “Configurations.”) A typical model for one of the and2
gates shown in Figure 6-2 might look like this:
LIBRARY IEEE;
USE IEEE.std_logic_1164.ALL;
ENTITY and2 IS
GENERIC ( a_ipd, b_ipd, c_opd : TIME );
162
Figure 6-2
Gate Array Logic
with Input and Output Delays.
Chapter Six
c_opd
a_ipd
b_ipd
c_opd
and2
a_ipd
b_ipd
c_opd
and2
a_ipd
b_ipd
c_opd
and2
PORT ( a, b : IN std_logic;
c: OUT std_logic);
END and2;
ARCHITECTURE int_signals OF and2 IS
SIGNAL inta, intb : std_logic;
BEGIN
inta <= TRANSPORT a AFTER a_ipd;
intb <= TRANSPORT b AFTER b_ipd;
c <= inta AND intb AFTER c_opd;
END int_signals;
ARCHITECTURE attr OF and2 IS
BEGIN
c <= a’DELAYED(a_ipd) AND b’DELAYED(b_ipd) AFTER c_opd;
END attr;
In the preceding example, two architectures for entity and2 show two
different methods of delaying the input signals by the path delay. The first
method uses transport-delayed internal signals to delay the input signals.
These delayed signals are then ANDed together and assigned to output
port c.
The second method makes use of the predefined signal attribute
’DELAYED. Input signals a and b are delayed by the path delay generic value
a_ipd (a input path delay) and b_ipd (b input path delay). The values of
the delayed signals are ANDed together and assigned to output port c.
Predefined Attributes
163
If the optional time expression for attribute ’DELAYED is not specified,
0 ns is assumed. A signal delayed by 0 ns is delayed by one delta. (Delta
delay is discussed in Chapter 2.)
Another application for the ’DELAYED attribute is to perform a hold-check.
Earlier in this chapter, we discussed what setup and hold times were and
how to implement the setup check using ’LAST_EVENT. Implementing the
hold-check requires the use of a delayed version of the clk signal. The
example shown earlier has been modified to include the hold-check function
as shown here:
LIBRARY IEEE;
USE IEEE.std_logic_1164.ALL;
ENTITY dff IS
GENERIC ( setup_time, hold_time : TIME );
PORT( d, clk : IN std_logic;
PORT( q : OUT std_logic);
BEGIN
setup_check : PROCESS ( clk )
BEGIN
IF ( clk = ’1’ ) and ( clk’EVENT ) THEN
ASSERT ( d’LAST_EVENT >= setup_time )
REPORT “setup violation”
SEVERITY ERROR;
END IF;
END PROCESS setup_check;
hold_check : PROCESS (clk’DELAYED(hold_time))
BEGIN
IF ( clk’DELAYED(hold_time) = ’1’ ) and
( clk’DELAYED(hold_time)’EVENT ) THEN
ASSERT ( d’LAST_EVENT = 0 ns ) OR ( d’LAST_EVENT >
hold_time )
REPORT “hold violation”
SEVERITY ERROR;
END IF;
END PROCESS hold_check;
END dff;
ARCHITECTURE dff_behave OF dff IS
BEGIN
dff_process : PROCESS ( clk )
BEGIN
IF ( clk = ’1’ ) AND ( clk’EVENT ) THEN
q <= d;
END IF;
END PROCESS dff_process;
END dff_behave;
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Chapter Six
A delayed version of the clk input is used to trigger the hold-check. The
clk input is delayed by the amount of the hold-check. If the data input
changes within the hold time, d’LAST_EVENT returns a value that is less
than the hold time. When d changes exactly at the same time as the
delayed clk input, d’LAST_EVENT returns 0 ns. This is a special case and
is legal so it must be handled specially.
An alternative method for checking the hold time of a device is to trigger
the hold-check process when the d input changes and then look back at the
last change on the clk input. However, this is more complicated and
requires the designer to manually keep track of the last reference edge
on the clk input.
Another interesting feature of attributes that this model pointed out is
the cascading of attributes. In the preceding example, the delayed version
of the clk signal was checked for an event. This necessitated the use of
clk’DELAYED (hold_time) ’EVENT. The return value from this attribute
is true whenever the signal created by the ’DELAYED attribute has an
event during the current delta time point. In general, attributes can be
cascaded any level if the values returned from the previous attribute are
appropriate for the next attribute.
Attribute ’STABLE
Attribute ’STABLE is used to determine the relative activity level of a
signal. It can be used to determine if the signal just changed or has not
changed in a specified period of time. The resulting value output is itself
a signal that can be used to trigger other processes.
Following is an example of how attribute ’STABLE works:
LIBRARY IEEE;
USE IEEE.std_logic_1164.ALL;
ENTITY pulse_gen IS
PORT( a : IN std_logic;
PORT( b : OUT BOOLEAN);
END pulse_gen;
ARCHITECTURE pulse_gen OF pulse_gen IS
BEGIN
b <= a’STABLE( 10 ns );
END pulse_gen;
Figure 6-3 shows the resulting waveform b when waveform a is presented to the model.
Predefined Attributes
Figure 6-3
Example Showing
’DELAYED (10ns).
165
A
B
0
10
20
30
40
50
60
70
80
At the first two changes in signal a (10 ns and 30 ns), signal b immediately changes to false (actually at the next delta). Then when signal a
has been stable for 10 ns, signal b changes to true. At time 55 ns, signal
a changes value again, so signal b changes to false. Because signal a
changes 5 ns later (60 ns), signal a has not been stable long enough to
allow output b to go to a true value. Only at 10 ns after the last change
on signal a (60 ns) is the input signal a stable long enough to allow signal
b to change to true.
If the time value specified for the ’STABLE attribute is 0 ns, or not specified, then the ’STABLE attribute is false for 1 delta whenever the signal
that the attribute is attached to changes. An example of this scenario is
shown in Figure 6-4.
When used in this method, the resulting signal value has the same
timing but opposite value as function attribute ’EVENT. A statement to
detect the rising edge of a clock could be written in two ways, as shown
here:
IF (( clk’EVENT ) AND ( clk = ’1’ ) AND
( clk’LAST_VALUE = ’0’ )) THEN
.
. -- DO PROCESSING
.
END IF;
IF (( NOT( clk’STABLE) ) AND ( clk = ’1’ ) AND
(
clk’LAST_VALUE = ’0’ )) THEN
.
. --- DO PROCESSING
.
END IF;
166
Figure 6-4
Example Showing
’DELAYED (0 ns).
Chapter Six
A
B
One Delta wide
In both cases, the IF statement detects the rising edge; but the IF statement using ’EVENT is more efficient in memory space and speed. The reason for this is that attribute ’STABLE creates an extra signal in the design
that uses more memory to store, and whenever the value for the new
signal needs to be updated, it must be scheduled. Keeping track of signal
events costs memory and time.
Attribute ’QUIET
Attribute ’QUIET has the same functionality as ’STABLE, except that
’QUIET is triggered by transactions on the signal that it is attached to in
addition to events. Attribute ’QUIET creates a BOOLEAN signal that is true
whenever the signal it is attached to has not had a transaction or event
for the time expression specified.
Typically, models that deal with transactions involve complex models
of devices at the switch level or the resolution of driver values. Following
is an interesting application using the attribute ’QUIET:
ARCHITECTURE test OF test IS
TYPE t_int is (int1, int2, int3, int4, int5 );
SIGNAL int, intsig1, intsig2, intsig3 : t_int;
SIGNAL lock_out : BOOLEAN;
BEGIN
int1_proc: PROCESS
BEGIN
.
.
.
WAIT ON trigger1; -- outside trigger signal
WAIT UNTIL clk = ’1’;
IF NOT(lock_out) THEN
intsig1 <= int1;
END IF;
Predefined Attributes
167
END PROCESS int1_proc;
int2_proc: PROCESS
BEGIN
.
.
.
WAIT ON trigger2;-- outside trigger signal
WAIT UNTIL clk = ’1’;
IF NOT(lock_out) THEN
intsig2 <=int2;
END IF;
END PROCESS int2_proc;
int3_proc: PROCESS
BEGIN
.
.
.
WAIT ON trigger3;-- outside trigger signal
WAIT UNTIL clk = ’1’;
IF NOT(lock_out) THEN
intsig3 <=int3;
END IF;
END PROCESS int3_proc;
int <=intsig1 WHEN NOT(intsig1’QUIET) ELSE
intsig2 WHEN NOT(intsig2’QUIET) ELSE
intsig3 WHEN NOT(intsig3’QUIET) ELSE
int;
int_handle : PROCESS
BEGIN
WAIT ON int’TRANSACTION;-- described next
lock_out <= TRUE;
WAIT FOR 10 ns;
CASE int IS
WHEN int1 =>
.
.
WHEN int2 =>
.
.
WHEN int3 =>
.
.
WHEN int4 =>
.
.
WHEN int5 =>
.
.
END CASE;
lock_out <= false;
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Chapter Six
END PROCESS;
END test;
This example shows how a priority mechanism could be modeled for an
interrupt handler. Process int1_proc has the highest priority, and process
int3_proc has the lowest. Whenever one of the processes is triggered, the
appropriate interrupt handler is placed on signal int, and the interrupt
handler for that interrupt is called.
The model consists of three processes that drive the interrupt signal
int, and another process to call the appropriate interrupt handling function. Signal int is not a resolved signal and therefore cannot have multiple drivers. If a resolution function is written for signal int, the order of
the drivers cannot be used to determine priority. Therefore, the approach
shown in the preceding was taken.
In this approach, three internal signals intsig1, intsig2, and intsig3
are driven by each of the processes, respectively. These signals are then
combined, using a conditional signal assignment statement. The conditional signal assignment statement makes use of the predefined attribute
’QUIET to determine when a transaction has been assigned to a driver of
a signal. It is required that transactions are detected on the internal signals,
because the process always assigns the same value so an event only occurs
on the first assignment.
The priority mechanism is controlled by the conditional signal assignment statement. When a transaction occurs on intsig1, intsig2, or
intsig3, the assignment statement evaluates and assigns the appropriate
value to signal int based on the signal(s) that had a transaction. If a
transaction occurred only on intsig2, intsig2’QUIET would be false,
causing the conditional signal assignment statement to place the value of
intsig2 on signal int. But what happens if intsig3 and intsig2 occur
at the same time? The conditional signal assignment statement evaluates,
and the first clause that has a WHEN expression return true does the
assignment and then exits the rest of the statement. For this example, the
value for intsig2 is returned, because it is first in the conditional signal
assignment statement. The priority of the inputs is determined by the order of the WHEN clauses in the conditional signal assignment statement.
Attribute ’TRANSACTION
The process that implemented the interrupt handling for the previous
example uses the ’TRANSACTION attribute in a WAIT statement. This
Predefined Attributes
169
attribute is another of the attributes that creates a signal where it is
used. Attribute ’TRANSACTION creates a signal of type BIT that toggles
from ‘1’ or ‘0’ for every transaction of the signal that it is attached to.
This attribute is useful for invoking processes when transactions occur
on signals.
In the preceding example, the interrupt handler process needs to be
executed whenever a transaction occurs on signal int. This is true because
the same interrupt could happen twice or more in sequence. If this occurred,
a transaction, not an event would be generated on signal int. Without
the attribute ’TRANSACTION, WAIT statements are sensitive to events. By
using the attribute ’TRANSACTION, the value of int’TRANSACTION toggles for every transaction causing an event to occur, thus activating the
WAIT statement.
Type Kind Attributes
Type attributes return values of kind type. There is only one type
attribute, and it must be used with another value or function type attribute.
The only type attribute available in VHDL is the attribute t’BASE.
This attribute returns the base type of a type or subtype. This attribute
can only be used as the prefix of another attribute, as shown in the
following example:
do_nothing : PROCESS(x)
TYPE color IS (red, blue, green, yellow, brown, black);
SUBTYPE color_gun IS color RANGE red TO green;
VARIABLE a : color;
BEGIN
a := color_gun’BASE’RIGHT;
a := color’BASE’LEFT;
-- a = black
-- a = red
-- a = yellow
a := color_gun’BASE’SUCC(green);
END PROCESS do_nothing;
In the first assignment to variable a, color_gun’BASE returns type
color, the base type of color_gun. The statement color’RIGHT then
returns the value black. In the second assignment statement, the base
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Chapter Six
type of type color is type color. The statement color’LEFT returns the
value red. In the last assignment, color_gun’BASE returns type color,
and color’SUCC(green) returns yellow.
Range Kind Attributes
The last two predefined attributes in VHDL return a value kind of range.
These attributes work only with constrained array types and return the
index range specified by the optional input parameter. Following are the
attribute notations:
■ a’RANGE[(n)]
■ a’REVERSE_RANGE[(n)]
Attributes ’RANGE return the nth range denoted by the value of parameter n. Attribute ’RANGE returns the range in the order specified, and
’REVERSE_RANGE returns the range in reverse order.
Attributes ’RANGE and ’REVERSE_RANGE can be used to control the
number of times that a loop statement loops. Following is an example:
FUNCTION vector_to_int(vect: std_logic_vector) RETURN
INTEGER IS
VARIABLE result : INTEGER := 0;
BEGIN
FOR i IN vect’RANGE LOOP
result := result * 2;
IF vect(i) = ’1’ THEN
result := result + 1;
END IF;
END LOOP;
RETURN result;
END vector_to_int;
This function converts an array of bits into an integer value. The number of times that the loop needs to be executed is determined by the
number of bits in the input argument vect. When the function call is made,
the input argument cannot be an unconstrained value; therefore, the
attribute ’RANGE can be used to determine the range of the input vector.
The range can then be used in the loop statement to determine the number
of times to execute the loop and finish the conversion.
Predefined Attributes
171
The ’REVERSE_RANGE attribute works similar to the ’RANGE attribute, except that the range is returned in the reverse order. For a type
shown in the following, the ’RANGE attribute returns 0 TO 15, and the
’REVERSE_RANGE attribute returns 15 DOWNTO 0:
TYPE array16 IS ARRAY(0 TO 15) OF BIT;
VHDL attributes extend the language to provide some very useful functionality. They make models much easier to read and maintain.
SUMMARY
In this chapter, we discussed the following:
■ The different kinds of attributes and how some just return values,
while others create new signals.
■ How ’LEFT, ’RIGHT, ’LENGTH, ’HIGH, and ’LOW can be used to get
the bounds of a type or array.
■ How ’POS, ’VAL, ’SUCC, ’PRED, ’LEFTOF, and ’RIGHTOF can be used
to manipulate enumerated types.
■ How ’ACTIVE, ’EVENT, ’LAST_ACTIVE, ’LAST_EVENT, and
’LAST_VALUE can be used to return information about when events
occur.
■ How ’DELAYED, ’STABLE, ’QUIET, and ’TRANSACTION create new
signals that return information about other signals.
■ How range attributes ’RANGE and ’REVERSE_RANGE can be used to
control statements over the exact range of a type.
In the next chapter, we examine configurations, the method of binding
architectures to entities.
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CHAPTER
7
Configurations
Configurations are a primary design unit used to bind
component instances to entities. For structural models,
configurations can be thought of as the parts list for the
model. For component instances, the configuration specifies
from many architectures for an entity which architecture
to use for a specific instance. When the configuration for
an entity-architecture combination is compiled into the
library, a simulatable object is created.
Configurations can also be used to specify generic values
for components instantiated in the architecture configured
by the configuration. This mechanism, for example, provides a late-binding capability for delay values. Delay values
calculated from a physical layout tool, such as a printed
circuit board design system or a gate array layout system,
can be inserted in a configuration to provide a simulation
model with actual delays in the design.
If the designer wants to use a component in an architecture that has different port names from the architecture component declaration, the new component can have
its ports mapped to the appropriate signals. With this
functionality, libraries of components can be mixed and
matched easily.
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The configuration can also be used to provide a very fast substitution
capability. Multiple architectures can exist for a single entity. One architecture might be a behavioral model for the entity, while another architecture might be a structural model for the entity. The architecture used in
the containing model can be selected by specifying which architecture to
use in the configuration, and recompiling only the configuration. After
compilation, the simulatable model uses the specified architecture.
Default Configurations
The simplest form of explicit configuration is the default configuration.
(The simplest configuration is none at all in which the last architecture
compiled is used for an entity.) This configuration can be used for models
that do not contain any blocks or components to configure. The default
configuration specifies the configuration name, the entity being configured,
and the architecture to be used for the entity. Following is an
example of two default configurations shown by configurations big_count
and small_count:
LIBRARY IEEE;
USE IEEE.std_logic_1164.ALL;
ENTITY counter IS
PORT(load, clear, clk : IN std_logic;
PORT(data_in : IN INTEGER;
PORT(data_out : OUT INTEGER);
END counter;
ARCHITECTURE count_255 OF counter IS
BEGIN
PROCESS(clk)
VARIABLE count : INTEGER := 0;
BEGIN
IF clear = ‘1’ THEN
count := 0;
ELSIF load = ‘1’ THEN
count := data_in;
ELSE
IF (clk’EVENT) AND (clk = ‘1’) AND
(clk’LAST_VALUE = ‘0’) THEN
IF (count = 255) THEN
count := 0;
ELSE
count := count + 1;
END IF;
Configurations
175
END IF;
END IF;
data_out <= count;
END PROCESS;
END count_255;
ARCHITECTURE count_64k OF counter IS
BEGIN
PROCESS(clk)
VARIABLE count : INTEGER := 0;
BEGIN
IF clear = ‘1’ THEN
count := 0;
ELSIF load = ‘1’ THEN
count := data_in;
ELSE
IF (clk’EVENT) AND (clk = ‘1’) AND
(clk’LAST_VALUE = ‘0’) THEN
IF (count = 65535) THEN
count := 0;
ELSE
count := count + 1;
END IF;
END IF;
END IF;
data_out <= count;
END PROCESS;
END count_64k;
CONFIGURATION small_count OF counter IS
FOR count_255
END FOR;
END small_count;
CONFIGURATION big_count OF counter IS
FOR count_64k
END FOR;
END big_count;
This example shows how two different architectures for a counter
entity can be configured using two default configurations. The entity for the
counter does not specify any bit width for the data to be loaded into
the counter or data from the counter. The data type for the input and output
data is INTEGER. With a data type of integer, multiple types of counters can
be supported up to the integer representation limit of the host computer
for the VHDL simulator.
The two architectures of entity counter specify two different-sized
counters that can be used for the entity. The first architecture, count_255,
specifies an 8-bit counter. The second architecture, count_64k, specifies a
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16-bit counter. The architectures specify a synchronous counter with a
synchronous load and clear. All operations for the device occur with respect
to the clock.
Each of the two configurations for the entity specifies a different
architecture for the counter entity. Let’s examine the first configuration
in more detail. The configuration design unit begins with the keyword
CONFIGURATION and is followed by the name of the configuration. In this
example, the name of the configuration is small_count. The keyword OF
precedes the name of the entity BEGIN configured (counter). The next line
of the configuration starts the block configuration section. The keyword
FOR is followed by a name of the architecture to use for the entity being
configured or the name of the block of the architecture that will be configured. Any component or block configuration information then exists
between the FOR ARCHITECTURE clause and the matching END FOR.
In this architecture, there are no blocks or components to configure;
therefore, the block configuration area from the FOR clause to the END
FOR clause is empty, and the default is used. The configuration is called
the default configuration, because the default is used for all objects in
the configuration.
The first configuration is called small_count and binds architecture
count_255 with entity counter to form a simulatable object. The second
configuration binds architecture count_64k with entity counter and
forms a simulatable object called big_count.
Component Configurations
In this section, we discuss how architectures that contain instantiated
components can be configured. Architectures that contain other components are called structural architectures. These components are configured through component configuration statements.
Let’s first look at some very simple examples of component configurations, and then at some progressively more complex examples. The first
example is a simple 2 to 4 decoder device. Figure 7-1 shows the symbol
for the decoder, and Figure 7-2 shows the schematic.
The components used in the design are defined using the VHDL description shown here:
LIBRARY IEEE;
USE IEEE.std_logic_1164.ALL;
Configurations
Figure 7-1
Symbol for Decoder
Example.
177
Decode
Q0
A
Q1
B
Q2
EN
Q3
ENTITY inv IS
PORT( a : IN std_logic;
PORT( b : OUT std_logic);
END inv;
ARCHITECTURE behave OF inv IS
BEGIN
b <= NOT(a) AFTER 5 ns;
END behave;
CONFIGURATION invcon OF inv IS
FOR behave
END FOR;
END invcon;
LIBRARY IEEE; USE IEEE.std_logic_1164.ALL;
ENTITY and3 IS
PORT( a1, a2, a3 : IN std_logic;
PORT( o1 : OUT std_logic);
END and3;
ARCHITECTURE behave OF and3 IS
BEGIN
o1 <= a1 AND a2 AND a3 AFTER 5 ns;
END behave;
CONFIGURATION and3con OF and3 IS
FOR behave
END FOR;
END and3con;
Next, the entity and architecture for decode are shown:
LIBRARY IEEE; USE IEEE.std_logic_1164.ALL;
ENTITY decode IS
PORT( a, b, en : IN std_logic;
PORT( q0, q1, q2, q3 : OUT std_logic);
END decode;
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EN
Figure 7-2
Gate Level Schematic
for Decoder.
Q0
nota
Q1
A
notb
Q2
B
Q3
ARCHITECTURE structural OF decode IS
COMPONENT inv
PORT( a : IN std_logic;
PORT( b : OUT std_logic);
END COMPONENT;
COMPONENT and3
PORT( a1, a2, a3 : IN std_logic;
PORT( o1 : OUT std_logic);
END COMPONENT;
SIGNAL nota, notb : std_logic;
BEGIN
I1 : inv
PORT MAP(a, nota);
I2 : inv
PORT MAP(b, notb);
A1 : and3
PORT MAP(nota, en, notb, Q0);
A2 : and3
PORT MAP(a, en, notb, Q1);
A3 : and3
Configurations
179
PORT MAP(nota, en, b, Q2);
A4 : and3
PORT MAP(a, en, b, Q3);
END structural;
When all of the entities and architectures have been compiled into the
working library, the circuit can be simulated. The simulator uses the last
compiled architecture to build the executable design for the simulator
because it is the default. Using the last compiled architecture for an
entity to build the simulator works fine in a typical system, until more
than one architecture exists for an entity. Then it can become confusing
as to which architecture was compiled last. A better method is to specify
exactly which architecture to use for each entity. The component configuration binds architectures to entities.
Two different styles can be used for writing a component configuration for an entity. The lower-level configuration style specifies lowerlevel configurations for each component, and the entity-architecture
style specifies entity-architecture pairs for each component. The word
style is used to describe these two different configurations because
there is no hard-and-fast rule about how to use them. Lower-level configurations can be mixed with entity-architecture pairs, creating a
mixed-style configuration.
Lower-Level Configurations
Let’s examine the configuration for the lower-level configuration style
first. Following is an example of such a configuration for the decode entity:
CONFIGURATION decode_llcon OF decode IS
FOR structural
FOR I1 : inv USE CONFIGURATION WORK.invcon;
END FOR;
FOR I2 : inv USE CONFIGURATION WORK.invcon;
END FOR;
FOR ALL : and3 USE CONFIGURATION WORK.and3con;
END FOR;
END FOR;
END decode_llcon;
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This configuration specifies which configuration to use for each component in architecture structural of entity decode. The specified lower-level
configuration must already exist in the library for the current configuration to compile. Each component being configured has a FOR clause to begin
the configuration and an END FOR clause to end the configuration specification for the component. Each component can be specified with the
component instantiation label directly, as shown for component I1, or
with an ALL or OTHERS clause as shown by the and3 components.
After the component is uniquely specified by label or otherwise, the USE
CONFIGURATION clause specifies which configuration to use for this
instance of the component. In the preceding example, the configuration
specification for component I1 uses the configuration called invcon, from
the working library. For configuration decode_llcon to compile, configuration invcon must have been already compiled into library WORK.
Notice that the names of the entities, architectures, and configurations
reflect a naming convention. In general, this is a good practice. It helps
distinguish the different types of design units from one another when they
all exist in a library.
The advantage of this style of configurations is that most configurations are easy to write and understand. The disadvantage is not being
able to change the configuration of a lower-level component, without
implementing a two-step or more process of recompilation when hierarchy
levels increase.
Entity-Architecture Pair Configuration
The other style of component configurations is the entity-architecture pair
style. Following is an example of a configuration that uses the same
entity and architectures as the previous example:
CONFIGURATION decode_eacon OF decode IS
FOR structural
FOR I1 : inv USE ENTITY WORK.inv(behave);
END FOR;
FOR OTHERS : inv USE ENTITY WORK.inv(behave);
END FOR;
FOR A1 : and3 USE ENTITY WORK.and3(behave);
END FOR;
FOR OTHERS : and3 USE ENTITY WORK.and3(behave);
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181
END FOR;
END FOR;
END decode_eacon;
This configuration looks very similar to the lower-level configuration style
except for the USE clause in the component specification. In the previous
example, a configuration was specified, but in this style, an entityarchitecture pair is specified. The architecture is actually optional. If no
architecture is specified, the last compiled architecture for the entity is used.
Let’s take another look at the FOR clause for the first inverter, I1. In
the preceding example, the component is still specified by the label or by
an ALL or OTHERS clause. In this example, a USE ENTITY clause follows.
This clause specifies the name of the entity to use for this component.
The entity can have a completely different name than the component
being specified. The component name comes from the component declaration in the architecture, while the entity name comes from the actual
entity that has been compiled in the library specified. Following the entity
is an optional architecture name that specifies which architecture to use
for the entity.
Notice that the OTHERS clause is used for the second inverter in this
example. The first inverter is configured from its label I1, and all components that have not yet been configured are configured by the OTHERS
clause. This capability allows component I1 to use an architecture that
is different from the other components to describe its behavior. This
concept allows mixed-level modeling to exist. One component can be modeled at the switch or gate level, and the other can be modeled at the behavior level.
To change the architecture used for a component with the first configuration, decode_llcon requires modifying the lower-level configuration
and recompiling, then recompiling any higher-level configurations that
depend on it. With the second configuration decode_eacon, changing the
architecture for a component involves modifying configuration
decode_eacon and recompiling. No other configurations need be recompiled.
Port Maps
In the last two examples of component configurations, default mapping of
entity ports and component ports was used. When the port names for an
entity being configured to a component match the component port names,
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no other mapping needs to take place. The default mapping causes the
ports to match. What happens when the component ports do not match
the entity being mapped to the component instance? Without any further
information, the compiler cannot figure out which ports to map to which
and produces an error. However, more information can be passed to the
compiler with the configuration port map clause.
The configuration port map clause looks exactly like the component
instantiation port map clause used in an architecture. The configuration
port map clause specifies which of the component ports map to the actual
ports of the entity. If the port names are different, then the port map
clause specifies the mapping.
Let’s change the port names of the inv component used in the previous
example and see what the effect is in the configuration:
LIBRARY IEEE;
USE IEEE.std_logic_1164.ALL;
ENTITY inv IS
PORT( x : IN std_logic;
PORT( y : OUT std_logic);
END inv;
ARCHITECTURE behave OF inv IS
BEGIN
y <= NOT(x) AFTER 5 ns;
END behave;
CONFIGURATION invcon OF inv IS
FOR behave
END FOR;
END invcon;
The entity and architecture for decode stays exactly the same, including
the component declaration. The configuration, however, needs to add the
port map clause, as shown in the following example:
CONFIGURATION decode_map_con OF decode IS
FOR structural
FOR I1 : inv USE ENTITY WORK.inv(behave);
PORT MAP( x => a, y => b );
END FOR;
FOR I2 : inv USE ENTITY WORK.inv(behave);
PORT MAP( x => a, y => b );
END FOR;
FOR ALL : and3 USE ENTITY WORK.and3(behave);
Configurations
183
END FOR;
END FOR;
END decode_map_con;
The port map clause maps the port names of the component declarations, called the formal ports, to the port names of the entities from the
library. The term used for the ports of the entities from the library being
mapped are actuals. The ports are mapped using named association. The
rules for mapping ports using named association in the configuration port
map clause are the same rules as used in the component instantiation
port map clause.
In the preceding example, component declaration inv, port a, is mapped
to entity inv, port x, of the actual entity. Component declaration inv, port
b, is mapped to entity inv, port y, of the actual entity. Using the configuration port map clause can allow entities with completely different port
names to be mapped into existing architectures.
Mapping Library Entities
Not only can the ports be mapped with the configuration statement, but
entities from libraries can be mapped to components as well. This capability allows the names of components to differ from the actual entities
being mapped to them. The designer can easily switch the entity used for
each component in the architecture from one entity to another. This feature
allows the designer to map component instances to different entities.
Let’s assume that one AND gate of the decoder needs to be implemented differently from the others due to physical constraints of the device.
For instance instead of using a 3-input AND gate, a 3-input AND gate is
built using 2-input AND gates. Let’s start with the 2-input AND gate model
as shown below:
LIBRARY IEEE; USE IEEE.std_logic_1164.ALL;
ENTITY and2 IS
PORT( a, b : in std_logic;
c : out std logic );
END and2;
ARCHITECTURE behave OF and2 IS
BEGIN
c <= a and b;
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Chapter Seven
END behave;
CONFIGURATION and2con OF and2 IS
FOR behave
END FOR;
END and2con;
Two of these can be connected with the entity architecture shown below to form a structural representation of the 3-input AND gate.
LIBRARY IEEE;
USE IEEE.std_logic_1164.ALL;
ENTITY
struc_and3 IS
PORT( I1, I2, I3 : IN std_logic;
PORT( O1 : OUT std_logic);
END struc_and3;
ARCHITECTURE structural OF struc_and3 IS
COMPONENT and2
PORT( a, b : in std logic;
c : out std logic );
END COMPONENT;
SIGNAL s1, s2 : std_logic;
BEGIN
X1 : and2
PORT MAP( a => I1, b => I2, c => s1 );
X2 : and2
PORT MAP( a => I3, b => s1, c => O1 );
END structural;
This architecture can then be configured with the following configuration:
CONFIGURATION and3strc con OF struc and3 IS
FOR structural
FOR X1 : and2 USE CONFIGURATION WORK.and2con;
END FOR;
FOR X2 : and2 USE CONFIGURATION WORK.and2con;
END FOR;
END FOR;
END and3strc con;
Now, configuration decode_map_con of entity decode, described earlier,
can be modified as follows:
Configurations
185
CONFIGURATION decode_map_con OF decode IS
FOR structural
FOR ALL : inv USE ENTITY WORK.inv(behave);
END FOR;
FOR A1 : and3 USE ENTITY WORK.struc and3 (structural)
PORT MAP ( I1 => a1, I2 => a2, I3 => a3, o1 => O1;
END FOR;
FOR OTHERS : and3 USE ENTITY WORK.and3(behave);
END FOR;
END FOR;
END decode_map_con;
This configuration maps the first inverter, 3 input AND gate, A1, to entity struc_3, and other 3 input AND gates, A2–A4, to the behavioral entity, and3. Also the I1, I2, I3 and O1 ports of struc_and3 are mapped
to ports a1, a2, a3 and o2 of the component declaration for component
and3.
Generics in Configurations
Generics are parameters that are used to pass information into entities.
Typical applications include passing in a generic value for the rise and
fall delay of output signals of the entity. Other applications include passing in temperature, voltage, and loading to calculate delay values in the
model. (Modeling efficiency delay calculations should be done prior to simulation and the calculated delay values can then be passed back into the
model through generics.) A description of generics can be found in Chapter 3, “Sequential Processing.” This section concentrates on how configurations can be used to specify the value of generics.
Generics can be declared in entities, but can have a value specified in
a number of places, as listed in the following:
■ A default value can be specified in the generic declaration.
■ A value can be mapped in the architecture, in the component
instantiation.
■ A default value can be specified in the component declaration.
■ A value can be mapped in the configuration for the component.
Default values specified in the generic declaration, or the component
declaration, can be overridden by mapped values in the architecture or
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configuration sections. If no overriding values are present, the default
values are used; but if a value is mapped to the generic with a generic
map, the default value is overridden.
To see an example of this, let’s modify the decoder example, used previously in this chapter, to include two generics. The first specifies a timing mode to run the simulation, and the second is a composite type
containing the delay values for the device. These two types are declared
in the package p_time_pack, as shown in the following:
LIBRARY IEEE;
USE IEEE.std_logic_1164.ALL;
PACKAGE p_time_pack IS
TYPE t_time_mode IS (minimum, typical, maximum);
TYPE t_rise_fall IS
RECORD
rise : TIME;
fall : TIME;
END RECORD;
TYPE t_time_rec IS ARRAY(t_time_mode’LOW TO
t_time_mode’HIGH) OF t_rise_fall;
FUNCTION calc_delay(newstate : IN std_logic; mode : IN
t_time_mode;
delay_tab : IN t_time_rec ) return time;
END p_time_pack;
PACKAGE BODY p_time_pack IS
FUNCTION calc_delay(newstate : IN std_logic; mode : IN
t_time_mode;
delay_tab : IN t_time_rec ) return time IS
BEGIN
CASE f_state(newstate) IS
WHEN ‘0’ =>
RETURN delay_tab(mode).fall;
WHEN ‘1’ =>
RETURN delay_tab(mode).rise;
WHEN ‘X’ =>
IF (delay_tab(mode).rise <= delay_tab(mode).fall) THEN
RETURN delay_tab(mode).rise;
ELSE
RETURN delay_tab(mode).fall;
END IF;
END CASE;
END calc_delay;
END p_time_pack;
This package declares types t_time_mode and t_time_rec, which are
used for the generics of the inverter and 3-input AND gates. It also includes
Configurations
187
a new function, calc_delay, which is used to retrieve the proper delay
value from the delay table, depending on the type of transition occurring.
The and3 and inv gates of the decoder example have been rewritten to
include the generics discussed previously, as well as the delay calculation
function. Following are the new models:
LIBRARY IEEE;
USE IEEE.std_logic_1164.ALL;
USE WORK.p_time_pack.ALL;
ENTITY inv IS
GENERIC( mode : t_time_mode;
delay_tab : t_time_rec :=
(( 1 ns, 2 ns),
-- min
(( ( 2 ns, 3 ns),
-- typ
(( ( 3 ns, 4 ns))); -- max
PORT( a : IN std_logic;
PORT( b : OUT std_logic);
END inv;
ARCHITECTURE inv_gen OF inv IS
BEGIN
inv_proc : PROCESS(a)
VARIABLE state : std_logic;
BEGIN
state := NOT(a);
b <= state after calc_delay( state, mode, delay_tab);
END PROCESS inv_proc;
END inv_gen;
LIBRARY IEEE; USE IEEE.std_logic_1164.ALL;
USE WORK.p_time_pack.ALL;
ENTITY and3 IS
GENERIC( mode : t_time_mode;
delay_tab : t_time_rec :=
(( 2 ns, 3 ns), -- min
(( ( 3 ns, 4 ns), -- typ
(( ( 4 ns, 5 ns))); -- max
PORT( a1, a2, a3 : IN std_logic;
PORT( o1 : OUT std_logic);
END and3;
ARCHITECTURE and3_gen OF and3 IS
BEGIN
and3_proc : PROCESS( a1, a2, a3 )
VARIABLE state : std_logic;
BEGIN
state := a1 AND a2 AND a3;
o1 <= state after calc_delay( state, mode, delay_tab);
END PROCESS and3_proc;
END and3_gen;
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After the entities and architectures for the gates have been defined,
configurations that provide specific values for the generics are defined.
These models can have their generic values specified by two methods.
The first method is to specify the generic values in the architecture where
the components are being instantiated. The second method is to specify
the generic values in the configuration for the model, where the components are instantiated.
Generic Value Specification in
Architecture
Specifying the generic values in the architecture of an entity allows the
designer to delay the specification of the generic values until the architecture of the entity is created. Different generic values can be specified
for each instance of an entity allowing one entity to represent many different physical devices. Following is an example of an architecture with
the generic values specified in it:
ARCHITECTURE structural OF decode IS
COMPONENT inv
GENERIC( mode : t_time_mode;
GENERIC( delay_tab : t_time_rec);
PORT( a : IN std_logic;
PORT( b : OUT std_logic);
END COMPONENT;
COMPONENT and3
GENERIC( mode : t_time_mode;
GENERIC( delay_tab : t_time_rec);
PORT( a1, a2, a3 : IN std_logic;
PORT( o1 : OUT std_logic);
END COMPONENT;
SIGNAL nota, notb : std_logic;
BEGIN
I1 : inv
GENERIC MAP( mode => maximum,
delay_tab => ((1.3 ns, 1.9 ns),
delay_tab => ((2.1 ns, 2.9 ns),
delay_tab => ((3.2 ns, 4.1 ns)))
PORT MAP( a, nota );
I2 : inv
Configurations
189
GENERIC MAP( mode
delay_tab
delay_tab
delay_tab
PORT MAP( b, notb
=>
=>
=>
=>
);
minimum,
((1.3 ns, 1.9 ns),
((2.1 ns, 2.9 ns),
((3.2 ns, 4.1 ns)))
A1 : and3
GENERIC MAP( mode => typical,
delay_tab => ((1.3 ns, 1.9 ns),
delay_tab => ((2.1 ns, 2.9 ns),
delay_tab => ((3.2 ns, 4.1 ns)))
PORT MAP( nota, en, notb, q0 );
A2 : and3
GENERIC MAP( mode => minimum,
delay_tab => ((1.3 ns, 1.9 ns),
delay_tab => ((2.1 ns, 2.9 ns),
delay_tab => ((3.2 ns, 4.1 ns)))
PORT MAP( a, en, notb, q1 );
A3 : and3
GENERIC MAP( mode => maximum,
delay_tab => ((1.3 ns, 1.9 ns),
delay_tab => ((2.1 ns, 2.9 ns),
delay_tab => ((3.2 ns, 4.1 ns)))
PORT MAP( nota, en, b, q2 );
A4 : and3
GENERIC MAP( mode => maximum,
delay_tab => ((2.3 ns, 2.9 ns),
delay_tab => ((3.1 ns, 3.9 ns),
delay_tab => ((4.2 ns, 5.1 ns)))
PORT MAP( a, en, b, q3 );
END structural;
Generics are treated in the same manner as ports with respect to how
they are mapped. If a component port in a component declaration has a
different name than the actual entity compiled into the library, then a port
map clause is needed in the configuration specification, for the containing entity. The same is true for a generic. If a generic declaration
in a component declaration has a different name than the actual
generic for the component, then a generic map clause is needed to make
the appropriate mapping.
In the preceding example, the generic names are the same in the
entity declaration and the component declaration; therefore, the default
mapping provides the appropriate connection between the two.
The configuration for the preceding example needs only to specify
which actual entities will be used for the component instantiations in
the architecture. No generic information needs to be provided, because the
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generics have been mapped in the architecture. The configuration can be
specified as shown in the following:
CONFIGURATION decode_gen_con2 OF decode IS
FOR structural
FOR i1, i2 : inv USE ENTITY WORK.inv(inv_gen);
END FOR;
FOR a1, a2, a3, a4 : and3 USE ENTITY
WORK.and3(and3_gen);
END FOR;
END FOR;
END decode_gen_con2;
The lower-level configuration cannot specify values for the generics if
the architecture has mapped values to the generics in the architecture.
Generic Specifications in
Configurations
The method of specifying generic values with the most flexibility is to
specify generic values in the configuration for the entity. This method
allows the latest binding of all the methods for specifying the values for
generics. Usually, the later the values are specified, the better. Late binding
allows back-annotation of path delay generics to occur in the configuration.
For instance, there are a number of steps involved in the design of an
ASIC. Following are the steps required:
1. Create the logic design model of a device.
2. Simulate the model.
3. Add estimated delays to device model.
4. Simulate model.
5. Create physical layout of the model.
6. Calculate physical delays from the layout.
7. Feed back physical delays to the device model.
8. Resimulate using actual delays.
The process of feeding back the physical delays into the model can be
accomplished by modifying the architecture or by creating a configuration
Configurations
191
to map the delays back to the model. Modifying the architecture involves
changing the values in all of the generic map clauses used to map the
delays in the architecture. This method has a big drawback. Modifying the
architecture that contains the component instantiation statements
requires recompilation of the architecture and the configuration for the
design unit. This can be an expensive proposition in a very large design.
The second method, which creates a configuration that maps all of the
delays to the generics of the entity, is much more efficient. A configuration
of this type contains a generic map value for each generic to be specified
in the configuration. Any generics not specified in the configuration are
mapped in the architecture or defaulted.
Let’s use the decoder example again but now assume that it represents
part of an ASIC that has delays back-annotated to it. The inv and and3
devices have an intrinsic propagation delay through the device that is
based on the internal characteristics of the device, and these devices have
an external delay that is dependent on the driver path and device loading.
The intrinsic and external delays are passed into the model as generic
values. The intrinsic delay is passed into the model to allow a single model
to be used for model processes. The external delay is passed to the
model, because it may vary for every instance, as loading may be different for each instance. (A more accurate model of delays is obtained using input delays.)
The entity and architecture for the inv and and3 gates look like this:
LIBRARY IEEE;
USE IEEE.std_logic_1164.ALL;
ENTITY inv IS
GENERIC(int_rise, int_fall, ext_rise,
GENERIC(ext_fall : time);
PORT( a: IN std_logic; b: OUT std_logic);
END inv;
ARCHITECTURE inv_gen1 OF inv IS
BEGIN
inv_proc : PROCESS(a)
VARIABLE state : std_logic;
BEGIN
state := NOT(a);
IF state = ‘1’ THEN
b <= state AFTER (int_rise + ext_rise);
ELSIF state = ‘0’ THEN
b <=state AFTER (int_fall + ext_fall);
ELSE
b <= state AFTER (int_fall + ext_fall);
END IF;
END PROCESS inv_proc;
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END inv_gen1;
------------------------------------------------LIBRARY IEEE; USE IEEE.std_logic_1164.ALL;
ENTITY and3 IS
GENERIC(int_rise, int_fall, ext_rise, ext_fall : time);
PORT( a1, a2, a3: IN std_logic;
PORT( o1: OUT std_logic);
END and3;
ARCHITECTURE and3_gen1 OF and3 IS
BEGIN
and3_proc : PROCESS(a1, a2, a3)
VARIABLE state : std_logic;
BEGIN
state := a1 AND a2 AND a3;
IF state = ‘1’ THEN
o1 <= state AFTER (int_rise + ext_rise);
ELSIF state = ‘0’ THEN
o1 <= state AFTER (int_fall + ext_fall);
ELSE
o1 <= state AFTER (int_fall + ext_fall);
END IF;
END PROCESS and3_proc;
END and3_gen1;
There are no local configurations specified at this level in the design
because this has nearly the same effect of mapping the generic values in
the architecture. Instead, a full configuration for entity decode is specified
that maps the generics at all levels of the decoder. The entity and
architecture for the decoder, as shown in the following, are very similar
to the original example used earlier:
LIBRARY IEEE;
USE IEEE.std_logic_1164.ALL;
ENTITY decode IS
PORT( a, b, en : IN std_logic;
PORT( q0, q1, q2, q3 : OUT std_logic);
END decode;
ARCHITECTURE structural OF decode IS
COMPONENT inv
PORT( a : IN std_logic;
PORT( b : OUT std_logic);
END COMPONENT;
COMPONENT and3
PORT( a1, a2, a3 : IN std_logic;
PORT( o1 : OUT std_logic);
END COMPONENT;
Configurations
193
SIGNAL nota, notb : std_logic;
BEGIN
I1 : inv
PORT MAP( a, nota);
I2 : inv
PORT MAP( b, notb);
AN1 : and3
PORT MAP( nota, en, notb, q0);
AN2 : and3
PORT MAP( a, en, notb, q1);
AN3 : and3
PORT MAP( nota, en, b, q2);
AN4 : and3
PORT MAP( a, en, b, q3);
END structural;
Notice that the component declarations for components inv and and3
in the architecture declaration section do not contain the generics
declared in the entity declarations for entities inv and and3. Because the
generics are not being mapped in the architecture, there is no need to
declare the generics for the components in the architecture.
Following is the configuration to bind all of these parts together into
an executable model:
CONFIGURATION decode_gen1_con OF decode IS
FOR structural
FOR I1 : inv USE ENTITY WORK.inv(inv_gen1)
GENERIC MAP( int_rise => 1.2 ns,
GENERIC MAP( int_fall => 1.7 ns,
GENERIC MAP( ext_rise => 2.6 ns,
GENERIC MAP( ext_fall => 2.5 ns);
END FOR;
FOR I2 :
GENERIC
GENERIC
GENERIC
GENERIC
END FOR;
inv USE ENTITY WORK.inv(inv_gen1)
MAP( int_rise => 1.3 ns,
MAP( int_fall => 1.4 ns,
MAP( ext_rise => 2.8 ns,
MAP( ext_fall => 2.9 ns);
FOR AN1 : and3 USE ENTITY
GENERIC MAP( int_rise =>
GENERIC MAP( int_fall =>
GENERIC MAP( ext_rise =>
GENERIC MAP( ext_fall =>
END FOR;
WORK.and3(and3_gen1)
2.2 ns,
2.7 ns,
3.6 ns,
3.5 ns);
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Chapter Seven
FOR AN2 : and3 USE ENTITY
GENERIC MAP( int_rise =>
GENERIC MAP( int_fall =>
GENERIC MAP( ext_rise =>
GENERIC MAP( ext_fall =>
END FOR;
WORK.and3(and3_gen1)
2.2 ns,
2.7 ns,
3.1 ns,
3.2 ns);
FOR AN3 : and3 USE ENTITY
GENERIC MAP( int_rise =>
GENERIC MAP( int_fall =>
GENERIC MAP( ext_rise =>
GENERIC MAP( ext_fall =>
END FOR;
WORK.and3(and3_gen1)
2.2 ns,
2.7 ns,
3.3 ns,
3.4 ns);
FOR AN4 : and3 USE ENTITY
GENERIC MAP( int_rise =>
GENERIC MAP( int_fall =>
GENERIC MAP( ext_rise =>
GENERIC MAP( ext_fall =>
END FOR;
END FOR;
END decode_gen1_con;
WORK.and3(and3_gen1)
2.2 ns,
2.7 ns,
3.0 ns,
3.1 ns);
Each component instance is configured to the correct entity and architecture, and the generics of the entity are mapped with a generic map
clause. Using this type of configuration allows each instance to have
unique delay characteristics. Of course, the generics passed into the device
can represent any type of data the designer wants, but typically the generics are used to represent delay information. VITAL uses generics to pass
delay information to library components. We examine this more closely in
later chapters.
The power of this type of configuration is realized when the delay values
are updated. For instance, in the ASIC example, the estimated delays are
included in the configuration initially, but after the ASIC device has been
through the physical layout process, the actual delay information can be
determined. This information can be fed back into the configuration so
that the configuration has the actual delay information calculated from
the layout tool. Building a new simulatable device, including the new delay
information, requires only a recompile of the configuration. The entities
and architectures do not need to be recompiled.
If the delay information was included in the architecture for the device,
then a lot more of the model would need to be recompiled to build the
simulatable entity. All of the architectures that included the generics
would need to be recompiled, and so would the configuration for the entity.
A lot of extra code would be recompiled unnecessarily.
Configurations
195
The information in this section on generics can be summarized by the
charts shown in Figures 7-3 and 7-4. (These charts were originally created
by Paul Krol.)
These charts shows the effect of the declarations and mapping of
generics on the values actually obtained in the model. The first four
columns of Figure 7-3 describe where a particular generic, G, can be
declared and mapped to a value. The next column describes the
error/warning number returned from a particular combination of declaration and mapping. The next two columns describe the values obtained
by the generic, G, and any other generics for the entity for a particular
declaration and mapping combination. At the bottom of Figure 7-3 and in
Figure 7-4, are the tables of translations used to translate the character
values used to the appropriate action taken.
Board-Socket-Chip Analogy
A good analogy for describing how entity declarations, architectures, component declarations, and configuration specifications all interact is the
Figure 7-3
Configuration
Generic Table.
Declaration
Mapping
Entity
Component
Instance
D
D
A
D
N
A
D
N
N
D
Error /
Warning
Configuration
A
A
N
D
A
X
D/N
A
X
D/N
X
D/N
A
2
D/N
X
A
3
D/N
X
Generic Values
Same
Other
I
E
I
E
C
E
I
M
C
M
C
E
1
2
A
A
A
4
X
X
5
196
Figure 7-4
Configuration
Generic Table Translations.
Chapter Seven
Declarations / Mapping
D
Declared, with default value
N
Declared, with no default value
X
Not Declared
A
Actual Mapped
Errors / Warnings
1
Can only map generic in configuration if declared in the entity
2
Generic declared in component but not in entity, hence it is not used
3
Can only map generic in component instance if declared in component declarations
4
Can’t map a generic in the component instance and the configuration
5
Must map at least one generic to get the default value for other generics
Generic Values
E
Default taken from entity
M
Default taken from configuration
I
Actual taken from component instance
C
Actual taken from configuration
board-socket-chip analogy. (This analogy was originally presented to me
by Dr. Alec Stanculescu.) In this analogy, the architecture of the top-level
entity represents the board being modeled. The component instance represents a socket on the board, and the lower-level entity being instantiated
in the architecture represents the chip.
This analogy helps describe how the ports and generics are mapped at
each level. At the board (architecture) level component socket pins are
interconnected with signals. The chip pins are then connected to socket
pins when the chip is plugged into the socket. Following is an example of
how this works:
LIBRARY IEEE;
USE IEEE.std_logic_1164.ALL;
Configurations
197
ENTITY board IS
GENERIC (qdelay, qbdelay : time);
PORT( clk, reset, data_in : IN std_logic;
PORT( data_out : OUT std_logic);
END board;
ARCHITECTURE structural OF board IS
COMPONENT dff
GENERIC( g1, g2 : time);
PORT( p1, p2, p3, p4 : IN std_logic;
p5, p6 : OUT std_logic);
END COMPONENT;
SIGNAL ground : std_logic := ‘1’;
SIGNAL int1, nc : std_logic;
BEGIN
U1 : dff
GENERIC MAP( g1 => qdelay,
GENERIC MAP( g2 => qbdelay)
PORT MAP( p1 => clk,
PORT MAP( p2 => data_in,
PORT MAP( p3 => reset,
PORT MAP( p4 => ground,
PORT MAP( p5 => int1,
PORT MAP( p6 => nc);
U2 : dff
GENERIC MAP( g1
GENERIC MAP( g2
PORT MAP( p1 =>
PORT MAP( p2 =>
PORT MAP( p3 =>
PORT MAP( p4 =>
PORT MAP( p5 =>
PORT MAP( p6 =>
END structural;
=> qdelay,
=> qbdelay)
clk,
int1,
reset,
ground,
data_out,
nc);
The entity and architecture shown are a simple 2-bit shift register
made from two D flip-flop (DFF) component instantiations. This example,
though relatively simple, shows how ports and generics are mapped at
different levels.
The component instance for component DFF in the architecture statement part acts like a socket in the architecture for the board. When a
component instance is placed in the architecture, signals are used to connect the component to the board, which is the architecture. The actual
chip is not connected to the socket until a configuration is specified for the
board entity. If all of the names of the socket ports and generics match
the names of the actual entity being used, then no mapping is needed. The
default mapping connects the chip to the socket. If the names are different, or the number of ports are not the same, for the component instanti-
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Chapter Seven
ation and the actual entity, then a mapping between the socket (component instantiation) and the chip (actual entity) is needed.
The actual chip to be mapped is described by the entity and architecture
shown here:
LIBRARY IEEE;
USE IEEE.std_logic_1164.ALL;
ENTITY dff IS
GENERIC( q_out, qb_out : time);
PORT( preset, clear, din,
PORT( clock : IN std_logic;
PORT( q, qb : OUT std_logic);
END dff;
ARCHITECTURE behave OF dff IS
BEGIN
dff_proc : PROCESS(preset, clear, clock)
VARIABLE int_q : std_logic;
BEGIN
IF preset = ‘0’ and clear = ‘0’ THEN
IF (clock’EVENT) AND (clock = ‘1’) THEN
int_q := din;
END IF;
ELSIF preset = ‘1’ AND clear = ‘0’ THEN
int_q := ‘1’;
ELSIF clear = ‘1’ AND preset = ‘0’ THEN
int_q := ‘0’;
ELSE
int_q := ‘X’;
END IF;
q <= int_q after q_out;
int_q := not(int_q);
qb <= int_q after qb_out;
END PROCESS dff_proc;
END behave;
The names of the ports and generics are completely different than the
component declaration; therefore, mapping is required. Following is a
configuration that places the actual chip in the socket (maps the ports
and generics):
CONFIGURATION board_con OF board IS
FOR structural
Configurations
199
FOR U1,U2: dff USE WORK.dff(behave)
GENERIC MAP( q_out => g1, qb_out => g2)
PORT MAP( preset => ground, clear => p3,
PORT MAP( din => p2, clock => p1,
PORT MAP( q => p5, qb => p6);
END FOR;
END FOR;
END board_con;
Block Configurations
When an architecture contains block statements, the configuration must
reflect this fact. (Block statements are discussed in Chapter 2, “Behavioral
Modeling.”) Blocks act like another level of hierarchy between the containing architecture and any components being configured. The configuration must specify which block of a configuration is being configured when
the architecture is being configured.
Following shows an architecture fragment that contains three blocks:
LIBRARY IEEE;
USE IEEE.std_logic_1164.ALL;
ENTITY cpu IS
PORT( clock : IN std_logic;
PORT( addr : OUT std_logic_vector(0 to 3);
PORT( data : INOUT std_logic_vector(0 to 3);
PORT( interrupt : IN std_logic;
PORT( reset : IN std_logic);
END cpu;
ARCHITECTURE fragment OF cpu IS
COMPONENT int_reg
PORT( data : IN std_logic;
PORT( regclock : IN std_logic;
PORT( data_out : OUT std_logic);
END COMPONENT;
COMPONENT alu
PORT( a, b : IN std_logic;
PORT( c, carry : OUT std_logic);
END COMPONENT;
SIGNAL a, c, carry : std_logic_vector(0 TO 3);
BEGIN
reg_array : BLOCK
BEGIN
R1 : int_reg
PORT MAP( data(0), clock, data(0));
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Chapter Seven
R2 : int_reg
PORT MAP( data(1), clock, data(1));
R3 : int_reg
PORT MAP( data(2), clock, data(2));
R4 : int_reg
PORT MAP( data(3), clock, data(3));
END BLOCK reg_array;
shifter : BLOCK
BEGIN
A1 : alu
PORT MAP( a(0), data(0), c(0), carry(0));
A2 : alu
PORT MAP( a(1), data(1), c(1), carry(1));
A3 : alu
PORT MAP( a(2), data(2), c(2), carry(2));
A4 : alu
PORT MAP( a(3), data(3), c(3), carry(3));
shift_reg : BLOCK
BEGIN
R1 : int_reg
PORT MAP( data, shft_clk, data_out);
END BLOCK shift_reg;
END BLOCK shifter;
END fragment;
The architecture consists of three blocks, each containing component
instantiations. The first block contains four int_reg components, and the
second contains an alu component, plus another BLOCK statement. The last
block contains a single int_reg component.
The configuration for this architecture must take into account the fact
that BLOCK statements exist in the architecture. Following is a simple
configuration for the architecture:
CONFIGURATION cpu_con OF cpu IS
FOR fragment
FOR reg_array
FOR ALL: int_reg USE CONFIGURATION WORK.int_reg_con;
END FOR;
END FOR;
Configurations
201
FOR shifter
FOR A1 : alu USE CONFIGURATION WORK.alu_con;
END FOR;
FOR shift_reg
FOR R1 : int_reg USE CONFIGURATION WORK.int_reg_con;
END FOR;
END FOR;
END FOR;
END FOR;
END cpu_con;
In the configuration cpu_con of entity cpu, architecture fragment is
used for the entity. Inside of block reg_array, all (R1 through R4) of the
int_reg components use configuration int_reg_con. In block shifter,
the alu component (A1) uses configuration alu_con. For block shift_reg
inside of block shifter, the int_reg component uses configuration
int_reg_con.
Architecture Configurations
The last type of configuration we discuss is the architecture configuration.
This configuration exists in the architecture declarative region and
specifies the configurations of parts used in the architecture. If this type
of configuration is used, a separate configuration declaration is not
needed to configure the components used in the architecture.
The next example configuration is for a very high-level description of an
autopilot. The autopilot block diagram is shown in Figure 7-5. Following
is an example of this type of configuration:
PACKAGE ap IS
TYPE alt IS INTEGER RANGE 0 TO 50000;
TYPE hdg IS INTEGER RANGE 0 TO 359;
TYPE vdir IS INTEGER RANGE 0 TO 9;
TYPE hdir IS INTEGER RANGE 0 TO 9;
TYPE control IS INTEGER RANGE 0 TO 9;
END ap;
USE WORK.ap.ALL;
ENTITY autopilot IS
PORT( altitude : IN alt;
altitude_set : IN alt;
heading : IN hdg;
heading_set : IN hdg;
rudder : OUT control;
202
Figure 7-5
Block Diagram of
Autopilot Example.
Chapter Seven
Alt_Comp
Indicated
Altitude
Alt_Ctrl
Up/Down
Elevator
Desired
Altitude
Hdg_Comp
Indicated
Heading
Hdg_Ctrl
Left/Right
Desired
Heading
Aileron
Rudder
aileron : OUT control;
elevator : OUT control);
END autopilot;
ARCHITECTURE block_level OF autopilot IS
COMPONENT alt_compare
PORT( alt_ref : IN alt;
PORT( alt_ind : IN alt;
PORT( up_down : OUT vdir);
END COMPONENT;
COMPONENT hdg_compare
PORT( hdg_ref : IN hdg;
PORT( hdg_ind : IN hdg;
PORT( left_right : OUT hdir);
END COMPONENT;
COMPONENT hdg_ctrl
PORT( left_right : IN hdir;
PORT( rdr : OUT control;
PORT( alrn : OUT control);
END COMPONENT;
COMPONENT alt_ctrl
PORT( up_down : IN vdir;
PORT( elevator : OUT control);
END COMPONENT;
SIGNAL up_down : vdir;
SIGNAL left_right : hdir;
FOR M1 : alt_compare USE CONFIGURATION WORK.alt_comp_con;
Configurations
203
FOR M2 : hdg_compare USE CONFIGURATION WORK.hdg_comp_con;
FOR M3 : hdg_ctrl USE ENTITY WORK.hdg_ctrl(behave);
FOR M4 : alt_ctrl USE ENTITY WORK.alt_ctrl(behave);
BEGIN
M1 : alt_compare
PORT MAP( alt_ref => altitude,
PORT MAP( alt_ind => alt_set,
PORT MAP( up_down => up_down);
M2 : hdg_compare
PORT MAP( hdg_ref => heading,
PORT MAP( hdg_ind => hdg_set,
PORT MAP( left_right => left_right);
M3 : hdg_ctrl
PORT MAP( left_right => left_right,
PORT MAP( rdr => rudder,
PORT MAP( alrn => aileron);
M4 : alt_ctrl
PORT MAP( up_down => up_down,
PORT MAP( elevator => elevator);
END block_level;
This model is a top-level description of an autopilot. There are four
instantiated components that provide the necessary functionality of the
autopilot. This model demonstrates how component instantiations can be
configured in the architecture declaration section of an architecture.
Notice that after the component declarations in the architecture declaration
section of architecture block_level, there are four statements
similar to the following:
FOR M1 : alt_compare USE CONFIGURATION WORK.alt_comp_con;
These statements allow the designer to specify either the configuration
or the entity-architecture pair to use for a particular component type. This
type of configuration does not provide the same flexibility to the designer as the separate configuration declaration, but it is useful for small
designs.
Configurations are a useful tool for managing large designs. With
proper use of configurations, a top-down design approach can be implemented that allows all levels of description of the design to be used for
the most efficient model needed at any point in the design process.
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Chapter Seven
SUMMARY
In this chapter, we discussed the following:
■ How default configurations can be used to bind architectures to
entities.
■ How component configurations can be used to specify which entity
to use for each component instantiation.
■ How port maps within configurations allow mapping entities with
different names to component instances.
■ How generics can be specified in configurations to allow late
binding of generic information.
■ How block configurations can be used to configure architectures
with block statements in them.
■ How architecture configurations allow specification of configurations
for component instantiations in the architecture declaration section.
The basic features of VHDL have now been introduced. In the next
chapter, we examine some of the more esoteric but useful features that
exist in VHDL.
CHAPTER
8
Advanced Topics
In this chapter, some of the more esoteric features of
VHDL are discussed. Some of the features may be useful
for certain types of designs, and not for others. Typical
usage examples are presented to show how these features
might be taken advantage of.
Some of the features discussed include overloading,
qualified expressions, user-defined attributes, generate
statements, aliases, and TextIO. All of these features provide the user with an advanced environment with which
to do modeling.
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Chapter Eight
Overloading
Overloading allows the designer to write much more readable code. An
object is overloaded when the same object name exists for multiple subprograms or type values. The VHDL compiler selects the appropriate
object to use in each instance.
In VHDL, a number of types of overloading are possible. Subprograms
can be overloaded, operators can be overloaded, and enumeration types can
be overloaded. Overloading subprograms allows subprograms to operate
on objects of different types. Overloading an operator allows the operator to perform the same operation on multiple types. Overloading frees
the designer from the necessity of generating countless unique names for
subprograms that do virtually the same operation. The result of using
overloaded subprograms and operators is models that are easier to read
and maintain.
Subprogram Overloading
Subprogram overloading allows the designer to write multiple subprograms
with the same name, but the number of arguments, the type of arguments,
and return value (if any) can be different. The VHDL compiler, at compile
time, selects the subprogram that matches the subprogram call. If no subprogram matches the call, an error is generated.
The following example illustrates how a subprogram can be overloaded
by the argument type:
LIBRARY IEEE;
USE IEEE.std_logic_1164.ALL;
PACKAGE p_shift IS
TYPE s_int IS RANGE 0 TO 255;
TYPE s_array IS ARRAY(0 TO 7) OF std_logic;
FUNCTION shiftr( a : s_array) return s_array;
FUNCTION shiftr( a : s_int) return s_int;
END p_shift;
PACKAGE BODY p_shift IS
FUNCTION shiftr( a : s_array) return s_array IS
VARIABLE result : s_array;
BEGIN
FOR i IN a’RANGE LOOP
IF i = a’HIGH THEN
result(i) := ‘0’;
Advanced Topics
207
ELSE
result(i) := a(i + 1);
END IF;
END LOOP;
RETURN result;
END shiftr;
FUNCTION shiftr( a : s_int) return s_int IS
BEGIN
RETURN (a/2);
END shiftr;
END p_shift;
The package p_shift contains two functions both named shiftr. Both
functions provide a right-shift capability, but each function operates on a
specific type. One function works only with type s_int, and the other
works only with type s_array. The compiler picks the appropriate function
based on the calling argument(s) and return argument.
In the following example, different types of function calls are shown,
and the results obtained with each call:
USE WORK.p_shift.ALL;
ENTITY shift_example IS
END shift_example;
ARCHITECTURE test OF shift_example IS
SIGNAL int_signal : s_int;
SIGNAL array_signal : s_array;
BEGIN
-- picks function that works with s_int type
int_signal <= shiftr(int_signal);
-- picks function that works with
-- s_array type
array_signal <= shiftr(array_signal);
-- produces error because no function
-- will match
array_signal <= shiftr(int_signal);
END test;
The architecture test contains three calls to function shiftr. The first
calls shiftr with an argument type of s_int and a return type of s_int.
This call uses the second function described in package body p_shift, the
function with input arguments, and return type of s_int.
The second call to shiftr uses the array type s_array, and therefore
picks the first function defined in package p_shift. Both the input
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Chapter Eight
argument(s) type(s) and return type must match for the function to
match the call.
The third call to function shiftr shows an example of a call where the
input argument matches the s_int type function, but the return type of
the function does not match the target signal. With the functions currently described in package p_shift, no function matches exactly, and
therefore the compilation of the third line produces an error.
To make the third call legal, all that is needed is to define a function
that matches the types of the third call. An example of the function declaration is shown in the following code line. The function body for this
function is left as an exercise for the reader:
FUNCTION shiftr( a : s_int) return s_array;
OVERLOADING SUBPROGRAM ARGUMENT TYPES To overload
argument types, the base type of the subprogram parameters or return
value must differ. For example, base types do not differ when two subtypes
are of the same type. Two functions that try to overload these subtypes produce a compile error. Following is an example:
PACKAGE type_error IS
SUBTYPE log4 IS BIT_VECTOR( 0 TO 3);
SUBTYPE log8 IS BIT_VECTOR( 0 TO 7);
-- this function is Ok
FUNCTION not( a : log4) return integer;
-- this function declaration will cause an
-- error
FUNCTION not( a : log8) return integer;
END type_error;
This package declares two subtypes log4 and log8 of the unconstrained BIT_VECTOR type. Two functions named not are then declared
using these subtypes. The first function declaration is legal, but the second
function declaration causes an error. The error is that two functions have
been declared for the same base type. The two types log4 and log8 are
not distinct, because they both belong to the same base type.
All of the examples shown so far have been overloading of functions.
Overloading of procedures works in the same manner.
SUBPROGRAM PARAMETER OVERLOADING Two or more subprograms with the same name can have a different number of parameters.
Advanced Topics
209
The types of the parameters can be the same, but the number of
parameters can be different. This is shown by the following example:
LIBRARY IEEE;
USE IEEE.std_logic_1164.ALL;
PACKAGE p_addr_convert IS
FUNCTION convert_addr(a0, a1 : std_logic) return integer;
FUNCTION convert_addr(a0, a1, a2 : std_logic)return
integer;
FUNCTION convert_addr(a0, a1, a2, a3 : std_logic) return
integer;
END p_addr_convert;
PACKAGE BODY p_addr_convert IS
FUNCTION convert_addr(a0, a1 : std_logic) RETURN
INTEGER IS
VARIABLE result : INTEGER := 0;
BEGIN
IF (a0 = ‘1’) THEN
result := result + 1;
END IF;
IF (a1 = ‘1’) THEN
result := result + 2;
END IF;
RETURN result;
END convert_addr;
FUNCTION convert_addr(a0, a1, a2 : std_logic) RETURN
INTEGER IS
VARIABLE result : INTEGER := 0;
BEGIN
result := convert_addr(a0, a1);
IF (a2 = ‘1’) THEN
result := result + 4;
END IF;
RETURN result;
END convert_addr;
FUNCTION convert_addr(a0, a1, a2, a3 : std_logic) RETURN
INTEGER IS
VARIABLE result : INTEGER := 0;
BEGIN
result := convert_addr(a0, a1, a2);
IF (a3 = ‘1’) THEN
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Chapter Eight
result := result + 8;
END IF;
RETURN result;
END convert_addr;
END p_addr_convert;
This package declares three functions that convert 2, 3, or 4 input bits
into integer representation. Each function is named the same, but the
appropriate function is called depending on the number of input arguments
that are passed to the function. If 2 bits are passed to the function, then
the function with two arguments is called. If 3 bits are passed, the function with three arguments is called, and so on.
Following is an example using these functions:
LIBRARY IEEE;
USE IEEE.std_logic_1164.ALL;
USE WORK.p_addr_convert.ALL;
ENTITY test IS
PORT(i0, i1, i2, i3 : in std_logic);
END test;
ARCHITECTURE test1 OF test IS
SIGNAL int1, int2, int3 : INTEGER;
BEGIN
-- uses first function
int1 <= convert_addr(i0, i1);
-- uses second function
int2 <= convert_addr(i0, i1, i2);
-- uses third function
int3 <= convert_addr(i0, i1, i2, i3);
END test1;
The first call to the convert_addr function has only two arguments in
the argument list, and therefore the first function in package
p_addr_convert is used. The second call has three arguments in its
argument list and calls the second function. The last call matches the
third function from package p_addr_convert.
Overloading Operators
One of the most useful applications of overloading is the overloading of
operators. The need for overloading operators arises because the operators
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211
supplied in VHDL only work with specific types. For instance, the + operator only works with integer, real, and physical types, while the & (concatenation) operator only works with array types. If a designer wants to use
a particular operator on a user-defined type, then the operator must be
overloaded to handle the user type. A complete listing of the operators and
the types supported by them can be found in the VHDL Language Reference Manual.
An example of a typical overloaded operator is the + operator. The +
operator is defined for the numeric types, but if the designer wants to add
two BIT_VECTOR objects, the + operator does not work. The designer must
write a function that overloads the operator to accomplish this operation.
The following package shows an overloaded function for operator + that
allows addition of two objects of BIT_VECTOR types:
PACKAGE math IS
FUNCTION “+”( l,r : BIT_VECTOR) RETURN INTEGER;
END math;
PACKAGE BODY math IS
FUNCTION vector_to_int( S : BIT_VECTOR) RETURN INTEGER IS
VARIABLE result : INTEGER := 0;
VARIABLE prod : INTEGER := 1;
BEGIN
FOR i IN s’RANGE LOOP
IF s(i) = ‘1’ THEN
result := result + prod;
END IF;
prod := prod * 2;
END LOOP;
RETURN result;
END vector_to_int;
FUNCTION “+”(l,r : BIT_VECTOR) RETURN INTEGER IS
BEGIN
RETURN ( vector_to_int(l) + vector_to_int(r));
END;
END math;
Whenever the + operator is used in an expression, the compiler calls
the + operator function that matches the types of the operands. When the
operands are of type INTEGER, the built-in + operator function is called. If
the operands are of type BIT_VECTOR, then the function from package math
is called. The following example shows uses for both functions:
USE WORK.math.ALL;
ENTITY adder IS
PORT( a, b : IN BIT_VECTOR(0 TO 7);
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PORT( c : IN INTEGER;
PORT( dout : OUT INTEGER);
END adder;
ARCHITECTURE test OF adder IS
SIGNAL internal : INTEGER;
BEGIN
internal <= a + b;
dout <= c + internal;
END test;
This example illustrates how overloading can be used to make very
readable models. The value assigned to signal internal is the sum of
inputs a and b. Since a and b are of type BIT_VECTOR, the overloaded
operator function that has two BIT_VECTOR arguments is called. This function adds the values of a and b together and returns an integer value to
be assigned to signal internal.
The second addition uses the standard built-in addition function that
is standard in VHDL because both operands are of type INTEGER. This
model could have been written as shown in the following, but would still
function in the same manner:
PACKAGE math IS
FUNCTION addvec( l,r : bit_vector) RETURN INTEGER;
END math;
PACKAGE BODY math IS
FUNCTION vector_to_int( S : bit_vector) RETURN INTEGER IS
VARIABLE result : INTEGER := 0;
VARIABLE prod : INTEGER := 1;
BEGIN
FOR i IN s’RANGE LOOP
IF s(i) = ‘1’ THEN
result := result + prod;
END IF;
prod := prod * 2;
END LOOP;
RETURN result;
END vector_to_int;
FUNCTION addvec(l,r : bit_vector) RETURN INTEGER IS
BEGIN
RETURN ( vector_to_int(l) + vector_to_int(r));
END addvec;
END math;
USE WORK.math.ALL;
ENTITY adder IS
PORT( a, b : IN BIT_VECTOR(0 TO 7);
PORT( c : IN INTEGER;
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213
PORT( dout : OUT INTEGER);
END adder;
ARCHITECTURE test2 OF adder IS
SIGNAL internal : INTEGER;
BEGIN
internal <= addvec(a,b);
dout <= c + internal;
END test2;
In this example, a function called advec is used to add a and b. Both
coding styles give exactly the same results, but the first example using
the overloaded + operator is much more readable and easier to maintain.
If another person besides the designer of a model takes over the maintenance of the model, it is much easier for the new person to understand
the model if overloading was used.
OPERATOR ARGUMENT TYPE OVERLOADING Arguments to
overloaded operator functions do not have to be of the same type, as the
previous two examples have shown. The parameters to an overloaded
operator function can be of any type. In some cases, it is preferable to
write two functions so that the order of the arguments is not important.
Let’s examine the functions for an overloaded logical operator that mixes
signals of type BIT and signals of a nine-state value system:
PACKAGE p_logic_pack IS
TYPE t_nine_val IS (Z0, Z1, ZX,
TYPE t_nine_val IS (R0, R1, RX,
TYPE t_nine_val IS (F0, F1, FX);
FUNCTION “AND”( l, r : t_nine_val) RETURN BIT;
FUNCTION “AND”( l : BIT; r : t_nine_val) RETURN BIT;
FUNCTION “AND”( l : t_nine_val; r : BIT) RETURN BIT;
END p_logic_pack;
PACKAGE BODY p_logic_pack IS
FUNCTION nine_val_2_bit( t : IN t_nine_val) RETURN BIT IS
TYPE t_nine_val_conv IS ARRAY(t_nine_val) OF BIT;
CONSTANT nine_2_bit : t_nine_val_conv :=
(‘0’,
— Z0
‘1’,
— Z1
‘1’,
— ZX
‘0’,
— R0
‘1’,
— R1
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‘1’,
‘0’,
‘1’,
‘1’);
—
—
—
—
RX
F0
F1
FX
BEGIN
RETURN nine_2_bit(t);
END nine_val_2_bit;
FUNCTION “AND”(l,r : t_nine_val) RETURN BIT IS
BEGIN
RETURN (nine_val_2_bit(l) AND nine_val_2_bit(r));
END;
FUNCTION “AND”(l :BIT; r : t_nine_val) RETURN BIT IS
BEGIN
RETURN ( l AND nine_val_2_bit(r));
END;
FUNCTION “AND”(l : t_nine_val; r : BIT) RETURN BIT IS
BEGIN
RETURN (nine_val_2_bit(l) AND r);
END;
END p_logic_pack;
The package p_logic_pack declares three overloaded functions for the
AND operator. In one function, both input types are type t_nine_val. In
the other two functions, only one input is type t_nine_val, and the other
input is type BIT. All functions return a result of type BIT. Notice that, to
overload the AND operator, the syntax is the same as overloading the +
operator from the previous example.
When the AND operator is used in a model, the appropriate function is
called based on the types of the operands. In the following code fragments,
we can see the differences:
SIGNAL a, b : t_nine_val;
SIGNAL c,e : bit;
e <= a AND b;
-- calls first function
e <= a AND c;
-- calls third function
e <= c AND b;
-- calls second function
By having three functions called AND, we do not need to worry about
which side of the operator an expression resides on. All of the possible
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215
combinations of operator order are covered with three functions, because
the function for two inputs of type BIT are built in.
Aliases
An alias creates a new name for all or part of the range of an array type.
It is very useful for naming parts of a range as if they were subfields. For
example, in a CPU model, an instruction is fetched from memory. The
instruction may be an array of 32 bits that is interpreted as a number of
smaller fields to represent the instruction opcode, source register 1, source
register 2, and so on. Aliases provide a mechanism to name each of the
subfields of the instruction and to reference these fields directly by the
alias names. This is illustrated by the following example:
SIGNAL instruction : BIT_VECTOR(31 DOWNTO 0);
ALIAS opcode : BIT_VECTOR(3 DOWNTO 0) IS instruction(31
DOWNTO 28);
ALIAS src_reg : BIT_VECTOR(4 DOWNTO 0) IS instruction(27
DOWNTO 23);
ALIAS dst_reg : BIT_VECTOR(4 DOWNTO 0) IS instruction(22
DOWNTO 18);
In this example, the aliases have been created for a signal object.
Using the alias name in an assignment or referencing operation is the
same as using the piece of the instruction object being aliased, but much
more convenient.
Remember that the semantics in place for the object being aliased are
applied to the alias as well. If an alias is created for a constant object, the
alias cannot have an assignment for the same reasons that a constant
cannot have an assignment.
Qualified Expressions
One of the side effects of overloading is that multiple functions or procedures
may match in a particular instance because the types are ambiguous.
For the compiler to figure out which subprogram to use, a qualified
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expression may be required. A qualified expression states the exact type
that the expression should attain. For instance, when evaluating an
expression containing a mixture of overloaded subprograms and constant
values, the designer may need to qualify an expression to produce correct
results. Following is an example of such a situation:
PACKAGE p_qual IS
TYPE int_vector IS ARRAY(NATURAL RANGE <>) OF INTEGER;
FUNCTION average( a : int_vector) RETURN INTEGER;
FUNCTION average( a : int_vector) RETURN REAL;
END p_qual;
USE WORK.p_qual.ALL;
ENTITY normalize IS
PORT( factor : IN REAL;
PORT( points : IN int_vector;
PORT( result : OUT REAL);
END normalize;
ARCHITECTURE qual_exp OF normalize IS
BEGIN
result <= REAL’(average(points)) * factor;
END qual_exp;
Package p_qual defines two overloaded functions named average and
an unconstrained type, int_vector. The package body is left as an exercise
for the reader.
Architecture qual_exp has a single concurrent signal assignment statement that calls function average. Because there are two functions named
average, there are two possible functions that can be used by this call. To
clarify which function to use, the expression has been qualified to return
a REAL type. The keyword REAL followed by a ’ specifies that the expression inside the parentheses return a type REAL.
The expression was qualified to make sure that the average function
returning a REAL number was called instead of the average function that
returns an INTEGER. In this example, the expression required a qualified
expression to allow the architecture to compile. The compiler does not
make any random guesses about which function to use. The designer must
specify exactly which one to use in cases where more than one function
can match; otherwise, an error is generated.
Another use for a qualified expression is to build the source value for
an assignment statement. Based on the type of the signal assignment target, the source value can be built. Following is an example:
Advanced Topics
217
PACKAGE p_qual_2 IS
TYPE vector8 IS ARRAY( 0 TO 7) OF BIT;
END p_qual_2;
USE WORK.p_qual_2.ALL;
ENTITY latch IS
PORT( reset, clock : IN BIT;
PORT( data_in : IN vector8;
PORT( data_out : OUT vector8);
END latch;
ARCHITECTURE behave OF latch IS
BEGIN
PROCESS(clock)
BEGIN
IF (clock = ‘1’) THEN
IF (reset = ‘1’) THEN
data_out <= vector8’(others => ‘0’);
ELSE
data_out <= data_in;
END IF;
END IF;
END PROCESS;
END behave;
This example is an 8-bit transparent latch, with a reset line to set the
latch to zero. When the clock input is a ‘1’ value, the latch is transparent, and input values are reflected on the output. When the clock input
is ‘0’, the data_in value is latched. When reset is a ‘1’ value while clock
input is a ‘1’, the latch is reset. This is accomplished by assigning all
‘0’s to data_out. One method to assign all ‘0’s to data_out is to use an
aggregate assignment. Because data_out is 8 bits, the following aggregate
assignment sets data_out to all ‘0’s:
data_out <= (‘0’, ‘0’, ‘0’, ‘0’, ‘0’, ‘0’, ‘0’, ‘0’);
This aggregate works fine unless the type of data_out changes. If the
type of output data_out was suddenly changed to 16 bits instead of 8, the
aggregate could no longer be used.
Another method to accomplish the assignment to output data_out is to
use a qualified expression. The assignment to data_out when reset = ‘1’
in the preceding example shows how this might be done. The following expression:
(others => ‘0’)
can be qualified with the type of the target signal (data_out). This allows
the compiler to determine how large the target signal is and how large to
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make the source being assigned to the signal. Now, whenever the target
signal type is changed, the source changes to match.
User-Defined Attributes
VHDL user-defined attributes are a mechanism for attaching data to VHDL
objects. The data attached can be used during simulation or by
another tool that reads the VHDL description. Data such as the disk file
name of the model, loading information, driving capability, resistance,
capacitance, physical location, and so on can be attached to objects. The
type and value of the data is completely user-definable. The value, when
specified, is constant throughout the simulation.
User-defined attributes can behave similar to entity generic values,
with one exception. Generics are only legal on entities, but user-defined
attributes can be assigned to the following list of objects:
■ Entity
■ Architecture
■ Configuration
■ Procedure
■ Function
■ Package
■ Type and Subtype
■ Constant
■ Signal
■ Variable
■ Component
■ Label
To see how user-defined attributes operate, let’s examine the following
description:
PACKAGE p_attr IS
TYPE t_package_type IS ( leadless,
TYPE t_package_type IS ( pin_grid,
TYPE t_package_type IS ( dip);
ATTRIBUTE package_type : t_package_type;
Advanced Topics
219
ATTRIBUTE location : INTEGER;
END p_attr;
USE WORK.p_attr.ALL;
ENTITY board IS
PORT(
.
.
.
);
END board;
ARCHITECTURE cpu_board OF board IS
COMPONENT mc68040
GENERIC(
. . . . . . );
PORT(
.
.
.
);
END COMPONENT;
SIGNAL a : INTEGER;
SIGNAL b : t_package_type;
ATTRIBUTE package_type OF mc68040 : COMPONENT IS pin_grid;
ATTRIBUTE location OF mc68040 : COMPONENT IS 20;
BEGIN
a <= mc68040’location;
-- returns 20
b <= mc68040’package_type;
-- returns pin_grid
END cpu_board;
This is a very simple example of how attributes can be attached to
objects. Much more complicated types and attributes can be created. What
this example shows is a code fragment of a CPU board design in which
the package type and location information are specified as attributes of
the single microprocessor used in the design.
The package_type attribute is used to hold the kind of packaging
used for the microprocessor. Attributes that have values specified do not
have to be used in the simulation. Other tools such as physical layout
tools or fault simulation can make use of attributes that a logic simulator cannot.
In this example, a physical layout tool could read the package type
information from the package_type attribute and, based on the value
assigned to the attribute, fill in the value for the location attribute.
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Chapter Eight
The package p_attr defines the type used for one of the attributes and
contains the attribute declarations for two attributes. The attribute
declarations make the name and type of the attribute visible to any
object for use if needed.
In the architecture cpu_board of entity board are the attribute specifications. The attribute specification describes the attribute name to be
used, the name of the object to which the attribute is attached, the object
kind, and finally the value of the attribute.
To access the value of a user-defined attribute, use the same syntax for
a predefined attribute. In the signal assignment statements of architecture cpu_board, the attribute value is retrieved by specifying the name of
the object, followed by a ’ and the attribute name.
Generate Statements
Generate statements give the designer the ability to create replicated
structures, or select between multiple representations of a model. Generate
statements can contain IF-THEN and looping constructs, nested to any
level, that create concurrent statements.
Typical applications include memory arrays, registers, and so on.
Another application is to emulate a conditional compilation mechanism
found in other languages such as C.
Following is a simple example showing the basics of generate statements:
LIBRARY IEEE;
USE IEEE.std_logic_1164.ALL;
ENTITY shift IS
PORT( a, clk : IN std_logic;
PORT( b : OUT std_logic);
END shift;
ARCHITECTURE gen_shift OF shift IS
COMPONENT dff
PORT( d, clk : IN std_logic;
PORT( q : OUT std_logic);
END COMPONENT;
SIGNAL z : std_logic_vector( 0 TO 4 );
BEGIN
z(0) <= a;
g1 : FOR i IN 0 TO 3 GENERATE
dffx : dff PORT MAP( z(i), clk, z(i + 1));
Advanced Topics
221
END GENERATE;
b <= z(4);
END gen_shift;
This example represents the behavior for a 4-bit shift register. Port a
is the input to the shift register, and port b is the output. Port clk shifts
the data from a to b.
Architecture gen_shift of entity shift contains two concurrent signal
assignment statements and one GENERATE statement. The signal assignment statements connect the internal signal z to input port a and output
port b. The generate statement in this example uses a FOR scheme to generate four DFF components. The resultant schematic for this architecture
is shown in Figure 8-1.
The FOR in the generate statement acts exactly like the FOR loop
sequential statement in that variable i need not be declared previously,
i is not visible outside the generate statement, and i cannot be assigned
inside the generate statement.
The result of the generate statement is functionally equivalent to the
following architecture:
ARCHITECTURE long_way_shift OF shift IS
COMPONENT dff
PORT( d, clk : IN std_logic;
PORT( q : OUT std_logic);
END COMPONENT;
SIGNAL z : std_logic_vector( 0 TO 4 );
BEGIN
z(0) <= a;
dff1:
dff2:
dff3:
dff4:
Figure 8-1
Schematic Representing Generate Statement.
dff
dff
dff
dff
PORT
PORT
PORT
PORT
Z(0)
Z(1)
D
Q
CLK
CLK
z(0),
z(1),
z(2),
z(3),
clk,
clk,
clk,
clk,
DFFX(1)
DFFX(0)
A
MAP(
MAP(
MAP(
MAP(
);
);
);
);
Q
DFFX(3)
DFFX(2)
Z(2)
D
CLK
CLK
z(1)
z(2)
z(3)
z(4)
Z(3)
D
Q
CLK
Z(4)
D
Q
CLK
B
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Chapter Eight
b <= z(4);
END long_way_shift;
The difference between the two architectures is that architecture
gen_shift could be specified with generic parameters such that different-sized
shift registers could be generated based on the value of the generic parameters. Architecture long_way_shift is fixed in size and cannot be changed.
Irregular Generate Statement
The last example showed how a regular structure could be generated, but
in practice most structures are not completely regular. Most regular
structures have irregularities at the edges. This is shown by Figure 8-2.
In the last example, the irregularities were handled by the two
concurrent signal assignment statements. Following is another way to
handle the irregularities:
LIBRARY IEEE;
USE IEEE.std_logic_1164.ALL;
ENTITY shift IS
GENERIC ( len : INTEGER);
PORT( a, clk : IN std_logic;
PORT( b : OUT std_logic);
END shift;
ARCHITECTURE if_shift OF shift IS
COMPONENT dff
PORT( d, clk : IN std_logic;
PORT( q : OUT std_logic);
END COMPONENT;
SIGNAL z : std_logic_vector( 1 TO (len -1) );
BEGIN
g1 : FOR i IN 0 TO (len -1) GENERATE
Figure 8-2
Irregular Generate
Statement Representation.
DFFX(1)
DFFX(0)
Z(1)
A
D
Q
CLK
CLK
Z(2)
D
Q
CLK
CLK
DFFX(N)
DFFX(2)
Z(N)
D
Q
CLK
D
Q
CLK
B
Advanced Topics
223
IF i = 0 GENERATE
dffx : dff PORT MAP( a, clk, z(i + 1));
END GENERATE;
IF i = (len -1) GENERATE
dffx : PORT MAP( z(i), clk, b );
END GENERATE;
IF (i > 0) AND i < (len -1) GENERATE
dffx : PORT MAP( z(i), clk, Z(i + 1) );
END GENERATE;
END GENERATE;
END if_shift;
This example uses a shift register that has a configurable size. Generic
len passed in specifies the length of the shift register. (Generic len must be
at least 2 for the shift register to work properly.) Generic len is used in
the specification of the length of signal array z. This type of array is known
as a generically constrained array because the size of the array is specified
through one or more generics.
The FOR clause of the generate also uses generic len to specify the
maximum number of DFF components to be generated. Notice that this
generate statement uses the conditional form of the generate statement.
If the condition is true, the concurrent statements inside the generate
statement are generated; otherwise, nothing is generated.
The first IF-THEN condition checks for the first flip-flop in the shift
register. If this is the first flip-flop, notice that the port map clause maps
the input signal a directly to the flip-flop instead of through an intermediate signal. The same is true of the next IF-THEN condition. It checks for
the last flip-flop of the shift register and maps the last output to output
port b. Any other flip-flops in the shift register are generated by the third
conditional generate statement.
Following is another interesting example using the conditional generate statement:
PACKAGE gen_cond IS
TYPE t_checks IS ( onn, off);
END gen_cond;
USE WORK.gen_cond.ALL;
LIBRARY IEEE;
USE IEEE.std_logic_1164.ALL;
ENTITY dff IS
GENERIC( timing_checks : t_checks;
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GENERIC( setup, qrise, qfall, qbrise, qbfall : time);
PORT( din, clk : IN std_logic;
PORT( q, qb : OUT std_logic);
END dff;
ARCHITECTURE condition OF dff IS
BEGIN
G1 : IF (timing_checks = onn) GENERATE
ASSERT ( din’LAST_EVENT >>setup)
REPORT “setup violation”
SEVERITY ERROR;
END GENERATE;
PROCESS(clk)
VARIABLE int_qb : std_logic;
BEGIN
IF (clk = ‘1’) AND (clk’EVENT) AND (clk’LAST_VALUE =
‘0’) THEN
int_qb := not din;
q <= din AFTER f_delay( din, qrise, qfall);
qb <= int_qb AFTER f_delay( int_qb, qbrise, qbfall);
END IF;
END PROCESS;
END condition;
In this example, a DFF component is modeled using a generate statement to control whether or not a timing check statement is generated for
the architecture. The generic, timing_checks, can be passed a value of onn
or off. (Note the spelling of onn. We cannot use a value of on because it
is a reserved word.) If the value is onn, then the generate statement
generates a concurrent assertion statement. If the value of generic
timing_checks is off, then no assertion statement is generated. This
functionality emulates the conditional compilation capability of some
programming languages, such as C and Pascal.
TextIO
One of the predefined packages that is supplied with VHDL is the
Textual Input and Output (TextIO) package. The TextIO package contains
procedures and functions that give the designer the ability to read from
and write to formatted text files. These text files are ASCII files of any
format that the designer desires. (VHDL does not impose any limits of
format, but the host machine might impose limits.) TextIO treats these
Advanced Topics
225
ASCII files as files of lines, where a line is a string, terminated by a
carriage return. There are procedures to read a line and write a line and
a function that checks for end of file.
The TextIO package also declares a number of types that are used
while processing text files. Type line is declared in the TextIO package
and is used to hold a line to write to a file or a line that has just been read
from the file. The line structure is the basic unit upon which all TextIO
operations are performed. For instance, when reading from a file, the first
step is to read in a line from the file into a structure of type line. Then
the line structure is processed field by field.
The opposite is true for writing to a file. First, the line structure is built
field by field in a temporary line data structure, then the line is written
to the file.
Following is a very simple example of a TextIO behavior:
USE WORK.TEXTIO.ALL;
ENTITY square IS
PORT( go : IN std_logic);
END square;
ARCHITECTURE simple OF square IS
BEGIN
PROCESS(go)
FILE infile : TEXT IS IN “/doug/test/example1”;
FILE outfile : TEXT IS OUT “/doug/test/outfile1”;
VARIABLE out_line, my_line : LINE;
VARIABLE int_val : INTEGER;
BEGIN
WHILE NOT( ENDFILE(infile)) LOOP
-- read a line from the input file
READLINE( infile, my_line);
-- read a value from the line
READ( my_line, int_val);
-- square the value
int_val := int_val **2;
-- write the squared value to the line
WRITE( out_line, int_val);
-- write the line to the output file
WRITELINE( outfile, out_line);
END LOOP;
END PROCESS;
END simple;
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Chapter Eight
This example shows how to read a single integer value from a line,
square the value, and write the squared value to another file. It illustrates
how TextIO can be used to read values from files and write values to files.
The process statement is executed whenever signal go has an event
occur. The process then loops until an end-of-file condition occurs on the
input file infile. The READLINE statement reads a line from the file and
places the line in variable my_line. The next executable line contains a
READ procedure call that reads a single integer value from my_line into
variable int_val. Procedure READ is an overloaded procedure that reads
different type values from the line, depending on the type of the argument
passed to it.
After the value from the file has been read into variable int_val, the
variable is squared, and the squared value is written to another variable
of type line, called out_line. Procedure WRITE is also an overloaded
procedure that writes a number of different value types, depending on the
type of the argument passed to it.
The last TextIO procedure call made is the WRITELINE procedure call. This
procedure writes out the line variable out_line to the output file outfile.
If the following input file is used as input to this architecture, the second
file shown reflects the output generated:
10
20
50
16#A
1_2_3
87
<— hex input
<— underscores ignored
52 <— second argument ignored
The output from the input file would look like this:
100
400
2500
100
15129
7569
The first value in the input file is 10. It is squared to result in 100 and
written to the output file. The same is true for the values 20 and 50. The
next value in the file is specified in hexadecimal notation. A hexadecimal
A value is 10 base ten, which squared results in 100.
The next example in the file shows a number with embedded underscore
characters. The underscores are used to separate fields of a number and are
ignored in the value of the number. The number 1_2_3 is the same as 123.
Advanced Topics
227
The last entry in the input file shows a line with two input values on
the line. When the line is read into the my_line variable, both values
exist in the line, but because there is only one READ procedure call, only
the first value is read from the line.
More than one data item can be read from a single line, as well as data
items of any types. For instance, a TextIO file could be a list of instructions
for a microprocessor. The input file could contain the type of instruction, a
source address, and a destination address. This is shown by the following
simple example:
USE WORK.TEXTIO.ALL;
PACKAGE p_cpu IS
TYPE t_instr IS (jump, load,
TYPE t_instr IS (store, addd,
TYPE t_instr IS (subb, test, noop);
FUNCTION convertstring( s : STRING) RETURN t_instr;
END p_cpu;
PACKAGE BODY p_cpu IS
FUNCTION convertstring( s : STRING) RETURN t_instr IS
SUBTYPE twochar IS string(1 to 2);
VARIABLE val : twochar;
BEGIN
val := s(1 to 2);
CASE val IS
WHEN “ju” =>
RETURN jump;
WHEN “lo” =>
RETURN load;
WHEN “st” =>
RETURN store;
WHEN “ad” =>
RETURN addd;
WHEN “su” =>
RETURN subb;
WHEN “te” =>
RETURN test;
WHEN “no” =>
RETURN noop;
WHEN others =>
RETURN noop;
END CASE;
END convertstring;
END p_cpu;
USE WORK.p_cpu.ALL;
USE WORK.TEXTIO.ALL;
ENTITY cpu_driver IS
228
Chapter Eight
PORT( next_instr : IN BOOLEAN;
PORT( instr : OUT t_instr;
PORT( src : OUT INTEGER;
PORT( dst : OUT INTEGER);
END cpu_driver;
ARCHITECTURE a_cpu_driver OF cpu_driver IS
FILE instr_file : TEXT IS IN “instfile”;
BEGIN
read_instr : PROCESS( next_instr)
VARIABLE aline : LINE;
VARIABLE a_instr : STRING(1 to 4);
VARIABLE asrc, adst : INTEGER;
BEGIN
IF next_instr THEN
IF ENDFILE(instr_file) THEN
ASSERT FALSE
REPORT “end of instructions”
SEVERITY WARNING;
ELSE
READLINE( instr_file, aline);
READ( aline, a_instr);
READ( aline, asrc);
READ( aline, adst);
END IF;
instr <= convertstring(a_instr);
src <= asrc;
dst <= adst;
END IF;
END PROCESS read_instr;
END a_cpu_driver;
Package p_cpu defines type t_instr, the enumerated type that represents CPU instructions to be executed. The package also defines a function,
convert_string, that is used to convert the string value read in using
TextIO procedures into a t_instr type. The conversion is necessary because
the TextIO package does not contain any procedures for reading in userdefined types. (However, a designer can write a user-defined overloaded
procedure that has the same basic interface as the procedures in the
TextIO package.) This process is usually very straightforward, as seen by
the convert_string procedure.
Entity cpu_driver is the entity that reads in the file of instructions. It
has a single input port called next_instr which is used to signal the
entity to read in the next instruction. When a true event occurs on input
port next_instr, process read_instr executes. If the file is at the end
already, the ASSERT statement is called, and a warning message is issued.
Advanced Topics
229
If we are not at the end of the file, the process reads in a line from the file
into variable aline.
Successive reads on variable aline retrieve the appropriate fields from
the line. All of the reads return the value into internal variables, but variables asrc and adst are not really needed because there exists a TextIO
procedure for reading integer values. Variable ainstr is used to allow the
string read in to be converted into the enumerated type t_instr before
being assigned to the output port instr.
SUMMARY
In this chapter we discussed the following:
■ Overloading functions, arguments, operators to make VHDL
models more readable.
■ How aliases can be used to name sections of an object.
■ How qualified expressions are used to direct conversion.
■ How user-defined attributes can be used to add information to
objects.
■ How generate statements can be used to replicate entity
instantiations.
■ How TextIO is used to read and write text files.
This chapter showed some of the more esoteric features of VHDL. This
chapter concludes the discussion of VHDL features. The next two chapters
concentrate on the synthesis process and how to write VHDL for
synthesis. The next few chapters then guide the reader through a topdown description of a device.
This page intentionally left blank.
CHAPTER
9
Synthesis
One of the best uses of VHDL today is to synthesize ASIC
and FPGA devices. This chapter and the next focus on
how to write VHDL for synthesis.
Synthesis is an automatic method of converting a higher
level of abstraction to a lower level of abstraction. There
are several synthesis tools available currently, including
commercial as well as university-developed tools. In this
discussion, the examples use the commercially available
Exemplar Logic Leonardo Sectrum synthesis tool.
The current synthesis tools available today convert
Register Transfer Level (RTL) descriptions to gate level
netlists. These gate level netlists consist of interconnected
gate level macro cells. Models for the gate level cells are
contained in technology libraries for each type of technology
supported.
232
Chapter Nine
Figure 9-1
Gate Level Netlist
Synthesis.
Technology
Library
RTL
Description
Synthesis
Gate Level
Netlists
Constraints
These gate level netlists currently can be optimized for area, speed,
testability, and so on. The synthesis process is shown in Figure 9-1.
The inputs to the synthesis process are an RTL (Register Transfer
Level) VHDL description, circuit constraints and attributes for the design,
and a technology library. The synthesis process produces an optimized gate
level netlist from all of these inputs. In the next few sections, each of these
inputs is described, and we discuss the synthesis process in more detail.
Register Transfer Level
Description
A register transfer level description is characterized by a style that specifies all of the registers in a design, and the combinational logic between.
This is shown by the register and cloud diagram in Figure 9-2. The registers are described either explicitly through component instantiation or
implicitly through inference. The registers are shown as the rectangular
objects connected to the clock signal. The combinational logic is described
by logical equations, sequential control statements (CASE, IF then ELSE,
and so on), subprograms, or through concurrent statements, which are
represented by the cloud objects between registers.
Synthesis
233
Register
Figure 9-2
Register and Cloud
Diagram.
Register
Datain
Dataout
Combinational
Logic
CLK
CLK
Clock
RTL descriptions are used for synchronous designs and describe the
clock-by-clock behavior of the design. Following is an example of an RTL
description that uses component instantiation:
ENTITY datadelay IS
PORT( clk, din, en : IN BIT;
PORT( dout : OUT BIT);
END datadelay;
ARCHITECTURE synthesis OF datadelay IS
COMPONENT dff
PORT(clk, din : IN BIT;
PORT(q,qb : OUT BIT);
END COMPONENT;
SIGNAL q1, q2, qb1, qb2 : BIT;
BEGIN
r1 : dff PORT MAP(clk, din, q1, qb1);
r2 : dff PORT MAP(clk, q1, q2, qb2);
dout <= q1 WHEN en = ‘1’ ELSE
q2;
END synthesis;
This example is the circuit for a selectable data delay circuit. The
circuit delays the input signal din by 1 or 2 clocks depending on the value
of en. If en is a 1, then input din is delayed by 1 clock. If en is a 0, input
din is delayed by 2 clocks.
Figure 9-3 shows a schematic representation of this circuit. The clock
signal connects to the clk input of both flip-flops, while the din signal
connects only to the first flip-flop. The q output of the first flip-flop is then
234
Chapter Nine
r1
Figure 9-3
Register Transfer
Level with Component Instances.
r2
DIN
D
CLK
D
Q
CLK
QB
Q
QB
CLK
EN
Assignment Statement to
Dout
DOUT
connected to the d input of the next flip-flop. The selected signal assignment to signal dout forms a mux operation that selects between the two
flip-flop outputs.
This example could be rewritten as follows using register inference:
ENTITY datadelay IS
PORT( clk, din, en : IN BIT;
PORT( dout : OUT BIT);
END datadelay;
ARCHITECTURE inference OF datadelay IS
SIGNAL q1, q2 : BIT;
BEGIN
reg_proc: PROCESS
BEGIN
WAIT UNTIL clk’EVENT and clk = ‘1’;
q1 <= din;
q2 <= q1;
END PROCESS;
dout <= q1 WHEN en = ‘1’ ELSE
q2;
Synthesis
235
END inference;
In the first version, the registers are instantiated using component
instantiation statements that instantiate r1 and r2.
In this version, the dff components are not instantiated, but are
inferred through the synthesis process. Register inference is discussed
more in Chapter 10, “VHDL Synthesis.” Process reg_proc has a WAIT
statement that is triggered by positive edges on the clock. When the WAIT
statement is triggered, signal q1 is assigned the value of din, and q2 is
assigned the previous value of q1. This, in effect, creates two flip-flops.
One flip-flop for signal q1, and the other for signal q2.
This is a register transfer level description because registers r1 and r2
from the first version form the registers, and the conditional signal
assignment for port dout forms the combinational logic between registers.
In the second version, the inferred registers form the register description,
while the conditional signal assignment still forms the combinational logic.
The advantage of the second description is that it is technology independent. In the first description, actual flip-flop elements from the technology library were instantiated, thereby making the description technology dependent. If the designer should decide to change technologies, all of the
instances of the flip-flops would need to be changed to the flip-flops from
the new technology. In the second version of the design, the
designer did not specify particular technology library components, and the
synthesis tools are free to select flip-flops from whatever technology
library the designer is currently using, as long as these flip-flops match
the functionality required.
After synthesis, both of these descriptions produce a gate level description, as shown in Figure 9-4.
Notice that the gate level description has two registers (FDSR1) with
mux (Mux21S) logic controlling the output signal from each register. Depending on the technology library selected and the constraints, the mux
logic varies widely from and-or-invert gates to instantiated 2-input
multiplexers.
Following is the netlist generated by the Exemplar Logic Leonardo
Spectrum synthesis tool for the same design:
- -- Definition of
- -- - - -
datadelay
236
Chapter Nine
dout
Figure 9-4
A Gate Level Description.
A
B
S
en
q2
clk
CP
z
dout
MUX21S
Q
D
q1
CP
din
FDSR1
Q
D
FDSR1
library IEEE, EXEMPLAR;
use IEEE.STD_LOGIC_1164.all;
use EXEMPLAR.EXEMPLAR_1164.all;
entity datadelay is
port (
clk : IN std_logic ;
din : IN std_logic ;
en : IN std_logic ;
dout : OUT std_logic) ;
end datadelay ;
architecture inference of datadelay is
component FDSR1
port (
Q : OUT std_logic ;
D : IN std_logic ;
CP : IN std_logic) ;
end component ;
component MU21S
port (
Z : OUT std_logic ;
A : IN std_logic ;
B : IN std_logic ;
S : IN std_logic) ;
end component ;
signal q2, q1: std_logic ;
begin
q2_XMPLR : FDSR1 port map ( Q=>q2, D=>q1, CP=>clk);
q1_XMPLR : FDSR1 port map ( Q=>q1, D=>din, CP=>clk);
dout_XMPLR_XMPLR : MU21S port map ( Z=>dout, A=>q2, B=>q1,
S=>en);
end inference ;
Synthesis
237
The netlist matches the gate level generated schematic. The netlist contains two instantiated flip-flops (FDSR1) and one instantiated 2-input multiplexer (Mux21S).
This very simple example shows how RTL synthesis can be used to
create technology-specific implementations from technology-independent
VHDL descriptions. In the next few sections, we examine much more complex examples. But first, let’s look at some of the ways to control how the
synthesized design is created.
Constraints
Constraints are used to control the output of the optimization and mapping process. They provide goals that the optimization and mapping
processes try to meet and control the structural implementation of the
design. They represent part of the physical environment that the design
has to interface with. The constraints available in synthesis tools today
include area, timing, power, and testability constraints. In the future, we
will probably see packaging constraints, layout constraints, and so on.
Today, the most common constraints in use are timing constraints.
A block diagram of a design with some possible constraints is shown in
Figure 9-5. Again, the design is shown using the cloud notation. The combinational logic between registers is represented as clouds, with wires
going in and out representing the interconnection to the registers.
Delay Constraint
Figure 9-5
Register and Cloud
Diagram with Constraints.
Register
Register
Datain
Dataout
max_delay 5
Combinational
Logic
CLK
CLK
Area = 100
Clock
clock 0 10 10
Area Constraint
Clock Constraint
238
Chapter Nine
There are a number of constraints shown on the diagram including
required time constraints, late arrival constraints, and clock cycle constraints.
Required time constraints specify the latest time that a signal can occur.
Clock constraints are used to specify the operating frequency of the clock.
From the clock constraint, required time constraints of each signal feeding
a clocked register can be calculated. Each of these constraints is further
described in the next sections.
Timing Constraints
Typical uses for timing constraints are to specify maximum delays for
particular paths in a design. For instance, a typical timing constraint is
the required time for an output port. The timing constraint guides the
optimization and mapping to produce a netlist that meets the timing
constraint. Meeting timing is usually one of the most difficult tasks when
designing an ASIC or FPGA using synthesis tools. There may be no design
that meets the timing constraints specified. A typical delay constraint in
Leonardo synthesis format is shown here:
set_attribute -port data_out -name required_time -value 25
This constraint specifies that the maximum delay for signal data_out
should be less than or equal to 25 library units. A library unit can be
whatever the library designer used when describing the technology from
a synthesis point of view. Typically, it is nanoseconds, but can be picoseconds
or some other time measurement depending on the technology.
Clock Constraints
One method to constrain a design is to add a required_time constraint
to every flip-flop input with the value of a clock cycle. The resulting
design would be optimized to meet the one clock cycle timing constraint.
An easier method, however, is to add a clock constraint to the design. A
clock constraint effectively adds an input required_time constraint to
every flip-flop data input. An example clock constraint is shown here:
set_attribute -port clk -name clock_cycle -value 25
Synthesis
239
This example sets a clock cycle constraint on port clk with a value of
25 library units.
Some synthesis tools (such as Exemplar Logic Leonardo) do a static
timing analysis to calculate the delay for each of the nodes in the design. The
static timing analyzer uses a timing model for each element connected in
the netlist. The timing analyzer calculates the worst and best case timing
for each node by adding the contribution of each cell that it traverses.
The circuit is checked to see if all delay constraints have been met. If
so, the optimization and mapping process is done; otherwise, alternate
optimization strategies may be applied—such as adding more parallelism
or more buffered outputs to the slow paths—and the timing analysis is
executed again. More detail about the typical timing analysis is discussed
later in the section “Technology Libraries.”
Attributes
Attributes are used to specify the design environment. For instance,
attributes specify the loading that output devices have to drive, the drive
capability of devices driving the design, and timing of input signals. All
of this information is taken into account by the static timing analyzer to
calculate the timing through the circuit paths. A cloud diagram showing
attributes is shown in Figure 9-6.
Figure 9-6
Register and Cloud
Diagram with Attributes.
Data2
drive
Register
late arrival
load
Register
Datain
Dataout
Combinational
Logic
CLK
CLK
setup/hold
Clock
240
Chapter Nine
Load
Each output can specify a drive capability that determines how many
loads can be driven within a particular time. Each input can have a load
value specified that determines how much it will slow a particular driver.
Signals that are arriving later than the clock can have an attribute that
specifies this fact.
The Load attribute specifies how much capacitive load exists on a
particular output signal. This load value is specified in the units of the
technology library in terms of pico-farads, or standard loads, and so on.
For instance, the timing analyzer calculates a long delay for a weak driver
and a large capacitive load, and a short delay for a strong driver and a
small load. An example of a load specification in Leonardo synthesis format
is shown here:
set_attribute -port xbus -name input_load -value 5
This attribute specifies that signal xbus will load the driver of this
signal with 5 library units of load.
Drive
The Drive attribute specifies the resistance of the driver, which controls
how much current it can source. This attribute also is specified in the units
of the technology library. The larger a driver is the faster a particular path
will be, but a larger driver takes more area, so the designer needs to trade
off speed and area for the best possible implementation. An example of a
drive specification in Leonardo synthesis format is shown here:
set_attribute -port ybus -name output_drive -value 2.7
This attribute specifies that signal ybus has 2.7 library units of drive
capability.
Arrival Time
Some synthesis tools (such as Exemplar Logic Leonardo) use a static
timing analyzer during the synthesis process to check that the logic being
created matches the timing constraints the user has specified. Setting the
arrival time on a particular node specifies to the static timing analyzer
Synthesis
241
when a particular signal will occur at a node. This is especially important
for late arriving signals. Late arriving signals drive inputs to the current
block at a later time, but the results of the current block still must meet
its own timing constraints on its outputs. Therefore, the path to the output
of the late arriving input must be faster than any other inputs, or the
timing constraints of the current block cannot be met.
Technology Libraries
Technology libraries hold all of the information necessary for a synthesis
tool to create a netlist for a design based on the desired logical behavior,
and constraints on the design. Technology libraries contain all of the
information that allows the synthesis process to make the correct choices
to build a design. Technology libraries contain not only the logical function of an ASIC cell, but the area of the cell, the input to output timing of
the cell, any constraints on fanout of the cell, and the timing checks that
are required for the cell. Other information stored in the technology
library may be the graphical symbol of the cell for use in schematics.
Following is an example technology library description of a 2-input
AND gate written in Synopsys .lib format:
library (xyz) {
cell (and2) {
area : 5;
pin (a1, a2) {
direction : input;
capacitance : 1;
}
pin (o1) {
direction : output;
function : “a1 * a2”;
timing () {
intrinsic_rise : 0.37;
intrinsic_fall : 0.56;
rise_resistance : 0.1234;
fall_resistance : 0.4567;
related_pin : “a1 a2”;
}
}
}
}
This technology library describes a library named xyz with one library
cell contained in it. The cell is named and2 and has two input pins a1 and
242
Chapter Nine
a2 and one output pin o1. The cell requires 5 units of area, and the input
pins have 1 unit of loading capacitance to the driver driving them. The
intrinsic rise and fall delays listed with pin o1 specify the delay to the
output with no loading. The timing analyzer uses the intrinsic delays plus
the rise and fall resistance with the output loading to calculate the delay
through a particular gate. Notice that the function of pin o1 is listed as
the AND of pins a1 and a2. Also, notice that pin o1 is related to pins a1 and
a2 in that the timing delay through the device is calculated from pins a1
and a2 to pin o1.
Most synthesis tools have fairly complicated delay models to calculate
timing through an ASIC cell. These models include not only intrinsic rise
and fall time, but output loading, input slope delay, and estimated wire
delay. A diagram illustrating this is shown in Figure 9-7.
The total delay from gate A1 to gate C1 is:
intrinsic_delay
+ loading_delay + wire_delay + slope_delay
The intrinsic delay is the delay of the gate without any loading. The
loading delay is the delay due to the input capacitance of the gate being
driven. The wire delay is an estimated delay used to model the delay
through a typical wire used to connect cells together. It can be a statistical
model of the wire delays usually based on the size of the chip die. Given
a particular die size, the wire loading effect can be calculated and added
to the overall delay. The final component in the delay equation is the
extra delay needed to handle the case of slowly rising input signals due
to heavy loading or light drive.
Figure 9-7
Delay Effects Used in
Delay Model.
Loading
Delay
Slope Delay
A1
B1
Intrinsic
Delay
C1
Wire Delay
Synthesis
243
In the preceding technology library, the intrinsic delays are given in the
cell description. The loading delay is calculated based on the load applied
to the output pin o1 and the resistance values in the cell description. The
value calculated for the wire delay depends on the die size selected by the
user. Selecting a wire model scales the delay values. Finally, the input
slope delay is calculated by the size of the driver, in this example, A1, and
the capacitance of the gate being driven. The capacitance of the gate
being driven is in the technology library description.
Technology libraries can also contain data about how to scale delay
information with respect to process parameters and operating conditions.
Operating conditions are the device operating temperature and power
supply voltage applied to the device.
Synthesis
To convert the RTL description to gates, three steps typically occur. First,
the RTL description is translated to an unoptimized boolean description
usually consisting of primitive gates such as AND and OR gates, flip-flops,
and latches. This is a functionally correct but completely unoptimized
description. Next, boolean optimization algorithms are executed on this
boolean equivalent description to produce an optimized boolean equivalent
description. Finally, this optimized boolean equivalent description is
mapped to actual logic gates by making use of a technology library of the
target process. This is shown in Figure 9-8.
Translation
The translation from RTL description to boolean equivalent description
is usually not user controllable. The intermediate form that is generated
is usually a format that is optimized for a particular tool and may not
even be viewable by the user.
All IF, CASE, and LOOP statements, conditional signal assignments, and
selected signal assignment statements are converted to their boolean
equivalent in this intermediate form. Flip-flops and latches can either be
instantiated or inferred; both cases produce the same flip-flop or latch
entry in the intermediate description.
244
Figure 9-8
Synthesis Process.
Chapter Nine
Created by
User
VHDL RTL
Description
Translate
Unoptimized Boolean
Description
Optimize
Optimized Boolean
Description
Map to Gates
Gate Level
Netlist
Boolean Optimization
The optimization process takes an unoptimized boolean description and
converts it to an optimized boolean description. In many designers’ eyes,
this is where the real work of synthesis gets done. The optimization
process uses a number of algorithms and rules to convert the unoptimized
boolean description to an optimized one. One technique is to convert the
unoptimized boolean description to a very low-level description (a pla
format), optimize that description (using pla optimization techniques),
Synthesis
245
and then try to reduce the logic generated by sharing common terms (introducing intermediate variables).
Flattening
The process of converting the unoptimized boolean description to a pla
format is known as flattening, because it creates a flat signal representation of only two levels: an AND level and an OR level. The idea is to get
the unoptimized boolean description into a format in which optimization
algorithms can be used to optimize the logic. A pla structure is a very easy
description in which to perform boolean optimization, because it has a
simple structure and the algorithms are well known. An example of a
boolean description is shown here:
Original equations
a = b and c;
b = x or (y and z);
c = q or w;
This description shows an output a that has three equations describing
its function. These equations use two intermediate variables b and c to
hold temporary values which are then used to calculate the final value
for a. These equations describe a particular structure of the design that
contains two intermediate nodes or signals, b and c. The flattening
process removes these intermediate nodes to produce a completely flat
design, with no intermediate nodes. For example, after removing intermediate variables:
a = (x and q) or (q and y and z) or (w and x) or (w and y
and z);
This second description is the boolean equivalent of the first, but it has
no intermediate nodes. This design contains only two levels of logic gates:
an AND plane and an OR plane. This should result in a very fast design
because there are very few logic levels from the input to the output. In
fact, the design is usually very fast. There are, however, a number of problems with this type of design.
First, this type of design can actually be slower than one that has more
logic levels. The reason is that this type of design can have a tremendous
fanout loading on the input signals because inputs fan out to every term.
Second, this type of design can be very large, because there is no sharing
between terms. Every term has to calculate its own functionality. Also,
246
Chapter Nine
there are a number of circuits that are difficult to flatten, because the
number of terms created is extremely large. An equation that only contains AND functions produces one term. A function that contains a large
XOR function can produce hundreds or even thousands of terms. A 2-input
XOR has the terms A and (not B) or B and (not A). An N-input XOR has
2**(N-1) terms. For instance, a 16-input XOR has 32,768 terms and a 32bit XOR has over 2 billion terms. Clearly, designs with these types of functions cannot be flattened.
Flattening gets rid of all of the implied structure of design whether it
is good or not. Flattening works best with small pieces of random control
logic that the designer wants to minimize. Used in conjunction with structuring, a minimal logic description can be generated.
Usually, the designer wants a design that is nearly as fast as the flattened design, but is much smaller in area. To reduce the fanout of the
input pins, terms are shared. Some synthesis vendors call this process
structuring or factoring.
Factoring
Factoring is the process of adding intermediate terms to add structure to a
description. It is the opposite of the flattening process. Factoring is usually
desirable because, as was mentioned in the last section, flattened designs
are usually very big and may be slower than a factored design because of
the amount of fanouts generated. Following is a design before factoring:
x = a and b or a and d;
y = z or b or d;
After factoring the common term, (b or d), is factored out to a separate
intermediate node. The results are shown here:
x = a and q;
y = z or q;
q = b or d;
Factoring usually produces a better design but can be very designdependent. Adding structure adds levels of logic between the inputs and
outputs. Adding levels of logic adds more delay. The net result is a smaller
design, but a slower design. Typically, the designer wants a design that is
nearly as fast as the flattened design if it was driven by large drivers, but
as small as the completely factored design. The ideal case is one in which
Synthesis
247
the critical path was flattened for speed and the rest of the design was
factored for small area and low fanout.
After the design has been optimized at the boolean level, it can be
mapped to the gate functions in a technology library.
Mapping to Gates
The mapping process takes the logically optimized boolean description
created by the optimization step and uses the logical and timing information from a technology library to build a netlist. This netlist is targeted
to the user’s needs for area and speed. There are a number of possible
netlists that are functionally the same but vary widely in speed and area.
Some netlists are very fast but take a lot of library cells to implement, and
others take a small number of library cells to implement but are very slow.
To illustrate this point, let’s look at a couple of netlists that implement
the same functionality. Following is the VHDL description:
LIBRARY IEEE;
USE IEEE.std_logic_1164.ALL;
USE IEEE.std_logic_unsigned.ALL;
ENTITY adder IS
PORT( a,b : IN std_logic_vector(7 DOWNTO 0);
PORT( c : OUT std_logic_vector(7 DOWNTO 0)
PORT( );
END adder;
ARCHITECTURE test OF adder IS
BEGIN
c <= a + b;
END test;
Both of the examples implement an 8-bit adder, but the first implementation is a small but slower design, and the second is a bigger but fast
design. The small but slower design is an 8-bit ripple carry adder shown
in Figure 9-9. The bigger but faster design is an 8-bit lookahead adder
shown in Figure 9-10.
Both of these netlists implement the same function, an 8-bit adder. The
ripple carry adder takes less cells to implement but is a slower design
because it has more logic levels. The lookahead adder takes more cells to
implement but is a faster design because more of the boolean operations are
calculated in parallel. The additional logic to calculate the functionality
in parallel adds extra logic to the design making the design bigger.
248
Figure 9-9
Smaller but Slower 8-Bit Ripple Carry Adder.
Figure 9-10
Bigger but Faster 8-Bit Lookahead Adder.
249
250
Chapter Nine
In most synthesis tools, the designer has control over which type of
adder is selected through the use of constraints. If the designer wants to
constrain the design to a very small area and doesn’t need the fastest
possible speed, then the ripple carry adder probably works. If the designer
wants the design to be as fast as possible and doesn’t care as much about
how big the design gets, then the lookahead adder is the one to select.
The mapping process takes as input the optimized boolean description,
the technology library, and the user constraints, and generates an optimized netlist built entirely from cells in the technology library. During the
mapping process, cells are inserted that implement the boolean function
from the optimized boolean description. These cells are then locally optimized to meet speed and area requirements. As a final step, the synthesis
tool has to make sure that the output does not violate any of the rules of
the technology being used to implement the design, such as the maximum
number of fanouts a particular cell can have.
SUMMARY
In this chapter, we discussed some of the basic principles of the synthesis
process. In the next chapter, we take a closer look at how to write models
that can be synthesized.
CHAPTER
10
VHDL Synthesis
In this chapter, we focus on how to write VHDL that can
be read by synthesis tools. We start out with some simple
combinational logic examples, move on to some sequential
models, and end the chapter with a state machine description.
All of the examples are synthesized with the Exemplar
Logic Leonardo synthesis environment. The technology library used is an example library from Exemplar Logic. All
of the output data should be treated as purely sample outputs and not representative of how well the Exemplar
Logic tools work with real design data and real constraints.
252
Chapter Ten
Simple Gate—Concurrent
Assignment
The first example is a simple description for a 3-input OR gate:
LIBRARY IEEE;
USE IEEE.std_logic_1164.ALL;
ENTITY or3 IS
PORT (a, b, c : IN std_logic;
PORT (d : OUT std_logic);
END or3;
ARCHITECTURE synth OF or3 IS
BEGIN
d <= a OR b OR c;
END synth;
This model uses a simple concurrent assignment statement to describe
the functionality of the OR gate. The model specifies the functionality
required for this entity, but not the implementation. The synthesis tool
can choose to implement this functionality in a number of ways, depending
on the cells available in the technology library and the constraints on
the model. For instance, the most obvious implementation is shown in
Figure 10-1.
This implementation uses a 3-input OR gate to implement the functionality specified in the concurrent signal assignment statement contained
in architecture synth.
What if the technology library did not contain a 3-input OR device? Two
other possible implementations are shown in Figures 10-2 and 10-3.
The first implementation uses a 3-input NOR gate followed by an inverter.
The synthesis tool may choose this implementation if there are no 3-input
OR devices in the technology library. Alternatively, if there are no 3input devices, or if the 3-input devices violate a speed constraint, the
Figure 10-1
Model
Implementation.
in [1]
c
out
in [0]
a
b
in [0]
out
in [1]
d
VHDL Synthesis
253
INBUF
Figure 10-2
3-Input OR.
b
PAD
Y
A
INBUF
c
PAD
Y
PAD
OUTBUF
NANDOC
Y
D
PAD
d
C
INBUF
a
B
Y
3-input OR function could be built from four devices, as shown in Figure
10-3. Given a technology library of parts, the functionality desired, and
design constraints, the synthesis tool is free to choose among any of the
implementations that satisfy all the requirements of a design, if such a
design exists. There are lots of cases where the technology or constraints
are such that no design can meet all of the design requirements.
IF Control Flow Statements
In the next example, control flow statements such as IF THEN ELSE are
used to demonstrate how synthesis from a higher level description is
accomplished. This example forms the control logic for a household alarm
system. It uses sensor input from a number of sensors to determine
whether or not to trigger different types of alarms. Following is the input
description:
LIBRARY IEEE;
USE IEEE.std_logic_1164.ALL;
ENTITY alarm_cntrl IS
PORT( smoke, front_door, back_door, side_door,
PORT( alarm_disable, main_disable,
PORT( water_detect : IN std_logic;
PORT( fire_alarm, burg_alarm,
PORT( water_alarm : OUT std_logic);
END alarm_cntrl;
ARCHITECTURE synth OF alarm_cntrl IS
BEGIN
PROCESS(smoke, front_door, back_door, side_door,
PROCESS(alarm_disable, main_disable,
PROCESS(water_detect)
BEGIN
254
Figure 10-3
Another 3-Input OR
Implementation.
Chapter Ten
in [1]
c
out
d
in [0]
a
b
in [0]
out
in [1]
IF ((smoke = ‘1’) AND (main_disable = ‘0’)) THEN
fire_alarm <= ‘1’;
ELSE
fire_alarm <= ‘0’;
END IF;
IF (((front_door = ‘1’) OR (back_door = ‘1’) OR
(side_door = ‘1’)) AND
((alarm_disable = ‘0’) AND (main_disable =
‘0’))) THEN
burg_alarm <= ‘1’;
ELSE
burg_alarm <= ‘0’;
END IF;
IF ((water_detect = ‘1’) AND (main_disable = ‘0’))
THEN
water_alarm <= ‘1’;
ELSE
water_alarm <= ‘0’;
END IF;
END PROCESS;
END synth;
The input description contains a number of sensor input ports such as
a smoke detector input, a number of door switch inputs, a basement water
detector, and two disable signals. The main_disable port is used to disable
all alarms, while the alarm_disable port is used to disable only the
burglar alarm system.
The functionality is described by three separate IF statements. Each
IF statement describes the functionality of one or more output ports. Notice that the functionality could also be described very easily with equations, as in the first example. Sometimes, however, the IF statement style
is more readable. For instance, the first IF statement can be described by
the following equation:
VHDL Synthesis
255
fire_alarm <= smoke and not(main_disable);
Because the three IF statements are separate and they generate
separate outputs, we can expect that the resulting logic would be three separate pieces of logic. However, the main_disable signal is shared between
the three pieces of logic. Any operations that make use of this signal may
be shared by the other logic pieces. How this sharing takes place is determined by the synthesis tool and is based on the logical functionality of the
design and the constraints. Speed constraints may force the logical operations to be performed in parallel.
A sample synthesized output is shown in Figure 10-4. Notice that
signal main_disable connects to all three output gates, while signal
alarm_disable only connects to the alarm control logic. The logic for
the water alarm and smoke detector turn out to be quite simple, but we
could have guessed that because our equations were so simple. The next
example is not so simple.
Y
Figure 10-4
A sample synthesized
output.
GND
back_door
D0
Y
D1
D2
D3
CM8
S00
S01
S10
S11
Y
VCC
alarm_disable
A
Y
A
Y
burg_alarm
side_door
front_door
smoke
main_disable
water_detect
A
B
NOR2A
Y
fire_alarm
A
B
NOR2A
Y
water_alarm
256
Chapter Ten
Case Control Flow Statements
The next example is an implementation of a comparator. There are two 8bit inputs to be compared and a CTRL input that determines the type of
comparison made. The possible comparison types are A > B, A < B, A B,
A ≠ B, A B, and A B. The design contains one output port for each of
the comparison types. If the desired comparison output is true, then the output value on that output port is a ‘1’. If false, the output port value is a
‘0’. Following is a synthesizable VHDL description of the comparator:
PACKAGE comp_pack IS
TYPE bit8 is range 0 TO 255;
TYPE t_comp IS (greater_than, less_than, equal,
not_equal, grt_equal, less_equal);
END comp_pack;
LIBRARY IEEE;
USE IEEE.std_logic_1164.ALL;
USE WORK.comp_pack.ALL;
ENTITY compare IS
PORT( a, b : IN bit8;
PORT( ctrl : IN t_comp;
PORT( gt, lt, eq, neq, gte, lte : OUT std_logic);
END compare;
ARCHITECTURE synth OF compare IS
BEGIN
PROCESS(a, b, ctrl)
BEGIN
gt <= ‘0’; lt <= ‘0’; eq <= ‘0’; neq <= ‘0’; gte <=
‘0’; lte <= ‘0’;
CASE ctrl IS
WHEN greater_than =>
IF (a > b) THEN
gt <= ‘1’;
END IF;
WHEN less_than =>
IF (a < b) THEN
lt <= ‘1’;
END IF;
WHEN equal =>
IF (a = b) THEN
eq <= ‘1’;
END IF;
WHEN not_equal =>
IF (a /= b) THEN
neq <= ‘1’;
END IF;
WHEN grt_equal =>
IF (a >= b) THEN
VHDL Synthesis
257
gte <= ‘1’;
END IF;
WHEN less_equal =>
IF (a > b) THEN
lte <= ‘1’;
END IF;
END CASE;
END PROCESS;
END synth;
Notice that, in this example, the equations of the inputs and outputs are
harder to write because of the comparison operators. It is still possible to
do, but is much less readable than the case statement shown earlier.
When synthesizing a design, the complexity of the design is related
to the complexity of the equations that describe the design function.
Typically, the more complex the equations, the more complex the design
created. There are exceptions to this rule, especially when the equations
reduce to nothing.
A sample synthesized output from the preceding description is shown
in Figure 10-5. The inputs are shown on the left of the schematic diagram,
and the outputs are shown in the lower right of the schematic. The equations for the comparison operators have all been shared and combined
together to produce an optimal design. This design is a very small number
of gates for the operation performed.
There are still a number of cases where hand design can create smaller
designs, but in most cases today the results of synthesis are very good;
and you get the added benefit of using a higher level design language for
easier maintainability and a shorter design cycle.
Simple Sequential Statements
Let’s take a closer look at an example that we already discussed in the
last chapter. This is the inferred D flip-flop. Inferred flip-flops are created
by WAIT statements or IF THEN ELSE statements, which are surrounded
by sensitivities to a clock. By detecting clock edges, the synthesis tool can
locate where to insert flip-flops so that the design that is ultimately built
behaves as the simulation predicts.
Following is an example of a simple sequential design using a WAIT
statement:
LIBRARY IEEE;
USE IEEE.std_logic_1164.ALL;
ENTITY dff IS
258
Chapter Ten
Figure 10-5
A Sample Synthesized
Output.
PORT( clock, din : IN std_logic;
PORT( dout : OUT std_logic);
END dff;
ARCHITECTURE synth OF dff IS
BEGIN
PROCESS
BEGIN
WAIT UNTIL ((clock’EVENT) AND (clock = ‘1’));
dout <= din;
END PROCESS;
END synth;
VHDL Synthesis
259
The description contains a synthesizable entity and architecture representing a D flip-flop. The entity contains the clock, din, and dout ports
needed for a D flip-flop, while the architecture contains a single process
statement with a single WAIT statement. When the clock signal has a rising
edge occur, the contents of din are assigned to dout. Effectively, this is how
a D flip-flop operates.
The synthesized output of this design matches the functionality of the
RTL description. It is very important for the synthesis and simulation
results to agree. Otherwise, the resulting synthesized design may not
work as planned. Part of the synthesis methodology should require that
a final gate level simulation of the design is executed to verify that the
gate level functionality is correct. (We perform this step in an example
later on.)
The output of the Leonardo synthesis tool is shown in Figure 10-6.
As expected, the output of the synthesis tool produced a single D flipflop. The synthesis tool connected the ports of the entity to the proper
ports of actual FPGA library macro so that the device works as expected
in the design.
Asynchronous Reset
In a number of instances, D flip-flops are required to have an asynchronous
reset capability. The previous D flip-flop did not have this capability. How
would we generate a D flip-flop with an asynchronous reset? Remember
the simulation and synthesis results must agree. Following is one way to
accomplish this:
LIBRARY IEEE;
USE IEEE.std_logic_1164.ALL;
ENTITY dff_asynch IS
Figure 10-6
The Output of the
Leonardo Synthesis
Tool.
din
D
clock
>
S
R
Q
dout
260
Chapter Ten
PORT( clock, reset, din : IN std_logic;
PORT( dout : OUT std_logic);
END dff_asynch;
ARCHITECTURE synth OF dff_asynch IS
BEGIN
PROCESS(reset, clock)
BEGIN
IF (reset = ‘1’) THEN
dout <= ‘0’;
ELSEIF (clock’EVENT) AND (clock = ‘1’) THEN
dout <= din;
END IF;
END PROCESS;
END synth;
The ENTITY statement now has an extra input, the reset port, which
is used to asynchronously reset the D flip-flop. Notice that reset and
clock are in the process sensitivity list and cause the process to be evaluated. If an event occurs on signals clock or reset, the statements inside
the process are executed.
First, signal reset is tested to see if it has an active value (‘1’). If active,
the output of the flip-flop is reset to ‘0’. If reset is not active (‘0’), then
the clock signal is tested for a rising edge. If signal clock has a rising
edge, then input din is assigned as the new flip-flop output.
The fact that the reset signal is tested first in the IF statement gives
the reset signal a higher priority than the clock signal. Also, because the
reset signal is tested outside of the test for a clock edge, the reset signal
is asynchronous to the clock.
The Leonardo synthesis tool produces a D flip-flop with an asynchronous
reset input, as shown in Figure 10-7. The resulting design has an extra
inverter (IVP component) in the circuit because the only flip-flop macro
that would match the functionality required had a reset input that was
active low.
Figure 10-7
The Leonardo
Synthesis Tool
Produces a
D Flip-Flop.
din
D
clock
>
reset
S
R
Q
dout
VHDL Synthesis
261
Asynchronous Preset and Clear
Is it possible to describe a flip-flop with an asynchronous preset and
clear? As an attempt, we can use the same technique as in the asynchronous reset example. The following example illustrates an attempt to
describe a flip-flop with an asynchronous preset and clear inputs:
LIBRARY IEEE;
USE IEEE.std_logic_1164.ALL;
ENTITY dff_pc IS
PORT( preset, clear, clock, din : IN std_logic;
PORT( dout : OUT std_logic);
END dff_pc;
ARCHITECTURE synth OF dff_pc IS
BEGIN
PROCESS(preset, clear, clock)
BEGIN
IF (preset = ‘1’) THEN
dout <= ‘1’;
ELSEIF (clear = ‘1’) THEN
dout <= ‘0’;
ELSEIF (clock’EVENT) AND (clock = ‘1’) THEN
dout <= din;
END IF;
END PROCESS;
END synth;
The entity contains a preset signal that sets the value of the flip-flop
to a ‘1’, a clear signal that sets the value of the flip-flop to a ‘0’, and the
normal clock and din ports used for the clocked D flip-flop operation. The
architecture contains a single process statement with a single IF statement to describe the flip-flop behavior. The IF statement assigns a ‘1’ to
the output for a ‘1’ value on the preset input and a ‘0’ to the output
for a ‘1’ on the clear input. Otherwise, the clock input is checked for a
rising edge, and the din value is clocked to the output dout.
What does the output of the synthesis process produce for this VHDL
input? The output is shown in Figure 10-8. We were expecting the output
of the synthesis tool in which the design preset input was connected to
the preset input of the flip-flop, and the design clear input was connected to the clear input of the flip-flop. The output from the synthesis
tool is a design in which the design preset and clear inputs are separated from the flip-flop preset and clear inputs by some logic.
262
Figure 10-8
Output of synthesis
process.
Chapter Ten
din
S
Q
D
dout
preset
clock
>
R
clear
in [0]
out
in [1]
in
out
This logic circuitry performs a prioritization of the preset signal with
respect to the clear signal. Because the preset signal occurs before the
clear signal in the IF statement, the preset signal is tested before the
clear signal. If the preset signal is active, the flip-flop presets regardless of the state of the clear input. Effectively, the preset signal has a
higher priority than the clear signal. There is currently no way to write
a VHDL description to generate a design in which the preset and clear
inputs have the same priority.
More Complex Sequential
Statements
The next example is a more complex sequential design of a 4-bit counter.
This example makes use of a two-process description style. This style works
very well for some synthesis tools, producing very good synthesis results.
Each process has a particular function. One process is clocked and the
other is not. The clocked process is used to maintain the present state of the
counter, while the unclocked process calculates the next state of the counter.
Following is an example of a counter written in this way:
USE IEEE.std_logic_1164.ALL;
USE IEEE.std_logic_unsigned.ALL;
PACKAGE count_types IS
SUBTYPE bit4 IS std_logic_vector(3 DOWNTO 0);
VHDL Synthesis
263
END count_types;
LIBRARY IEEE;
USE IEEE.std_logic_1164.ALL;
USE IEEE.std_logic_unsigned.ALL;
USE WORK.count_types.ALL;
ENTITY count IS
PORT(clock, load, clear : IN std_logic;
din : IN bit4;
dout : INOUT bit4);
END count;
ARCHITECTURE synth OF count IS
SIGNAL count_val : bit4;
BEGIN
PROCESS(load, clear, din, dout)
BEGIN
IF (load = ‘1’) THEN
count_val <= din;
ELSEIF (clear = ‘1’) THEN
count_val <= “0000”;
ELSE
count_val <= dout + “0001”;
END IF;
END PROCESS;
PROCESS
BEGIN
WAIT UNTIL clock’EVENT and clock = ‘1’;
dout <= count_val;
END PROCESS;
END synth;
The description contains a package that defines a 4-bit range that
causes the synthesis tools to generate a 4-bit counter. Changing the size of
the range causes the synthesis tools to generate different-sized counters.
By using a constrained universal integer range, the model can take advantage of the built-in arithmetic operators for type universal integer. The
other alternative is to define a type that is 4 bits wide and then create a
package that overloads the arithmetic operators for the 4-bit type.
The entity contains a clock input port to clock the counter, a load
input port that allows the counter to be synchronously loaded, a clear
input port that synchronously clears the counter, a din input port that
allows values to be loaded into the counter, and an output port dout that
presents the current value of the counter to the outside world.
The architecture for the counter contains two processes. The process
labeled synch is the process that maintains the current state of the
264
Chapter Ten
counter. It is the process that is clocked by the clock and transfers the new
calculated output count_val to the current output dout.
The other process contains a single IF statement that determines
whether the counter is being loaded, cleared, or is counting up.
A sample synthesized output is shown in Figure 10-9. In this example,
the generated results are as expected. The left side of the schematic shows
the inputs to the counter; the right side of the schematic has the counter
output. Notice that the design contains four flip-flops (FDSR1), exactly as
specified. Also, notice that the logic generated for the counter is very
small. This design was optimized for area; thus, the number of levels of
logic are probably higher than a design optimized for speed.
Four-Bit Shifter
Another sequential example is a 4-bit shifter. This shifter can be loaded
with a value and can be shifted left or right one bit at a time. Following
is the model for the shifter:
LIBRARY IEEE;
USE IEEE.std_logic_1164.ALL;
PACKAGE shift_types IS
SUBTYPE bit4 IS std_logic_vector(3 downto 0);
END shift_types;
USE WORK.shift_types.ALL;
LIBRARY IEEE;
USE IEEE.std_logic_1164.ALL;
ENTITY shifter IS
PORT( din : IN bit4;
clk, load, left_right : IN std_logic;
dout : INOUT bit4);
END shifter;
ARCHITECTURE synth OF shifter IS
SIGNAL shift_val : bit4;
BEGIN
nxt: PROCESS(load, left_right, din, dout)
BEGIN
IF (load = ‘1’) THEN
shift_val <= din;
ELSEIF (left_right = ‘0’) THEN
shift_val(2 downto 0) <= dout(3 downto 1);
shift_val(3) <= ‘0’;
ELSE
shift_val(3 downto 1) <= dout(2 downto 0);
VHDL Synthesis
265
Figure 10-9
A Sample Synthesized
Output.
shift_val(0) <= ‘0’;
END IF;
END PROCESS;
current: PROCESS
BEGIN
WAIT UNTIL clk’EVENT AND clk = ‘1’;
dout <= shift_val;
END PROCESS;
END synth;
The 4-bit type used for the input and output of the shifter is declared
in package shift_types. This package is used by entity shifter to declare ports din and dout. Ports clk, load, and left_right are std_logic
signals used to control the functions of the shifter.
266
Chapter Ten
The architecture is organized similarly to the last example, with two
processes used to describe the functionality of the architecture. One process
keeps track of the current value of the shifter, and the other calculates
the next value based on the last value and the control inputs.
Process current is used to keep track of the current value of the shifter.
It is a process that has a single WAIT statement and a single signal assignment statement. When the clk signal has a rising edge occur, the signal
assignment statement is activated and the next calculated value of the
shifter (shift_val) is written to the signal that holds the current state of
the shifter (dout).
Process nxt is used to calculate the next value of shift_val to be
written into dout. Load is the highest priority input and, if equal to ‘1’,
causes shift_val to receive the value of din. Otherwise, signal
left_right is tested to see if the shifter is shifting left or right. Because
this shifter does not contain a carryin or carryout, ‘0’ values are
written into the bits whose value has been shifted over. (A good exercise
is to write a shifter that contains a carryin and carryout.)
The synthesis tool produces a schematic for this input description as
shown in Figure 10-10. By counting the flip-flops (FDSR1) on the page, it
can be seen that this is indeed a 4-bit shifter.
State Machine Example
The next example is a simple state machine used to control a voicemail
system. (This example does not represent any real system in use and is
necessarily simple to make it easier to fit in the book.) The voicemail
controller allows the user to send messages, review messages, save
messages, and erase messages. A state diagram showing the possible
state transitions is shown in Figure 10-11.
The normal starting state is state main. From main, the user can select
whether to review messages or send messages. To get to the Review menu,
the user presses the 1 key on the touch-tone phone. To select the Send
Message menu, the user presses the 2 key on the touch-tone phone. After
the user has selected either of these options, further menu options allow the
user to perform other functions such as Save and Erase. For instance, if
the user first selects the Review menu by pressing key 1, then pressing key
2 allows the user to save a reviewed message when reviewing is complete.
Following is the VHDL description for the voicemail controller:
VHDL Synthesis
267
Figure 10-10
The Synthesis Tool
Produces a Schematic.
PACKAGE vm_pack IS
TYPE t_vm_state IS (main_st, review_st, repeat_st,
TYPE t_vm_state IS (save_st,
TYPE t_vm_state IS (erase_st, send_st,
TYPE t_vm_state IS (address_st, record_st,
TYPE t_vm_state IS (begin_rec_st, message_st);
TYPE t_key IS (‘0’,’1’,’2’,’3’,’4’,’5’,’6’,’7’,’8’,’9’,
TYPE t_key IS (’*’,’#’);
END vm_pack;
USE WORK.vm_pack.ALL;
LIBRARY IEEE;
USE IEEE.std_logic_1164.ALL;
ENTITY control IS
PORT( clk : in std_logic;
PORT( key : in t_key;
268
Chapter Ten
Figure 10-11
State Transition
Diagram for
Voicemail Controller.
1
#
1
Review
2
Main
3
#
Send
2
Repeat
Save
Erase
Address
#
Record
5
Begin
Record
Record
Message
#
PORT( play, recrd, erase, save, address
: out std_logic);
END control;
ARCHITECTURE synth OF control IS
SIGNAL next_state, current_state :
t_vm_state;
BEGIN
VHDL Synthesis
269
PROCESS(current_state, key)
BEGIN
play <= ‘0’;
save <= ‘0’;
erase <= ‘0’;
recrd <= ‘0’;
address <= ‘0’;
CASE current_state IS
WHEN main_st =>
IF (key = ‘1’) THEN
next_state <= review_st;
ELSEIF (key = ‘2’) THEN
next_state <= send_st;
ELSE
next_state <= main_st;
END IF;
WHEN review_st =>
IF (key = ‘1’) THEN
next_state <= repeat_st;
ELSEIF (key = ‘2’) THEN
next_state <= save_st;
ELSEIF (key = ‘3’) THEN
next_state <= erase_st;
ELSEIF (key = ‘#’) THEN
next_state <= main_st;
ELSE
next_state <= review_st;
END IF;
WHEN repeat_st =>
play <= ‘1’;
next_state <= review_st;
WHEN save_st =>
save <= ‘1’;
next_state <= review_st;
WHEN erase_st =>
erase <= ‘1’;
next_state <= review_st;
WHEN send_st =>
next_state <= address_st;
WHEN address_st =>
address <= ‘1’;
IF (key = ‘#’) THEN
next_state <= record_st;
ELSE
next_state <= address_st;
270
Chapter Ten
END IF;
WHEN record_st =>
IF (key = ‘5’) THEN
next_state <= begin_rec_st;
ELSE
next_state <= record_st;
END IF;
WHEN begin_rec_st =>
recrd <= ‘1’;
next_state <= message_st;
WHEN message_st =>
recrd <= ‘1’;
IF (key = ‘#’) THEN
next_state <= send_st;
ELSE
next_state <= message_st;
END IF;
END CASE;
END PROCESS;
PROCESS
BEGIN
WAIT UNTIL clk = ‘1’ AND clk’EVENT;
current_state <= next_state;
END PROCESS;
END synth;
Package vm_types contains the type declarations for the state values
and keys allowed by the voicemail controller. Notice that the states are all
named something meaningful as opposed to S1, S2, S3, and so on. This
makes the model much more readable.
This package is used by the entity to declare local signals and the key
input port. The entity only has one input, the key input, which represents
the possible key values from a touch-tone phone keypad. All of the other
ports of the entity are output ports (except clk) and are used to control
the voicemail system operations.
This model uses the two-process style to describe the operation of the
state machine. This style is very useful for describing state machines as
one process represents the current state register, and the other process
represents the next state logic.
The next state process starts by initializing all of the output signals to
‘0’. The reason for this is to provide the synthesis tool with a default value
to assign the signal if the signal was not assigned in the CASE statement.
VHDL Synthesis
271
The rest of the next state process consists of one CASE statement. This
CASE statement describes the action to occur based on the current state
of the state machine and any inputs that affect the state machine. The
condition that the CASE statement keys from is the current state. The state
machine can be placed in a different state depending on the inputs that
are being tested by the current state. For instance, if the current state is
main_st, when the key input is ‘1’, the next state is review_st; when
the key input is ‘2’, the next state is send_st.
When this description is synthesized using the Leonardo synthesis tool,
the schematic shown in Figure 10-12 is generated. The key and clk inputs
are shown coming into the left side of the schematic and outputs save,
recrd, address, erase, and play are shown coming out of the right side
of the schematic. Intermixed in the design are the state flip-flops that
are used to hold the current state of the voicemail controller and the
logic used to generate the next state of the controller. This type of output
is indicative of state machine descriptions.
Figure 10-12
Generated Using the
Leonardo Synthesis
Tool.
272
Chapter Ten
SUMMARY
In this chapter, we looked at a number of different VHDL synthesis examples. They ranged from simple gate level descriptions to more complex
examples that contained state machines. In the next few chapters, we look
at a more complex example that requires a number of state machines, and
we follow the process from start to finish.
CHAPTER
11
High-Level
Design Flow
This chapter describes the design flow used to create complex FPGA and ASIC devices. The designer starts with a
design specification, creates an RTL description, verifies
that description, synthesizes the description to gates, uses
place and route tools to implement the design in the chip,
and then verifies that the final result is correct in terms
of function and timing. The high-level design flow is
shown in Figure 11-1.
The first step in a high-level design flow is the design
specification process. This process involves specifying the
behavior expected of the final design. The designer puts
enough detail into the specification so that the design can
be built. The specification is usually written in the
designer’s native language and specifies the expected
function and behavior of the design using textual
description and graphic elements.
274
Figure 11-1
High-Level Design
Flow.
Chapter Eleven
Design Specification
HDL Capture
RTL Simulation
RTL Synthesis
Functional
Gate Simulation
Place and Route
Post Layout Timing
Simulation
Static Timing Analysis
After the specification has been completed, the designer or designers can
begin the process of implementation. Some design teams create a high- level
behavioral or algorithmic description of the design to verify design intent,
then convert that description to RTL (Register Transfer Level) later. However, most design teams skip the behavioral description and implement the
RTL directly. The RTL is created during the HDL capture step. The de-
High-Level Design Flow
275
signer creates the VHDL RTL description that describes the clock-by-clock
behavior of the design. The designer most likely uses a common text editor
such as Emacs, or vi, whatever is available on the designer’s computer. Some
designers also use high-level entry tools that contain block editors and state
machine editors that automatically create the VHDL code.
The designer enters the VHDL code for entities of the design and
checks them for correct syntax. After the syntax errors have been
removed, the designer can begin the process of verifying the correctness
of the VHDL using RTL simulation.
RTL Simulation
The RTL simulation step is used to verify the correctness of the RTL
VHDL description. The designer has described the clock-by-clock behavior
of the design. Now, the designer uses stimulus that represents the design
environment to drive the design and check to make sure that the results
are correct. A standard VHDL simulator can be used to read the RTL
VHDL description and verify the correctness of the design.
The VHDL simulator reads the VHDL description, compiles it into an
internal format, and then executes the compiled format using test vectors.
The designer can look at the output of the simulation and determine
whether or not the design is working properly.
The usual RTL simulation step looks like Figure 11-2.
The designer creates the VHDL as described earlier and compiles the
VHDL RTL description to remove any syntax errors. After the syntax
errors have been removed, the design is simulated to verify the correctness
of the design. After the simulation has completed, the designer analyzes
the results of the simulation to determine if the design is correct or not.
If not, the designer must fix the VHDL code and compile and simulate the
design again. This process continues until all errors are removed.
The designer loads the compiled VHDL description into the simulator
and applies stimulus to the design. This may be a file of input stimulus,
a set of commands the designer enters, or an automatic testbench that
applies the stimulus and checks the results. (These are discussed in
Chapter 14, “RTL Simulation.”) After the stimulus has been entered, the
designer runs the simulation for as long as needed to generate enough
output data to determine if the design is correct. At the beginning of the
design process, this may be only a few vectors to make sure that the
design resets properly. But later, more and more of the vectors are run as
the design starts to function properly.
276
Figure 11-2
RTL Simulation Flow.
Chapter Eleven
Create VHDL
Compile VHDL
Run RTL Simulation
no
Results
OK
yes
After the simulation has been run, the simulator will have generated
output data that can be analyzed. The designer usually has a number of
ways to analyze the data. Most common are waveform output and text
tabular output. A sample waveform output is shown in Figure 11-3.
A waveform display shows the values of the signals of the design over
time. The designer can see the relationships between signal transitions
very easily. Using the waveform display, the designer can determine when
system clock edges occur and if the proper signal transitions are present.
The text tabular output is the same data as the waveform display, but
in a different format. A sample output is shown in Figure 11-4.
All of the signal transitions are shown from top to bottom instead of left
to right. It is also easier to read some of the signal values when the signal
High-Level Design Flow
277
Figure 11-3
Sample Waveform
Output.
has a lot of changes in a short amount of time and the signal values are
represented by a number of text characters. Most text table outputs can
also filter the output data using a number of different mechanisms such as
only on Print on Change or Print on Strobe.
While the output data is being analyzed, the user finds errors in the
design description. The user uses the waveform and tabular displays to
trace down the source of the errors in the VHDL code, make a change to the
VHDL to fix the problem, recompile the design again, and rerun the test.
If the problem is fixed, the designer tries to find the next problem, until
all problems have been found.
When the designer is happy with the behavior of the design, the
designer can start the process of building the real hardware device. To
implement the design, the designer uses VHDL synthesis tools. The next
step in the process is the VHDL synthesis step.
VHDL Synthesis
The goal of the VHDL synthesis step is to create a design that implements
the required functionality and matches the designer’s constraints in
speed, area, or power.
The VHDL synthesis tools convert the VHDL description into a netlist
in the target FPGA or ASIC technology. For the VHDL synthesis tool to
perform this step properly, the VHDL code must be written in a particular
style, as discussed in Chapter 10, “VHDL Synthesis.”
278
Chapter Eleven
Figure 11-4
Text Tabular Output.
To synthesize a VHDL description, the designer reads the verified
VHDL description into the VHDL synthesis tool in the same way that the
designer read the design into the VHDL simulator. The VHDL synthesis
tool reports syntax errors and synthesis errors. Synthesis errors usually
result from the designer using constructs that are not synthesizable. For
instance, ACCESS types in VHDL are not synthesizable, because they could
specify hardware that is dynamic in nature. Of course, syntax errors
result from improper VHDL syntax being read by the VHDL synthesis
tool. Presumably, most all of these errors will already have been taken
care of because the VHDL code has already been verified with the VHDL
simulator. The VHDL synthesis tool also reports warnings of constructs
that have the possibility of generating mismatches between the RTL simulation results and the output netlist simulation results.
The designer reads the VHDL design into the VHDL synthesis tool. If
there are no syntax errors, the designer can synthesize the design and map
the design to the target technology. If the designer had to make changes to
the VHDL description, then the VHDL description needs to be simulated
again and the output validated for correctness. First, the designer needs to
make sure that the synthesizer is producing an output in the target technology that looks reasonable. The designer looks at the synthesizer output
to determine whether or not the synthesizer produced a good result.
The synthesizer produces an output netlist in the target technology
and a number of report files. By looking at the netlist, the designer can
High-Level Design Flow
279
determine whether or not the design looks reasonable. For most reasonable size designs, however, it can be very difficult to determine how well
the synthesizer implemented the function. The designer looks at the report files to determine the quality of the synthesis output. The most common output files are the timing report and the area report. Most synthesis tools produce a number of other reports such as hierarchy reports,
instance reports, net reports, power reports, and others. The most useful
reports initially are the timing and area reports, because these are usually
the most critical factors.
Following is a sample area report:
*******************************************************
Cell: adder
View: test
Library: work
*******************************************************
Total accumulated area :
Number of LCs :
Number of CARRYs :
Number
Number
Number
Number
of
of
of
of
8
7
ports :
nets :
instances :
references to this view :
Cell
Library
GND
OUTBUF
INBUF
CARRY
OR2
AND2
LCELL
XOR2
flex10
flex10
flex10
flex10
flex10
flex10
flex10
flex10
24
107
91
0
References
1
8
16
7
14
21
8
16
x
x
x
x
x
x
x
x
Total Area
1
1
1
1
1
1
1
1
1
8
16
7
14
21
8
16
GND
OUTBUF
INBUF
CARRYs
OR2
AND2
LCs
XOR2
The area report tells the designer the size of the implemented design.
The units of measure are determined by the units used when the synthesis library was implemented. For instance, the typical unit for ASIC
designs is equivalent 2-input NAND gates, or gate equivalents. Using this
measurement, a 2-input NAND gate would consume one gate equivalent,
a 2-input AND gate would also consume one gate equivalent. A 4-input
NAND gate would consume two gate equivalents. For FPGA designs,
equivalent gate measurements vary from manufacturer to manufacturer
280
Chapter Eleven
because each has a different-sized basic cell. In the preceding sample area
report, the design produced 8 LC (Logic Cells) and 7 Carry devices. A
typical LC is 10 to 12 logic gates; the Carry device is 2 to 3 gates. So, this
example would represent about 90 to 120 gates.
The area report shows the designer how much of the resources of the
chip the design has consumed. The designer can tell if the design is too
big for a particular chip and the designer needs to target a larger chip, if
the design should go into a smaller chip, or if the current chip will work
fine. The designer can also get a relative size of the design to use in later
stages of the design process.
The timing report shows the timing of critical paths or specified paths of
the design. The designer examines the timing of the critical paths closely because these paths ultimately determine how fast the design can run. If the
longest path is a timing critical part of the design and is not meeting the
speed requirements of the designer, then the designer may have to modify
the VHDL code or try new timing constraints to make the path meet timing.
The following is a sample timing report:
Critical Path Report
Critical path #1, (unconstrained path)
NAME
GATE
ARRIVAL
LOAD
—————————————————————————————————————————————————————————————————————————————
a(0)/
0.00 up
0.00
ix30/OUT
INBUF
2.40 up
0.00
modgen_0_l1_l0_l0_0_l0_c1/Y
AND2
2.40 up
0.00
modgen_0_l1_l0_l0_0_l0_c3/Y
OR2
2.40 up
0.00
modgen_0_l1_l0_l0_0_l0_c4/Y
OR2
2.40 up
0.00
modgen_0_l1_l0_l0_0_l0_c5/Y
CARRY
2.90 up
0.00
modgen_0_l1_l0_l0_1_l0_c1/Y
AND2
2.90 up
0.00
modgen_0_l1_l0_l0_1_l0_c3/Y
OR2
2.90 up
0.00
modgen_0_l1_l0_l0_1_l0_c4/Y
OR2
2.90 up
0.00
modgen_0_l1_l0_l0_1_l0_c5/Y
CARRY
3.40 up
0.00
modgen_0_l1_l0_l0_2_l0_c2/Y
AND2
3.40 up
0.00
modgen_0_l1_l0_l0_2_l0_c4/Y
OR2
3.40 up
0.00
modgen_0_l1_l0_l0_2_l0_c5/Y
CARRY
3.90 up
0.00
modgen_0_l1_l0_l0_3_l0_c1/Y
AND2
3.90 up
0.00
modgen_0_l1_l0_l0_3_l0_c3/Y
OR2
3.90 up
0.00
modgen_0_l1_l0_l0_3_l0_c4/Y
OR2
3.90 up
0.00
modgen_0_l1_l0_l0_3_l0_c5/Y
CARRY
4.40 up
0.00
modgen_0_l1_l0_l0_4_l0_c1/Y
AND2
4.40 up
0.00
modgen_0_l1_l0_l0_4_l0_c3/Y
OR2
4.40 up
0.00
modgen_0_l1_l0_l0_4_l0_c4/Y
OR2
4.40 up
0.00
modgen_0_l1_l0_l0_4_l0_c5/Y
CARRY
4.90 up
0.00
modgen_0_l1_l0_l0_5_l0_c1/Y
AND2
4.90 up
0.00
modgen_0_l1_l0_l0_5_l0_c3/Y
OR2
4.90 up
0.00
modgen_0_l1_l0_l0_5_l0_c4/Y
OR2
4.90 up
0.00
High-Level Design Flow
281
NAME
GATE
ARRIVAL
LOAD
—————————————————————————————————————————————————————————————————————————————
modgen_0_l1_l0_l0_5_l0_c5/Y
CARRY
5.40 up
0.00
modgen_0_l1_l0_l0_6_l0_c1/Y
AND2
5.40 up
0.00
modgen_0_l1_l0_l0_6_l0_c3/Y
OR2
5.40 up
0.00
modgen_0_l1_l0_l0_6_l0_c4/Y
OR2
5.40 up
0.00
modgen_0_l1_l0_l0_6_l0_c5/Y
CARRY
5.90 up
0.00
modgen_0_l1_l0_l0_7_l0_sum0/Y
XOR2
5.90 up
0.00
modgen_0_l1_l0_l0_7_l0_sum1/Y
XOR2
5.90 up
0.00
modgen_0_l1_l0_l0_7_l0_sum2/Y
LCELL
10.00 up
0.00
ix39/OUT
OUTBUF
13.80 up
0.00
c(7)/
13.80 up
0.00
data arrival time
13.80
In this report, the worst-case path is listed shown with estimated time
values for each node traversed in the design. The timing analyzer calculates the time for a path from an input pin to a flip-flop or output, or from
a flip-flop output to a flip-flop input, or output pin.
The designer has the ability to ask for the timing for particular paths
of interest, or of the paths that have the longest timing value, and how
many to display. As mentioned previously, the worst-case paths ultimately
determine the speed of the design. For instance, in this case, the worstcase path is 13.8 nanoseconds; therefore, the fastest this design would be
able to run is about 72 MHz.
The last type of output data that the designer can examine is the
netlist for the design in the target technology. This output is a gate or
macro-level output in a format compatible with the place and route
tools that are used to implement the design in the target chip. For instance, most place and route tools for FPGA technologies take in an
EDIF netlist as an input format. The primitives used in the netlist are
those used in the synthesis library to describe the technology. The place
and route tools understand what to do with these primitives in terms
of how to place a primitive and how to route wires to them. The
following example uses a VHDL netlist for ease of understanding. To
save space (and boredom), this is not a complete netlist, but gives the
reader an idea of how a netlist is structured. The complete netlist can
be found on the included CD:
--- Definition of
---
adder
library IEEE, EXEMPLAR; use IEEE.STD_LOGIC_1164.all; use
282
Chapter Eleven
EXEMPLAR.EXEMPLAR_1164.all;
-- Library use clause for technology cells
library altera ;
use altera.all ;
entity adder is
port (
a : IN std_logic_vector (7 DOWNTO 0) ;
b : IN std_logic_vector (7 DOWNTO 0) ;
c : OUT std_logic_vector (7 DOWNTO 0)) ;
end adder ;
architecture test of adder is
component XOR2
port (
Y : OUT std_logic ;
IN1 : IN std_logic ;
IN2 : IN std_logic) ;
end component ;
component LCELL
port (
Y : OUT std_logic ;
IN1 : IN std_logic) ;
end component ;
component AND2
port (
Y : OUT std_logic ;
IN1 : IN std_logic ;
IN2 : IN std_logic) ;
end component;
.
.
.
signal c_dup0_7, c_dup0_6, c_dup0_5, c_dup0_4, c_dup0_3,
c_dup0_2,
c_dup0_1, c_dup0_0, modgen_0_l1_l0_c_int_7,
modgen_0_l1_l0_c_int_6,
modgen_0_l1_l0_c_int_5, modgen_0_l1_l0_c_int_4,
modgen_0_l1_l0_c_int_3,
modgen_0_l1_l0_c_int_2, modgen_0_l1_l0_c_int_1,
modgen_0_l1_l0_l0_0_l0_s1, modgen_0_l1_l0_l0_0_l0_s2,
modgen_0_l1_l0_l0_0_l0_w1, modgen_0_l1_l0_l0_0_l0_w2,
modgen_0_l1_l0_l0_0_l0_w3, modgen_0_l1_l0_l0_0_l0_w4,
b_2_int, b_1_int, b_0_int, U_0: std_logic ;
.
.
.
begin
modgen_0_l1_l0_l0_0_l0_sum0 : XOR2 port map ( Y=>
modgen_0_l1_l0_l0_0_l0_s1, IN1=>a_0_int, IN2=>U_0);
modgen_0_l1_l0_l0_0_l0_sum1 : XOR2 port map ( Y=>
High-Level Design Flow
283
modgen_0_l1_l0_l0_0_l0_s2,
IN1=>modgen_0_l1_l0_l0_0_l0_s1, IN2=>
b_0_int);
modgen_0_l1_l0_l0_0_l0_sum2 : LCELL port map (
Y=>c_dup0_0, IN1=>
modgen_0_l1_l0_l0_0_l0_s2);
modgen_0_l1_l0_l0_0_l0_c0 : AND2 port map (
Y=>modgen_0_l1_l0_l0_0_l0_w1,
IN1=>a_0_int, IN2=>b_0_int);
modgen_0_l1_l0_l0_0_l0_c1 : AND2 port map (
Y=>modgen_0_l1_l0_l0_0_l0_w2,
IN1=>a_0_int, IN2=>U_0);
modgen_0_l1_l0_l0_0_l0_c2 : AND2 port map (
Y=>modgen_0_l1_l0_l0_0_l0_w3,
IN1=>U_0, IN2=>b_0_int);
.
.
.
ix43 : OUTBUF port map ( \OUT\=>c(3), \IN\=>c_dup0_3);
ix44 : OUTBUF port map ( \OUT\=>c(2), \IN\=>c_dup0_2);
ix45 : OUTBUF port map ( \OUT\=>c(1), \IN\=>c_dup0_1);
ix46 : OUTBUF port map ( \OUT\=>c(0), \IN\=>c_dup0_0);
U_0_XMPLR : GND port map ( Y=>U_0);
end test ;
Notice that all of the other interconnect signal names have names such
as modgen_0_11_xx or ix123. There is no corresponding signal name in
the source file to specify the signal name; therefore, the synthesis tool generates names for these signals. The netlist can be used to figure out how
well the synthesizer implemented a part of the design, or to track down
a problem net. It can be very useful to find out why a critical path was
implemented too slowly.
When the netlist meets the designer’s timing, area, power, and other
constraints, the next step is to pass the netlist to the gate level simulator.
This simulator checks the functionality of the synthesized design.
Functional Gate-Level Verification
Some designers might want to do a quick check on the output of the synthesis tool to make sure that the synthesis tool produced a design that is
functionally correct. If proper design rules are followed for the input
VHDL description, the synthesis tool should never generate an output
that is functionally different from the RTL VHDL input, unless the tool
has a bug. However, if some of the warnings or errors are ignored or some
284
Chapter Eleven
part of the design is written using a strange VHDL style, the synthesizer
can produce an output netlist that does not exactly match the RTL input
in terms of functionality. Most designers like to run a quick check on the
results of the synthesis tool to make sure the synthesis tool produced a
functionally correct output.
To do this, the designer runs a functional gate-level verification. The
designer reads the output VHDL netlist from the synthesis tool plus a
library of the synthesis primitives into the VHDL simulator and runs the
simulation using the RTL verification vectors. If the design matches,
then the synthesis tool did not produce logic mismatches; if it does not
match, the designer needs to debug the VHDL RTL description to see
what is wrong.
The most common method for performing this step is to run a VITAL
simulation of the netlist from the synthesis tool. For a completely functional
simulation, no timing is back-annotated. If the synthesis tool supports
estimated timing and SDF file generation, the synthesis tool could write
the VHDL netlist and an SDF timing file for the design. The
designer could use these two files to run a VITAL simulation with estimated timing. After the design has been functionally verified, it is passed
to the place and route tools to implement the design.
Place and Route
Place and route tools are used to take the design netlist and implement
the design in the target technology device. The place and route tools place
each primitive from the netlist into an appropriate location on the target
device and then route signals between the primitives to connect the
devices according to the netlist. Place and route tools are typically very
architecture and device dependent. These tools are tuned to take advantage of each architectural and routing advantage the device contains.
FPGA vendors provide these tools because the differences in architectures
are large enough that writing a common tool for all architectures would
be very difficult. Place and route tools for ASIC devices can be obtained
from the ASIC vendor or EDA (Electronic Design Automation) vendors.
ASIC architectures do not have as wide a variation between architectures
as FPGA architectures and, therefore, place and route tools exist that can
handle lots of different ASIC architectures.
High-Level Design Flow
Figure 11-5
Place and Route
Data Flow.
285
Netlist
Device
Information
Timing
Constraints
Placement
Constraints
Place and
Route
Device
Implementation
Figure 11-5 shows a dataflow diagram of the place and route tools.
Inputs to the place and route tools are the netlist in EDIF or another
netlist format, and possibly timing constraints. The format of the netlist
input file varies from manufacturer to manufacturer. Some tools use
EDIF; others use proprietary formats such as XNF.
Another input to some place and route tools is the timing constraints,
which give the place and route tools an indication about which signals
have critical timing associated with them and to route these nets in the
most timing efficient manner. These nets are typically identified during
the static timing analysis process during synthesis. These constraints tell
the place and route tool to place the primitives in close proximity to one
another and to use the fastest routing. The closer the cells are, the shorter
the routed signals will be and the shorter the time delay.
Some place and route tools allow the designer to specify the placement
of large parts of the design. This process is also known as floorplanning.
Floorplanning allows the user to pick locations on the chip for large blocks
of the design so that routing wires are as short as possible. The designer lays
out blocks on the chip as general areas. The floorplanner feeds this information to the place and route tools so that these blocks are placed properly.
After the cells are placed, the router makes the appropriate connections.
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Chapter Eleven
After all the cells are placed and routed, the output of the place and
route tools consists of data files that can be used to implement the chip.
In the case of FPGAs, these files describe all of the connections needed to
make the FPGA macrocells implement the functionality required. Antifuse FPGAs use this information to burn the appropriate fuses, while
reprogrammable devices download this information to the device to turn
on the appropriate transistor connections.
The other output from the place and route software is a file used to
generate the timing file. This file describes the actual timing of the programmed FPGA device or the final ASIC device. This timing file, as much
as possible, describes the timing extracted from the device when it is
plugged into the system for testing. The most common format of this file
for most simulators is SDF (Standard Delay Format). Sometimes, proprietary formats are generated and later translated to SDF. SDF is used
to back-annotate the post route timing information from place and route
tools into the post layout timing simulation.
Post Layout Timing Simulation
After the place and route process has completed, the designer will want
to verify the results of the place and route process. There are a number
of methods to accomplish this task but the most common is to use post
route gate-level simulation. This simulation combines the netlist used for
place and route with the timing file from the place and route process into
a simulation that checks both functionality and timing of the design. The
designer can run the simulation and generate accurate output waveforms
that show whether or not the device is operating properly and if the timing
is being met.
If the design has been properly structured, the same test vectors used
for the RTL simulation can be used for the post route gate-level simulation. In this way, the designer is saved the process of generating a new
set of vectors to check the gate-level design and verifying the new vector
output values.
Post route gate-level simulation, if done properly, also uses the same
simulator as the RTL simulation. For VHDL simulations, this requires a
VITAL-compliant (standard way of describing designs with designs that
allow SDF timing back-annotation) VHDL simulator. VHDL simulators
that are not VITAL-compliant do not accelerate the execution of the gatelevel primitives and cannot accept SDF to back annotate the timing.
High-Level Design Flow
287
Static Timing
For designs of 10,000 gates to 100,000 gates, post route timing simulation
can be a good method of verifying design functionality and timing. However, as designs get larger, or if the designer does not have test vectors,
the designer can use static timing analysis to make sure the design meets
the timing requirements. A static timing analyzer traces each path in
the design and keeps track of the timing from a clock edge or an input. A
timing report is then generated in a number of formats. For instance, the
designer can ask for all paths and get an enormous listing of every path
in the design. A more intelligent method, however, is to ask for the most
timing critical paths in the design and make sure the timing constraints
have been met.
Typical static timing analyzers have a number of report types that can
be generated so that the designer can make sure the critical paths of the
design can be found and verified to be within the required specifications.
If paths are not within the specifications, the static timing analyzer shows
the entire path so that the designer can try to fix the problem.
SUMMARY
In this chapter, the complete VHDL design process using synthesis was
described. This process is very similar no matter which VHDL synthesis
or simulation tool is used. The designer must follow a number of steps
that add more detail to the design. At each step, the designer has checks
to make sure that the correct behavior is being implemented. At the
beginning of the process, RTL simulation is used to verify correctness.
After synthesis, the netlist, timing report, and area report are all examined to make sure the design fits the designer’s constraints. Functional
simulation is then run to verify that the synthesis tool produced a functionally correct design. The design is put through the place and route
process to implement the design in the target technology. The final check
is then to verify using post route gate level simulation that the design is
functionally correct and meets timing.
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CHAPTER
12
Top-Level
System Design
In the last few chapters, we have discussed VHDL language
features and the VHDL synthesis process. In the next few
chapters, we tie all of these ideas together by developing
a top-down design for a small CPU design, verify its functionality, verify that it can be synthesized, and implement
the design in an FPGA device.
290
Figure 12-1
CPU Block Diagram.
Chapter Twelve
OpRegsel
OpReg
Reg0
Instrsel
InstrReg
Reg1
Reg2
Alusel
Reg3
•
Compsel
Comp
ALU
Regsel
Compout
•
Shifter
Shiftsel
Reg7
OutReg
Clock
Outsel
Reset
ProgCnt
Control
Progsel
Addrsel
AddrReg
Addr(15:0)
Data(15:0)
Ready
R/W
VMA
CPU Design
The example is a small, 16-bit microprocessor. A block diagram is shown
in Figure 12-1.
The processor contains a number of basic pieces. There is a register
array of eight 16-bit registers, an ALU (Arithmetic Logic Unit), a shifter,
a program counter, an instruction register, a comparator, an address register, and a control unit. All of these units communicate through a common, 16-bit tristate data bus.
Top-Level System Operation
The top-level design consists of the processor block and a memory block
communicating through a bidirectional databus, an address bus, and a few
control lines. The processor fetches instructions from the external memory
and executes these instructions to run a program. These instructions are
Top-Level System Design
291
stored in the instruction register and decoded by the control unit. The
control unit causes the appropriate signal interactions to make the
processor unit execute the instruction.
If the instruction is an add of two registers, the control unit would
cause the first register value to be written to register OpReg for temporary
storage. The second register value would then be placed on the data bus.
The ALU would be placed in add mode and the result would be stored in
register OutReg. Register OutReg would store the resulting value until it
is copied to the final destination.
When executing an instruction, a number of steps take place. The program counter holds the address in memory of the current instruction. After
an instruction has finished execution, the program counter is advanced to
where the next instruction is located. If the processor is executing a linear
stream of instructions, this is the next instruction. If a branch was taken,
the program counter is loaded with the next instruction location directly.
The control unit copies the program counter value to the address register, which outputs the new address on the address bus. At the same
time, the control unit sets the R/W (read write signal) to a ‘0’ value for
a read operation and sets signal VMA (Valid Memory Address) to a ‘1’,
signaling the memory that the address is now valid. The memory decodes
the address and places the memory data on the data bus. When the data
has been placed on the data bus, the memory has set the READY signal
to a ‘1’ value indicating that the memory data is ready for consumption.
The control unit causes the memory data to be written into the instruction register. The control unit now has access to the instruction and decodes
the instruction. The decoded instruction executes, and the process starts
over again.
Instructions
Instructions can be divided into a number of different types as follows:
■ Load—These instructions load register values from other registers,
memory locations, or with immediate values given in the instruction.
■ Store—These instructions store register values to memory locations.
■ Branch—These instructions cause the processor to go to another
location in the instruction stream. Some branch instructions test
values before branching; others branch without testing.
292
Chapter Twelve
■ ALU—These instructions perform arithmetic and logical operations such as ADD, SUBTRACT, OR, AND, and NOT.
■ Shift—These instructions use the shift unit to perform shift
operations on the data passed to it.
Sample Instruction Representation
Instructions share common attributes, but come in a number of flavors.
Sample instructions are shown in Figure 12-2.
All instructions contain the opcode in the five most significant bits of
the instruction. Single-word instructions also contain two 3-bit register
fields in the lowest 6 bits of the instruction. Some instructions, such as
INC (Increment), only use one of the fields, but other instructions, such
as MOV (Move), use both register fields to specify the From register and the
To register. In double-word instructions, the first word contains the opcode
and destination register address, and the second word contains the immediate instruction location or data value to be loaded. For instance, a
LoadI (Load Immediate) instruction would look like this:
LoadI 1, 16#15
Figure 12-2
Instruction Words.
15
14
13
DST
SRC
Opcode
12
5
11
4
3
2
1
0
Single Word
DST
Opcode
15
14
13
12
11
2
1
0
2
1
0
Address or Data
15
14
13
12
5
11
Double Word
4
3
Top-Level System Design
293
DST
Opcode
Figure 12-3
Instruction Data.
0
0
1
0
0
0
0
0
1
0
1
1
LoadI
0
0
0
0
0
0
0
0
0
0
0
0
1
1
0
1
5
This instruction loads the hex value 15 into register 1. The instruction
words look like those shown in Figure 12-3.
When the control unit decodes the opcode of the first word, it determines that the instruction is two words long and loads the second word
to complete the instruction.
The instructions implemented in the processor and their opcodes are
listed in Figure 12-4.
Not all of the possible instructions have been implemented in this
processor example to limit the complexity for ease of publication. Typical
commercial processors are much more complicated and have pipelined
instruction streams for faster execution. To reduce complexity, this example
is not pipelined.
CPU Top-Level Design
The next few sections contain the VHDL description for each of the CPU
components. First of all, a top-level package cpu_lib.vhd is needed to describe the signal types that are used to communicate between the CPU
components. Following is this package:
library IEEE;
use IEEE.std_logic_1164.all;
use IEEE.std_logic_arith.all;
package cpu_lib is
type t_shift is (shftpass, shl, shr, rotl, rotr);
subtype t_alu is unsigned(3 downto 0);
constant alupass : unsigned(3 downto 0) := “0000”;
294
Figure 12-4
Opcode Table.
Chapter Twelve
OPCODE
INSTRUCTION
NOTE
00000
00001
00010
00011
00100
00101
00110
00111
01000
01001
01010
01011
01100
01101
01110
01111
10000
10001
10010
10011
10100
10101
10110
10111
11000
11001
11010
11011
11100
11101
NOP
LOAD
STORE
MOVE
LOADI
BRANCHI
BRANCHGTI
INC
DEC
AND
OR
XOR
NOT
ADD
SUB
ZERO
BRANCHLTI
BRANCHLT
BRANCHNEQ
BRANCHNEQI
BRANCHGT
BRANCH
BRANCHEQ
BRANCHEQI
BRANCHLTEI
BRANCHLTE
SHL
SHR
ROTR
ROTL
No operation
Load register
Store register
Move value to register
Load register with immediate value
Branch to immediate address
Branch greater than to immediate address
Increment
Decrement
And two registers
Or two registers
Xor two registers
Not a register value
Add two registers
Subtract two registers
Zero a register
Branch less than to immediate address
Branch less than
Branch not equal
Branch not equal to immediate address
Branch greater than
Branch all the time
Branch if equal
Branch if equal to immediate address
Branch if less or equal to immediate address
Branch if less or equal
Shift left
Shift right
Rotate right
Rotate left
constant
constant
constant
constant
constant
constant
constant
constant
constant
andOp : unsigned(3 downto 0) := “0001”;
orOp : unsigned(3 downto 0) := “0010”;
notOp : unsigned(3 downto 0) := “0011”;
xorOp : unsigned(3 downto 0) := “0100”;
plus : unsigned(3 downto 0) := “0101”;
alusub : unsigned(3 downto 0) := “0110”;
inc : unsigned(3 downto 0) := “0111”;
dec : unsigned(3 downto 0) := “1000”;
zero : unsigned(3 downto 0) := “1001”;
type t_comp is (eq, neq, gt, gte, lt, lte);
subtype t_reg is std_logic_vector(2 downto 0);
type state is (reset1, reset2, reset3, reset4, reset5,
type state is (reset6, execute, nop, load, store, move,
Top-Level System Design
type
type
type
type
type
type
type
type
295
state
state
state
state
state
state
state
state
is
is
is
is
is
is
is
is
(load2, load3, load4, store2, store3,
(store4, move2, move3, move4,incPc, incPc2,
(incPc3, incPc4, incPc5, incPc6, loadPc,
(loadPc2,loadPc3, loadPc4, bgtI2, bgtI3,
(bgtI4, bgtI5, bgtI6, bgtI7,bgtI8, bgtI9,
(bgtI10, braI2, braI3, braI4, braI5, braI6,
(loadI2,loadI3, loadI4, loadI5, loadI6,
(inc2, inc3, inc4);
subtype bit16 is std_logic_vector(15 downto 0);
end cpu_lib;
This package describes a number of types that are used to specify the ALU
functionality, the shifter operation, and the states needed for the control
of the CPU.
The highest level of the design is described by the file top.vhd as
shown in the following:
library IEEE;
use IEEE.std_logic_1164.all;
use work.cpu_lib.all;
entity top is
end top;
architecture behave of top is
component mem
port (addr : in bit16;
port (sel, rw : in std_logic;
port (ready : out std_logic;
port (data : inout bit16);
end component;
component cpu
port(clock, reset, ready : in std_logic;
port(addr : out bit16;
port(rw, vma : out std_logic;
port(data : inout bit16);
end component;
signal addr, data : bit16;
signal vma, rw, ready : std_logic;
signal clock, reset : std_logic := ‘0’;
begin
clock <= not clock after 50 ns;
reset <= ‘1’, ‘0’ after 100 ns;
m1 : mem port map (addr, vma, rw, ready, data);
u1 : cpu port map(clock, reset, ready, addr, rw, vma,
u1 : cpu port map(data);
end behave;
296
Chapter Twelve
This model instantiates components cpu and mem and specifies the necessary signals to connect the components, as shown in Figure 12-5.
Component mem is a memory device and contains the instructions and
data for the CPU to execute. Component cpu is an RTL implementation
of the CPU device that is simulated for correctness and synthesized to
implement the design.
Let’s now take a look at the description for the memory component to
see how it works. The memory is described in file mem.vhd shown in the
following:
library IEEE;
use IEEE.std_logic_1164.all;
use IEEE.std_logic_arith.all;
use IEEE.std_logic_unsigned.all;
Top
Figure 12-5
Top Level of CPU
Design.
VMA
Ready
R/W
Mem
Cpu
Addr
Data
Clock
Reset
Top-Level System Design
297
use work.cpu_lib.all;
entity mem is
port (addr : in bit16;
port (sel, rw : in std_logic;
port (ready : out std_logic;
port (data : inout bit16);
end mem;
architecture behave of mem is
begin
memproc: process(addr, sel, rw)
type t_mem is array(0 to 63) of bit16;
variable mem_data : t_mem :=
(“0010000000000001”, --- 0 loadI 1, #
-- load source
address
“0000000000010000”, --- 1 10
“0010000000000010”, --- 2 loadI 2, #
-- load
destination address
“0000000000110000”, --- 3 30
“0010000000000110”, --- 4 loadI 6, #
-- load data
end address
“0000000000101111”, --- 5 2F
“0000100000001011”, --- 6 load 1, 3
-- load reg3
with source element
“0001000000011010”, --- 7 store 3, 2
-- store reg3
at destination
“0011000000001110”, --- 8 bgtI 1, 6, # -- compare to
see if at end of data
“0000000000000000”, --- 9 00
-- if so just
start over
“0011100000000001”, --- A inc 1
-- move source
address to next
“0011100000000010”, --- B inc 2
-- move
destination address to next
“0010100000001111”, --- C braI #
-- go to the
next element to copy
“0000000000000110”, --- D 06
“0000000000000000”, --- E
“0000000000000000”, --- F
“0000000000000001”, --- 10
--- Start of source
array
“0000000000000010”, --- 11
“0000000000000011”, --- 12
“0000000000000100”, --- 13
“0000000000000101”, --- 14
“0000000000000110”, --- 15
“0000000000000111”, --- 16
“0000000000001000”, --- 17
“0000000000001001”, --- 18
“0000000000001010”, --- 19
“0000000000001011”, --- 1A
“0000000000001100”, --- 1B
“0000000000001101”, --- 1C
“0000000000001110”, --- 1D
298
Chapter Twelve
“0000000000001111”,
“0000000000010000”,
“0000000000000000”,
“0000000000000000”,
“0000000000000000”,
“0000000000000000”,
“0000000000000000”,
“0000000000000000”,
“0000000000000000”,
“0000000000000000”,
“0000000000000000”,
“0000000000000000”,
“0000000000000000”,
“0000000000000000”,
“0000000000000000”,
“0000000000000000”,
“0000000000000000”,
“0000000000000000”,
“0000000000000000”,
array
“0000000000000000”,
“0000000000000000”,
“0000000000000000”,
“0000000000000000”,
“0000000000000000”,
“0000000000000000”,
“0000000000000000”,
“0000000000000000”,
“0000000000000000”,
“0000000000000000”,
“0000000000000000”,
“0000000000000000”,
“0000000000000000”,
“0000000000000000”,
“0000000000000000”);
---------------------------------------
1E
1F
20
21
22
23
24
25
26
27
28
29
2A
2B
2C
2D
2E
2F
30
-------------------------------
31
32
33
34
35
36
37
38
39
3A
3B
3C
3D
3E
3F
--- start of destination
begin
data <= “ZZZZZZZZZZZZZZZZ”;
ready <= ‘0’;
if sel = ‘1’ then
if rw = ‘0’ then
data <= mem_data(CONV_INTEGER(addr(15 downto 0)))
after 1 ns;
ready <= ‘1’;
elsif rw = ‘1’ then
mem_data(CONV_INTEGER(addr(15 downto 0))) := data;
end if;
else
data <= “ZZZZZZZZZZZZZZZZ” after 1 ns;
end if;
end process;
end behave;
Top-Level System Design
299
Entity mem is a large array with a simple bus interface to allow reading
and writing to the memory. A memory location is selected for read by
placing the appropriate address of the location on signal addr, setting
input rw (read write) to a ‘0’ and putting the value ‘1’ on signal sel
(select). The value of the memory location appears on signal data, and
signal ready is set to a ‘1’ value signaling that the memory information
is available.
To write a location in the memory, the address is placed on signal addr,
set signal rw to a ‘1’ value, set signal sel to a ‘1’ value, and put the data
to be written on input data.
The memory is divided into two separate sections. The first section is
the instruction area, and the second is the data area. The instruction area
contains the instructions to be executed, and the second section contains
the data area for the instructions to manipulate. The CPU instructions
start at location 00 and end at location 0D. The data area starts at location 10 and ends at location 3F, the end of the array. The instructions
stored in the memory device are a simple algorithm for moving a block of
data from one location to another. This type of program could also be
considered a block copy operation.
Block Copy Operation
A diagram showing how a block copy operation looks is shown in Figure 12-6.
The copy operation starts when the CPU gets a reset signal. A reset
signal causes the CPU to reset its internal state and start processing
instructions at location 00 of the memory. The first few instructions set
up the appropriate CPU registers so that the block copy operation can
proceed. Register 1 contains the starting address, or the address of the
first element of the memory block to be copied. Register 2 contains the
starting address for the destination of the memory block. Register 6 contains
the ending address of the memory block to be copied.
The first instruction at location 00 loads register 1 with the starting
address of the memory block to be copied. The actual address is contained
in mem location 01. The value is hexadecimal 10 or 16 decimal. The block
copy program starts the copy operation from location 10. The first instruction is a double-word instruction. The first word specifies the instruction
opcode and the registers to be used in the instruction. The second word
contains the absolute address to be used in the operation.
The next instruction is at memory location 02. The first instruction
advanced the program counter past location 01, which contained the
300
Chapter Twelve
Memory Data
Figure 12-6
Block Copy
Operation.
Source
Destination
starting address. This instruction loads register 2 with the destination
address for the block copy. The destination is at 30 hex or 48 decimal.
These load instructions are load-immediate instructions, which load the
next memory location into the register specified in the instruction.
The final setup instruction is at mem location 04. This instruction loads
the last address of the memory block to be copied. This register signals
the block copy routine when to stop the copy operation. After this instruction has been executed, all of the registers have been set up for the block
copy operation, and the copy loop can start.
Instruction 6 is the start of a loop of instructions that perform the copy
operation. Instruction 6 copies the contents of the memory location whose
address is contained in register 1 to register 3. Instruction 7 copies the
data in register 3 to the memory location specified in register 2. After
Top-Level System Design
301
these two instructions have executed the first time, the first memory
element of the block will have been copied.
After the copy operation, the program needs to check if it is done.
Instruction 8 compares the address in register 1 versus the end address
in register 6 to see if the copy operation has completed the last element.
If so, the program should exit because the copy operation has completed.
However, in this simple example, there is no other program to execute, so
this program branches to instruction 00 and starts the process over again.
If the copy operation is not completed, the CPU executes the instruction at 0A, which increments register 1. This instruction increments
register 1 so that it points to the next element to be copied. Instruction
0B increments register 2, which moves the destination address to the
next location.
Finally, instruction 0C branches back to instruction 06 and continues
the next copy operation. Figures 12-7 and 12-8 show the memory array
before the copy and after.
Figure 12-7
The Memory Array
Before the Copy.
302
Chapter Twelve
Figure 12-8
The Memory Array
After the Copy.
In Figure 12-7, location 48 is all zeroes. In Figure 12-8, location 48 is
no longer all zeroes, but has the first value from the block copy operation.
If the simulation is run completely, one by one the data from the source
array (location 16) is copied to the destination (location 48). After the last
location is copied, the program repeats the same steps.
SUMMARY
In this chapter, we examined the top level of a design that consisted of a
CPU, a memory array, and the top-level instantiation of those components.
In the next chapter, we examine the CPU in more detail.
CHAPTER
1
13
Basic Synthesis
Concepts
CPU:
Description
This chapter answers the question, “What are abstract data
types?” The idea of using well-designed abstract data types
(ADTs) to simplify the development life cycle and to create
reusable code is well established. This chapter covers the baIn this chapter, we further refine the CPU description and
sics of designing and implementing ADTs in an object-oriexamine the RTL (Register Transfer Level) description of
ented programming language. As a foundation to exploring
the CPU. The CPU is described by a number of lower-level
data abstraction, we will take a look inside Java and excomponents that are instantiated to form the CPU design.
plore some of the internal workings of the Java runtime sysAt the top of the CPU design is an architecture that
tem. Java reference objects will be explained. The passing
instantiates all of the lower-level components to form
of reference and value types as arguments and how each
the CPU. The CPU block diagram is shown in Figure 13-1.
type of argument passing is used in the Java programming
language will be discussed. Near the end of this chapter, exercises are provided to stimulate understanding in the use
of reference objects.
304
Figure 13-1
CPU Block Diagram.
Chapter Thirteen
OpRegsel
OpReg
Reg0
Instrsel
InstrReg
Reg1
Reg2
Alusel
Reg3
•
Compsel
Comp
ALU
Regsel
Compout
•
Shifter
Shiftsel
Reg7
OutReg
Clock
Outsel
Reset
ProgCnt
Control
Progsel
Addrsel
AddrReg
Addr(15:0)
Data(15:0)
Ready
R/W
VMA
Following is an implementation of this block diagram, shown by file
cpu.vhd:
library IEEE;
use IEEE.std_logic_1164.all;
use work.cpu_lib.all;
entity cpu is
port(clock, reset, ready : in std_logic;
port(addr : out bit16;
port(rw, vma : out std_logic;
port(data : inout bit16);
end cpu;
architecture rtl of cpu is
component regarray
port( data : in bit16;
port( sel : in t_reg;
port( en : in std_logic;
port( clk : in std_logic;
port( q : out bit16);
end component;
CPU: Synthesis Description
component reg
port( a : in bit16;
port( clk : in std_logic;
port( q : out bit16);
end component;
component trireg
port( a : in bit16;
port( en : in std_logic;
port( clk : in std_logic;
port( q : out bit16);
end component;
component control
port( clock : in std_logic;
port( reset : in std_logic;
port( instrReg : in bit16;
port( compout : in std_logic;
port( ready : in std_logic;
port( progCntrWr : out std_logic;
port( progCntrRd : out std_logic;
port( addrRegWr : out std_logic;
port( outRegWr : out std_logic;
port( outRegRd : out std_logic;
port( shiftSel : out t_shift;
port( aluSel : out t_alu;
port( compSel : out t_comp;
port( opRegRd : out std_logic;
port( opRegWr : out std_logic;
port( instrWr : out std_logic;
port( regSel : out t_reg;
port( regRd : out std_logic;
port( regWr : out std_logic;
port( rw : out std_logic;
port( vma : out std_logic
port( );
end component;
component alu
port( a, b : in bit16;
port( sel : in t_alu;
port( c : out bit16);
end component;
component shift
port ( a : in bit16;
port( sel : in t_shift;
port( y : out bit16);
end component;
component comp
port( a, b : in bit16;
305
306
Chapter Thirteen
port( sel : in t_comp;
port( compout : out std_logic);
end component;
signal
signal
signal
signal
signal
signal
signal
signal
begin
opdata, aluout, shiftout, instrregOut : bit16;
regsel : t_reg;
regRd, regWr, opregRd, opregWr, outregRd, outregWr,
addrregWr, instrregWr, progcntrRd, progcntrWr,
compout : std_logic;
alusel : t_alu;
shiftsel : t_shift;
compsel : t_comp;
ra1 : regarray port map(data, regsel, regRd, regWr, data);
opreg: trireg port map (data, opregRd, opregWr, opdata);
alu1: alu port map (data, opdata, alusel, aluout);
shift1: shift port map (aluout, shiftsel, shiftout);
outreg: trireg port map (shiftout, outregRd, outregWr,
data);
addrreg: reg port map (data, addrregWr, addr);
progcntr: trireg port map (data, progcntrRd, progcntrWr,
data);
comp1: comp port map (opdata, data, compsel, compout);
instr1: reg port map (data, instrregWr, instrregOut);
con1: control port map (clock, reset, instrregOut, com
pout, ready, progcntrWr, progcntrRd, addrregWr, out
regWr, outregRd, shiftsel, alusel, compsel, opre
gRd, opregWr, instrregWr, regsel, regRd, regWr, rw,
vma);
end rtl;
Architecture rtl of entity cpu is a structural implementation of the
block diagram. Architecture rtl contains the component declarations of all
of the components used to build the design, the signals used to connect the
components, and the component instantiations to create the functionality.
After the component and signal declarations are the component instantiation statements that instance the components and connect the appropriate signals. In the next few sections, each of the VHDL component
descriptions is described in more detail.
ALU
The first entity described is the ALU. This entity performs a number of
arithmetic or logical operations on one or more input busses. A symbol for
the ALU is shown in Figure 13-2.
CPU: Synthesis Description
Figure 13-2
ALU Interface.
307
a
b
sel
ALU
c
Inputs a and b are the two input busses upon which the ALU operations
are performed. Output bus c returns the result of the ALU operation. Input
sel determines which operation is performed as specified by Figure 13-3.
As we can see, the ALU can perform a number of arithmetic operations,
such as add and subtract, and some logical operations, such as AND, OR,
and XOR. Following is a VHDL description of the ALU entity:
library IEEE;
use IEEE.std_logic_1164.all;
use IEEE.std_logic_unsigned.all;
use work.cpu_lib.all;
entity alu is
port( a, b : in bit16;
port( sel : in t_alu;
port( c : out bit16);
end alu;
architecture rtl of alu is
begin
aluproc: process(a, b, sel)
begin
case sel is
when alupass =>
c <= a after 1 ns;
when andOp =>
c <= a and b after 1 ns;
when orOp =>
c <= a or b after 1 ns;
when xorOp =>
c <= a xor b after 1 ns;
308
Figure 13-3
ALU Function Table.
Chapter Thirteen
Sel Input
Operation
0000
C=A
0001
C = A AND B
0010
C = A OR B
0011
C = NOT A
0100
C = A XOR B
0101
C=A+B
0110
C=A–B
0111
C=A+1
1000
C = A –1
1001
C=0
when notOp =>
c <= not a after 1 ns;
when plus =>
c <= a + b after 1 ns;
when alusub =>
c <= a - b after 1 ns;
when inc =>
c <= a + “0000000000000001” after 1 ns;
when dec =>
c <= a - “0000000000000001” after 1 ns;
when zero =>
c <= “0000000000000000” after 1 ns;
when others =>
c <= “0000000000000000” after 1 ns;
end case;
end process;
end rtl;
The architecture uses a large case statement on input sel to determine
which of the arithmetic or logical operations to perform. The possible
values of signal sel are determined by type t_alu described in package
cpu_lib in file cpulib.vhd. After the new value for output c is calculated,
all of the resulting values are assigned with a 1-nanosecond time delay
to eliminate delta delay problems during RTL simulation.
CPU: Synthesis Description
309
Comp
The next component described is the comparator entity comp. This entity
compares two values and returns either a ‘1’ or ‘0’ depending on the
type of comparison requested and the values being compared. A symbol
showing the ports of the comparator is shown in Figure 13-4.
The comparison type is determined by the value on input port sel. For
instance, to compare if inputs a and b are equal, apply the value eq to port
sel. If ports a and b have the same value, port compout returns ‘1’. If the
values are not equal, ‘0’ is returned. The types of comparisons allowed are
described by type t_comp in package cpu_lib in file cpulib.vhd described
earlier. The full table of comparison types and values is shown in Figure 13-5.
All operations work on two input values and return a single bit result.
This bit is used to control the flow of operation within the processor while
executing instructions. Following is a VHDL description of the comp entity:
library IEEE;
use IEEE.std_logic_1164.all;
use IEEE.std_logic_arith.all;
Figure 13-4
Comp Interface.
a
b
sel
Comp
compout
Figure 13-5
Comp Operation
Table.
Sel input value
Comparison
EQ
Compout = 1 when a equals b
NEQ
Compout = 1 when a is not equal to b
GT
Compout = 1 when a is greater than b
GTE
Compout = 1 when a is greater than or equal to b
LT
Compout = 1 when a is less than b
LTE
Compout = 1 when a is less than or equal to b
310
Chapter Thirteen
use work.cpu_lib.all;
entity comp is
port( a, b : in bit16;
sel : in t_comp;
compout : out std_logic);
end comp;
architecture rtl of comp is
begin
compproc: process(a, b, sel)
begin
case sel is
when eq =>
if a = b then
compout <= ‘1’ after
else
compout <= ‘0’ after
end if;
when neq =>
if a /= b then
compout <= ‘1’ after
else
compout <= ‘0’ after
end if;
when gt =>
if a > b then
compout <= ‘1’ after
else
compout <= ‘0’ after
end if;
when gte =>
if a >= b then
compout <= ‘1’ after
else
compout <= ‘0’ after
end if;
when lt =>
if a < b then
compout <= ‘1’ after
else
compout <= ‘0’ after
end if;
when lte =>
if a <= b then
compout <= ‘1’ after
else
compout <= ‘0’ after
end if;
end case;
end process;
end rtl;
1 ns;
1 ns;
1 ns;
1 ns;
1 ns;
1 ns;
1 ns;
1 ns;
1 ns;
1 ns;
1 ns;
1 ns;
CPU: Synthesis Description
311
The comparator consists of a large case statement where each branch
of the case statement contains an IF. If the condition tested is true, a ‘1’
value is assigned; otherwise, a ‘0’ is assigned. Again, each assignment
occurs after 1 nanosecond to remove delta delay problems.
Control
The control entity provides the necessary signal interactions to make the
data flow properly through the CPU and perform the expected functions.
Architecture rtl contains a state machine that causes all appropriate signal
values to update based on the current state and input signals and produce
a next state for the state machine. A symbol for the control block is shown
in Figure 13-6.
The control symbol has only a few inputs, but a lot of outputs. The
control block provides all of the control signals to regulate data traffic for
the CPU. Following is the VHDL description for the CPU:
library IEEE;
use IEEE.std_logic_1164.all;
use work.cpu_lib.all;
entity control is
port( clock : in std_logic;
port( reset : in std_logic;
port( instrReg : in bit16;
port( compout : in std_logic;
port( ready : in std_logic;
port( progCntrWr : out std_logic;
port( progCntrRd : out std_logic;
port( addrRegWr : out std_logic;
port( addrRegRd : out std_logic;
port( outRegWr : out std_logic;
port( outRegRd : out std_logic;
port( shiftSel : out t_shift;
port( aluSel : out t_alu;
port( compSel : out t_comp;
port( opRegRd : out std_logic;
port( opRegWr : out std_logic;
port( instrWr : out std_logic;
port( regSel : out t_reg;
port( regRd : out std_logic;
port( regWr : out std_logic;
port( rw : out std_logic;
port( vma : out std_logic
port);
end control;
312
Figure 13-6
Control Symbol.
Chapter Thirteen
ProgCntrWr
Reset
ProgCntrRd
AddrRegWr
Clock
AddrRegRd
InstrReg
OutRegWr
OutRegRd
Compout
ShiftSel
AluSel
Ready
CompSel
Control
OpRegRd
OpRegWr
InstrWr
RegSel
RegRd
RegWr
Rw
Vma
architecture rtl of control is
signal current_state, next_state : state;
begin
nxtstateproc: process( current_state, instrReg, compout,
nxtstateproc: process( ready)
begin
progCntrWr <= ‘0’;
progCntrRd <= ‘0’;
addrRegWr <= ‘0’;
outRegWr <= ‘0’;
outRegRd <= ‘0’;
shiftSel <= shftpass;
aluSel <= alupass;
compSel <= eq;
opRegRd <= ‘0’;
opRegWr <= ‘0’;
instrWr <= ‘0’;
regSel <= “000”;
regRd <= ‘0’;
regWr <= ‘0’;
rw <= ‘0’;
CPU: Synthesis Description
313
vma <= ‘0’;
case current_state is
when reset1 =>
aluSel <= zero after 1 ns;
shiftSel <= shftpass;
next_state <= reset2;
when reset2 =>
aluSel <= zero;
shiftSel <= shftpass;
outRegWr <= ‘1’;
next_state <= reset3;
when reset3 =>
outRegRd <= ‘1’;
next_state <= reset4;
when reset4 =>
outRegRd <= ‘1’;
progCntrWr <= ‘1’;
addrRegWr <= ‘1’;
next_state <= reset5;
when reset5 =>
vma <= ‘1’;
rw <= ‘0’;
next_state <= reset6;
when reset6 =>
vma <= ‘1’;
rw <= ‘0’;
if ready = ‘1’ then
instrWr <= ‘1’;
next_state <= execute;
else
next_state <= reset6;
end if;
when execute =>
case instrReg(15 downto 11) is
when “00000” =>
next_state <= incPc;
--- nop
when “00001” =>
--- load
regSel <= instrReg(5 downto 3);
regRd <= ‘1’;
next_state <= load2;
when “00010” =>
--- store
regSel <= instrReg(2 downto 0);
regRd <= ‘1’;
314
Chapter Thirteen
next_state <= store2;
when “00011” =>
----- move
regSel <= instrReg(5 downto 3);
regRd <= ‘1’;
aluSel <= alupass;
shiftSel <= shftpass;
next_state <= move2;
when “00100” =>
progcntrRd <= ‘1’;
alusel <= inc;
shiftsel <= shftpass;
next_state <= loadI2;
---- loadI
when “00101” =>
progcntrRd <= ‘1’;
alusel <= inc;
shiftsel <= shftpass;
next_state <= braI2;
---- BranchImm
when “00110” =>
---- BranchGTImm
regSel <= instrReg(5 downto 3);
regRd <= ‘1’;
next_state <= bgtI2;
when “00111” =>
------- inc
regSel <= instrReg(2 downto 0);
regRd <= ‘1’;
alusel <= inc;
shiftsel <= shftpass;
next_state <= inc2;
when others =>
next_state <= incPc;
end case;
when load2 =>
regSel <= instrReg(5 downto 3);
regRd <= ‘1’;
addrregWr <= ‘1’;
next_state <= load3;
when load3 =>
vma <= ‘1’;
rw <= ‘0’;
next_state <= load4;
when load4 =>
vma <= ‘1’;
rw <= ‘0’;
regSel <= instrReg(2 downto 0);
regWr <= ‘1’;
CPU: Synthesis Description
315
next_state <= incPc;
when store2 =>
regSel <= instrReg(2 downto 0);
regRd <= ‘1’;
addrregWr <= ‘1’;
next_state <= store3;
when store3 =>
regSel <= instrReg(5 downto 3);
regRd <= ‘1’;
next_state <= store4;
when store4 =>
regSel <= instrReg(5 downto 3);
regRd <= ‘1’;
vma <= ‘1’;
rw <= ‘1’;
next_state <= incPc;
when move2 =>
regSel <= instrReg(5 downto 3);
regRd <= ‘1’;
aluSel <= alupass;
shiftsel <= shftpass;
outRegWr <= ‘1’;
next_state <= move3;
when move3 =>
outRegRd <= ‘1’;
next_state <= move4;
when move4 =>
outRegRd <= ‘1’;
regSel <= instrReg(2 downto 0);
regWr <= ‘1’;
next_state <= incPc;
when loadI2 =>
progcntrRd <= ‘1’;
alusel <= inc;
shiftsel <= shftpass;
outregWr <= ‘1’;
next_state <= loadI3;
when loadI3 =>
outregRd <= ‘1’;
next_state <= loadI4;
when loadI4 =>
outregRd <= ‘1’;
progcntrWr <= ‘1’;
addrregWr <= ‘1’;
316
Chapter Thirteen
next_state <= loadI5;
when loadI5 =>
vma <= ‘1’;
rw <= ‘0’;
next_state <= loadI6;
when loadI6 =>
vma <= ‘1’;
rw <= ‘0’;
if ready = ‘1’ then
regSel <= instrReg(2 downto 0);
regWr <= ‘1’;
next_state <= incPc;
else
next_state <= loadI6;
end if;
when braI2 =>
progcntrRd <= ‘1’;
alusel <= inc;
shiftsel <= shftpass;
outregWr <= ‘1’;
next_state <= braI3;
when braI3 =>
outregRd <= ‘1’;
next_state <= braI4;
when braI4 =>
outregRd <= ‘1’;
progcntrWr <= ‘1’;
addrregWr <= ‘1’;
next_state <= braI5;
when braI5 =>
vma <= ‘1’;
rw <= ‘0’;
next_state <= braI6;
when braI6 =>
vma <= ‘1’;
rw <= ‘0’;
if ready = ‘1’ then
progcntrWr <= ‘1’;
next_state <= loadPc;
else
next_state <= braI6;
end if;
when bgtI2 =>
regSel <= instrReg(5 downto 3);
regRd <= ‘1’;
opRegWr <= ‘1’;
CPU: Synthesis Description
317
next_state <= bgtI3;
when bgtI3 =>
opRegRd <= ‘1’;
regSel <= instrReg(2 downto 0);
regRd <= ‘1’;
compsel <= gt;
next_state <= bgtI4;
when bgtI4 =>
opRegRd <= ‘1’ after 1 ns;
regSel <= instrReg(2 downto 0);
regRd <= ‘1’;
compsel <= gt;
if compout = ‘1’ then
next_state <= bgtI5;
else
next_state <= incPc;
end if;
when bgtI5 =>
progcntrRd <= ‘1’;
alusel <= inc;
shiftSel <= shftpass;
next_state <= bgtI6;
when bgtI6 =>
progcntrRd <= ‘1’;
alusel <= inc;
shiftsel <= shftpass;
outregWr <= ‘1’;
next_state <= bgtI7;
when bgtI7 =>
outregRd <= ‘1’;
next_state <= bgtI8;
when bgtI8 =>
outregRd <= ‘1’;
progcntrWr <= ‘1’;
addrregWr <= ‘1’;
next_state <= bgtI9;
when bgtI9 =>
vma <= ‘1’;
rw <= ‘0’;
next_state <= bgtI10;
when bgtI10 =>
vma <= ‘1’;
rw <= ‘0’;
if ready = ‘1’ then
progcntrWr <= ‘1’;
318
Chapter Thirteen
next_state <= loadPc;
else
next_state <= bgtI10;
end if;
when inc2 =>
regSel <= instrReg(2 downto 0);
regRd <= ‘1’;
alusel <= inc;
shiftsel <= shftpass;
outregWr <= ‘1’;
next_state <= inc3;
when inc3 =>
outregRd <= ‘1’;
next_state <= inc4;
when inc4 =>
outregRd <= ‘1’;
regsel <= instrReg(2 downto 0);
regWr <= ‘1’;
next_state <= incPc;
when loadPc =>
progcntrRd <= ‘1’;
next_state <= loadPc2;
when loadPc2 =>
progcntrRd <= ‘1’;
addrRegWr <= ‘1’;
next_state <= loadPc3;
when loadPc3 =>
vma <= ‘1’;
rw <= ‘0’;
next_state <= loadPc4;
when loadPc4 =>
vma <= ‘1’;
rw <= ‘0’;
if ready = ‘1’ then
instrWr <= ‘1’;
next_state <= execute;
else
next_state <= loadPc4;
end if;
when incPc =>
progcntrRd <= ‘1’;
alusel <= inc;
shiftsel <= shftpass;
next_state <= incPc2;
when incPc2 =>
CPU: Synthesis Description
319
progcntrRd <= ‘1’;
alusel <= inc;
shiftsel <= shftpass;
outregWr <= ‘1’;
next_state <= incPc3;
when incPc3 =>
outregRd <= ‘1’;
next_state <= incPc4;
when incPc4 =>
outregRd <= ‘1’;
progcntrWr <= ‘1’;
addrregWr <= ‘1’;
next_state <= incPc5;
when incPc5 =>
vma <= ‘1’;
rw <= ‘0’;
next_state <= incPc6;
when incPc6 =>
vma <= ‘1’;
rw <= ‘0’;
if ready = ‘1’ then
instrWr <= ‘1’;
next_state <= execute;
else
next_state <= incPc6;
end if;
when others =>
next_state <= incPc;
end case;
end process;
controlffProc: process(clock, reset)
begin
if reset = ‘1’ then
current_state <= reset1 after 1 ns;
elsif clock’event and clock = ‘1’ then
current_state <= next_state after 1 ns;
end if;
end process;
end rtl;
Architecture rtl contains two processes. The first is a combinational
process (not clocked) that examines the current state and all inputs and
produces output control values and next state output. The second is a
sequential process (has a clock) that is used to store the current state and
320
Chapter Thirteen
copy the next state to the current state. The next state transitions occur
on rising edges of the clock input. The control block is a very large state
machine that contains a number of states for each instruction. Executing
all of the states for an instruction performs the necessary steps to complete the instruction.
If the reset signal is high, the sequential process labeled controlffproc
sets signal current_state to state value reset1. This is the first state of
the reset sequence for the CPU. This state starts the process of getting
the CPU ready to execute instructions.
If the reset signal is not ‘1’ and there is a rising edge on the clock
signal, then the controlffproc process copies the next_state signal
generated by the combinational process to signal current_state. This is
the method for the state machine to advance from one state to another.
After the reset signal is set to a value other than ‘1’, the state machine
is in state reset1. This state causes the alu entity to output the value 0,
the shift entity to pass the value with no modification, and the next state
signal to be updated with the value reset2. This can be seen in the VHDL
description for entity control in the case statement starting at the when
clause for state reset1. At the next clock edge, the state machine advances
to state reset2. State reset2 leaves the control signals for the alu and
shift entities as before, but also sets the OutRegWr signal to a ‘1’,
causing the 0 value on the data bus to be written to register OutReg. The
goal of the reset sequence is to set up the program counter to start reading
instructions from memory.
After state reset2, the state machine next goes to state reset3 on
the next clock edge. This state sets signal OutRegRd to a ‘1’, causing
entity OutReg to output its value to the data bus. The state machine
then advances to state reset4. During reset4, the value from OutReg
is copied into register ProgCntr and also to register AddrReg. The state
machine advances to state reset5, sets output signal RW (read write) to
‘0’ (read mode), and signals VMA (Valid Memory Address) to a ‘1’.
This causes memory entity mem to output the data at location 0 to
the data bus. The state machine advances to state reset6 and, depending
on the value of the ready signal from the memory, either stays in
reset6 or writes the memory data value to register InstrReg and goes
to state execute.
At this point, the state machine has reset the state of the CPU to a
known state and loaded the first instruction into register InstrReg. From
this point forward, the state machine changes state depending on the instructions encountered.
CPU: Synthesis Description
321
The reset of the description for the state machine contains the state
transitions for the rest of the instructions that have been implemented.
As mentioned previously, not all of the instructions have been implemented and are left as an exercise for the reader.
Reg
The reg entity is used for the address register and the instruction register.
These registers need to be able to capture the input data on a rising edge
of the clk input and drive output q with the captured data. The value of
input a is assigned to output q when a rising edge occurs on input clk.
The assignment is delayed by 1 nanosecond to remove delta delay problems
during simulation. A symbol for the reg entity is shown in Figure 13-7.
The reg symbol contains three ports. Port a is the data input port, port
q is the data output port, and port clk controls when the data is stored
in the reg entity. Following is the VHDL description for entity reg:
library IEEE;
use IEEE.std_logic_1164.all;
use work.cpu_lib.all;
entity reg is
port( a : in bit16;
port( clk : in std_logic;
port( q : out bit16);
end reg;
architecture rtl of reg is
begin
regproc: process
begin
wait until clk’event and clk = ‘1’;
q <= a after 1 ns;
end process;
end rtl;
Figure 13-7
Reg Symbol.
a
clk
Reg
q
322
Chapter Thirteen
Process regproc is triggered when a rising edge occurs on input clk.
When the process is triggered, input a is copied to output q.
Regarray
The regarray entity is used to model the set of registers within the CPU
that are used to store intermediate values during instruction processing.
These registers are read from and written to during the execution of
instructions. The set of registers is modeled as a RAM of eight 16-bit
words. The symbol for the regarray entity is shown in Figure 13-8.
To write a location in the regarray, set input sel to the location to be
written, input data with the data to be written, and put a rising edge on
input clk. To read a location from regarray, set input sel to the location
to be read and set input en to a ‘1’; the data is output on port q.
The register array is modeled as two separate processes as shown in
the following:
library IEEE;
use IEEE.std_logic_1164.all;
use IEEE.std_logic_unsigned.all;
use work.cpu_lib.all;
entity regarray is
port( data : in bit16;
port( sel : in t_reg;
port( en : in std_logic;
port( clk : in std_logic;
port( q : out bit16);
end regarray;
Figure 13-8
RegArray Symbol.
data
sel
en
RegArray
clk
q
CPU: Synthesis Description
323
architecture rtl of regarray is
type t_ram is array (0 to 7) of bit16;
signal temp_data : bit16;
begin
process(clk,sel)
variable ramdata : t_ram;
begin
if clk’event and clk = ‘1’ then
ramdata(conv_integer(sel)) := data;
end if;
temp_data <= ramdata(conv_integer(sel)) after 1 ns;
end process;
process(en, temp_data)
begin
if en = ‘1’ then
q <= temp_data after 1 ns;
else
q <= “ZZZZZZZZZZZZZZZZ” after 1 ns;
end if;
end process;
end rtl;
The first process models the part of the RAM that stores the data. This
process contains a local variable ramdata that is used to store the data
written to the regarray entity. When the clk signal has a rising edge, the
location selected by input sel is updated with the new value. This process
also writes the location to a signal called temp_data to pass the value to
the second process. The reason for this is that this model was written
using VHDL 87, and variables cannot be shared between processes. In
VHDL 93, sharing variables between processes is legal but has other synthesis ramifications.
The second process is used to read data from the regarray. Whenever
input sel changes, the first process updates the value of temp_data. Signal temp_data is passed to the second process to pass the memory data.
The second process outputs the value of temp_data if the en signal is ‘1’;
otherwise, it puts out Z values. The Z values signify that the regarray
entity is not driving the output when the en input is unasserted.
A smart synthesis tool reading this design can realize that the regarray
entity can be implemented by a RAM device in the target technology and
provide the proper mapping. For instance, if the design were to be mapped
to an FPGA technology that included RAM in the architecture, the synthesis tool could map the regarray entity to an onboard RAM device. Using such an implementation instead of a set of flip-flops and gates creates
a smaller and faster implementation.
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Chapter Thirteen
Shift
The next device to be described is the shift entity. The shift entity is
used to perform shifting and rotation operations within the CPU. The
shift entity has a 16-bit input bus, a 16-bit output bus, and a sel input
that determines which shift operation to perform. This is shown by the
symbol in Figure 13-9.
The types of shift operations that can be performed by the shift entity
are shown in Figures 13-10 and 13-11.
As can be seen by the figures, the shift entity can perform a shift left,
shift right, rotate left, and rotate right operation. One operation that is
not shown by the figures is a pass through operation in which all input
bits are passed through to the output unchanged. Following is an entity
that performs these operations:
library IEEE;
use IEEE.std_logic_1164.all;
use work.cpu_lib.all;
entity shift
port ( a :
port( sel
port( y :
end shift;
is
in bit16;
: in t_shift;
out bit16);
architecture rtl of shift is
begin
shftproc: process(a, sel)
begin
case sel is
when shftpass =>
y <= a after 1 ns;
when shl =>
y <= a(14 downto 0) & ‘0’ after 1 ns;
when shr =>
y <= ‘0’ & a(15 downto 1) after 1 ns;
a
Figure 13-9
Shift Symbol.
sel
Shift
y
CPU: Synthesis Description
Figure 13-10
Shift Operations.
0
325
0
0
0
0
0
0
0
0
0
0
1
0
1
0
1
0
1
0
1
0
1
0
0
0
1
0
1
0
1
0
0
1
0
1
0
•••••••••
0
0
0
0
0
0
0
0
0
'0'
Shift Left
0
0
0
0
0
0
0
0
0
•••••••••
'0'
0
0
0
0
0
0
0
0
0
0
Shift Right
when rotl =>
y <= a(14 downto 0) & a(15) after 1 ns;
when rotr =>
y <= a(0) & a(15 downto 1) after 1 ns;
end case;
end process;
end rtl;
The shftpass mode allows the shifter to pass the input data to the
output without any shift operations. This mode is quite common because
all of the ALU operations flow through the shift entity, and very few
instructions are actually performing a shift operation.
326
Figure 13-11
Rotate Operations.
Chapter Thirteen
0
0
0
0
0
0
0
0
0
0
0
1
0
1
0
1
1
0
1
0
1
0
0
1
0
1
0
1
0
0
1
0
1
0
•••••••••
0
0
0
0
0
0
0
0
0
0
Rotate Left
0
0
0
0
0
0
0
0
0
0
•••••••••
1
0
0
0
0
0
0
0
0
0
Rotate Right
The shl and shr selections perform shift left and shift right operations,
respectively. The rotl and rotr selections perform rotate left and rotate
right operations, respectively.
Trireg
The last component of the CPU is the tristate register component, trireg.
The tristate register is connected to the main data bus and can store
information from the data bus as well as drive information to the data
bus. The trireg entity has four ports as shown in Figure 13-12.
Input a is the data input to the register, and port q is the data output
from the register. Input clk is used to store a new value into the register.
CPU: Synthesis Description
Figure 13-12
Trireg Symbol.
327
a
en
clk
Trireg
q
When a rising edge is applied to input clk, the data on input a is stored
in the register.
Input en is used to control output q. When en is a ‘1’ value, the register
state is driven to output q. When en is a ‘0’, output q is a high impedance
value and not driving. This functionality is implemented by entity trireg
shown in the following:
library IEEE;
use IEEE.std_logic_1164.all;
use work.cpu_lib.all;
entity trireg is
port( a : in bit16;
port( en : in std_logic;
port( clk : in std_logic;
port( q : out bit16);
end trireg;
architecture rtl of trireg is
signal val : bit16;
begin
triregdata: process
begin
wait until clk’event and clk = ‘1’;
val <= a;
end process;
trireg3st: process(en, val)
begin
if en = ‘1’ then
q <= val after 1 ns;
elsif en = ‘0’ then
q <= “ZZZZZZZZZZZZZZZZ” after 1 ns;
-- exemplar_translate_off
else
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Chapter Thirteen
q <= “XXXXXXXXXXXXXXXX” after 1 ns;
-- exemplar_translate_on
end if;
end process;
end rtl;
The functionality is described by two processes that use a signal to
communicate much like the regarray entity. The first process controls
when signal val is written. Signal val is written only on the rising edge
of input clk. The second process transfers the value of signal val only
when input en is a ‘1’ value; otherwise, a value of ‘Z’ is output.
SUMMARY
When all of these entities are connected together correctly, the functionality of the CPU results. The next two chapters focus on simulating the
design for proper operation and synthesizing the design to a target device.
CHAPTER
14
CPU:
RTL Simulation
In this chapter, a VHDL simulator is used to verify the
functionality of the CPU VHDL RTL description. The
VHDL RTL description of the CPU is simulated with a
standard VHDL simulator to verify that the description
is correct.
A simulator needs two inputs: the description of the
design and stimulus to drive the design. Sometimes designs
are self-stimulating and do not need any external stimulus,
but in most cases, VHDL designers use a VHDL testbench
of one kind or another to drive the design being tested.
The structure of the design looks like Figure 14-1.
The top-level design description instantiates two
components: the first being the design under test (DUT)
and the second the stimulus driver. These components are
connected with signals that represent the external environment of the DUT. The top level of the design does not
contain any external ports, just internal signals that connect the two instantiated components.
330
Chapter Fourteen
Top Level
Figure 14-1
Top-Level Design
Structure.
Stimulus
Driver
Design
Under
Test
Testbenches
A testbench is used to verify the functionality of a design. The testbench
allows the design to verify the functionality of the design at each step
in the HDL synthesis-based methodology. When the designer makes a
small change to fix an error, the change can be tested to make sure that
it did not affect other parts of the design. New versions of the design can
be verified against known good results to verify compatibility.
A testbench is at the highest level in the hierarchy of the design. The
testbench instantiates the design under test (DUT). The testbench provides
the necessary input stimulus to the DUT and examines the output from
the DUT. Figure 14-2 shows a block diagram of how this process appears.
The testbench encapsulates the stimulus driver, known good results,
and DUT, and contains internal signals to make the proper connections.
The stimulus driver drives inputs into the DUT. The DUT responds to the
input signals and produces output results. Finally, a compare function
within the testbench compares the results from the DUT against those
known good results and reports any discrepancies. That is the basic
function of a testbench, but there are a number of methods of writing a
testbench and each method has advantages and disadvantages.
CPU: RTL Simulation
331
Testbench
Figure 14-2
Testbench Block
Diagram.
Stimulus Driver
DUT
OK
Errors
Good Results
Kinds of Testbenches
There is a myriad of ways to write a testbench, but some of the most common are described in this section. The following are the most common
testbench types:
■ Stimulus only—Contains only the stimulus driver and DUT; does
not contain any results verification.
■ Full testbench—Contains stimulus driver, known good results, and
results comparison.
■ Simulator specific—Testbench is written in a simulator-specific
format.
■ Hybrid testbench—Combines techniques from more than one
testbench style.
■ Fast testbench—Testbench written to get ultimate speed from
simulation.
To show the different types of testbenches, a common example is used.
To make it simple to understand the stimulus and response, a counter
example is used. The following description is the package, entity, and architecture for an 8-bit counter:
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Chapter Fourteen
PACKAGE count_types IS
SUBTYPE bit8 is INTEGER RANGE 0 to 255;
END count_types;
LIBRARY IEEE;
USE IEEE.std_logic_1164.all;
USE work.count_types.all;
ENTITY count IS
PORT (clk
: IN std_logic;
ld
: IN std_logic;
up_dwn : IN std_logic;
clk_en : IN std_logic;
din : IN bit8;
qout : INOUT bit8);
END count;
ARCHITECTURE synthesis OF count IS
SIGNAL count_val : bit8;
BEGIN
PROCESS(ld, up_dwn, din, qout)
BEGIN
IF ld = ‘1’ THEN
count_val <= din;
ELSIF up_dwn = ‘1’ THEN
IF (qout >= 255) THEN
count_val <= 0;
ELSE
count_val <= count_val + 1;
END IF;
ELSE
IF (qout <= 0) THEN
count_val <= 255;
ELSE
count_val <= count_val - 1;
END IF;
END IF;
END PROCESS;
PROCESS
BEGIN
WAIT UNTIL clk’EVENT AND clk = ‘1’;
IF clk_en = ‘1’ THEN
qout <= count_val;
END IF;
END PROCESS;
END synthesis;
Package count_types contains the type declaration for the 8-bit signal
type used in the counter. The counter is loadable, counts up and down, and
contains a clock enable. The counter is implemented as two processes: a
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333
combinational process and a sequential process. The combinational
process calculates the next state of the counter, and the sequential process
keeps track of the current state of the counter and updates the next state
of the counter on a rising edge of the clk input. We use the counter to discuss a number of different types of testbenches.
Stimulus Only
The stimulus only testbench contains the stimulus driver and DUT blocks
of a testbench. The verification process is left to the designer. This type of
testbench is useful at the beginning of a design project when no known
good vectors exist, or for a quick check of an entity.
Following is an example stimulus only testbench:
ENTITY testbench IS END;
------------------------------------------------------------ STIMULUS ONLY
-- testbench for 8-bit loadable counter
-- reads from file “counter.txt”
----------------------------------------------------------LIBRARY ieee;
USE ieee.std_logic_1164.ALL;
USE std.textio.ALL;
USE ieee.std_logic_textio.all;
USE WORK.count_types.all;
ARCHITECTURE stimonly OF testbench IS
------------------------------------ component declaration for counter
----------------------------------COMPONENT count
PORT (clk
: IN std_logic;
ld
: IN std_logic;
up_dwn : IN std_logic;
clk_en : IN std_logic;
din : IN bit8;
qout : INOUT bit8);
END COMPONENT;
SIGNAL clk, ld, up_dwn, clk_en : std_logic;
SIGNAL qout, din : bit8;
BEGIN
-- instantiate the component
uut: count PORT MAP(clk => clk,
ld => ld,
up_dwn => up_dwn,
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Chapter Fourteen
clk_en => clk_en,
din => din,
qout => qout);
-- provide stimulus and check the result
test: PROCESS
VARIABLE tmpclk, tmpld, tmpup_dwn, tmpclk_en :
std_logic;
VARIABLE tmpdin : integer;
FILE vector_file : text IS IN “counter.txt”;
VARIABLE l : line;
VARIABLE vector_time : time;
VARIABLE r : real;
VARIABLE good_number, good_val : boolean;
VARIABLE space : character;
BEGIN
WHILE NOT endfile(vector_file) LOOP
readline(vector_file, l);
-- read the time from the beginning of the line
-- skip the line if it doesn’t start with a number
read(l, r, good => good_number);
NEXT WHEN NOT good_number;
vector_time := r * 1 ns;
IF (now < vector_time) THEN
-- convert real
number to time
-- wait until the
vector time
WAIT FOR vector_time - now;
END IF;
read(l, space);
--- skip a space
-- read clk value
read(l, tmpclk, good_val);
assert good_val REPORT “bad clk value “;
-- read ld value
read(l,tmpld, good_val);
assert good_val REPORT “bad ld value “;
-- read up_dwn value
read(l,tmpup_dwn, good_val);
assert good_val REPORT “bad up_dwn value “;
-- read clk_en value
read(l,tmpclk_en, good_val);
assert good_val REPORT “bad clk_en value “;
read(l, space);
--- skip a space
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335
-- read din value
read(l, tmpdin, good_val);
assert good_val REPORT “bad din value “;
clk <= tmpclk;
ld <= tmpld;
up_dwn <= tmpup_dwn;
clk_en <= tmpclk_en;
din <= tmpdin;
END LOOP;
ASSERT false REPORT “Test complete”;
WAIT;
END PROCESS;
END;
The beginning of the testbench declares entity testbench as an entity
with no ports. This is completely legal as the testbench is the topmost entity and does not interract with any other entities.
Next is the architecture declaration. The architecture uses a number
of packages including IEEE standard packages and counter. The next
section in the model declares the component for the DUT (Device Under
Test), the counter. The ports and types on this component should match
the DUT. Next, the local interconnect signals are declared. After the architecture declaration section, the DUT component is instantiated and connected to the local interconnect signals.
A process called test is declared which contains the stimulus generation
capability. First, a number of local variables are declared to receive data
from the TextIO procedures used to read the stimulus information from
a file. TextIO can only assign to variable objects not signals; therefore, local
variables are assigned by the TextIO procedures, and these variables are
assigned to the internal interconnect signals.
Inside the process is a single while loop that reads data from the
stimulus file until an end-of-file condition is reached. Each pass through
the loop reads another line from the file and reads the appropriate data
from that line.
The first data read from the line is the time that this vector is to be
applied. The process checks to make sure that the value read is a valid
number. If not, the line is discarded because it does not represent a
valid stimulus line. This allows comment lines to be inserted in the vector
files. If a valid number was not read, the process skips this iteration
through the loop and goes to the next iteration using the next clause.
If the value read was a good number, then the vector is assumed to be
valid. The process reads each data value from the vector and applies the
values to the locally declared variables.
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Chapter Fourteen
In the counter example, the first value read is the clk signal. The TextIO statement reads a STD_LOGIC value from line l and assigns the value
read to variable tmpclk. Later, the tmpclk variable is assigned to the
signal clk.
The process continues to read a line, read a time value, wait until that
time value occurs, read all vector values, and apply vector values until the
end of the file is reached. When the end of the file is reached, the loop
terminates, an assertion message is written to standard output, and the
process waits forever. The WAIT statement after the assertion at the end
of the loop doesn’t have a termination condition and, therefore, waits
forever, effectively stopping execution of this process.
The TEXTIO readline statement inside the while loop reads a vector line
from a vector file. Following is an example vector file:
--- vector file for counter
-- time clk ld up_dwn clk_en din
10 0001 0
20 1101 50
30 0001 0
40 1001 0
50 0001 0
60 1001 0
70 0001 0
80 1001 0
90 0001 0
100 1101 10
110 0001 0
120 1001 0
130 0001 0
140 1001 0
150 0001 0
160 1001 0
The first two lines of the vector file do not start with valid numbers and
are treated as comment lines. Comment lines can be embedded anywhere
in the file. Comments can also be placed at the end of a vector because
any data after the last field of the vector are ignored.
Each vector line starts with a time value and then contains a string of
values to be assigned to the DUT at that time. Spaces can be embedded
between vector values if a corresponding read function exists in the while
loop to skip the space.
For the stimulus only testbench, the test process reads a vector from
the file and applies the stimulus to the DUT. The stimulus only testbench
does not check the output results of the DUT in reaction to the applied
stimulus. The stimulus only testbench is most useful for a quick check of
a piece of a design that is easy for the designer to verify manually or for
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337
early in the design process when no known good results exist to verify
against. When the results are verified, these results become the known
good results to verify future versions or minor changes to the design.
Full Testbench
A full testbench is very similar to a stimulus only testbench except that the
full testbench also includes the capability to check the output of the DUT.
The full testbench applies the stimulus to the design and then examines the
outputs of the design to see if the output results of the DUT match known
good results.
Following is a full testbench for the counter:
ENTITY testbench IS END;
------------------------------------------------------------- FULL TESTBENCH
-- testbench for counter
-- reads from file “counter.txt”
-----------------------------------------------------------LIBRARY ieee;
USE ieee.std_logic_1164.ALL;
USE std.textio.ALL;
USE ieee.std_logic_textio.all;
USE WORK.count_types.all;
ARCHITECTURE full OF testbench IS
------------------------------------ component declaration for counter
----------------------------------COMPONENT count
PORT (clk
: IN std_logic;
ld
: IN std_logic;
up_dwn : IN std_logic;
clk_en : IN std_logic;
din : IN bit8;
qout : INOUT bit8);
END COMPONENT;
SIGNAL clk, ld, up_dwn, clk_en : std_logic;
SIGNAL qout, din : bit8;
BEGIN
-- instantiate the component
uut: count
PORT MAP(clk => clk,
ld => ld,
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Chapter Fourteen
up_dwn => up_dwn,
clk_en => clk_en,
din => din,
qout => qout);
-- provide stimulus and check the result
test: PROCESS
VARIABLE tmpclk, tmpld, tmpup_dwn, tmpclk_en :
std_logic;
VARIABLE tmpqout, tmpdin : bit8;
FILE vector_file : text IS IN “counter.txt”;
VARIABLE l : line;
VARIABLE vector_time : time;
VARIABLE r : real;
VARIABLE good_number, good_val : boolean;
VARIABLE space : character;
BEGIN
WHILE NOT endfile(vector_file) LOOP
readline(vector_file, l);
-- read the time from the beginning of the line
-- skip the line if it doesn’t start with a number
read(l, r, good => good_number);
NEXT WHEN NOT good_number;
vector_time := r * 1 ns;
IF (now < vector_time) THEN
-- convert real
number to time
-- wait until the
vector time
WAIT FOR vector_time - now;
END IF;
read(l, space);
--- skip a space
-- read clk value
read(l, tmpclk, good_val);
assert good_val REPORT “bad clk value”;
-- read ld value
read(l, tmpld, good_val);
assert good_val REPORT “bad ld value”;
-- read up_dwn value
read(l, tmpup_dwn, good_val);
assert good_val REPORT “bad up_dwn value”;
-- read clk_en value
read(l, tmpclk_en, good_val);
assert good_val REPORT “bad clk_en value”;
CPU: RTL Simulation
339
read(l, space);
--- skip a space
-- read din value
read(l, tmpdin, good_val);
assert good_val REPORT “bad din value”;
read(l, space);
--- skip a space
---- the difference in the file is below
-- read good output value
read(l, tmpqout, good_val);
assert good_val REPORT “bad qout value”;
-- Compare outputs
assert tmpqout = qout REPORT “vector mismatch”;
clk <= tmpclk;
ld <= tmpld;
up_dwn <= tmpup_dwn;
clk_en <= tmpclk_en;
din <= tmpdin;
END LOOP;
ASSERT false REPORT “Test complete”;
WAIT;
END PROCESS;
END full;
The full testbench looks exactly the same as the stimulus only testbench for most of the file. The full testbench has a top-level entity with
no ports, an architecture that instantiates the DUT, and a while loop that
reads a vector file. The differences are in the while loop itself. The first
part of the while loop is exactly the same. The process reads a time value
and waits for that time value to occur. The full testbench is different in that,
not only does the full testbench read the input values, but it also reads the
output values and then performs a compare operation between the output
values from the DUT versus the values read from the file. If a mismatch
is found, an assertion message is generated to let the designer know that
the output results did not match the known good results.
The full testbench also reads from a vector file to get the stimulus for
the design and the expected results. The vector file contains a time value,
the input values, and the expected output values. Following is the full
testbench vector file:
--- vector file for counter
-- time clk ld up_dwn clk_en din dout
0 0001 0 0
10 1001 0 255
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Chapter Fourteen
20 0101 10 255
30 1001 0 10
40 0001 0 10
50 1001 0 8
60 0001 0 8
70 1001 0 7
80 0001 0 7
90 1001 0 6
100 0101 100 100
110 1001 0 100
120 0001 0 100
130 1001 0 98
140 0001 0 98
150 1001 0 97
160 0001 0 97
Notice that the vector file looks nearly the same as the stimulus only
vector file except for the extra columns for the expected results.
The full testbench can be used to verify that a DUT matches a specification. To do so, the specification must include a set of known good results
that the testbench can match against.
The full testbench can also be used to verify that a small change or
optimization still matches the known good results. A designer may find a
small error during verification that only requires a small localized
change to the design. The designer can make the change and rerun the
testbench to make sure that the change did not affect the rest of the design,
and that the design still functions properly.
Testbenches can also be used to sign off designs. After the design
matches the testbench results, the design is ready to be put into production,
or be signed off.
The stimulus only and full testbench are only a couple examples of
the many ways that a testbench can be written. Another example is the
simulator-specific testbench.
Simulator Specific
The simulator-specific testbench is written specifically for one brand of
simulator. Most simulators include a command language that allows the
designer to control the simulator. The designer can compile designs, load
designs, create libraries, set breakpoints, run the simulation, and lots of
other tasks using the simulator command language. Most of these simulators also allow the designer to set signals to new values. Using command languages, the designer can write a testbench. Following is an example of a simulator-specific testbench:
CPU: RTL Simulation
341
-- setup the clock
force -repeat 20 clk 0 0, 1 10
-- log the results to a file
list *
-- setup initial signal conditions
force ld 0
force up_dwn 0
force clk_en 1
force din 16#00
-- run the simulation
run 100
--- set next signal conditions
force ld 1
force up_dwn 0
force clk_en 1
force din 16#AA
--- run the simulation
run 200
--- set next signal conditions
force ld 1
force up_dwn 0
force clk_en 1
force din 16#55
--- run the simulation
run 200
write list data.out
quit -f
The command language used for this testbench is the Model Technology
ModelSim command language. This simulator has a very rich command
language that allows the designer to perform all of the necessary operations
to compile designs, load designs, debug designs, save designs, and so on. The
ModelSim simulator also has the capability to generate repeating clock signals to drive the design. The first command in the testbench file creates a
repeating clock for signal clk. The clock repeats every 20 time units and is
set to a ‘0’ value at time 0 and a ‘1’ value at time 10.
The next command (list *) allows the designer to write all the signal
values to an output file. The * specifies that all signals be written to
the file.
The next few commands in the file set up stimulus values on the counter
input signals. The force command sets the signal to a value until it is
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Chapter Fourteen
changed by another force command. The input signals are all set to an
initial value and the run command advances simulation time and runs
the simulation. All of the input values are propagated appropriately
through the design.
After the run command has finished, the new input stimulus values are
set up with more force commands, and the simulation is run again. This
process continues until all stimulus has been run through the design. The
write command near the end of the file writes the results of the simulation
to a file. The designer can analyze the output file to determine if the design
is correct or use a file compare facility to automatically compare the DUT
results to known good results.
The advantages of a simulator-specific testbench are that it is fairly
quick and easy to generate, and it can be loaded and reloaded into the
simulator without shutting the simulator down and starting over every
time. A simulation can be run, the results analyzed, simulation time reset
to 0, a stimulus file loaded, and the simulator run again.
The disadvantage of the simulator-specific testbench is that the testbench is specific to one simulator and cannot be easily migrated. If the
design is to be passed to another design group using another simulator,
the testbenches need to be rewritten in the new command language.
Hybrid Testbenches
Hybrid testbenches do not utilize only one technique, but a combination
of a number of techniques. Hybrid testbenches can use a full testbench
approach but have some of the stimulus data generated in the testbench rather than read from a file. Hybrid testbenches can also mix
simulator-specific commands with stimulus read from a file.
Following is a sample hybrid testbench:
ENTITY testbench IS END;
------------------------------------------------------------- HYBRID Testbench
-- testbench for 8-bit loadable updown counter
-- reads from file “counter.txt”
-----------------------------------------------------------LIBRARY ieee;
USE ieee.std_logic_1164.ALL;
USE std.textio.ALL;
USE ieee.std_logic_textio.all;
CPU: RTL Simulation
343
USE WORK.count_types.all;
ARCHITECTURE hybrid OF testbench IS
------------------------------------ component declaration for counter
----------------------------------COMPONENT count
PORT (clk
: IN std_logic;
ld
: IN std_logic;
up_dwn : IN std_logic;
clk_en : IN std_logic;
din : IN bit8;
qout : INOUT bit8);
END COMPONENT;
SIGNAL ld, up_dwn, clk_en : std_logic;
SIGNAL clk : std_logic := ‘0’;
SIGNAL qout, din : bit8;
BEGIN
-- instantiate the component
uut: count
PORT MAP(clk => clk,
ld => ld,
up_dwn => up_dwn,
clk_en => clk_en,
din => din,
qout => qout);
-- Generate the system clock
clk <= not clk after 10 ns;
-- provide stimulus and check the result
test: PROCESS
VARIABLE tmpclk, tmpld, tmpup_dwn, tmpclk_en :
std_logic;
VARIABLE tmpqout, tmpdin : bit8;
FILE vector_file : text IS IN “counter.txt”;
VARIABLE l : line;
VARIABLE vector_time : time;
VARIABLE r : real;
VARIABLE good_number, good_val : boolean;
VARIABLE space : character;
BEGIN
WHILE NOT endfile(vector_file) LOOP
readline(vector_file, l);
-- read the time from the beginning of the line
-- skip the line if it doesn’t start with a number
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Chapter Fourteen
read(l, r, good => good_number);
NEXT WHEN NOT good_number;
vector_time := r * 1 ns;
IF (now < vector_time) THEN
-- convert real
number to time
-- wait until the
vector time
WAIT FOR vector_time - now;
END IF;
read(l, space);
--- skip a space
-- read ld value
read(l,tmpld, good_val);
assert good_val REPORT “bad ld value”;
-- read up_dwn value
read(l,tmpup_dwn, good_val);
assert good_val REPORT “bad up_dwn value”;
-- read clk_en value
read(l,tmpclk_en, good_val);
assert good_val REPORT “bad clk_en value”;
read(l, space);
--- skip a space
-- read din value
read(l, tmpdin, good_val);
assert good_val REPORT “bad din value”;
ld <= tmpld;
up_dwn <= tmpup_dwn;
clk_en <= tmpclk_en;
din <= tmpdin;
END LOOP;
ASSERT false REPORT “Test complete”;
WAIT;
END PROCESS;
END;
The hybrid testbench example looks very similar to the stimulus only
testbench example except that, right after the counter component instantiation, the system clock is generated by a signal assignment statement.
Signal clk is assigned the value of not clk after 10 nanoseconds. This
statement creates a periodic waveform with a period of 20 nanoseconds.
The testbench does not read signal clock from the vector file. The vector
file contains changes only on signals other than clock. This results in a
much smaller file that can be read much faster. Following is the hybrid
vector file:
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345
--- vector file for counter
-- time ld up_dwn clk_en din
10 001 0
20 101 50
30 001 0
100 101 0
110 001 0
250 101 35
260 001 0
If this example were a full testbench, the vector file would not be shorter
because a vector would be needed on each clock transition to specify the
output results for comparison.
The advantage of the hybrid testbench is that less data needs to be read
from a vector file. Stimulus data is instead provided by either simulator
command language commands or generated in the testbench.
The disadvantage of the hybrid testbench is that it is more difficult to
change data from run to run when the hybrid testbench generates the
stimulus in the testbench. In the case where simulator command language
commands are used to generate stimulus, the testbench is less portable.
Fast Testbench
All of the testbench styles discussed so far have one common trait: They can
become the limiting factor in how fast a simulation can run. This is especially
true of the testbenches that read data from vector files. These files can become
very large, and the time it takes to read a vector and process the vector can
be the limiting factor in how fast the simulator executes. The same can be true
of the simulator-specific testbench if the simulator does not read the entire
command file in at the start of simulation. If the file is read in chunks, the
file read operation can significantly slow the simulation.
To get around these problems, a designer can elect to use a fast testbench. The fast testbench is optimized for speed and typically does not
limit the speed of the simulation, unless the design is very small.
Following is an example fast testbench:
ENTITY testbench IS END;
------------------------------------------------------------- FAST Testbench
-- testbench for 8-bit loadable updown counter
-----------------------------------------------------------LIBRARY ieee;
USE ieee.std_logic_1164.ALL;
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Chapter Fourteen
USE WORK.count_types.all;
ARCHITECTURE fast OF testbench IS
------------------------------------ component declaration for counter
----------------------------------COMPONENT count
PORT (clk
: IN std_logic;
ld
: IN std_logic;
up_dwn : IN std_logic;
clk_en : IN std_logic;
din : IN bit8;
qout : INOUT bit8);
END COMPONENT;
SIGNAL clk, ld, up_dwn, clk_en : std_logic := ‘0’;
SIGNAL qout, din : bit8;
BEGIN
-- instantiate the component
uut: count
PORT MAP(clk => clk,
ld => ld,
up_dwn => up_dwn,
clk_en => clk_en,
din => din,
qout => qout);
-- generate the clock in the testbench
clk <= not clk after 10 ns;
-- provide stimulus and check the result
test: PROCESS
TYPE stim_vec is
RECORD
event_time : time;
ld : std_logic;
up_dwn : std_logic;
clk_en : std_logic;
din : bit8;
qout : bit8;
END RECORD;
TYPE vec_array is array(0 to 8) of stim_vec;
VARIABLE stim_array : vec_array := (
(0 ns,
‘0’, ‘0’, ‘1’, 10, 10),
(20 ns, ‘1’, ‘0’, ‘1’, 100, 2),
(30 ns, ‘0’, ‘0’, ‘1’, 0,
0),
(100 ns, ‘1’, ‘0’, ‘1’, 55, 8),
(110 ns, ‘0’, ‘0’, ‘1’, 0,
0),
(150 ns, ‘1’, ‘0’, ‘1’, 150, 58),
(160 ns, ‘0’, ‘0’, ‘1’, 0,
151),
CPU: RTL Simulation
347
(250 ns, ‘1’, ‘0’, ‘1’, 201, 160),
(260 ns, ‘0’, ‘0’, ‘1’, 0,
161));
VARIABLE ev_time : time;
BEGIN
FOR i in stim_array’RANGE LOOP
ev_time := stim_array(i).event_time;
IF (now < ev_time) THEN
-- wait until the
vector time
WAIT FOR ev_time - now;
END IF;
-- assign ld value
ld <= stim_array(i).ld;
-- assign up_dwn value
up_dwn <= stim_array(i).up_dwn;
-- assign clk_en value
clk_en <= stim_array(i).clk_en;
-- assign din value
din <= stim_array(i).din;
-- check qout value
assert qout = stim_array(i).qout REPORT “vector
mismatch”;
END LOOP;
ASSERT false REPORT “Test complete”;
WAIT;
END PROCESS;
END;
The fast testbench looks similar to the other testbench styles in that it
has a top-level entity that instantiates a DUT and a process that generates
the stimulus. What’s different is that, instead of reading the stimulus
vectors from a file, the vectors are compiled into the testbench model.
The testbench declares a record type that contains a field for each
input signal (and output signal, if a full testbench is being modeled).
Next, the model declares an array of the record type that contains the
vector values. A variable of the array type is declared and then initialized
with the vector values. A while loop reads each record of the array, waits
until the vector time is active, and applies the vector values to the design
inputs, similar to the way the file was read using TextIO. Notice that array
and record indexing is used to select each signal value.
348
Chapter Fourteen
The advantages of the fast testbench are that it executes extremely fast
and doesn’t suffer from the operating system file overhead of reading a file.
A disadvantage is that the compiled model can get very large if the
number of vectors is large, making compile time long and simulator
memory usage excessive. Another disadvantage of the fast testbench is
that the model is not easily changed between simulation runs. Changing
the testbench requires a recompilation step. Therefore, the fast testbench
is most useful for models that need fast vector application and the vectors
can be run in a small- or medium-sized loop where the vectors are applied
again and again.
The advantages and disadvantages of each kind of testbench type are
shown in Figure 14-3.
Notice that the stimulus only and full testbenches use TextIO. This can
limit their speed if the DUT requires a lot of vector input. However, the
advantages of using TextIO is the ease of changing the input data. No recompilation step is required to change the stimulus data. All that is required to make a change to the input stimulus is to change the input file
and restart the simulation.
The simulator-specific testbench is also very easy to change because
it is typically an interpreted command language. Interpreted command
languages don’t need a separate compile step. Updating the command language file and reloading it in the simulator is all that is required to make
a change. The price of this flexibility, however, may be slow execution
speed. An interpreted command language doesn’t need to be compiled, but
may not execute fast depending on how many vectors are needed how
quickly. A design that needs a lot of vectors very quickly may be limited
by the speed of the interpreter.
The fast testbench really excels at going fast, but is much more difficult to change quickly than some of the other testbench types. To make
a change, the vectors must be updated and the testbench recompiled. If
the vector file is large, this process can take an excessive amount of time.
Now that we have discussed testbenches, let’s use one to simulate the
CPU for correctness.
Figure 14-3
Testbench
Advantages and
Disadvantages.
Stimulus Only
Full
Simulator Specific
Hybrid
Fast
Speed
Slow
Slow
Medium
Medium
Extremely Fast
Flexibility
High
High
High
Medium
Low
Portability
High
High
Low
High
High
CPU: RTL Simulation
349
CPU Simulation
Simulating the CPU design is different from most other entities because
the CPU design doesn’t need much outside stimulus. The memory device
provides the input data for the CPU much as a stimulus file would for
other entities. The CPU reads its program from the memory device. The
CPU need only have the clk signal and reset signal stimulated properly,
and the CPU reads and executes instructions from that point forward.
The only stimulus needed to start the operation of the CPU is a uniform
signal applied to the clk input and a pulse applied to the reset input for
at least 2 clock cycles. This starts the CPU into the reset sequence. After
the reset sequence has been started, the CPU is initialized and starts
executing the CPU instructions from the mem entity.
The CPU is simulated as stimulus only initially to verify that the device
seems to be functioning. More complex testbenches need to be created that
include comparison against a known good result to verify correctness. The
simplest method for doing this is to manually verify the results the first
time, capture the output results, and then use them for comparison later.
The first step in simulating the CPU is to compile all the files that
make up the design into a format that the simulator can use. The compiled format is loaded into the simulator, and the simulation is executed.
The ModelSim simulator from Model Technology is used for the simulation process.
The first step in compiling all of the files in the design is to create one
or more libraries to store the compiled data. The default library to store
the compiled data is a library called work. The name work is the logical
name of the library; the physical location of the library can be anywhere.
To create a library, the VLIB command is used as shown here:
vlib work
This creates the work library in the current working directory of the
current disk. After the library has been created, the VHDL source files for
the design can be compiled into the target library. To compile each of the
files, the VCOM command must be run either from the GUI (Graphical User
Interface) or from the command line. Most of the operations of the simulator have a GUI method of performing the command line command. This
allows casual users as well as expert users to effectively use the simulator. Normally, casual users use the GUI and experts use the command line
and script interface.
350
Chapter Fourteen
To compile a file from the GUI, the file is selected in the compile dialog
box as shown in Figure 14-4.
The GUI includes a file browser that allows the designer to select the
files to compile and then click the Compile button to compile the file.
To compile a file from the command line interface, the following command
is issued:
vcom cpu_lib.vhd
This checks that the VHDL syntax is correct and converts the VHDL
syntax to the binary format needed to simulate the design. Following is a
complete script that compiles all of the files in the proper order:
vcom
vcom
vcom
vcom
vcom
vcom
Figure 14-4
Compile VHDL
Source Dialog Box.
cpu_lib.vhd
alu.vhd
comp.vhd
reg.vhd
shift.vhd
control.vhd
CPU: RTL Simulation
vcom
vcom
vcom
vcom
vcom
351
regarray.vhd
trireg.vhd
cpu.vhd
mem.vhd
top.vhd
After all of the files have been compiled, the design can be loaded into
the simulator for verification. This can be initiated from the GUI or from
the command line with the following command:
vsim -lib work top behave
This command specifies the library (work), entity (top), and architecture
(behave) or configuration to simulate. After the design has been loaded,
the simulator needs stimulus for the design and specification of what data
to monitor. For this simulation, the current_state, the memory interface,
program counter, and other signals are monitored. Figure 14-5 shows a
waveform display of the reset sequence of the CPU.
From this display, we can verify that the CPU is functioning properly.
At time 0, the reset signal is set to a ‘1’ value, which puts the CPU into
state reset1, the first state of the reset sequence. After the reset signal
is set to ‘0’, the CPU can begin performing the reset sequence. The two
most interesting signals to examine are current_state and next_state.
Notice that, while the reset input is a ‘1’, the CPU remains in state
reset1. After signal reset is set to a ‘0’, on the next rising edge of signal
clock, current_state advances to state reset2.
Each clock rising edge after that causes the CPU to advance to the next
state. At state reset3, the data bus receives the value 0000 to be used as
the starting address for the first instruction. At state reset4, register
Figure 14-5
Waveform Display of
the Reset Sequence.
352
Chapter Fourteen
addreg is loaded with the data bus value so that the 0000 value can be
used to drive the addr bus. At state reset5, the data bus is driven with
the instruction data from component mem at address 0. This data is then
loaded into register instrreg in reset6 so that the control entity can use
the instruction contents.
The next state after reset6 is the first execution step of the instruction
that was just fetched from the memory. Looking back at the description
of the mem entity, we can see that the first instruction loads register 1 with
the source address of the copy operation. Figure 14-6 shows the waveform
display after the reset sequence has completed and the first instruction
has started to execute.
This instruction is a LoadI (Load Immediate) instruction that uses two
words of the memory. The instruction is shown here:
LoadI 1, #
10
The first word of the instruction specifies the behavior of the instruction,
and the second specifies the data to be loaded into the register specified
by the instruction. This instruction first puts the program counter value
to the data bus so that the value can be incremented. The program
counter is then able to read the second word of the instruction that contains
the data to be loaded into reg 1.
During state execute, the program counter is incremented and the
incremented value can be found as the output of the ALU aluout. During
states loadi2, loadi3, and loadi4, this value is transferred to register
addreg and data is read from mem entity in state loadi5. During state
loadi6, data from memory is loaded into register 1.
Figure 14-6
Waveform Display
after the Reset
Sequence Has
Completed.
CPU: RTL Simulation
353
After the load instruction has executed all of the states, to complete the
load instruction, the CPU advances to a set of states that increments
the program counter register to point to the next instruction.
The CPU performs three load instructions to load the proper CPU
registers before the block copy can proceed. A final load instruction is
performed which loads the value to be copied into register 3. At this point,
the CPU program counter is pointing to address 7, a store instruction.
This instruction uses the address in reg 2 to store the value in reg 3 to
the new location. A waveform display showing the store instruction is
shown in Figure 14-7.
During state execute, the value of reg2 is read to the data bus where it
is copied to the address register in state store2. During store3, register
array (3) drives the data bus with the data to be stored. During state
store4, the value is written to the mem address.
After the store instruction is completed, the CPU checks to see if the
block copy operation has completed. This is accomplished by the instruction
at location 8, which branches back to instruction 00 if reg 1 is greater than
reg 6. This instruction execution is shown Figure 14-8.
The first step is to read the value of register 1. This value is stored to
register opreg during state bgti2. Next, the value of reg6 is read and a
comparison is performed. Notice that signal compout stays a ‘0’ value
because the greater than operation failed; therefore, the branch operation
is be performed.
This set of instructions is performed a number of times until the
source array is copied to the destination array. The source array is shown
in Figure 14-9.
The array starts at location 16 and continues to location 31. The pattern
stored in the source array is a very simple one that starts at 1 and ends
Figure 14-7
A Waveform Display
Showing the Store
Instruction.
354
Figure 14-8
Branch Instruction
execution.
Figure 14-9
The Source Array.
Chapter Fourteen
CPU: RTL Simulation
355
at 16. Figure 14-10 shows the destination array before the copy operation
has completed.
The destination array starts at location 48 and ends at location 63. The
destination array is shown after two copy operations have been performed.
Notice that location 48 has the first value, and location 49 has the second
value. A complete simulation run completely copies one array to another.
All of the examples that allow the reader to duplicate the simulation of
the CPU are found on the CD that comes with this book.
SUMMARY
In this chapter, we examined what was necessary to perform a functional
verification of the CPU design and walked through one loop of the block
copy operation CPU simulation. In the next chapter, we synthesize the
CPU description to a target FPGA device for implementation.
Figure 14-10
The Destination Array
Before the Copy
Operation Has
Completed.
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CHAPTER
15
CPU Design:
Synthesis Results
After the CPU has been functionally verified, the design
can be implemented in actual hardware. This chapter
describes the synthesis process and synthesis results of
the CPU RTL description. The VHDL design description
is optimized and mapped to a programmable logic device.
As opposed to an ASIC device, these devices can be programmed by designers at their desks, and most can be
reprogrammed to fix errors later.
A synthesis tool is used to read in the VHDL description
and map the description to the target programmable logic
device. The synthesis tool reads all the VHDL source files,
links them together (elaborate), optimizes the design, and
then maps the optimized description to the target technology. The synthesis tool used is the Leonardo Spectrum
synthesis tool from Exemplar Logic. This is a popular synthesis tool in the FPGA (Field Programmable Gate Array)
market and produces very good results quickly.
358
Chapter Fifteen
Figure 15-1
Leonardo Spectrum
User Interface.
The first step in the synthesis process is to read all the files of the
design into the synthesis tool. This can be accomplished either by using
the synthesis tool GUI (Graphical User Interface) or by issuing command
language commands. First time or casual users will probably use the GUI
because no command language syntax knowledge is required, and all
operations can be accomplished through menu clicks. Everyday users of
the tool quickly learn the command language of the synthesis tool, create
scripts that build up the design, and run those scripts to create the
design. This provides a repeatable method of creating the design.
The Leonardo Spectrum GUI is shown in Figure 15-1.
Leonardo Spectrum contains a Quick Setup guide that allows the
designer to easily specify the source files for the design, the target technology, the target device and speed grade, the clock frequency, and the output file. Once this information has been specified, the flow can be run to create the netlist for the target device. This example will use the Advanced
GUI because the hierarchy will be preserved to get a better idea of the
size of each design block.
The first step is to select the Technology tab. In this section the target technology will be set: the device package and the speed grade of the
target technology. In this example we chose Altera Apex 20KE technology
CPU Design: Synthesis Results
359
Figure 15-2
Leonardo Spectrum
Technology Tab.
as the target technology. Then a default package is chosen, and the actual
package can be selected from a list. We use the EP20K200EFC484 package
because this design will be targeted to that device. The speed grade determines how fast the device will operate. In this example we use the fastest
device, the 2X speed grade. These settings are shown in Figure 15-2. Finally the target technology is loaded into the synthesis tool by selecting
the Load Library button.
Our next step is to read the VHDL files into the synthesis tool by
selecting the Input tab. The result is shown in Figure 15-3.
We then select the File Folder button next to the Working Directory
text. This brings up the working directory selection box (see Figure 15-4).
We choose the working directory by navigating to the directory containing
the design source files.
The source files can now be read into Leonardo Spectrum using the SET
Input Files dialog box. We select the file folder button next to the Open
Files text (see Figure 15-3). This dialog box allows us to select one or
more files to be added to the list of files for the design. In Figure 15-5 all the
design files are selected except the testbench (mem2.vhd) and top level that
instantiates the testbench (top.vhd). Clicking the Open button will add all
the files to the design file list.
360
Figure 15-3
Select Input Files Dialog Box.
Figure 15-4
Set Working Directory.
Chapter Fifteen
CPU Design: Synthesis Results
361
Figure 15-5
Select Input Files.
The order that the files are read in is determined by the order in the
list. The first to be read is the top of the list. In VHDL the order of reading
files is important so that package files are read in before they are used.
Also the top level of the design should be read in last so that the design
is properly elaborated. In Figure 15-6 the list of files has been reordered
so that the package file, cpulib.vhd, is now read first, and the design top
level, cpu.vhd, is now read last. Files are moved in the list by selecting
them, and dragging and dropping to the new location. Now that the files
are in the proper order all the design files can be read into the synthesis
tool by selecting the Read button.
Now that the design has been read into the system, constraints can
be placed on the design to control how the design is implemented. For instance timing constraints, input constraints, and output constraints can
all be entered at this point. For this example we will only enter a clock
constraint. The clock constraint will specify the minimum clock frequency
for the design. This will give the synthesis tool a target frequency with
which to implement all logic. The clock constraint is specified as shown
in Figure 15-7.
Now that the library has been loaded, the design files read in, and the
constraints specified, the design can be optimized. Select the Optimize tab
to invoke the optimization user interface. For this example optimizing for
area is used to create a small design. The hierarchy of the design will be
preserved to get an idea of the size of each block. In general if the design
362
Chapter Fifteen
Figure 15-6
Set Order of Input
Files.
is small enough, removing the hierarchy will create a smaller and faster
design. Finally IO pads will not be added to the design as the Altera place
and route tool will do this automatically. The optimize user interface with
all the switches set is shown in Figure 15-8. Selection of the Optimize
button will perform the optimization process and implement the specified
design with Apex 20KE technology primitives.
The Report tab is used to generate area and timing reports. An area
report gives the size of the design based on the design implementation
in the target technology. To generate a report, select the Report Area
CPU Design: Synthesis Results
363
Figure 15-7
Set Clock Constraint
to 30 Mhz.
button as shown in Figure 15-9. The report generated will look like the
one shown below:
->report_area -cell_usage -all_leafs
*******************************************************
Cell: cpu
View: rtl
Library: work
cpu
*******************************************************
Cell
Library
References Total Area
GND
apex20e
1 x
1
1 GND
364
Chapter Fifteen
Figure 15-8
Optimize Design.
TRI
VCC
alu
apex20e
apex20e
work
16 x
1 x
1 x
apex20_lcell_normal
comp
control
apex20e
work
work
33 x
1 x
1 x
1
1
1
156
1
26
108
1
1
384
16
1
1
156
33
26
108
1
1
384
TRIs
VCC
GND
LCs
LCs
LCs
LCs
GND
VCC
Memory Bits
CPU Design: Synthesis Results
365
Figure 15-9
Report Area.
reg
work
1 x
reg
work
1 x
regarray_notri
work
1 x
shift
trireg_notri
work
work
1 x
3 x
Number of ports :
11
1
16
1
1
128
1
16
1
37
11
1
16
1
1
128
1
48
3
LCs
GND
LCs
GND
VCC
Memory Bits
shift
LCs
GND
366
Chapter Fifteen
Number of nets :
Number of instances :
Number of references to this view:
198
61
0
total accumulated area:
DELAY
Number
Number
Number
Number
Number
Number
Number
of
of
of
of
of
of
of
flex10
GND :
LCs :
Memory Bits :
TRIs :
VCC :
SHIFT :
accumulated instances :
8 x
8
398
512
16
1
1
443
***********************************************
Device Utilization for EP20K200EFC484
***********************************************
Resource
Used
Avail
Utilization
----------------------------------------------IOs
37
376
9.84%
LCs
398
8320
4.78%
Memory Bits
512
106496
0.48%
----------------------------------------------Info, Command 'report_area' finished successfully
The last step in the synthesis process is to write out a gate-level
description for the optimized design. For this example the output format
used will be EDIF. The common term for this output file is a netlist,
because it describes the primitives used in the design and the signals (or
nets—short for networks) used to connect these primitives. To generate the
netlist select the Output tab, modify the name of the output file as necessary, and then select the Write button. This is shown in Figure 15-10.
This netlist will now be passed to the Altera place and route tools to
create the actual implementation of the device. This process is described
in the next chapter.
SUMMARY
In this chapter, we synthesized all of the VHDL RTL descriptions of the
CPU and analyzed the results. In the next chapter, we read the synthesized
netlist into the place and route tools, and run the place and route to implement the design in the target technology.
CPU Design: Synthesis Results
Figure 15-10
367
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CHAPTER
16
Place and Route
This chapter discusses the process of implementing the
synthesis netlist of the CPU design into a target FPGA
device. The place and route tools read the netlist, extract
the components and nets from the netlist, place the components on the target device, and interconnect the components
using the specified interconnections. After the place and
route process is complete, the designer has an implementation of the design in the target technology. The implementation still needs to be verified for logical and timing correctness.
370
Chapter Sixteen
Figure 16-1
Place and Route
Process.
No
Yes
Synthesis
Netlist
Placement
Routing
Constraints
Met?
Place and Route Process
The place and route process places each macro from the synthesis netlist
into an available location on the target silicon and connects the macros
using routing resources available on the target silicon. The place and
route process is shown in Figure 16-1.
The synthesis netlist is input to the placement process. The placement
process analyzes all of the macros used in the design and their connectivity
to try to determine an optimal placement for the macros. The placement
algorithms take into account a number of technology-specific factors of the
target technology to determine whether a particular placement is good or
not. After a trial placement and signal route is attempted, the design is
analyzed with respect to timing constraints. If the timing constraints are
not met, the place and route software continues to try different placements
and signal routing to try to meet the constraints.
Typical target devices have areas of the chip where logical functions
are placed, and areas where interconnect signals are routed to connect the
logical functions. This is shown in Figure 16-2.
The device is split into a number of logic areas with routing channels
that surround the logic areas. Logic areas contain the logical gates to
implement the boolean function of the design. Routing channels contain
the signals that are used to connect the logical gates together. For FPGA
devices, the routing channels contain programmable interconnect wires.
FPGA devices use an onboard RAM to store the value of programmable
switches that are used to form the signal interconnections. By enabling
the proper sets of pass transistor gates, signal interconnections between
logic gates can be formed as shown in the example in Figure 16-3.
To make a connection from logic block 1 to logic block 3, all of the
switches shown need to be enabled with a logic 1 value. The logic gates of
the devices are connected to local routing signals that can be connected
to more global routing signals by pass transistors that bridge the two
signals. The control signals of the pass transistors are stored in a loadable
Place and Route
Figure 16-2
FPGA Chip
Architecture.
371
Logic
Area
Logic
Area
Logic
Area
Logic
Area
Logic
Area
Logic
Area
Logic
Area
Logic
Area
Logic
Area
Routing Channels
Figure 16-3
Logic Block
Interconnection.
1
Logic
Block 1
1
1
Logic
Block 2
1
Logic
Block 3
RAM. The place and route tool generates the RAM image to be loaded into
the RAM on the device.
The routing channels contain vertical and horizontal lines. The horizontal wires connect devices within a row, while the vertical lines allow
connections across rows. Most routing channels contain wires of different
372
Figure 16-4
Vertical and Horizontal Routing.
Chapter Sixteen
1
2
3
4
5
6
10
11
12
13
14
15
7
8
9
17
18
Connects short
segments together
lengths that allow connections to adjacent logic areas. Sometimes, longer
connections are needed, and either a longer line must be used or shorter
lines must be connected together to form the connection. This is shown in
Figure 16-4.
The job of the place and route tool is to create the programming files
that will be used to specify the logic function of the logic macros in the logic
areas and the switch programming of the wires used to connect the macros
together. Too many switches on a routed signal can cause some negative
performance effects. Each switch adds capacitance and resistance to the
routed signal. After only a few connections, signals start to slow significantly because of the capacitance and resistance of the line.
The place and route tool, therefore, must try to minimize long connections
and the number of switches for a particular signal to create designs with
the highest speed. To get the highest utilization, the place and route
tools need to pack as many of the logical functions into a logic area as
possible and then use as much local routing resources as possible to connect
these functions.
The place and route tools can make tradeoffs if the speed-critical signals
are known ahead of time and are implemented using the highest speed
interconnect signals. The placement algorithm also tries to place logical
gates on the critical path close to each other so that local interconnect can
be used to connect the gates. Local interconnect is usually very fast because
the wires are short. Short wires have less capacitance and resistance and,
therefore, can operate at much higher speeds.
Place and Route
373
Placing and Routing the Device
The target device for the CPU design, as mentioned in earlier chapters,
is an FPGA device. The device used is the Apex 20KE architecture from
Altera. The place and route tools used with the Apex 20KE architecture
are in the Quartus toolset. Quartus is a set of tools that includes not only
place and route, but VHDL entry, VHDL simulation, gate-level simulation,
and timing analysis. The first step in the process is to compile the design
into the place and route environment.
Setting Up a Project
Most tools that work on a design with multiple data descriptions have a
project manager to keep all of the files for that design in one place. This
facilitates file management of the design. The first step in the place and
route process is to set up a project. In the case of the Quartus environment,
the project is usually named the same as the output EDIF file from synthesis. The Quartus user interface is shown in Figure 16-5.
Selecting the File Project Wizard menu item will bring up a wizard
that walks the user through the creation of a new project. The first pane
Figure 16-5
Quartus User
Interface.
374
Chapter Sixteen
of this wizard is shown in Figure 16-6. This introduces the concept of a
wizard to the user.
The next step is to select the directory that contains the EDIF file that
was generated by Leonardo Spectrum. In Figure 16-7, the directory of the
EDIF file, the name of the project, and the name of the top-level entity
are specified. It is usually a good idea to make the name of the project and
top-level entity the same.
The next step in the wizard is to add the EDIF file to the project.
Clicking the next button brings up the interface shown in Figure 16-8.
Using the button with the three dots, the file user can find the file and
add it to the project.
The next step is to specify the EDA settings so that the EDIF file can be
properly interpreted. Use the Project EDA Tool Settings menu item to
invoke the EDA Tool Settings Dialog box shown in Figure 16-9. The Design
entry/synthesis tool item needs to be changed to Leonardo Spectrum.
The next step is to select the physical device to implement.
Remember we will use the EP20K200EFC484 device. This device
is selected from the list of devices shown in Figure 16-10. The Chips and
Devices tab displays a list of devices to select from. The list includes the
device and the speed grade.
Finally the device pins need to be assigned to the ports of the VHDL
design. The cpu clock port must be assigned to a pad driver that is capable
Figure 16-6
New Project Wizard.
Place and Route
Figure 16-7
New Project Settings.
Figure 16-8
Specify Project Input
File.
375
376
Figure 16-9
EDA Tool Settings
Dialog Box
Figure 16-10
Specify Target
Device.
Chapter Sixteen
Place and Route
377
Figure 16-11
Specify Pin
Assignments.
of driving all the flip flops in the design. This requires a clock buffer
rather than a standard buffer. In Figure 16-11 the clock port is assigned
to a dedicated clock buffer port.
Now that all the parameters have been specified, the place and route
process can be run by selecting the Processing Start Compilation menu
item. After the compilation process completes, the results are displayed
(see Figure 16-12).
SUMMARY
In this chapter, the netlist output from the synthesis tool was read by the
place and route tool, and an implementation of the netlist was generated.
We examined the process required to run the place and route tool, the
inputs to the place and route tool, and the outputs from it. In the next
chapter, we examine how to verify that the design created from the place
and route tool meets our requirements.
378
Figure 16-12
Results after
Compilation.
Chapter Sixteen
CHAPTER
17
CPU:
VITAL Simulation
The last step in the high-density FPGA design process is
to run gate-level timing simulation of the design. Figure
17-1 shows the high-density FPGA design flow. The place
and route process produces a number of files that need to
be verified before the design is implemented. The gatelevel timing simulation process verifies that the design
from the place and route process is correct from a timing
and functional point of view.
Within VHDL, this process is implemented using VITAL.
VITAL is an IEEE standard that is used for modeling accurate timing at the gate level. VITAL is an acronym for the
VHDL Initiative Toward ASIC Libraries. VITAL specifies a
standard method of writing ASIC or FPGA libraries so
that timing can be back-annotated. VITAL libraries used
in concert with a VITAL-compliant VHDL simulator can
perform gate-level timing simulation of the target design.
380
Figure 17-1
High-Density Design
Flow.
Chapter Seventeen
Design Specification
HDL Capture
RTL Simulation
RTL Synthesis
Functional
Gate Simulation
Place and Route
Post Layout Timing
Simulation
The VITAL process is shown in Figure 17-2.
The place and route tools generate two VITAL-compliant simulator
input files. The first is a VHDL netlist that contains the interconnections
of all of the entities used to model the design. The second is a timingaccurate SDF back-annotation file used to input post-route timing into
the VITAL simulation. There is a third input needed to the simulation
process. The third input is the VITAL library that describes all of the
behavior of the entities used to implement the design. In the next few
sections, we examine each of these in more detail.
CPU:Vital Simulation
381
Figure 17-2
VITAL Data Flow.
Place and
Route
VHDL
Netlist
SDF
File
Vital
Library
Vital
Simulator
Waveforms
Tabular
Output
VITAL Library
One of the reasons VITAL was developed was because there were no
standard methods of describing timing behavior in VHDL. With no standard
method of describing timing, there was also no standard method of
describing timing back-annotation. VHDL was also inefficient at modeling
gate behavior when compared to gate-level simulators optimized for
gate-level performance.
For all these reasons, VITAL was created to allow near gate-level
simulation performance with timing accurate models. Some of the features
available with VITAL are as follows:
■ Accurate specification of delays—Delays can be specified pin to
pin, be dependent on state, or specified in relation to a particular
occurrence of a condition.
382
Chapter Seventeen
■ Accurate timing check support—Checks include setup checks, hold
checks, pulsewidth checks, period checks, and accurate glitch
detection.
■ Many ways to specify functionality—Functionality can be specified
with truth tables, state tables, boolean primitives, or a behavioral
description.
All of these features give the designer the ability to create timingaccurate FPGA or ASIC libraries.
VITAL Simulation Process
Overview
The place and route tool generates a number of output files, as we saw in
the last chapter. The VITAL simulation uses two of these files. The first
is the VHDL netlist. This is a file containing component declarations, signals, and component instantiations that connect all of the components
together with the declared signals to form the functionality of the design.
This file is read by the VITAL simulator and used to create the component connectivity in the database.
The second file is an SDF (Standard Delay Format) file that describes the
timing for the design. For each instance in the netlist, this file contains SDF
statements that describe the delays and timing checks for the instance. This
information is used during simulation to model the timing behavior.
To build the VITAL simulation database, the simulator needs to have
a VITAL library that contains components for the target technology and
the VHDL netlist and SDF timing file from the place and route tools. The
simulator uses the netlist to instantiate the proper instances from the
VITAL library in the internal database and then apply timing to the
instances with the SDF file. Each of the instances contains a number of
generics that receive the timing information. The timing data is used
within the model to provide the correct behavior of the underlying device.
VITAL Implementation
VITAL descriptions follow a standard style and make use of standard
functions and procedures from two VITAL packages. The VITAL Timing
CPU:Vital Simulation
383
Package contains procedures and functions for accurate delay modeling,
timing checks, and timing error reporting. The VITAL Primitives Package
contains built-in primitives that are optimized for simulator performance.
Most VITAL-compliant simulators build the primitives package into the
simulator for optimum performance.
VITAL contains two styles of modeling that can be back-annotated with
SDF timing data for timing-accurate simulation. The first style, VITAL
level 1, uses only VITAL primitives for modeling the behavior of the
design. The second, VITAL level 0, has the capability to back-annotate
timing, but uses behavioral statements to describe the functionality of the
design. VITAL level 1 descriptions can be accelerated by VITAL-compliant
simulators because the constructs used are built into the simulator. VITAL
level 0 descriptions may not be accelerated because these descriptions use
behavioral constructs which may not be built in.
Simple VITAL Model
To understand how the VITAL modeling process works, a simple VITAL
model is examined. The model describes the behavior of a 2-input AND
gate. The symbol for the AND gate is shown in Figure 17-3.
The AND gate has two inputs, in1 and in2, and an output y. When
modeled with VITAL, this device has an input delay on inputs in1 and
in1 Input Delay
Figure 17-3
VITAL AND Gate.
in1
y
in2
Output Delay
int2 -> y
384
Chapter Seventeen
in2, and pin-to-pin delays from input in1 to output y and from input in2
to output y.
Following is the VITAL model that implements the functionality of the
AND2 device:
----- CELL AND2 ----library IEEE;
use IEEE.STD_LOGIC_1164.all;
use IEEE.VITAL_Timing.all;
library alt_vtl;
use alt_vtl.SUPPORT.all;
-- entity declaration -entity AND2 is
generic(
TimingChecksOn: Boolean := True;
XGenerationOn: Boolean := False;
InstancePath: STRING := “*”;
tpd_IN1_Y : VitalDelayType01 :=
tpd_IN2_Y : VitalDelayType01 :=
tipd_IN1 : VitalDelayType01 :=
tipd_IN2 : VitalDelayType01 :=
DefPropDelay01;
DefPropDelay01;
DefPropDelay01;
DefPropDelay01);
port(
Y : out
STD_LOGIC;
IN1 : in
STD_LOGIC;
IN2 : in
STD_LOGIC);
attribute VITAL_LEVEL0 of AND2 : entity is TRUE;
end AND2;
-- architecture body -architecture AltVITAL of AND2 is
attribute VITAL_LEVEL1 of AltVITAL : architecture is
TRUE;
SIGNAL IN1_ipd : STD_ULOGIC := ‘U’;
SIGNAL IN2_ipd : STD_ULOGIC := ‘U’;
begin
---------------------- INPUT PATH DELAYs
--------------------WireDelay : block
begin
VitalWireDelay (IN1_ipd, IN1, tipd_IN1);
VitalWireDelay (IN2_ipd, IN2, tipd_IN2);
end block;
--------------------- BEHAVIOR SECTION
-------------------VITALBehavior : process (IN1_ipd, IN2_ipd)
CPU:Vital Simulation
385
-- functionality results
VARIABLE Results : STD_LOGIC_VECTOR(1 to 1) :=
VARIABLE Results : (others => ‘X’);
ALIAS Y_zd : STD_ULOGIC is Results(1);
-- output glitch detection variables
VARIABLE Y_GlitchData : VitalGlitchDataType;
begin
-------------------------- Functionality Section
------------------------Y_zd := (IN2_ipd) AND (IN1_ipd);
----------------------- Path Delay Section
---------------------VitalPathDelay01 (
OutSignal => Y,
OutSignalName => “Y”,
OutTemp => Y_zd,
Paths => (0 => (IN1_ipd’last_event, tpd_IN1_Y,
Paths => (0 => (TRUE),
Paths => (1 => (IN2_ipd’last_event, tpd_IN2_Y,
Paths => (1 => (TRUE)),
GlitchData => Y_GlitchData,
Mode => DefGlitchMode,
XOn => DefGlitchXOn);
end process;
end AltVITAL;
configuration CFG_AND2_VITAL of AND2 is
for AltVITAL
end for;
end CFG_AND2_VITAL;
The model looks like standard VHDL with some different packages
included. In fact, the model is standard VHDL. The entity contains declarations for the STD_1164 packages for the signal logic types, but also contains USE clauses for the VITAL timing package. The VITAL timing package is needed in the entity for AND2 to provide the type declarations for
the entity generics.
The entity statement contains four generics that are used to pass
delay information to the model. Each of the generics has a prefix that
represents the type of the delay. Generic tipd_in1 is an input delay for
input in1. Generic tipd_in2 is an input delay for input in2. Generic
386
Chapter Seventeen
tpd_in1_y models the pin-to-pin delay from input in1 to output y. Generic
tidp_in2_y models the pin-to-pin delay from input in2 to output y.
The timing information passed to these generics comes from the SDF
file generated by the place and route tool. Each of the delays passed to
the entity is instance specific.
Each of the generics has a type associated with it that represents how
many delay values can be held. In this example, the generic contains two
values. Delay tr01 represents the delay value when the signal changes
from a ‘0’ to ‘1’ value. Delay tr10 represents the delay when the signal
changes from a ‘1’ to ‘0’ value.
The entity also contains other generics that control functionality of the
VITAL model. This example contains a generic called TimingChecksOn
that controls whether or not the timing check functions in the VITAL
model are executed or not. Finally, the entity contains the input and
output ports for the model.
VITAL Architecture
The architecture for the VITAL model contains four distinct code areas.
These are the wire delay section, the timing violation section, the function
description section, and the path delay section. Not all models contain all
of these sections. Some models are purely combinational and do not need
timing check sections.
Wire Delay Section
The first section of the architecture is the wire delay section. The AND2
architecture starts with a number of library declarations; but notice that
the architecture also uses the VITAL primitives package. After the
architecture statement, the architecture declares two local signals,
in1_ipd and in2_ipd, and an attribute. The two signals are used to
delay the input signals to the entity. The delay values applied to the two
input signals represent the wiring delays to connect the physical gates
together. For instance, in Figure 17-4, gate U1 drives gates U2 and U3.
The wiring from gate U1 to gate U2 causes 8 nanoseconds of delay in
the path, but the wiring from U1 to U3 causes 10 nanoseconds of delay
in the path. With separate input delay values for each input, the wiring
delays can be modeled correctly.
CPU:Vital Simulation
387
U2
Figure 17-4
Wire Delay
Representation.
8 ns
U1
8 ns
U3
Attribute VitalLevel1 specifies that the VITAL model is level 1
compliant. Level 1 models are modeled only with VITAL primitives and
can be accelerated. Some simulators have compliance checkers that can
validate level 1 compliance.
The architecture contains a block labeled WireDelay which contains
the VHDL description that actually delays the input signals. The block
contains a call to the VitalWireDelay procedure for each input port. The
VitalWireDelay procedure delays the input ports by the value passed to
the appropriate generic used in the procedure call. In this example,
generic tipd_in1 is used to delay input in1, and generic tipd_in2 is used
to delay input in2.
After the wire delay section is the timing check section. This example has
no timing check section because it is a purely combinational gate model.
The next section is the functionality section. This section contains the
statements that model the behavior of the device. This section starts with
a process labeled VitalBehavior. Notice that the process is sensitive to
the delayed versions of the two input signals, in1_ipd and in2_ipd. There
are a number of local variables declared and a statement that performs
an AND function of the two inputs. This AND function can be built into the
simulator so that execution can be accelerated.
The last section of the architecture starts with the VitalPathDelay
procedure call. This section is the path delay section. This section schedules
the new logic values calculated in the functionality section to occur after
the appropriate delay. This section consists of a VitalPathDelay01 procedure call for each output from the entity.
388
Chapter Seventeen
The VitalPathDelay01 procedure has a number of parameters passed
to it. These parameters are used to control what kind of glitch behavior
is wanted, the delays to be used, and the temporary data used to store signal information.
In this example, the VitalPathDelay procedure is passed the following
parameters:
■ OutSignal —The signal to have the new value placed on it.
■ OutSignalName —The name of the output signal to be used in
glitch reporting or error reporting.
■ OutTemp —A temporary signal used to store the current value of
the signal for comparison.
■ Paths —An array used to store delay information. There is a table
entry for each delay arc through the device.
■ GlitchData —A temporary storage area used to store signal state
and transition information for use in calculating glitches.
■ Mode —Specifies the type of glitch behavior wanted.
■ GlitchKind —Specifies the kind of glitches generated, OnEvent or
OnDetect.
Flip-Flop Example
In this next section, we examine another VITAL model with more complexity. This example shows the VITAL model for a DFF device. This
device has sequential behavior and needs to have timing checks to check
for illegal timing conditions:
----- CELL DFF ----library IEEE;
use IEEE.STD_LOGIC_1164.all;
use IEEE.VITAL_Timing.all;
use IEEE.VITAL_Primitives.all;
library alt_vtl;
use alt_vtl.SUPPORT.all;
-- entity declaration -entity DFF is
generic(
TimingChecksOn: Boolean := True;
XGenerationOn: Boolean := False;
InstancePath: STRING := “*”;
CPU:Vital Simulation
389
tpd_PRN_Q_negedge : VitalDelayType01 :=
DefPropDelay01;
tpd_CLRN_Q_negedge : VitalDelayType01 :=
DefPropDelay01;
tpd_CLK_Q_posedge : VitalDelayType01 :=
DefPropDelay01;
tsetup_D_CLK_noedge_posedge : VitalDelayType :=
DefSetupHoldCnst;
tsetup_D_CLK_noedge_negedge : VitalDelayType :=
DefSetupHoldCnst;
thold_D_CLK_noedge_posedge :
VitalDelayType :=
DefSetupHoldCnst;
thold_D_CLK_noedge_negedge :
VitalDelayType :=
DefSetupHoldCnst;
tipd_D : VitalDelayType01 := DefPropDelay01;
tipd_CLRN : VitalDelayType01 := DefPropDelay01;
tipd_PRN : VitalDelayType01 := DefPropDelay01;
tipd_CLK : VitalDelayType01 := DefPropDelay01);
port(
Q : out
STD_LOGIC;
D : in
STD_LOGIC;
CLRN : in
STD_LOGIC;
PRN : in
STD_LOGIC;
CLK : in
STD_LOGIC);
attribute VITAL_LEVEL0 of DFF : entity is TRUE;
end DFF;
-- architecture body -architecture AltVITAL of DFF is
attribute VITAL_LEVEL1 of AltVITAL : architecture is
TRUE;
SIGNAL
SIGNAL
SIGNAL
SIGNAL
D_ipd : STD_ULOGIC := ‘U’;
CLRN_ipd : STD_ULOGIC := ‘U’;
PRN_ipd : STD_ULOGIC := ‘U’;
CLK_ipd : STD_ULOGIC := ‘U’;
begin
---------------------- INPUT PATH DELAYs
--------------------WireDelay : block
begin
VitalWireDelay (D_ipd, D, tipd_D);
VitalWireDelay (CLRN_ipd, CLRN, tipd_CLRN);
VitalWireDelay (PRN_ipd, PRN, tipd_PRN);
VitalWireDelay (CLK_ipd, CLK, tipd_CLK);
end block;
--------------------
390
Chapter Seventeen
-- BEHAVIOR SECTION
-------------------VITALBehavior : process (D_ipd, CLRN_ipd, PRN_ipd,
CLK_ipd)
-- timing check results
VARIABLE Tviol_D_CLK : STD_ULOGIC := ‘0’;
VARIABLE TimingData_D_CLK : VitalTimingDataType :=
VitalTimingDataInit;
-- functionality results
VARIABLE Violation : STD_ULOGIC := ‘0’;
VARIABLE PrevData_Q : STD_LOGIC_VECTOR(1 to 6) ;
VARIABLE D_delayed : STD_ULOGIC := ‘U’;
VARIABLE CLK_delayed : STD_ULOGIC := ‘U’;
VARIABLE Results : STD_LOGIC_VECTOR(1 to 1) :=
(others => ‘X’);
-- output glitch detection variables
VARIABLE Q_VitalGlitchData
: VitalGlitchDataType;
CONSTANT DFF_Q_tab : VitalStateTableType := (
-- Violation, CLRN_ipd, CLK_delayed, D_delayed, PRN_ipd,
CLK_ipd
( L, L, x, x, x, x, x, L ),
( L, H, L, H, x, H, x, H ),
( L, H, H, x, H, x, x, S ),
( L, H, x, x, L, x, x, H ),
( L, H, x, x, H, L, x, S ),
( L, x, L, L, H, H, x, L ) );
begin
------------------------- Timing Check Section
-----------------------if (TimingChecksOn) then
VitalSetupHoldCheck (
Violation
TimingData
TestSignal
TestSignalName
RefSignal
RefSignalName
SetupHigh
SetupLow
HoldHigh
HoldLow
=>
=>
=>
=>
=>
=>
=>
Tviol_D_CLK,
TimingData_D_CLK,
D_ipd,
“D”,
CLK_ipd,
“CLK”,
tsetup_D_CLK_noedge_
posedge,
=> tsetup_D_CLK_noedge_
posedge,
=> thold_D_CLK_noedge_
posedge,
=> thold_D_CLK_noedge_
posedge,
CPU:Vital Simulation
391
CheckEnabled
RefTransition
HeaderMsg
XOn
MsgOn
=> TO_X01(( (NOT PRN_ipd)
) OR ( (NOT
CLRN_ipd) ) ) /= ‘1’,
=> ‘/’,
=> InstancePath & “/DFF”,
=> DefTimingXon,
=> DefTimingMsgon );
end if;
-------------------------- Functionality Section
------------------------Violation := Tviol_D_CLK;
VitalStateTable(
StateTable => DFF_Q_tab,
DataIn => (
Violation, CLRN_ipd, CLK_delayed,
D_delayed, PRN_ipd, CLK_ipd),
Result => Results,
NumStates => 1,
PreviousDataIn => PrevData_Q);
D_delayed := D_ipd;
CLK_delayed := CLK_ipd;
----------------------- Path Delay Section
---------------------VitalPathDelay01 (
OutSignal => Q,
OutSignalName => “Q”,
OutTemp => Results(1),
Paths => (0 => (PRN_ipd’last_event,
tpd_PRN_Q_negedge, TRUE),
1 => (CLRN_ipd’last_event,
tpd_CLRN_Q_negedge, TRUE),
2 => (CLK_ipd’last_event,
tpd_CLK_Q_posedge, TRUE),
3 => (D_ipd’last_event,
tpd_CLK_Q_posedge, TRUE)),
GlitchData => Q_VitalGlitchData,
Mode => DefGlitchMode,
XOn => DefGlitchXOn);
end process;
end AltVITAL;
configuration CFG_DFF_VITAL of DFF is
for AltVITAL
end for;
end CFG_DFF_VITAL;
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Chapter Seventeen
The first thing to notice about this model is that there are quite a few
more generics used to pass timing information to the model. This is
because this model has more input ports; therefore, there are more input
delay generics and the model contains timing checks that need timing
information passed to them.
The wire delay section now delays four input ports instead of two. The D,
CLRN, PRN, and CLK inputs are delayed in the wire delay section. The architecture for the DFF also contains a number of local signals and variables used
to hold intermediate values for the timing check and functionality sections.
The final declaration item in the architecture declaration section is a table
that is used to model the behavior of the DFF. This DFF model uses a VITAL
State Table procedure to model the behavior of the device. This table is used
in the functionality section of the model by the VitalStateTable procedure
call. The signal values of the signals passed to the VitalStateTable procedure call are compared to the values in the table, and the new values for the
output signals and next state are predicted.
The timing check section for this example contains a VitalSetupHoldCheck procedure call. This procedure checks the setup and hold of data
changes versus the clock for the DFF device. The violation signal returned
by the VitalSetupHoldCheck procedure is used to affect the behavior of
the DFF device by the fact that its value is passed to the VitalStateTable
that controls the behavior of the DFF device.
The functionality section of the DFF device contains the single call to the
VitalStateTable procedure to calculate the value of the Q output based on
the values of the input ports, the previous state, and the violation signal
from the timing check procedures. Based on all of these inputs, a table row
matches, and the new Q output is passed to the path delay section.
The path delay section looks very similar to the path delay section for
the AND2 device discussed previously. The path delay section contains a
single call to the VitalPathDelay01 procedure, which schedules output Q
with the appropriate delay value.
To see how all of these VITAL functions and procedures are implemented, look at the VITAL packages included on the CD with the book or
visit www.vhdl.org/vital.
SDF File
The other piece of functionality needed to complete the VITAL simulation
picture is the SDF back-annotation file. This file is generated by the place
CPU:Vital Simulation
393
and route tools and contains accurate timing for the device. The SDF file
contains timing information for all of the generics in the VITAL library
that need data passed to them. Following is a sample SDF file:
(DELAYFILE
(SDFVERSION “2.1”)
(DESIGN “cpu”)
(DATE “10/25/97 10:59:58”)
(VENDOR “Altera”)
(PROGRAM “MAX+plus II”)
(VERSION “Version 7.2 RC2 2/14/97”)
(DIVIDER .)
(VOLTAGE :5:) (PROCESS “typical”) (TEMPERATURE
(TIMESCALE 100ps)
:25:)
(CELL
(CELLTYPE “DFF”)
(INSTANCE DFF_457)
(DELAY
(ABSOLUTE
(IOPATH (posedge CLK) Q (32:32:32) (32:32:32)))
(ABSOLUTE
(IOPATH (negedge PRN) Q (36:36:36) (36:36:36)))
(ABSOLUTE
(IOPATH (negedge CLRN) Q (37:37:37) (37:37:37)))
)
(TIMINGCHECK
(SETUP D (posedge CLK) (2:2:2))
(HOLD D (posedge CLK) (10:10:10))
))
)
The SDF file starts with a header section that describes the name of
the design the file will back-annotate, the vendor that generated the file, the
environment used to generate the timing numbers, and so on. After the
header, the file consists of a number of cells. Each cell in the SDF file represents an instance in the VHDL netlist produced by the place and route
tools. Each cell contains the type of cell, the instance name in the netlist,
and timing information to be back-annotated to the design. The VITALcompliant simulator reads the SDF file and matches the generics in the
VHDL source with the delay constructs in the SDF file. For instance, an
IOPATH construct in the SDF file specifies the rising and falling delays from
and input to an output signal. The IOPATH construct is converted into
generic names and values to be applied to the VITAL simulation. The designer of the VITAL model must ensure that the names used in the SDF
model and the names of the generics used in the VITAL model match so
that the generics can be properly matched with proper timing values.
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Chapter Seventeen
The last section is a timing check section that contains timing information for the timing checks of the cell, if they exist. The timing check
section of the SDF file is read by the VITAL simulator and extracts timing
information to plug into generics of the VITAL model. The timing check
generics control the timing values that are used in the timing checks of the
VITAL model while simulation is progressing.
The cell description in the preceding example is for the DFF model that
we looked at earlier. There are delay values for CLK to Q, PRN to Q, and CLRN
to Q, and values for the setup and hold check.
VITAL Simulation
To run the VITAL simulation, the designer first compiles the VITAL library into a simulator library. The device manufacturers supply VITAL
libraries for their devices. Next, the VITAL netlist is compiled to the
working library, and, finally, the SDF file is read in to back-annotate
the timing data into the design. After these steps have been completed,
the designer runs the VITAL simulation in the same manner as the RTL
simulation that we ran earlier.
The first step is to compile the VITAL library into a simulator library
so that it can be referenced. It is best if this library is compiled into the
location specified by the netlist from the place and route tool so that no
manual code modification is necessary. The following shows the first few
lines of the VITAL netlist generated by the MaxPlusII place and route
tool. The complete netlist is on the CD. Notice that the VITAL netlist
expects the VITAL component declarations, package VCOMPONENTS, to be
located in a library named alt_vtl:
-- MAX+plus II Version 7.2 RC2 2/14/97
-- Sat Oct 25 10:59:34 1997
-LIBRARY IEEE;
USE IEEE.std_logic_1164.all;
LIBRARY alt_vtl;
USE alt_vtl.VCOMPONENTS.all;
--ENTITY cpu IS
-PORT (
-addr : OUT std_logic_vector(15 downto 0);
-data : INOUT std_logic_vector(15 downto 0);
-clock : IN std_logic;
CPU:Vital Simulation
395
-ready : IN std_logic;
-reset : IN std_logic;
-rw : OUT std_logic;
-vma : OUT std_logic);
--END cpu;
ARCHITECTURE EPF10K10TC144_a3 OF cpu IS
SIGNAL gnd : std_logic := ‘0’;
SIGNAL vcc : std_logic := ‘1’;
SIGNAL
……
……….
To compile the VCOMPONENTS package into library alt_vtl, the following commands are executed in ModelSim:
vlib alt_vtl
vcom -work alt_vtl alt_vtl.cmp
Because there are no other library declarations for the actual vital
library, the vital library entities need to be compiled into the working
library to be visible. Following is the command to perform this step:
vcom alt_vtl.vhd
After these two files have been compiled, the VITAL netlist can be compiled into the working library. The following command compiles the
netlist:
vcom cpuout.vhd
We still need to simulate design TOP to verify the gate-level implementation of the CPU. However, this time, the CPU RTL description is replaced with a VITAL description of the CPU. This can be accomplished
by two different methods. The first involves compilation order, and the
second is by direct specification. Remember that the last architecture
compiled is used by default for an entity. By compiling architecture
EPF10K10TC144_a3 last, this architecture is used for entity cpu.
The other method is to write a configuration for architecture top that
specifies exactly which architecture is to be used. The following example
shows two configuration statements for the two different implementations
of the CPU:
configuration topconrtl of top is
for behave
for U1 : cpu use entity work.cpu(behave);
end for;
396
Chapter Seventeen
end for;
end topconrtl;
configuration topconstruct of top is
for behave
for U1 : cpu use entity work.cpu(EPF10K10TC144_a3);
end for;
end for;
end topconstruct;
Configuration topconrtl specifies the rtl implementation configuration
for entity top, and configuration topconstruct specifies the structural
implementation. Notice that the structural architecture was named the
same as the device that was implemented by the place and route tools.
To complete the simulation setup process, the final compilations
needed are shown here:
vcom top.vhd
vcom topconstruct.vhd
After these steps, the design is ready for simulation. To load the design
into the simulator, the following command is executed:
vsim topconstruct
The simulator brings up its windows and begins the simulation. If the
simulation is run ahead 500 nanoseconds, we can see the CPU start the
reset sequence as instructions are fetched. This is shown in Figure 17-5.
Figure 17-5
The Simulator
Window.
CPU:Vital Simulation
397
Running the simulation through the entire process verifies the functionality of the placed and routed design. To verify the timing and
functionality, we need to back-annotate the timing from place and route
to the simulation.
Back-Annotated Simulation
To run timing back-annotated simulation, we don’t need to recompile. We
only need to specify to the simulator which SDF file to read. This is done
by the following command:
vsim -sdfmax /u1=cpuout.sdf topconstruct
This command tells the simulator to back-annotate the VITAL simulation of the CPU design with SDF file cpuout.sdf created by the place
and route tools. After this command has executed, the simulation is
invoked, and the SDF file is back-annotated to component U1 (cpu) and
simulation started. Running the simulation produces the waveform shown
in Figure 17-6.
The back-annotated delays are seen on the waveforms for addr and
data around time 400 nanoseconds. Notice that, instead of one transition,
the waveforms have a number of transitions that finally settle out. Using
this timing information, the designer can now increase the clock speed
Figure 17-6
Simulation
Waveform.
398
Chapter Seventeen
until the design stops working to determine the maximum speed that
the design will run. By running the design through the entire simulation,
the functionality and timing of the design can be verified for correctness.
When the design meets the functionality and timing requirements, the
design can be signed off and built.
SUMMARY
In this chapter, we examined VITAL simulation and how to perform
VITAL simulation on the CPU design. The rest of the book contains useful appendices that describe some of the standard types, functions, and
procedures used throughout the book.
CHAPTER
18
At Speed
Debugging
Techniques
Throughout the book so far we have discussed a number
of techniques for implementing VHDL designs and ways
to make sure that the VHDL designs behave as expected.
These techniques include simulation, synthesis of the
design to an FPGA or ASIC, and gate-level simulation
using VITAL libraries. A new technique called At-Speed
Debugging is just becoming available that allows much
higher performance verification than a typical simulator,
yet provides the design visibility necessary to properly
debug a design. This technique provides designers with
the ability to debug their design in the target system, at
target speed, at the VHDL RTL level.
Figure 18-1 shows a block diagram of how this works.
The VHDL for the device is read into a tool that automatically creates and inserts a small debug core into the
device that probes internal signals. The debug core is created based on information from the designer about what
400
Chapter Eighteen
Hardware System
Figure 18-1
At-Speed Debugging
Overview.
FPGA or
FPGA
or
SOC
SOC
IICE
JTAG
HDL
Debugger
signals are to be probed. This debug core communicates through the
JTAG port on the device to an HDL debugger executing on a host platform. The HDL debugger sends and receives data from the debug core
and displays this data in context with the HDL for the design. Waveforms of the internal device data can also be displayed, providing the
ability to trace down problems in the design.
This technique works well for any design, but it works especially well
for designs where a tremendous amount of data must be processed by
the device to determine whether the device is working properly. For
instance, devices that process audio or video information require a
tremendous amount of data to be processed before it can be determined
that the device is working properly. A video processor might need to produce several minutes of high-quality video data to determine whether
the encryption decoding algorithm is working properly. Running at or
near speed will allow images to be generated quickly and the device
function to be analyzed for correctness.
The only system as of this writing that performs as described is the
Bridges2Silicon debugger from Bridges2Silicon. A block diagram of the
system is shown in Figure 18-2.
The Bridges2Silicon debugger contains two tools. The Bridges2Silicon
instrumentor reads the VHDL description and adds the debug core, called
an Intelligent In-Circuit Emulator (IICE) to the design. The Bridges2Silicon
debugger communicates with the JTAG port on the target device, reads
the database created by the instrumentor, and reads the original source
files created by the designer.
At Speed Debugging Techniques
401
Instrumented
HDL
Sources
Bridges2Silicon
Instrumentor
Figure 18-2
Bridges2Silicon
Debugger Overview.
HDL
Sources
Implement
(Synthesis, P&R)
B2S
Project
SOC
Bridges2Silicon
Debugger
JTAG
IICE
Instrumentor
The designer reads the VHDL design into the instrumentor and specifies
which signals to probe and which breakpoints to enable. The instrumentor
generates a new VHDL description of the design with the IICE core added
and connected to the appropriate places in the design. Once the new VHDL
description has been created, the designer synthesizes, and place and route
the new VHDL description. In an FPGA design environment, the device is
programmed with the new device file created by place and route.
Debugger
Once the board is powered up, and the FPGA device is programmed with
the new device file from place and route, the debugger can communicate
with the device through the JTAG port. The debugger also reads the database file created by the Instrumentor and the original VHDL source files.
The instrumentor database relates the real signals on the device to the
location of the signals in the original HDL.
Debug CPU Design
Let’s now look at the process of debugging the CPU design using the
Bridges2Silicon Debugger. The first step is to create a project containing
all of the HDL files for the design.
402
Chapter Eighteen
Create Project
To create a project, use the project editor invoked from the File
menu or from the toolbar. The Project Editor window is shown in Figure 18-3.
To add to the project, use the file navigator in the upper left to navigate to the location of files. First select the files and use the right arrow
key to add the files to the Design File list on the right of the project
editor window as shown in Figure 18-4. Now the design source files
need to be re-ordered so that the files are read in the proper order,
which is specified by the order of the list. The package cpulib.vhd
needs to be read first so that it is available to all the other design files.
The easiest way to move cpulib.vhd to the top of the list is to select
and drag it to the top of the list. The other file that needs to be moved
is the top level of the design, cpu.vhd. File cpu.vhd needs to be moved to
the bottom of the list so that it is the last file read.
Figure 18-3
Bridges2Silicon Project
Editor Window.
At Speed Debugging Techniques
403
Figure 18-4
Files added to Project
File List.
Specify Top-Level Parameters
Once all the files have been specified, the parameters for the top level
need to be specified so that the design elements can be properly linked.
There are two parameters that need to be specified: the TOP-LEVEL UNIT
and TOP-LEVEL LANGUAGE. TOP-LEVEL UNIT specifies which design unit is
the top level and will be linked. This value will be specified as CPU. TOPLEVEL LANGUAGE specifies the default language used to compile the design
and to write out the instrumented design. TOP-LEVEL LANGUAGE is specified
as VHDL by clicking the VHDL radio button. This is shown in Figure 18-5.
Now that we have specified all the needed parameters, click the OK button, which saves the project and also compiles the project. After compilation, Figure 18-6 shows the design loaded into the Instrumentor Window.
Specify Project Parameters
Once all the files have been added to the project, the device parameters need to be specified. These parameters determine how the device
404
Chapter Eighteen
will communicate with the JTAG port and the debugger on the host
platform. Use the dialog box shown in Figure 18-7 to set up the device
and IICE configuration settings.
1. Device family. We use the Altera Apex technology.
2. JTAG port. The choices are b2s and builtin. We will choose builtin
so that we can use JTAG communication with the JTAG tap controller
already present in the Altera device. This choice is used predominately
and b2s is only used when the board containing the device is not connected
to the JTAG chain on the board.
3. Type of RAM. This parameter specifies the type of RAM to be used for
the sample buffer that stores internal signal data. The choices are blockram, logic, and behavioral. This example will use blockram, the most common selection and the most efficient. This selection will build the sample
buffer from blockrams available on the FPGA device. If the logic choice is
selected, the sample buffer will be built from flip-flops in user logic. This
choice is used when there is limited blockram available and is not as efficient as blockram. The third choice is behavioral and will generate a
behavioral model for the sample buffer. This choice will let the synthesis
tool choose the sample buffer implementation based on available resources.
4. Sample clock. This parameter specifies a signal that will be used to
clock the data into the sample buffer. This signal can be any signal in the
design but must be a clocklike signal. For instance, this signal should be
the output of a register so that it does not contain glitches. In this example
the signal /clk will be used.
5. Sample depth. This parameter specifies how many samples are gathered when a trigger occurs. Depending on how much data are required
to find a bug, this value can be any power of 2 that will fit into the Buffer
Type specified for the device. In this example, the value 256 will be used.
Instrument Signals
Now that the design has been compiled and the communication parameters
specified, the signals to be instrumented can be selected. For this example
we are going to debug the control block. All breakpoints and signals in the
control block for the reset sequence will be instrumented for use later during debugging. The debugger GUI shows only the signals and breakpoints
that can be instrumented. Clicking the Radio button next to a signal or
breakpoint will instrument that signal or breakpoint. Figure 18-8 shows the
Radio buttons for the reset sequence selected for sampling and debugging.
At Speed Debugging Techniques
405
Write Instrumented Design
Once all the signals and breakpoints have been instrumented, the instrumented design can be written out. This design will include the original
design tree plus the IICE core added for debugging. The IICE core will be
connected in such a way as to probe all the instrumented signals in the
design. Select the File Save and Instrument menu items to write out the
instrumented design.
Implement New Design
The write instrumented design process will produce a new version of the
VHDL files for the CPU design. These files need to be synthesized as
described in earlier chapters. The output of the synthesis process is an
EDIF netlist. The EDIF netlist is placed and routed with the Altera
Quartus tools to produce a file that is programmed into the Altera device
Figure 18-5
Top Level Unit and
Language Specified.
406
Chapter Eighteen
as described earlier. Finally the new programming file is programmed
into the Altera device with the Altera programming software.
Start Debug
Now that the device has been programmed with the new netlist, the device
can be debugged with the Bridges2Silicon debugger. The debugger is
invoked, and the project created by the Bridges2Silicon instrumentor
is loaded into the debugger. This project loads the original source files for
the project into the debugger. The debugger now shows only instrumented
signals and breakpoints.
Enable Breakpoint
To enable a breakpoint, click on the radio button next to it. To put a watch
expression on a signal, click the signal and specify a signal expression value.
Figure 18-6
Compiled Project with
Debug Items Shown.
At Speed Debugging Techniques
407
A breakpoint is a simple way to specify a complex trigger expression.
A breakpoint behaves similar to a breakpoint in a software debugger except
that a breakpoint in the debugger does not stop the hardware. A breakpoint in the debugger represents a trigger condition that causes all instrumented signal values to be captured and sent to the debugger. Figure 18-9
shows the breakpoint selected at the source line where current_state
equals Load12.
To activate this trigger, click on the Run button.
Clicking the Run button will download this trigger selection to the
IICE hardware on the device and arm the trigger to sample data. When
signal current_state obtains the value Load12, the breakpoint triggers
and the sample buffer is sent to the debugger through the JTAG port of
the device. The actual signal values are displayed on the source code as
shown in Figure 18-10.
Notice that the value of current_state is Load12. All the rest of the
signals contain values captured when current_state changed to the value
Load12 and the trigger condition was met. With this capability a designer
can use breakpoints to see the actual behavior of the real hardware to
make sure that the design is behaving properly.
Figure 18-7
IICE Configuration
Dialog.
408
Chapter Eighteen
Figure 18-8
Signals and Breakpoints Selected for
Debug.
The sample buffer contains as many samples as specified by the sample
depth in the instrumentor setup. In this case 256 samples were captured
into the sample buffer and sent to the debugger when the trigger occurred.
Trigger Position
The position of the trigger in the sample buffer data can be controlled so
that more history before the trigger is shown, history and future data size
are the same, or less history and more future data is shown after the
trigger. In this example the history and future data is the same, which
corresponds to the middle trigger position.
Waveform Display
Another way to view the sample buffer data is via a waveform display. In
Figure 18-11 the sample buffer data are shown as a waveform. The vertical
At Speed Debugging Techniques
409
Figure 18-9
Breakpoint Enabled
on Loadl2.
cursor shows the position of the trigger. Also notice that the first two
signals in the waveform display are Sample Clock and Cycle. These are
added signals used to provide reference points in the waveform. The
sample clock is used to show the reference edges where sample data was
captured. The cycle number shows the relative position within the sample
buffer. Cycle number 0 is the trigger point, positive values are after the
trigger, and negative values are before the trigger.
Set Watchpoint
Another way to specify trigger conditions is to specify watchpoint expressions on signals. A watchpoint expression defines a value or transition that a signal will trigger. When the signal value equals the trigger
value or transitions from one value to another, the IICE will trigger and
cause the sample buffer to be transferred to the debugger. To set a watchpoint expression, click on a signal in the debugger source window. The
Watchpoint Expression dialog box appears (see Figure 18-12).
410
Chapter Eighteen
Figure 18-10
Trigger on Loadi2
and Captured Data
Displayed.
If only the first value is specified, the signal will cause a trigger when
that value is reached on the signal. When both values are specified, the
signal must transition from the first value to the second value to cause a
trigger. In Figure 18-13, signal current_state must transition from state
reset5 to state reset6 to cause a trigger.
Complex Triggers
Watchpoints and breakpoints can be combined to create more complex
triggers. Complex triggers allow the designer to zoom in on exactly the
area of the design to capture data from, or to locate more precisely the
cause of a design error. For instance, to specify exactly the case when the
instruction is written from the memory bus to the instruction register
after state reset6, the following watchpoints can be specified. Place a
watchpoint expression of reset6 on signal current_state, and a watchpoint expression of 1 on signal ready. When both of these expressions are
At Speed Debugging Techniques
Figure 18-11
Captured Data
Displayed as
Waveforms.
Figure 18-12
Watchpoint
Specification Dialog
Box.
411
412
Chapter Eighteen
Figure 18-13
Trigger on progcntrRd
Watchpoint.
true, the IICE will trigger, indicating that the instruction register is being written with a new value after reset.
SUMMARY
In this chapter we explored a new technique to debug designs using actual hardware implementations. The concepts behind instrumenting a design were introduced and techniques for instrumenting a design were
shown. Next we discussed techniques for setting trigger conditions to find
specific design states during debug. Finally we discussed how to specify
complex triggers to precisely define trigger behavior.
Appendix A
Standard Logic Package
This is a copy of the IEEE 1164 standard logic package. It is used in
all of the examples in the book and is listed here for reference.
------------------------------------------
-------------------------------------------------------Title
Library
: std_logic_1164 multi-value logic system
: This package shall be compiled into a
: library symbolically named IEEE.
:
Developers’ : IEEE model standards group (par 1164)
Purpose
: This package defines a standard for
: designers to use in describing the
: interconnection data types used in vhdl
: modeling.
:
Limitation : The logic system defined in this
: package may be insufficient for
: modeling switched transistors, since
: such a requirement is out of the scope
: of this effort. Furthermore,
: mathematics, primitives, timing
: standards, etc. are considered
: orthogonal issues as it relates to this
: package and are therefore beyond the
: scope of this effort.
:
Note
: No declarations or definitions shall be
: included in, or excluded from this
: package. The “package declaration”
: defines the types, subtypes and
: declarations of std_logic_1164. The
: std_logic_1164 package body shall be
: considered the formal definition of the
: semantics of this package. Tool
: developers may choose to implement the
: package body in the most efficient
: manner available to them.
:
-------------------------------------------------------modification history :
-------------------------------------------------------version | mod. date:|
v4.200 | 01/02/92 |
--------------------------------------------------------
PACKAGE std_logic_1164 IS
-------------------------------------------------------- logic state system (unresolved)
------------------------------------------------------TYPE std_ulogic IS ( ‘U’, -- Uninitialized
TYPE std_ulogic IS ( ‘X’, -- Forcing Unknown
414
Appendix A:
Standard Logic Package
TYPE std_ulogic IS ( ‘0’, -- Forcing 0
TYPE std_ulogic IS ( ‘1’, -- Forcing 1
TYPE std_ulogic IS ( ‘Z’, -- High Impedance
TYPE std_ulogic IS ( ‘W’, -- Weak
Unknown
TYPE std_ulogic IS ( ‘L’, -- Weak
0
TYPE std_ulogic IS ( ‘H’, -- Weak
1
TYPE std_ulogic IS ( ‘-’
-- Don’t care
TYPE std_ulogic IS );
-------------------------------------------------------- unconstrained array of std_ulogic for use with the
-- resolution function
------------------------------------------------------TYPE std_ulogic_vector IS ARRAY ( NATURAL RANGE <> )
OF std_ulogic;
-------------------------------------------------------- resolution function
------------------------------------------------------FUNCTION resolved ( s : std_ulogic_vector ) RETURN
std_ulogic;
-------------------------------------------------------- *** industry standard logic type ***
------------------------------------------------------SUBTYPE std_logic IS resolved std_ulogic;
-------------------------------------------------------- unconstrained array of std_logic for use in
-- declaring signal arrays
------------------------------------------------------TYPE std_logic_vector IS ARRAY ( NATURAL RANGE <>) OF
std_logic;
-------------------------------------------------------- common subtypes
------------------------------------------------------SUBTYPE X01
IS resolved std_ulogic RANGE ‘X’ TO
‘1’; -- (‘X’,’0’,’1’)
SUBTYPE X01Z
IS resolved std_ulogic RANGE ‘X’ TO
‘Z’; -- (‘X’,’0’,’1’,’Z’)
SUBTYPE UX01
IS resolved std_ulogic RANGE ‘U’ TO
‘1’; -- (‘U’,’X’,’0’,’1’)
SUBTYPE UX01Z
IS resolved std_ulogic RANGE ‘U’ TO
‘Z’; -- (‘U’,’X’,’0’,’1’,’Z’)
-------------------------------------------------------- overloaded logical operators
------------------------------------------------------FUNCTION “and”
RETURN UX01;
FUNCTION “nand”
RETURN UX01;
FUNCTION “or”
RETURN UX01;
FUNCTION “nor”
RETURN UX01;
FUNCTION “xor”
RETURN UX01;
-- function “xnor”
-- return ux01;
( l : std_ulogic; r : std_ulogic )
( l : std_ulogic; r : std_ulogic )
( l : std_ulogic; r : std_ulogic )
( l : std_ulogic; r : std_ulogic )
( l : std_ulogic; r : std_ulogic )
( l : std_ulogic; r : std_ulogic )
Appendix A:
Standard Logic Package
FUNCTION “not”
RETURN UX01;
415
( l : std_ulogic
)
-------------------------------------------------------- vectorized overloaded logical operators
------------------------------------------------------FUNCTION “and” ( l, r : std_logic_vector ) RETURN
std_logic_vector;
FUNCTION “and” ( l, r : std_ulogic_vector ) RETURN
std_ulogic_vector;
FUNCTION “nand” ( l, r : std_logic_vector ) RETURN
std_logic_vector;
FUNCTION “nand” ( l, r : std_ulogic_vector ) RETURN
std_ulogic_vector;
FUNCTION “or”
( l, r : std_logic_vector ) RETURN
std_logic_vector;
FUNCTION “or”
( l, r : std_ulogic_vector ) RETURN
std_ulogic_vector;
FUNCTION “nor” ( l, r : std_logic_vector ) RETURN
std_logic_vector;
FUNCTION “nor” ( l, r : std_ulogic_vector ) RETURN
std_ulogic_vector;
FUNCTION “xor” ( l, r : std_logic_vector ) RETURN
std_logic_vector;
FUNCTION “xor” ( l, r : std_ulogic_vector ) RETURN
std_ulogic_vector;
----------------
------------------------------------------------------Note : The declaration and implementation of the “xnor”
function is specifically commented until at which time
the VHDL language has been officially adopted as
containing such a function. At such a point, the
following comments may be removed along with this
notice without further “official” ballotting of this
std_logic_1164 package. It is the intent of this effort
to provide such a function once it becomes available
in the VHDL standard.
------------------------------------------------------function “xnor” ( l, r : std_logic_vector ) return
std_logic_vector;
function “xnor” ( l, r : std_ulogic_vector ) return
std_ulogic_vector;
FUNCTION “not” ( l : std_logic_vector ) RETURN
std_logic_vector;
FUNCTION “not” ( l : std_ulogic_vector ) RETURN
std_ulogic_vector;
-------------------------------------------------------- conversion functions
------------------------------------------------------FUNCTION To_bit
( s : std_ulogic;
xmap :
BIT := ‘0’) RETURN BIT;
FUNCTION To_bitvector ( s : std_logic_vector ; xmap :
BIT := ‘0’) RETURN BIT_VECTOR;
416
Appendix A:
Standard Logic Package
FUNCTION To_bitvector ( s : std_ulogic_vector; xmap :
BIT := ‘0’) RETURN BIT_VECTOR;
FUNCTION To_StdULogic
RETURN std_ulogic;
FUNCTION To_StdLogicVector
RETURN std_logic_vector;
FUNCTION To_StdLogicVector
RETURN std_logic_vector;
FUNCTION To_StdULogicVector
RETURN std_ulogic_vector;
FUNCTION To_StdULogicVector
RETURN std_ulogic_vector;
( b : BIT
)
( b : BIT_VECTOR
)
( s : std_ulogic_vector )
( b : BIT_VECTOR
)
( s : std_logic_vector
)
-------------------------------------------------------- strength strippers and type convertors
------------------------------------------------------FUNCTION
RETURN
FUNCTION
RETURN
FUNCTION
RETURN
FUNCTION
RETURN
FUNCTION
RETURN
FUNCTION
RETURN
To_X01 ( s : std_logic_vector
std_logic_vector;
To_X01 ( s : std_ulogic_vector
std_ulogic_vector;
To_X01 ( s : std_ulogic
X01;
To_X01 ( b : BIT_VECTOR
std_logic_vector;
To_X01 ( b : BIT_VECTOR
std_ulogic_vector;
To_X01 ( b : BIT
X01;
FUNCTION
RETURN
FUNCTION
RETURN
FUNCTION
RETURN
FUNCTION
RETURN
FUNCTION
RETURN
FUNCTION
RETURN
To_X01Z ( s : std_logic_vector
std_logic_vector;
To_X01Z ( s : std_ulogic_vector
std_ulogic_vector;
To_X01Z ( s : std_ulogic
X01Z;
To_X01Z ( b : BIT_VECTOR
std_logic_vector;
To_X01Z ( b : BIT_VECTOR
std_ulogic_vector;
To_X01Z ( b : BIT
X01Z;
FUNCTION
RETURN
FUNCTION
RETURN
FUNCTION
RETURN
FUNCTION
RETURN
FUNCTION
RETURN
FUNCTION
RETURN
To_UX01
std_logic_vector;
To_UX01
std_ulogic_vector;
To_UX01
UX01;
To_UX01
std_logic_vector;
To_UX01
std_ulogic_vector;
To_UX01
UX01;
( s : std_logic_vector
)
)
)
)
)
)
)
)
)
)
)
)
)
( s : std_ulogic_vector )
( s : std_ulogic
)
( b : BIT_VECTOR
)
( b : BIT_VECTOR
)
( b : BIT
)
-------------------------------------------------------- edge detection
-------------------------------------------------------
Appendix A:
Standard Logic Package
417
FUNCTION rising_edge (SIGNAL s : std_ulogic) RETURN
BOOLEAN;
FUNCTION falling_edge (SIGNAL s : std_ulogic) RETURN
BOOLEAN;
-------------------------------------------------------- object contains an unknown
------------------------------------------------------FUNCTION Is_X ( s : std_ulogic_vector ) RETURN
BOOLEAN;
FUNCTION Is_X ( s : std_logic_vector ) RETURN
BOOLEAN;
FUNCTION Is_X ( s : std_ulogic
) RETURN
BOOLEAN;
END std_logic_1164;
------------------------------------------
-------------------------------------------------------Title
Library
: std_logic_1164 multi-value logic system
: This package shall be compiled into a
: library symbolically named IEEE.
:
Developers’ : IEEE model standards group (par 1164)
Purpose
: This package defines a standard for
: designers to use in describing the
: interconnection data types used in vhdl
: modeling.
:
Limitation : The logic system defined in this
: package may be insufficient for modeling
: switched transistors, since such a
: requirement is out of the scope of this
: effort. Furthermore, mathematics,
: primitives, timing standards, etc. are
: considered orthogonal issues as it
: relates to this package and are
: therefore beyond the scope of this
: effort.
:
Note
: No declarations or definitions shall be
: included in, or excluded from this
: package. The “package declaration”
: defines the types, subtypes and
: declarations of std_logic_1164. The
: std_logic_1164 package body shall be
: considered the formal definition of the
: semantics of this package. Tool
: developers may choose to implement the
: package body in the most efficient
: manner available to them.
:
-------------------------------------------------------modification history :
-------------------------------------------------------version | mod. date:|
v4.200 | 01/02/91 |
--------------------------------------------------------
PACKAGE BODY std_logic_1164 IS
418
Appendix A:
Standard Logic Package
-------------------------------------------------------- local types
------------------------------------------------------TYPE stdlogic_1d IS ARRAY (std_ulogic) OF std_ulogic;
TYPE stdlogic_table IS ARRAY(std_ulogic, std_ulogic) OF
std_ulogic;
-------------------------------------------------------- resolution function
------------------------------------------------------CONSTANT resolution_table : stdlogic_table := (
-- ---------------------------------------------------------| U
X
0
1
Z
W
L
H
|
|
-- -------------------------------------------------------( ‘U’, ‘U’, ‘U’, ‘U’, ‘U’, ‘U’, ‘U’, ‘U’, ‘U’ ), -- | U |
( ‘U’, ‘X’, ‘X’, ‘X’, ‘X’, ‘X’, ‘X’, ‘X’, ‘X’ ), -- | X |
( ‘U’, ‘X’, ‘0’, ‘X’, ‘0’, ‘0’, ‘0’, ‘0’, ‘X’ ), -- | 0 |
( ‘U’, ‘X’, ‘X’, ‘1’, ‘1’, ‘1’, ‘1’, ‘1’, ‘X’ ), -- | 1 |
( ‘U’, ‘X’, ‘0’, ‘1’, ‘Z’, ‘W’, ‘L’, ‘H’, ‘X’ ), -- | Z |
( ‘U’, ‘X’, ‘0’, ‘1’, ‘W’, ‘W’, ‘W’, ‘W’, ‘X’ ), -- | W |
( ‘U’, ‘X’, ‘0’, ‘1’, ‘L’, ‘W’, ‘L’, ‘W’, ‘X’ ), -- | L |
( ‘U’, ‘X’, ‘0’, ‘1’, ‘H’, ‘W’, ‘W’, ‘H’, ‘X’ ), -- | H |
( ‘U’, ‘X’, ‘X’, ‘X’, ‘X’, ‘X’, ‘X’, ‘X’, ‘X’ ), -- | - |
);
FUNCTION resolved ( s : std_ulogic_vector ) RETURN
std_ulogic IS
VARIABLE result : std_ulogic := ‘Z’; -- weakest state
default
BEGIN
-- the test for a single driver is essential
-- otherwise the loop would return ‘X’ for a
-- single driver of ‘-’ and that would conflict
-- with the value of a single driver unresolved
-- signal.
IF
(s’LENGTH = 1) THEN
RETURN s(s’LOW);
ELSE
FOR i IN s’RANGE LOOP
result := resolution_table(result, s(i));
END LOOP;
END IF;
RETURN result;
END resolved;
-------------------------------------------------------- tables for logical operations
-------------------------------------------------------- truth table for “and” function
CONSTANT and_table : stdlogic_table := (
-- ---------------------------------------------------------|U
X
0
1
Z
W
L
H
| |
-- -------------------------------------------------------( ‘U’, ‘U’, ‘0’, ‘U’, ‘U’, ‘U’, ‘0’, ‘U’, ‘U’ ), -- | U |
( ‘U’, ‘X’, ‘0’, ‘X’, ‘X’, ‘X’, ‘0’, ‘X’, ‘X’ ), -- | X |
( ‘0’, ‘0’, ‘0’, ‘0’, ‘0’, ‘0’, ‘0’, ‘0’, ‘0’ ), -- | 0 |
( ‘U’, ‘X’, ‘0’, ‘1’, ‘X’, ‘X’, ‘0’, ‘1’, ‘X’ ), -- | 1 |
( ‘U’, ‘X’, ‘0’, ‘X’, ‘X’, ‘X’, ‘0’, ‘X’, ‘X’ ), -- | Z |
( ‘U’, ‘X’, ‘0’, ‘X’, ‘X’, ‘X’, ‘0’, ‘X’, ‘X’ ), -- | W |
( ‘0’, ‘0’, ‘0’, ‘0’, ‘0’, ‘0’, ‘0’, ‘0’, ‘0’ ), -- | L |
Appendix A:
Standard Logic Package
( ‘U’, ‘X’, ‘0’, ‘1’, ‘X’, ‘X’, ‘0’, ‘1’, ‘X’ ),
( ‘U’, ‘X’, ‘0’, ‘X’, ‘X’, ‘X’, ‘0’, ‘X’, ‘X’ ),
);
419
-- | H |
-- | - |
-- truth table for “or” function
CONSTANT or_table : stdlogic_table := (
-- ---------------------------------------------------------|U
X
0
1
Z
W
L
H
|
|
-- -------------------------------------------------------( ‘U’, ‘U’, ‘U’, ‘1’, ‘U’, ‘U’, ‘U’, ‘1’, ‘U’ ), -- | U |
( ‘U’, ‘X’, ‘X’, ‘1’, ‘X’, ‘X’, ‘X’, ‘1’, ‘X’ ), -- | X |
( ‘U’, ‘X’, ‘0’, ‘1’, ‘X’, ‘X’, ‘0’, ‘1’, ‘X’ ), -- | 0 |
( ‘1’, ‘1’, ‘1’, ‘1’, ‘1’, ‘1’, ‘1’, ‘1’, ‘1’ ), -- | 1 |
( ‘U’, ‘X’, ‘X’, ‘1’, ‘X’, ‘X’, ‘X’, ‘1’, ‘X’ ), -- | Z |
( ‘U’, ‘X’, ‘X’, ‘1’, ‘X’, ‘X’, ‘X’, ‘1’, ‘X’ ), -- | W |
( ‘U’, ‘X’, ‘0’, ‘1’, ‘X’, ‘X’, ‘0’, ‘1’, ‘X’ ), -- | L |
( ‘1’, ‘1’, ‘1’, ‘1’, ‘1’, ‘1’, ‘1’, ‘1’, ‘1’ ), -- | H |
( ‘U’, ‘X’, ‘X’, ‘1’, ‘X’, ‘X’, ‘X’, ‘1’, ‘X’ )
-- | - |
);
-- truth table for “xor” function
CONSTANT xor_table : stdlogic_table := (
-- ---------------------------------------------------------|U
X
0
1
Z
W
L
H
|
|
---------------------------------------------------( ‘U’, ‘U’, ‘U’, ‘U’, ‘U’, ‘U’, ‘U’, ‘U’, ‘U’ ), -- | U |
( ‘U’, ‘X’, ‘X’, ‘X’, ‘X’, ‘X’, ‘X’, ‘X’, ‘X’ ), -- | X |
( ‘U’, ‘X’, ‘0’, ‘1’, ‘X’, ‘X’, ‘0’, ‘1’, ‘X’ ), -- | 0 |
( ‘U’, ‘X’, ‘1’, ‘0’, ‘X’, ‘X’, ‘1’, ‘0’, ‘X’ ), -- | 1 |
( ‘U’, ‘X’, ‘X’, ‘X’, ‘X’, ‘X’, ‘X’, ‘X’, ‘X’ ), -- | Z |
( ‘U’, ‘X’, ‘X’, ‘X’, ‘X’, ‘X’, ‘X’, ‘X’, ‘X’ ), -- | W |
( ‘U’, ‘X’, ‘0’, ‘1’, ‘X’, ‘X’, ‘0’, ‘1’, ‘X’ ), -- | L |
( ‘U’, ‘X’, ‘1’, ‘0’, ‘X’, ‘X’, ‘1’, ‘0’, ‘X’ ), -- | H |
( ‘U’, ‘X’, ‘X’, ‘X’, ‘X’, ‘X’, ‘X’, ‘X’, ‘X’ )
-- | - |
);
-- truth table for “not” function
CONSTANT not_table: stdlogic_1d :=
-- ---------------------------------------------------------|U
X
0
1
Z
W
L
H
|
-- -------------------------------------------------------( ‘U’, ‘X’, ‘1’, ‘0’, ‘X’, ‘X’, ‘1’, ‘0’, ‘X’ );
------------------------------------------------------------- overloaded logical operators ( with optimizing hints )
-----------------------------------------------------------FUNCTION “and” ( l : std_ulogic; r : std_ulogic )
RETURN UX01 IS
BEGIN
RETURN (and_table(l, r));
END “and”;
FUNCTION “nand” ( l : std_ulogic; r : std_ulogic )
RETURN UX01 IS
BEGIN
RETURN (not_table ( and_table(l, r)));
END “nand”;
420
Appendix A:
Standard Logic Package
FUNCTION “or”
( l : std_ulogic; r : std_ulogic )
RETURN UX01 IS
BEGIN
RETURN (or_table(l, r));
END “or”;
FUNCTION “nor” ( l : std_ulogic; r : std_ulogic )
RETURN UX01 IS
BEGIN
RETURN (not_table ( or_table( l, r )));
END “nor”;
FUNCTION “xor” ( l : std_ulogic; r : std_ulogic )
RETURN UX01 IS
BEGIN
RETURN (xor_table(l, r));
END “xor”;
------
function “xnor” ( l : std_ulogic; r : std_ulogic )
return ux01 is
begin
return not_table(xor_table(l, r));
end “xnor”;
FUNCTION “not” ( l : std_ulogic ) RETURN UX01 IS
BEGIN
RETURN (not_table(l));
END “not”;
-------------------------------------------------------- and
------------------------------------------------------FUNCTION “and” ( l,r : std_logic_vector ) RETURN
std_logic_vector IS
ALIAS lv : std_logic_vector ( 1 TO l’LENGTH ) IS l;
ALIAS rv : std_logic_vector ( 1 TO r’LENGTH ) IS r;
VARIABLE result : std_logic_vector ( 1 TO
l’LENGTH );
BEGIN
IF ( l’LENGTH /= r’LENGTH ) THEN
ASSERT FALSE
REPORT “arguments of overloaded ‘and’ operator
are not of the same length”
SEVERITY FAILURE;
ELSE
FOR i IN result’RANGE LOOP
result(i) := and_table (lv(i), rv(i));
END LOOP;
END IF;
RETURN result;
END “and”;
------------------------------------------------------FUNCTION “and” ( l,r : std_ulogic_vector ) RETURN
std_ulogic_vector IS
ALIAS lv : std_ulogic_vector ( 1 TO l’LENGTH )
IS l;
ALIAS rv : std_ulogic_vector ( 1 TO r’LENGTH )
IS r;
VARIABLE result : std_ulogic_vector ( 1 TO
Appendix A:
Standard Logic Package
421
l’LENGTH );
BEGIN
IF ( l’LENGTH /= r’LENGTH ) THEN
ASSERT FALSE
REPORT “arguments of overloaded ‘and’ operator
are not of the same length”
SEVERITY FAILURE;
ELSE
FOR i IN result’RANGE LOOP
result(i) := and_table (lv(i), rv(i));
END LOOP;
END IF;
RETURN result;
END “and”;
-------------------------------------------------------- nand
------------------------------------------------------FUNCTION “nand” ( l,r : std_logic_vector ) RETURN
std_logic_vector IS
ALIAS lv : std_logic_vector ( 1 TO l’LENGTH )
IS l;
ALIAS rv : std_logic_vector ( 1 TO r’LENGTH )
IS r;
VARIABLE result : std_logic_vector ( 1 TO
l’LENGTH );
BEGIN
IF ( l’LENGTH /= r’LENGTH ) THEN
ASSERT FALSE
REPORT “arguments of overloaded ‘nand’
operator are not of the same length”
SEVERITY FAILURE;
ELSE
FOR i IN result’RANGE LOOP
result(i) := not_table(and_table (lv(i),
rv(i)));
END LOOP;
END IF;
RETURN result;
END “nand”;
------------------------------------------------------FUNCTION “nand” ( l,r : std_ulogic_vector ) RETURN
std_ulogic_vector IS
ALIAS lv : std_ulogic_vector ( 1 TO l’LENGTH )
IS l;
ALIAS rv : std_ulogic_vector ( 1 TO r’LENGTH )
IS r;
VARIABLE result : std_ulogic_vector ( 1 TO
l’LENGTH );
BEGIN
IF ( l’LENGTH /= r’LENGTH ) THEN
ASSERT FALSE
REPORT “arguments of overloaded ‘nand’
operator are not of the same length”
SEVERITY FAILURE;
ELSE
FOR i IN result’RANGE LOOP
result(i) := not_table(and_table (lv(i),
rv(i)));
END LOOP;
END IF;
422
Appendix A:
Standard Logic Package
RETURN result;
END “nand”;
-------------------------------------------------------- or
------------------------------------------------------FUNCTION “or” ( l,r : std_logic_vector ) RETURN
std_logic_vector IS
ALIAS lv : std_logic_vector ( 1 TO l’LENGTH )
IS l;
ALIAS rv : std_logic_vector ( 1 TO r’LENGTH )
IS r;
VARIABLE result : std_logic_vector ( 1 TO
l’LENGTH );
BEGIN
IF ( l’LENGTH /= r’LENGTH ) THEN
ASSERT FALSE
REPORT “arguments of overloaded ‘or’ operator
are not of the same length”
SEVERITY FAILURE;
ELSE
FOR i IN result’RANGE LOOP
result(i) := or_table (lv(i), rv(i));
END LOOP;
END IF;
RETURN result;
END “or”;
------------------------------------------------------FUNCTION “or” ( l,r : std_ulogic_vector ) RETURN
std_ulogic_vector IS
ALIAS lv : std_ulogic_vector ( 1 TO l’LENGTH )
IS l;
ALIAS rv : std_ulogic_vector ( 1 TO r’LENGTH )
IS r;
VARIABLE result : std_ulogic_vector ( 1 TO
l’LENGTH );
BEGIN
IF ( l’LENGTH /= r’LENGTH ) THEN
ASSERT FALSE
REPORT “arguments of overloaded ‘or’ operator
are not of the same length”
SEVERITY FAILURE;
ELSE
FOR i IN result’RANGE LOOP
result(i) := or_table (lv(i), rv(i));
END LOOP;
END IF;
RETURN result;
END “or”;
-------------------------------------------------------- nor
------------------------------------------------------FUNCTION “nor” ( l,r : std_logic_vector ) RETURN
std_logic_vector IS
ALIAS lv : std_logic_vector ( 1 TO l’LENGTH )
IS l;
ALIAS rv : std_logic_vector ( 1 TO r’LENGTH )
IS r;
VARIABLE result : std_logic_vector ( 1 TO
l’LENGTH );
BEGIN
Appendix A:
Standard Logic Package
423
IF ( l’LENGTH /= r’LENGTH ) THEN
ASSERT FALSE
REPORT “arguments of overloaded ‘nor’ operator
are not of the same length”
SEVERITY FAILURE;
ELSE
FOR i IN result’RANGE LOOP
result(i) := not_table(or_table (lv(i),
rv(i)));
END LOOP;
END IF;
RETURN result;
END “nor”;
------------------------------------------------------FUNCTION “nor” ( l,r : std_ulogic_vector ) RETURN
std_ulogic_vector IS
ALIAS lv : std_ulogic_vector ( 1 TO l’LENGTH )
IS l;
ALIAS rv : std_ulogic_vector ( 1 TO r’LENGTH )
IS r;
VARIABLE result : std_ulogic_vector ( 1 TO
l’LENGTH );
BEGIN
IF ( l’LENGTH /= r’LENGTH ) THEN
ASSERT FALSE
REPORT “arguments of overloaded ‘nor’ operator
are not of the same length”
SEVERITY FAILURE;
ELSE
FOR i IN result’RANGE LOOP
result(i) := not_table(or_table (lv(i),
rv(i)));
END LOOP;
END IF;
RETURN result;
END “nor”;
-------------------------------------------------------- xor
------------------------------------------------------FUNCTION “xor” ( l,r : std_logic_vector ) RETURN
std_logic_vector IS
ALIAS lv : std_logic_vector ( 1 TO l’LENGTH )
IS l;
ALIAS rv : std_logic_vector ( 1 TO r’LENGTH )
IS r;
VARIABLE result : std_logic_vector ( 1 TO
l’LENGTH );
BEGIN
IF ( l’LENGTH /= r’LENGTH ) THEN
ASSERT FALSE
REPORT “arguments of overloaded ‘xor’ operator
are not of the same length”
SEVERITY FAILURE;
ELSE
FOR i IN result’RANGE LOOP
result(i) := xor_table (lv(i), rv(i));
END LOOP;
END IF;
RETURN result;
424
Appendix A:
---------------------------------
Standard Logic Package
END “xor”;
------------------------------------------------------FUNCTION “xor” ( l,r : std_ulogic_vector ) RETURN
std_ulogic_vector IS
ALIAS lv : std_ulogic_vector ( 1 TO l’LENGTH )
IS l;
ALIAS rv : std_ulogic_vector ( 1 TO r’LENGTH )
IS r;
VARIABLE result : std_ulogic_vector ( 1 TO
l’LENGTH );
BEGIN
IF ( l’LENGTH /= r’LENGTH ) THEN
ASSERT FALSE
REPORT “arguments of overloaded ‘xor’ operator
are not of the same length”
SEVERITY FAILURE;
ELSE
FOR i IN result’RANGE LOOP
result(i) := xor_table (lv(i), rv(i));
END LOOP;
END IF;
RETURN result;
END “xor”;
-------------------------------------------------------- xnor
------------------------------------------------------------------------------------------------------------Note : The declaration and implementation of the “xnor”
function is specifically commented until at which time
the VHDL language has been officially adopted as
containing such a function. At such a point, the
following comments may be removed along with this
notice without further “official” ballotting of this
std_logic_1164 package. It is the intent of this effort
to provide such a function once it becomes available
in the VHDL standard.
------------------------------------------------------function “xnor” ( l,r : std_logic_vector ) return
std_logic_vector is
alias lv : std_logic_vector ( 1 to l’length )
is l;
alias rv : std_logic_vector ( 1 to r’length )
is r;
variable result : std_logic_vector ( 1 to
l’length );
begin
if ( l’length /= r’length ) then
assert false
report “arguments of overloaded ‘xnor’
operator are not of the same length”
severity failure;
else
for i in result’range loop
result(i) := not_table(xor_table (lv(i),
rv(i)));
end loop;
end if;
return result;
end “xnor”;
Appendix A:
Standard Logic Package
------------------
425
------------------------------------------------------function “xnor” ( l,r : std_ulogic_vector ) return
std_ulogic_vector is
alias lv : std_ulogic_vector ( 1 to l’length )
is l;
alias rv : std_ulogic_vector ( 1 to r’length )
is r;
variable result : std_ulogic_vector ( 1 to
l’length );
begin
if ( l’length /= r’length ) then
assert false
report “arguments of overloaded ‘xnor’
operator are not of the same length”
severity failure;
else
for i in result’range loop
result(i) := not_table(xor_table (lv(i),
rv(i)));
end loop;
end if;
return result;
end “xnor”;
-------------------------------------------------------- not
------------------------------------------------------FUNCTION “not” ( l : std_logic_vector ) RETURN
std_logic_vector IS
ALIAS lv : std_logic_vector ( 1 TO l’LENGTH )
IS l;
VARIABLE result : std_logic_vector ( 1 TO
l’LENGTH ) := (OTHERS => ‘X’);
BEGIN
FOR i IN result’RANGE LOOP
result(i) := not_table( lv(i) );
END LOOP;
RETURN result;
END;
------------------------------------------------------FUNCTION “not” ( l : std_ulogic_vector ) RETURN
std_ulogic_vector IS
ALIAS lv : std_ulogic_vector ( 1 TO l’LENGTH )
IS l;
VARIABLE result : std_ulogic_vector ( 1 TO
l’LENGTH ) := (OTHERS => ‘X’);
BEGIN
FOR i IN result’RANGE LOOP
result(i) := not_table( lv(i) );
END LOOP;
RETURN result;
END;
-------------------------------------------------------- conversion tables
------------------------------------------------------TYPE logic_x01_table IS ARRAY (std_ulogic’LOW TO
std_ulogic’HIGH) OF X01;
TYPE logic_x01z_table IS ARRAY (std_ulogic’LOW TO
426
Appendix A:
Standard Logic Package
std_ulogic’HIGH) OF X01Z;
TYPE logic_ux01_table IS ARRAY (std_ulogic’LOW TO
std_ulogic’HIGH) OF UX01;
-------------------------------------------------------- table name : cvt_to_x01
--- parameters :
-in : std_ulogic -- some logic value
-- returns
: x01
-- state value of logic
value
-- purpose
: to convert state-strength to state
only
--- example
: if (cvt_to_x01 (input_signal) = ‘1’ )
then ...
-------------------------------------------------------CONSTANT cvt_to_x01 : logic_x01_table := (
‘X’, -- ‘U’
‘X’, -- ‘X’
‘0’, -- ‘0’
‘1’, -- ‘1’
‘X’, -- ‘Z’
‘X’, -- ‘W’
‘0’, -- ‘L’
‘1’, -- ‘H’
‘X’
-- ‘-’
);
-------------------------------------------------------- table name : cvt_to_x01z
--- parameters :
-in : std_ulogic -- some logic value
-- returns
: x01z
-- state value of logic
value
-- purpose
: to convert state-strength to state
only
--- example
: if (cvt_to_x01z (input_signal) = ‘1’ )
then ...
-------------------------------------------------------CONSTANT cvt_to_x01z : logic_x01z_table := (
‘X’, -- ‘U’
‘X’, -- ‘X’
‘0’, -- ‘0’
‘1’, -- ‘1’
‘Z’, -- ‘Z’
‘X’, -- ‘W’
‘0’, -- ‘L’
‘1’, -- ‘H’
‘X’
-- ‘-’
);
-------------------------------------------------------- table name : cvt_to_ux01
--- parameters :
-in : std_ulogic
-- some logic value
Appendix A:
Standard Logic Package
-- returns
-- purpose
--- example
427
: ux01
-- state value of logic
value
: to convert state-strength to state
only
: if (cvt_to_ux01 (input_signal) = ‘1’ )
then ...
-------------------------------------------------------CONSTANT cvt_to_ux01 : logic_ux01_table := (
‘U’, -- ‘U’
‘X’, -- ‘X’
‘0’, -- ‘0’
‘1’, -- ‘1’
‘X’, -- ‘Z’
‘X’, -- ‘W’
‘0’, -- ‘L’
‘1’, -- ‘H’
‘X’
-- ‘-’
);
-------------------------------------------------------- conversion functions
------------------------------------------------------FUNCTION To_bit
( s : std_ulogic;
xmap :
BIT := ‘0’) RETURN BIT IS
BEGIN
CASE s IS
WHEN ‘0’ | ‘L’ => RETURN (‘0’);
WHEN ‘1’ | ‘H’ => RETURN (‘1’);
WHEN OTHERS => RETURN xmap;
END CASE;
END;
------------------------------------------------------FUNCTION To_bitvector ( s : std_logic_vector ; xmap :
BIT := ‘0’) RETURN BIT_VECTOR
IS
ALIAS sv : std_logic_vector ( s’LENGTH-1 DOWNTO
0 ) IS s;
VARIABLE result : BIT_VECTOR ( s’LENGTH-1 DOWNTO
0 );
BEGIN
FOR i IN result’RANGE LOOP
CASE sv(i) IS
WHEN ‘0’ | ‘L’ => result(i) := ‘0’;
WHEN ‘1’ | ‘H’ => result(i) := ‘1’;
WHEN OTHERS => result(i) := xmap;
END CASE;
END LOOP;
RETURN result;
END;
------------------------------------------------------FUNCTION To_bitvector ( s : std_ulogic_vector; xmap :
BIT := ‘0’) RETURN BIT_VECTOR
IS
ALIAS sv : std_ulogic_vector ( s’LENGTH-1 DOWNTO
0 ) IS s;
VARIABLE result : BIT_VECTOR ( s’LENGTH-1 DOWNTO
0 );
BEGIN
428
Appendix A:
Standard Logic Package
FOR i IN result’RANGE LOOP
CASE sv(i) IS
WHEN ‘0’ | ‘L’ => result(i) := ‘0’;
WHEN ‘1’ | ‘H’ => result(i) := ‘1’;
WHEN OTHERS => result(i) := xmap;
END CASE;
END LOOP;
RETURN result;
END;
------------------------------------------------------FUNCTION To_StdULogic
( b : BIT
)
RETURN std_ulogic IS
BEGIN
CASE b IS
WHEN ‘0’ => RETURN ‘0’;
WHEN ‘1’ => RETURN ‘1’;
END CASE;
END;
------------------------------------------------------FUNCTION To_StdLogicVector ( b : BIT_VECTOR
)
RETURN std_logic_vector
IS
ALIAS bv : BIT_VECTOR ( b’LENGTH-1 DOWNTO 0 )
IS b;
VARIABLE result : std_logic_vector ( b’LENGTH-1
DOWNTO 0 );
BEGIN
FOR i IN result’RANGE LOOP
CASE bv(i) IS
WHEN ‘0’ => result(i) := ‘0’;
WHEN ‘1’ => result(i) := ‘1’;
END CASE;
END LOOP;
RETURN result;
END;
------------------------------------------------------FUNCTION To_StdLogicVector ( s : std_ulogic_vector )
RETURN std_logic_vector
IS
ALIAS sv : std_ulogic_vector ( s’LENGTH-1 DOWNTO
0 ) IS s;
VARIABLE result : std_logic_vector ( s’LENGTH-1
DOWNTO 0 );
BEGIN
FOR i IN result’RANGE LOOP
result(i) := sv(i);
END LOOP;
RETURN result;
END;
------------------------------------------------------FUNCTION To_StdULogicVector ( b : BIT_VECTOR
)
RETURN std_ulogic_vector
IS
ALIAS bv : BIT_VECTOR ( b’LENGTH-1 DOWNTO 0 )
IS b;
VARIABLE result : std_ulogic_vector ( b’LENGTH-1
DOWNTO 0 );
BEGIN
FOR i IN result’RANGE LOOP
CASE bv(i) IS
Appendix A:
Standard Logic Package
429
WHEN ‘0’ => result(i) := ‘0’;
WHEN ‘1’ => result(i) := ‘1’;
END CASE;
END LOOP;
RETURN result;
END;
------------------------------------------------------FUNCTION To_StdULogicVector ( s : std_logic_vector )
RETURN std_ulogic_vector IS
ALIAS sv : std_logic_vector ( s’LENGTH-1 DOWNTO 0
) IS s;
VARIABLE result : std_ulogic_vector ( s’LENGTH-1
DOWNTO 0 );
BEGIN
FOR i IN result’RANGE LOOP
result(i) := sv(i);
END LOOP;
RETURN result;
END;
--------------------------------------------------------- strength strippers and type convertors
--------------------------------------------------------- to_x01
-------------------------------------------------------FUNCTION To_X01 ( s : std_logic_vector ) RETURN
std_logic_vector IS
ALIAS sv : std_logic_vector ( 1 TO s’LENGTH ) IS s;
VARIABLE result : std_logic_vector ( 1 TO s’LENGTH
);
BEGIN
FOR i IN result’RANGE LOOP
result(i) := cvt_to_x01 (sv(i));
END LOOP;
RETURN result;
END;
-------------------------------------------------------FUNCTION To_X01 ( s : std_ulogic_vector ) RETURN
std_ulogic_vector IS
ALIAS sv : std_ulogic_vector ( 1 TO s’LENGTH ) IS
s;
VARIABLE result : std_ulogic_vector ( 1 TO
s’LENGTH );
BEGIN
FOR i IN result’RANGE LOOP
result(i) := cvt_to_x01 (sv(i));
END LOOP;
RETURN result;
END;
-------------------------------------------------------FUNCTION To_X01 ( s : std_ulogic ) RETURN X01 IS
BEGIN
RETURN (cvt_to_x01(s));
END;
-------------------------------------------------------FUNCTION To_X01 ( b : BIT_VECTOR ) RETURN
std_logic_vector IS
ALIAS bv : BIT_VECTOR ( 1 TO b’LENGTH ) IS b;
VARIABLE result : std_logic_vector ( 1 TO b’LENGTH
);
430
Appendix A:
Standard Logic Package
BEGIN
FOR i IN result’RANGE LOOP
CASE bv(i) IS
WHEN ‘0’ => result(i) := ‘0’;
WHEN ‘1’ => result(i) := ‘1’;
END CASE;
END LOOP;
RETURN result;
END;
-------------------------------------------------------FUNCTION To_X01 ( b : BIT_VECTOR ) RETURN
std_ulogic_vector IS
ALIAS bv : BIT_VECTOR ( 1 TO b’LENGTH ) IS b;
VARIABLE result : std_ulogic_vector ( 1 TO b’LENGTH
);
BEGIN
FOR i IN result’RANGE LOOP
CASE bv(i) IS
WHEN ‘0’ => result(i) := ‘0’;
WHEN ‘1’ => result(i) := ‘1’;
END CASE;
END LOOP;
RETURN result;
END;
-------------------------------------------------------FUNCTION To_X01 ( b : BIT ) RETURN X01 IS
BEGIN
CASE b IS
WHEN ‘0’ => RETURN(‘0’);
WHEN ‘1’ => RETURN(‘1’);
END CASE;
END;
--------------------------------------------------------- to_x01z
-------------------------------------------------------FUNCTION To_X01Z ( s : std_logic_vector ) RETURN
std_logic_vector IS
ALIAS sv : std_logic_vector ( 1 TO s’LENGTH ) IS s;
VARIABLE result : std_logic_vector ( 1 TO s’LENGTH
);
BEGIN
FOR i IN result’RANGE LOOP
result(i) := cvt_to_x01z (sv(i));
END LOOP;
RETURN result;
END;
-------------------------------------------------------FUNCTION To_X01Z ( s : std_ulogic_vector ) RETURN
std_ulogic_vector IS
ALIAS sv : std_ulogic_vector ( 1 TO s’LENGTH ) IS
s;
VARIABLE result : std_ulogic_vector ( 1 TO s’LENGTH
);
BEGIN
FOR i IN result’RANGE LOOP
result(i) := cvt_to_x01z (sv(i));
END LOOP;
RETURN result;
END;
--------------------------------------------------------
Appendix A:
Standard Logic Package
431
FUNCTION To_X01Z ( s : std_ulogic ) RETURN X01Z IS
BEGIN
RETURN (cvt_to_x01z(s));
END;
-------------------------------------------------------FUNCTION To_X01Z ( b : BIT_VECTOR ) RETURN
std_logic_vector IS
ALIAS bv : BIT_VECTOR ( 1 TO b’LENGTH ) IS b;
VARIABLE result : std_logic_vector ( 1 TO b’LENGTH
);
BEGIN
FOR i IN result’RANGE LOOP
CASE bv(i) IS
WHEN ‘0’ => result(i) := ‘0’;
WHEN ‘1’ => result(i) := ‘1’;
END CASE;
END LOOP;
RETURN result;
END;
-------------------------------------------------------FUNCTION To_X01Z ( b : BIT_VECTOR ) RETURN
std_ulogic_vector IS
ALIAS bv : BIT_VECTOR ( 1 TO b’LENGTH ) IS b;
VARIABLE result : std_ulogic_vector ( 1 TO b’LENGTH
);
BEGIN
FOR i IN result’RANGE LOOP
CASE bv(i) IS
WHEN ‘0’ => result(i) := ‘0’;
WHEN ‘1’ => result(i) := ‘1’;
END CASE;
END LOOP;
RETURN result;
END;
-------------------------------------------------------FUNCTION To_X01Z ( b : BIT ) RETURN X01Z IS
BEGIN
CASE b IS
WHEN ‘0’ => RETURN(‘0’);
WHEN ‘1’ => RETURN(‘1’);
END CASE;
END;
--------------------------------------------------------- to_ux01
-------------------------------------------------------FUNCTION To_UX01 ( s : std_logic_vector ) RETURN
std_logic_vector IS
ALIAS sv : std_logic_vector ( 1 TO s’LENGTH ) IS s;
VARIABLE result : std_logic_vector ( 1 TO s’LENGTH
);
BEGIN
FOR i IN result’RANGE LOOP
result(i) := cvt_to_ux01 (sv(i));
END LOOP;
RETURN result;
END;
-------------------------------------------------------FUNCTION To_UX01 ( s : std_ulogic_vector ) RETURN
std_ulogic_vector IS
432
Appendix A:
Standard Logic Package
ALIAS sv : std_ulogic_vector ( 1 TO s’LENGTH ) IS
s;
VARIABLE result : std_ulogic_vector ( 1 TO s’LENGTH
);
BEGIN
FOR i IN result’RANGE LOOP
result(i) := cvt_to_ux01 (sv(i));
END LOOP;
RETURN result;
END;
-------------------------------------------------------FUNCTION To_UX01 ( s : std_ulogic ) RETURN UX01 IS
BEGIN
RETURN (cvt_to_ux01(s));
END;
-------------------------------------------------------FUNCTION To_UX01 ( b : BIT_VECTOR ) RETURN
std_logic_vector IS
ALIAS bv : BIT_VECTOR ( 1 TO b’LENGTH ) IS b;
VARIABLE result : std_logic_vector ( 1 TO b’LENGTH
);
BEGIN
FOR i IN result’RANGE LOOP
CASE bv(i) IS
WHEN ‘0’ => result(i) := ‘0’;
WHEN ‘1’ => result(i) := ‘1’;
END CASE;
END LOOP;
RETURN result;
END;
-------------------------------------------------------FUNCTION To_UX01 ( b : BIT_VECTOR ) RETURN
std_ulogic_vector IS
ALIAS bv : BIT_VECTOR ( 1 TO b’LENGTH ) IS b;
VARIABLE result : std_ulogic_vector ( 1 TO b’LENGTH
);
BEGIN
FOR i IN result’RANGE LOOP
CASE bv(i) IS
WHEN ‘0’ => result(i) := ‘0’;
WHEN ‘1’ => result(i) := ‘1’;
END CASE;
END LOOP;
RETURN result;
END;
-------------------------------------------------------FUNCTION To_UX01 ( b : BIT ) RETURN UX01 IS
BEGIN
CASE b IS
WHEN ‘0’ => RETURN(‘0’);
WHEN ‘1’ => RETURN(‘1’);
END CASE;
END;
--------------------------------------------------------- edge detection
-------------------------------------------------------FUNCTION rising_edge (SIGNAL s : std_ulogic) RETURN
BOOLEAN IS
Appendix A:
Standard Logic Package
433
BEGIN
RETURN (s’EVENT AND (To_X01(s) = ‘1’) AND
(To_X01(s’LAST_VALUE) = ‘0’));
END;
FUNCTION falling_edge (SIGNAL s : std_ulogic) RETURN
BOOLEAN IS
BEGIN
RETURN (s’EVENT AND (To_X01(s) = ‘0’) AND
(To_X01(s’LAST_VALUE) = ‘1’));
END;
--------------------------------------------------------- object contains an unknown
-------------------------------------------------------FUNCTION Is_X ( s : std_ulogic_vector ) RETURN BOOLEAN
IS
BEGIN
FOR i IN s’RANGE LOOP
CASE s(i) IS
WHEN ‘U’ | ‘X’ | ‘Z’ | ‘W’ | ‘-’ => RETURN
TRUE;
WHEN OTHERS => NULL;
END CASE;
END LOOP;
RETURN FALSE;
END;
------------------------------------------------------FUNCTION Is_X ( s : std_logic_vector ) RETURN BOOLEAN
IS
BEGIN
FOR i IN s’RANGE LOOP
CASE s(i) IS
WHEN ‘U’ | ‘X’ | ‘Z’ | ‘W’ | ‘-’ => RETURN
TRUE;
WHEN OTHERS => NULL;
END CASE;
END LOOP;
RETURN FALSE;
END;
-----------------------------------------------------------FUNCTION Is_X ( s : std_ulogic
) RETURN BOOLEAN
IS
BEGIN
CASE s IS
WHEN ‘U’ | ‘X’ | ‘Z’ | ‘W’ | ‘-’ => RETURN
TRUE;
WHEN OTHERS => NULL;
END CASE;
RETURN FALSE;
END;
END std_logic_1164;
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Appendix B
VHDL Reference Tables
This appendix focuses on tables of information that are useful when writing VHDL descriptions. Most of the information in the tables is available
in the text of the book, however, these tables consolidate the information
into one area for easy reference.
Table B-1 lists all of the different kinds of statements alphabetically
and includes an example usage.
Table B-1
Statement or Clause
Example(s)
Access Type
TYPE access_type IS ACCESS type_to_be_accessed;
Aggregate
record_type := (first, second, third);
Alias
ALIAS opcode : BIT_VECTOR (0 TO 3) IS
INSTRUCTION(10 TO 13);
Architecture
ARCHITECTURE architecture_name OF entity
name IS
-- declare some signals here
BEGIN
-- put some concurrent statements here
END architecture_name;
Array Type
TYPE array_type IS ARRAY (0 TO 7) OF BIT;
Assert
ASSERT x > 10 REPORT “x is too small”
SEVERITY ERROR;
Attribute Declaration
ATTRIBUTE attribute_name : attribute_type;
Attribute Specification
ATTRIBUTE attribute_name OF
entity_name : entity_class IS value;
Block Statement
block_name : BLOCK
-- declare some stuff here
BEGIN
- put some concurrent statements here
END BLOCK block_name;
Case Statement
CASE some_expression IS
WHEN some_value =>
-- do_some_stuff
WHEN some_other_value =>
-- do_some_other_stuff
WHEN OTHERS =>
-- do_some_default_stuff
END CASE;
436
Table B-1
Continued.
Appendix B: VHDL Reference Tables
Statement or Clause
Example(s)
Component Declaration
COMPONENT component_name
PORT (port1_name : port1_type;
port2_name : port2_type;
port3_name : port3_type);
END COMPONENT;
Component Instantiation
instance_name : component_name PORT MAP
(first_port, second_port, third_port);
instance_name : component_name PORT MAP
(formal1 => actual1, formal2 => actual2);
Conditional Signal Assignment
target <= first_value WHEN (x = y) ELSE
second_value WHEN a >= b ELSE
third_value;
Configuration Declaration
CONFIGURATION configuration_name OF
entity_name IS
FOR architecture_name
FOR instance_name : entity_name USE
ENTITY
library_name.entity_name
(architecture_name);
END FOR;
FOR instance_name : entity_name USE
CONFIGURATION
library_name.configuration_name;
END FOR;
END FOR;
END configuration_name;
Constant Declaration
CONSTANT constant_name : constant_type :=
value;
Entity Declaration
ENTITY entity_name IS
PORT (port1 : port1_type;
port2 : port2_type);
END entity_name;
Exit Statement
EXIT;
EXIT WHEN a <= b;
EXIT loop_label WHEN x = z;
File Type Declaration
TYPE file_type_name IS FILE OF data_type;
File Object Declaration
FILE file_object_name : file_type_name IS IN
“/absolute/path/name”;
For Loop
FOR loop_variable IN start TO end LOOP
-- do_some_stuff
END LOOP;
Appendix B: VHDL Reference Tables
Table B-1
437
Statement or Clause
Example(s)
Function Declaration
FUNCTION function_name (parameter1 :
parameter1_type;
parameter2 :
parameter2_type)
RETURN return_type;
Function Body
FUNCTION function_name (parameter1 :
parameter1_type;
parameter2 :
parameter2_type)
RETURN return_type IS
BEGIN
-- do some stuff
END function_name;
Generate Statement
generate_label : FOR gen_var IN start TO end
GENERATE label : component_name PORT
MAP (.........);
END GENERATE;
Generic Declaration
GENERIC (generic1_name : generic1_type;
generic2_name : generic2_type);
Generic Map
value2);
GENERIC MAP (generic1_name => value1,
Guarded Signal Assignment
g1 : BLOCK (clk = ‘1’ AND clk’EVENT)
BEGIN
q <= GUARDED d AFTER 5 NS;
END BLOCK;
IF Statement
IF x <= y THEN
-- some statements
END IF;
IF z > w THEN
-- some statements
ELSIF q < r THEN
-- some more statements
END IF;
IF a = b THEN
-- some statements
ELSIF c = d THEN
-- some more statements
ELSE
-- even more statements
END IF;
Incomplete Type
TYPE type_name;
438
Table B-1
Continued.
Appendix B: VHDL Reference Tables
Statement or Clause
Example(s)
Library Declaration
LIBRARY library_name;
Loop Statement
FOR loop_variable IN start TO end LOOP
-- do lots of stuff
END LOOP;
WHILE x < y LOOP
-- modify x and y and do other stuff
END LOOP;
Next Statement
IF i < 0 THEN
NEXT;
END IF;
Others Clause
WHEN OTHERS =>
-- do some stuff
Package Declaration
PACKAGE package_name IS
-- declare some stuff
END PACKAGE;
Package Body
PACKAGE BODY package_name IS
--put subprogram bodies here
END package_name;
Physical Type
TYPE physical_type_name IS RANGE start TO end
UNITS
unit1 ;
unit2 = 10 unit1;
unit3 = 10 unit2;
END UNITS;
Port Clause
PORT ( port1_name : port1_type; port2_name :
port2_type);
Port Map Clause
PORT MAP (port1_name => signal1, signal2);
Procedure Declaration
PROCEDURE procedure_name (parm1 : in
parm1_type; parm2 : out
parm2_type; parm3 :
inout parm3_type);
Procedure Body
PROCEDURE procedure_name (parm1 : in
parm1_type; parm2 : out
parm1_type; parm3 :
inout parm3_type) IS
BEGIN
-- do some stuff
END procedure_name;
Appendix B: VHDL Reference Tables
Table B-1
439
Statement or Clause
Example(s)
Process Statement
PROCESS (signal1, signal2, signal3)
-- declare some stuff
BEGIN
-- do some stuff
END PROCESS;
Record Type
TYPE record_type IS
RECORD
field1 : field1_type;
field2 : field2_type;
END RECORD;
Report Clause
ASSERT x = 10 REPORT “some string”;
Return Statement
RETURN;
RETURN (x + 10);
Selected Signal Assignment
WITH z SELECT
x <= 1 AFTER 5 NS WHEN 0,
2 AFTER 5 NS WHEN 1,
3 AFTER 5 NS WHEN OTHERS;
Severity Clause
ASSERT x > 5 REPORT “some string” SEVERITY
ERROR;
Signal Assignment
a <= b AFTER 20 NS;
Signal Declaration
SIGNAL x : xtype;
Subtype Declaration
SUBTYPE bit8 IS INTEGER RANGE 0 TO 255;
Transport Signal Assignment
x <= TRANSPORT y AFTER 50 NS;
Type Declaration
TYPE color is (red, yellow, blue, green, orange);
TYPE small_int is 0 to 65535;
Use Clause
USE WORK.my_package.all;
Variable Declaration
VARIABLE variable_name : variable_type;Wait
Statement WAIT ON a, b, c;
WAIT UNTIL clock’EVENT AND clock = ‘1’;
WAIT FOR 100 NS;
WAIT ON a, b UNTIL b > 10 FOR 50 NS;
While Loop
WHILE x > 15 LOOP
-- do some stuff
END LOOP;
Table B-2 lists all of the predefined attributes that retrieve information about VHDL type data. The descriptions are necessarily terse
to fit into the table cells; see Chapter 6, “Predefined Attributes” for
more detailed information.
440
Table B-2
Appendix B: VHDL Reference Tables
Attribute
Explanation
Examples
T’BASE
Returns the base type of
datatype it is attached to
NATURAL’BASE returns
INTEGER
T’LEFT
Returns left value specified in
type declaration
INTEGER’LEFT is -2147483647
BIT’LEFT is ‘0’
T’RIGHT
Returns right value specified
in type declaration
INTEGER’RIGHT is 2147483647
BIT’RIGHT is ‘1’
T’HIGH
Returns largest value specified
in declaration
TYPE bit8 is 255 downto 0
bit8’HIGH is 255
T’LOW
Returns smallest value
specified in declaration
TYPE bit8 is 255 downto 0
bit8’LOW is 0
T’POS(X)
Returns position number of
argument in type (first
position is 0)
TYPE color IS (red, green,
blue, orange);
color’POS(green) is 1
T’VAL(X)
Returns value in type at
specified position number
TYPE color IS (red, green,
blue, orange);
color’VAL(2) is blue
T’SUCC(X)
Returns the successor to the
value passed in
TYPE color IS (red,
green, blue, orange);
color’SUCC(green) is
blue
T’PRED(X)
Returns the predecessor to
the value passed in
TYPE color IS (red, green,
blue, orange);
color’PRED(blue) is green
T’LEFTOF(X)
Returns the value to the left
of the value passed in
TYPE color IS (red, green,
blue, orange);
color’LEFTOF(green) is
red
T’RIGHTOF(X)
Returns the value to the right
of the value passed in
TYPE color IS (red, green,
blue, orange);
color’RIGHTOF(blue) is
orange
Table B-3 lists all predefined attributes that return information about
array datatypes. The N parameter for all attributes specifies to which particular range the attribute is being applied. This only makes sense for
multidimensional arrays. For single-dimensional arrays, the parameter
can be ignored. For more detailed information, see Chapter 6, “Predefined
Attributes.”
Appendix B: VHDL Reference Tables
Table B-3
441
Attribute
Explanation
Example
A’LEFT(N)
Returns left array bound
of selected index range
a_type’LEFT(1) is 0
a_type’LEFT(2) is 7
A’RIGHT(N)
Returns right array bound
of selected index range
a_type’RIGHT(1) is 3
a_type’RIGHT(2) is 0
A’HIGH(N)
Returns largest array
bound value of selected
index range
a_type’HIGH(1) is 3
a_type’HIGH(2) is 7
A’LOW(N)
Returns smallest array
bound value of selected
index range
a_type’LOW(1) is 0
a_type’LOW(2) is 0
A’RANGE(N)
Returns selected index
range
a_type’RANGE(1) is 0
TO 3
a_type’RANGE(2) is 7
DOWNTO 0
A’REVERSE_RANGE(N)
Returns selected index
range reversed
a_type’REVERSE_RANGE(1)
is 3
DOWNTO 0
a_type’REVERSE_RANGE(2)
is 0 TO 7
A’LENGTH(N)
Returns size of selected
index range
a_type’LENGTH(1) is 4
a_type’LENGTH(2) is 8
All of the next examples apply to the following declaration:
TYPE a_type IS ARRAY(0 TO 3, 7 DOWNTO 0) OF BIT;
Table B-4 lists all predefined attributes that return information about
signals or create new signals. For more detailed information, see Chapter
6, “Predefined Attributes.”
Table B-5 lists all of the operators and their relative precedence.
Table B-6 lists all of the different types of literals and a sample usage.
In all cases, the _ character is ignored when interpreting the value of
a literal. The base that the exponent in the based integer and based real
examples is applied to is the base specified for interpreting the number.
Bit string literals are used to specify values for types that resemble the
BIT_VECTOR type.
442
Table B-4
Table B-5
Appendix B: VHDL Reference Tables
Attribute
Explanation
Example
S’DELAYED(T)
Creates a new signal delayed by T
clock’DELAYED(10 ns)
S’QUIET(T)
Creates a new signal that is true
when signal S has had no
transactions for time T; otherwise,
false
reset’QUIET(5 ns)
S’STABLE(T)
Creates a new signal that is true
when signal S has had no events
for time T; otherwise, false
clock’STABLE(1 ns)
S’TRANSACTION
Creates a signal of type BIT that
toggles for every transaction on
signal S
load’TRANSACTION
S’EVENT
Returns true when an event has
occurred for signal S this delta
clock’EVENT
S’ACTIVE
Returns true when a transaction
has occurred for signal S this delta
load’ACTIVE
S’LAST_EVENT
Returns the elapsed time since the
last event on signal S
data’LAST_EVENT
S’LAST_ACTIVE
Returns the elapsed time since the
last transaction on signal S
clock’LAST_ACTIVE
S’LAST_VALUE
Returns the previously assigned
value of signal S
data’LAST_VALUE
Precedence
Operator Class
Operator
Highest
Miscellaneous
**, ABS, NOT
Multiplying
*, /, MOD, REM
Sign
+, -
Adding
+, -, &
Relational
=, /=, <, <=, >, >=
Logical
AND, OR, NAND, NOR, XOR
Lowest
Appendix B: VHDL Reference Tables
Table B-6
443
Literal Type
Example
Decimal Integer
52
0
3E3
-- equals 3000
1_000_000 -- equals 1 million
Decimal Real
52.0
0.0
.178
1.222_333
Decimal Real with Exponent
1.2E+10
4.6E-9
Based Integer
16#FF# -- equals 255
8#777# -- equals 511
2#1101_0101# -- equals 213
16#FF#E1 -- equals 4080
Based Real
2#11.11#
16#AB.CD#E+2
8#77.66#E-10
Character
'a'
'*'
''
String
-- the space character
“this is a string”
""
-- empty string
“ABC” & “CDE” -- concatenation
Bit String
X”FFEF”
O”770770”
B”1111_0000_1111”
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Appendix C
Reading VHDL BNF
After the basic concepts of VHDL are understood, the designer might
want to try to write VHDL in a more elegant manner. To fully understand
how to apply all of the syntactic constructs available in VHDL, it is helpful
to know how to read the VHDL Bachus-Naur format (BNF) of the language. This format is in Appendix A of the IEEE Std 1076-1987 VHDL
Language Reference Manual (LRM), pages A1 to A17. This format
specifies which constructs are necessary versus optional, or repeatable
versus singular, and how constructs can be associated.
BNF is basically a hierarchical description method, where complex
constructs are made of successive specifications of lower-level constructs.
Our purpose for examining BNF is not to understand every nuance of the
BNF but to put the basics to use to help build complex VHDL constructs.
To this end, let us examine some BNF and discuss what it means.
Following is the BNF for the IF statement:
if_statement ::=
IF condition THEN
sequence_of_statements
{ELSIF condition THEN
sequence_of_statements}
[ELSE
sequence_of_statements]
END IF;
The first line of the BNF description specifies the name of the construct
being described. This line is read as follows: “The IF statement consists
of,” or “The IF statement is constructed from.” The rest of the description
represents the rules for constructing an IF statement.
The second line of the description specifies that the IF statement starts
with the keyword IF, is followed by a condition construct, and ends the
clause with the keyword THEN. The next line contains the construct
SEQUENCE_OF_STATEMENTS (which is discussed later in this appendix).
All of the constructs discussed so far are required for the IF statement
because the constructs are not enclosed in any kind of punctuation.
Statements enclosed in brackets [ ], as in lines 6 and 7, are optional
constructs. An optional construct can be specified or left out depending on
the functionality required. The ELSE clause of the IF statement is an
example of an optional construct. A legal IF statement may or may not
have an ELSE clause.
446
Appendix C: Reading VHDL BNF
Statements enclosed in curly braces { }, as in lines 4 and 5, are
optional and repeatable constructs. An optional and repeatable construct
can either be left out or have one or more of the construct exist. The
ELSIF clause is an example of an optional and repeatable construct. The IF
statement can be constructed without an ELSIF clause, or have one or
more ELSIF clauses, depending on the desired behavior.
The last line of the IF_STATEMENT description contains the END IF
clause. This is a required clause because it is not optional [ ] and is not
optional and repeatable { }.
The IF statement contains two other constructs that need more
description: the SEQUENCE_OF_STATEMENTS and the CONDITION. The
SEQUENCE_OF_STATEMENTS construct is described by the BNF shown here:
sequence_of_statements ::=
{sequential_statement}
The SEQUENCE_OF_STATEMENTS construct is described by one or more
sequential statements, where a sequential statement is described in the
following:
sequential_statement ::=
wait_statement
| assertion_statement
| signal_assignment_statement
| variable_assignment_statement
| procedure_call_statement
| if_statement
| case_statement
| loop_statement
| next_statement
| exit_statement
| return_statement
| null_statement
The | character means OR, such that a sequential statement can be a
WAIT statement, or an ASSERT statement, or a SIGNAL ASSIGNMENT statement, and so on. From this description, we can see that the statement part
of the IF statement can contain one or more sequential statements, such
as WAIT statements, ASSERT statements, and so on.
The CONDITION construct is specified with the BNF description shown
here:
condition ::= boolean_expression
Notice that the keyword boolean is italic. The italic indicates the type
of the expression required for the CONDITION. If a designer looks for a
Appendix C: Reading VHDL BNF
447
boolean expression construct to describe the syntax required, none will be
found. The reason is that all expressions share the same syntax description.
For our purposes, the boolean type of the expression is ignored, and the
construct description can be found under the following description:
expression ::=
relation {and relation}
|relation {or relation}
|relation {xor relation}
| relation [nand relation]
|relation [nor relation]
To summarize, curly braces { } are optional and repeatable constructs,
square brackets [ ] are optional constructs, and italic pieces of a construct can be ignored for purposes of finding descriptions.
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Appendix D
VHDL93 Updates
Early in 1993 the VHDL language standard was updated to reflect a
number of shortcomings with the VHDL 1076-1987 standard and to add
some new features to the language. This new standard is called VHDL
1076-1993. In this appendix the 1987 standard will be referred to as
VHDL87 and the 1993 standard as VHDL93.
The goal of this appendix is not to give the user a complete description
of every new or changed feature, but to give the reader an idea of the scope
of these changes and what effect they will have on future VHDL modeling efforts.
The goal of the update was to remain compatible with VHDL87 so that
VHDL87 models would work in a VHDL93 environment. This goal was
not entirely achieved as some of the new features were no longer compatible. The main reason for the incompatibility was the use of new keywords
in VHDL93, that may have been used as identifiers in VHDL87, and a major update of TEXTIO.
The rest of this appendix includes discussions of the VHDL87 features
that have either been added or changed. They are listed in alphabetical
order for easier access.
Alias
The alias clause has been generalized for VHDL93. In VHDL87 an alias
was used to give an alternate name to an object (see Chapter 8, “Advanced
Topics”). In VHDL93 the alias construct has been generalized to allow
aliasing not only types but functions and procedures as well.
A typical alias in VHDL87 would look as follows:
ALIAS opcode : BIT_VECTOR( 3 DOWNTO 0) IS instruction(31
DOWNTO 28);
Notice the type of the opcode needed to be specified (BIT_VECTOR( 3
DOWNTO 0)). In VHDL93 the type is now optional. This same alias can be
written in VHDL93 as follows:
ALIAS opcode IS instruction(27 DOWNTO 22);
450
Appendix D: VHDL93 Updates
Not only can objects be aliased in VHDL93 but functions can as well.
To specify a function alias requires a subprogram signature specification.
The signature specifies the types of the input parameters as well as the
type of the return parameter. An example is shown here:
ALIAS sub IS “-” [STD_LOGIC_VECTOR, STD_LOGIC_VECTOR,
RETURN STD_LOGIC_VECTOR];
This statement creates an alias called sub for an overloaded operator
function call that has two std_logic_vector input arguments and
returns a std_logic_vector.
Attribute Changes
There have been a number of new attributes added to VHDL93. They
reflect added functionality that was either difficult in VHDL87 or not possible. The following attributes have been added to VHDL93:
`ASCENDING
`DRIVING_VALUE
`IMAGE
`VALUE
`PATHNAME
`INSTANCE_NAME
`SIMPLE_NAME
`ASCENDING In VHDL87 it was tedious to find if a particular range
was ascending or descending. The `high and `low attributes of the type
had to be compared to determine if the range was truly ascending, a null
range, or a single value. Attribute `ascending will return true if the range
is ascending or false if not. An example is shown here:
SUBTYPE descend IS STD_LOGIC_VECTOR( 7 DOWNTO 0);
SUBTYPE ascend IS STD_LOGIC_VECTOR(0 TO 7);
descend`ASCENDING --> false
ascend`ASCENDING --> true
`DRIVING_VALUE In VHDL87 the value of an output port could not
be read. To do this required the port mode of the port to be inout, or the
use of an internal signal. These workarounds caused an increase in
complexity that typically was not warranted and therefore to get around
this inconvenience VHDL93 adds attribute `driving_value. Attribute
`driving_value allows the ability to read the value component of
Appendix D: VHDL93 Updates
451
the resolved value that a particular driver is driving so that it can be
further used in the model. In the example shown here the second component instantiation statement would cause an error because input
port a of U2 is trying to read the current value of dout. In VHDL93 the
`driving_value attribute gets around this problem by reading the driving
value of dout.
ENTITY invert IS
PORT( w: IN STD_LOGIC;
dout, doutb : OUT STD_LOGIC);
END invert;
ARCHITECTURE struct OF invert IS
COMPONENT inv
PORT( a : IN STD_LOGIC;
q : OUT STD_LOGIC);
END COMPONENT;
BEGIN
u1 : inv PORT MAP(a => w, q => dout);
--u2 : inv PORT MAP(a => dout, q => doutb);
-- won’t work because port
-- dout cannot be read
u2 : inv PORT MAP(a => dout`DRIVING_VALUE, q => doutb);
-- In VHDL93 this
-- will work
END struct;
`IMAGE AND `VALUE In VHDL87 it was difficult for an error message
to display the actual error value of a signal or a variable in a string. In
VHDL93 the attributes `IMAGE and `VALUE allow the modeler to convert to
and from type values into string values. Attribute `IMAGE converts a type
value into a string, and attribute `VALUE converts a string to a type value.
`PATHNAME, `INSTANCE_NAME, AND `SIMPLE_NAME The
other difficulty in VHDL87 of model error reporting was to uniquely
determine exactly which instance of a model was generating a message.
Most VHDL simulators had some mechanism of reporting the instance
information to the modeler, but this information was simulator-specific
and not standard. In VHDL93 three new attributes allow the modeler
access to all parts of the pathname that describes which instance a particular message is generated from.
■ `SIMPLE_NAME —returns a string which is the local name of the
calling entity.
■ `PATH_NAME —returns a string that describes the path to the entity
starting at the root of the design. The `PATH_NAME attribute does not
include the names of instantiated entities (`INSTANCE_NAME does)
452
Appendix D: VHDL93 Updates
■ `INSTANCE_NAME —returns a string that describes the path to the
entity starting at the root of the design. The `INSTANCE_NAME
attribute also includes the names of instantiated entities. These
entities are specified using a [email protected](architecture) syntax.
Bit String Literal
Bit string literals are a handy way in VHDL87 to assign bit_vector
values. For instance instead of having to explicitly enumerate each bit value
when assigning to a bit_vector an octal or hexadecimal notation can
be used as shown here:
SUBTYPE bit16 IS STD_LOGIC_VECTOR(15 DOWNTO 0);
..
VARIABLE bus_value : bit16;
-- these won’t work with VHDL87
bus_value := “0101010101010101”; --- or
bus_value := O”052525”; -- or
bus_value := X”5555”;
In VHDL93 this concept is extended to types std_logic_vector.
DELAY_LENGTH Subtype
In VHDL87 most time delays were specified with a type TIME. Type TIME
included negative and positive time values. Most uses of TIME required only
positive values of TIME. Therefore in VHDL93 a new type in package STANDARD
has been created, and called DELAY_LENGTH. It’s definition is shown here:
SUBTYPE DELAY_LENGTH IS TIME RANGE 0 FS TO TIME`HIGH;
As can be seen this type only includes the positive values of TIME.
Compiler writers can optimize the compilation and simulation processes
more with this knowledge.
Direct Instantiation
In VHDL87 an entity from a particular library could not be directly
instantiated in an architecture. A component was declared, instantiated,
Appendix D: VHDL93 Updates
453
and bound to an entity with a configuration. The component could have
been directly or implicitly configured.
In VHDL93 entities can be directly instantiated if they are visible. In
the example here entity adder from library work is directly instantiated
and configured in architecture struct.
ENTITY direct IS
PORT( i1 : IN STD_LOGIC;
o1 : OUT STD_LOGIC);
END ENTITY direct;
ARCHITECTURE struct OF direct IS
SIGNAL s1, s2 : STD_LOGIC;
BEGIN
U1 : ENTITY work.adder(behave)
GENERIC MAP(out_delay : delay_type)
PORT MAP(s1, s2, i1, o1);
END ARCHITECTURE struct;
A separate configuration is not necessary as the entity is uniquely
specified. This makes it very easy to describe designs structurally and
with a lot less lines of VHDL code. However, it can make design reuse
more difficult.
Extended Identifiers
In VHDL87 identifiers were limited to only characters a-z, A-Z, and 0-9.
This limited the number of identifiers that could be created. For manually
created VHDL this was not a major problem, but for VHDL that was
translated from some other format this caused some major problems.
Certain netlist formats contain identifiers that consist of operator symbols,
or start with a number. With VHDL93 the extended identifier allows the
user to specify identifiers in a much less restricted manner. Extended
identifiers can start with numbers or contain operator symbols.
Extended identifiers are specified by backslashes (\..\) around an
identifier. Extended identifiers can be used anywhere a normal identifier
can be used. An example using extended identifiers is shown here:
entity \74ls163\ is
port (clk : in std_logic;
\1n1\ : in std_logic;
reset : in std_logic;
q1 : out std_logic;
q2 : out std_logic;
q3 : out std_logic);
end \74ls163\;
454
Appendix D: VHDL93 Updates
In this example the entity name (\74ls163\), and one of the input ports
(\1n1\) are extended identifiers.
File Operations
One of the most welcome additions to VHDL93 is the ability to open
and close files. In VHDL87 files were declared in declarations and opened
implicitly by the elaboration process. VHDL93 adds the ability to specifically open and close files. This allows one subprogram or entity to create
a file which another subprogram or entity can read. In VHDL87 the
modeler would declare a file type to define the type, and later a file declaration that would ultimately open the file. This is shown here:
TYPE int_file IS FILE OF INTEGER; -- VHDL87
-FILE infile: int_file IS IN “/doug/test/example3”;
-- VHDL87 declares
-- and opens file
In VHDL93 the file type declarations remain the same, but the modeler
has a couple of ways to actually open the file. Probably the most common
will be to call the explicit FILE_OPEN procedure as shown here:
PROCEDURE FILE_OPEN(FILE infile: int_file;
EXTERNAL_NAME : IN “/doug/test/example3”;
OPEN_KIND : IN READ_MODE);
This will open the file for reading. If the file cannot be opened for some
reason a runtime error will be generated. An alternate way to open the
file is to call a different version of the FILE_OPEN procedure as shown here:
PROCEDURE FILE_OPEN(FILE_STATUS: FILE_OPEN_STATUS;
FILE : int_file;
EXTERNAL_NAME : IN “/doug/test/example3”;
OPEN_KIND : IN READ_MODE);
This procedure returns an output parameter called FILE_STATUS that
contains the status of the FILE_OPEN procedure call. A status value of
OPEN_OK means that the file is open and ready to be read. A value of
STATUS_ERROR means that the file object already has an external file
associated with it. A value of NAME_ERROR means that the external file does
not exist. A value of MODE_ERROR means that the external file cannot be
opened using the mode passed.
Appendix D: VHDL93 Updates
455
An alternate way of opening the file without calling the explicit
FILE_OPEN procedure is similar to the method used in VHDL87. This
method uses a file declaration similar to the one in VHDL87, that specifies
the name of the file object, the mode of the file object, and the external
filename to be associated with the file object as shown here:
FILE infile : int_file OPEN READ_MODE IS “/doug/test/
example3”;
This effectively calls the FILE_OPEN procedure as follows:
FILE_OPEN(infile, “/doug/test/example3”, READ_MODE);
When a file type declaration of a particular type_mark is declared the
following declarations are implicitly declared.
TYPE FT IS FILE OF type_mark;
PROCEDURE FILE_OPEN( FILE F : FT;
EXTERNAL_NAME : IN STRING;
OPEN_KIND : IN FILE_OPEN_KIND :=
READ_MODE);
PROCEDURE FILE_OPEN( STATUS : OUT FILE_OPEN_STATUS;
FILE F : FT;
EXTERNAL_NAME : IN STRING;
OPEN_KIND : IN FILE_OPEN_KIND :=
READ_MODE);
PROCEDURE FILE_CLOSE( FILE F : FT);
PROCEDURE READ( FILE F : FT; VALUE : OUT type_mark);
PROCEDURE WRITE( FILE F : FT; VALUE : OUT type_mark);
PROCEDURE ENDFILE( FILE F : FT) RETURN BOOLEAN;
The file type declaration declares a file of type type_mark. With the file
type declaration all of the above procedures are implicitly declared. Once
these procedures are declared they can be used to read and write files of
the type_mark.
Foreign Interface
In VHDL87 it was possible to call functions and procedures that were not
described using VHDL. It was possible but limited in scope and not very
well defined. The VHDL93 package standard now contains an attribute
456
Appendix D: VHDL93 Updates
called FOREIGN whose value is a string. This string value describes the
interface to the external function, procedure, or entity. The value of this
string is not standardized and depends on the type of the external code
being called. An example might look as follows:
FUNCTION beep( length : INTEGER) IS
ATTRIBUTE FOREIGN OF beep : FUNCTION IS
“sysbeep(length)”;
BEGIN
END FUNCTION beep;
In this example, a function called beep is declared that contains a
FOREIGN attribute. The FOREIGN attribute specifies that the body of this
function will be implemented by code other than VHDL. The string value
of the attribute declares the interface expected between function beep and
the foreign code to implement the function. However, the string value is
not defined in VHDL93 to be anything more than just a string.
Generate Statement Changes
In a minor addition, VHDL93 adds a declaration section to the generate
statement. Any declarations before the BEGIN clause are local only to the
generate statement.
g1: FOR k IN 0 TO 3 GENERATE
SIGNAL reset : STD_LOGIC;
BEGIN
dffx : dff PORT MAP( z(i), reset, clk, z(i + 1));
END GENERATE;
The generate statement above declares local signal reset. This signal
is local only to the generate statement.
Globally Static Assignment
VHDL93 adds a new feature that allows globally static values to be
assigned to port maps. In VHDL87 port maps could only bind formal parameters to signals. In VHDL93 this has been generalized to include expressions as well. These expressions have to be globally static, or known
at elaboration time.
Appendix D: VHDL93 Updates
457
u1: mux4 PORT MAP( k0 => s0, k1 => s1, k2 => s2, en =>
‘1’, q => outp);
In the example above the value 1 is mapped to port en. In VHDL87 a
separate signal would have to be created, assigned to the value 1, and
then mapped to port en.
The globally static value does not have to be just a simple value, it can
be any expression known at compile time that matches the type of the port.
Groups
It is sometimes useful while modeling to declare an attribute that is to
apply to more than one object. Especially in writing synthesizable models
some attributes are useful to describe behavior for an entire section of a
model. In VHDL87 there was no way to describe this type of attribute
structure. VHDL93 has the concept of groups which allows an attribute to
pertain to all objects in the group.
A group starts with a group template declaration such as shown here:
GROUP timing_arc IS (SIGNAL, SIGNAL);
This describes a group template called timing_arc that is a group of
two signal objects. After the group template is declared a group declaration
can be declared as shown here:
GROUP clk_to_q : timing_arc(clk, q);
GROUP rst_to_q : timing_arc(rst, q);
GROUP set_to_q : timing_arc(set, q);
These declarations show three separate group declarations named
clk_to_q, rst_to_q, and set_to_q. Each of these groups describe a group
object with two signals in the group. Once declared these groups can be
operated on as a single object. For instance, if the following attribute is
declared:
ATTRIBUTE prop_delay IS DELAY_LENGTH;
then the following attributes can be applied to the group.
ATTRIBUTE prop_delay OF clk_to_q : GROUP IS 2.3 NS;
ATTRIBUTE prop_delay OF rst_to_q : GROUP IS 3.1 NS;
ATTRIBUTE prop_delay OF set_to_q : GROUP IS 2.7 NS;
458
Appendix D: VHDL93 Updates
These attributes act on both signals in the group.
Another way to describe a group, especially a group that varies in size,
is shown here:
GROUP timing_arc IS (SIGNAL <>);
This syntax is similar to an unconstrained array and describes a group
consisting of one or more signal objects.
Incremental Binding
In VHDL87 the rules about binding were very restrictive. If a component
was bound in a configuration specification, it could not be bound in a
configuration declaration. This made back-annotation of timing delays
rather difficult because the back annotation program had to generate not
only the generic parameter values, but also the proper entity use clauses.
What the modeler would like to do is pick the proper entity to use with a
configuration specification in the architecture of the containing entity,
and use a configuration declaration to specify the values for the backannotated timing.
In VHDL87 this was not possible because the component could be configured in either place, but not both. In VHDL93 the incremental binding
feature allows the modeler to create models that behave as wanted.
An example is shown here:
ENTITY dff IS
GENERIC( delay : TIME;
PORT( din, clock : IN STD_LOGIC;
dout : OUT STD_LOGIC);
END ENTITY dff;
ENTITY top IS
PORT( z, clock : IN STD_LOGIC; qout : OUT STD_LOGIC);
END ENTITY top;
ARCHITECTURE struct OF top IS
COMPONENT dff IS
PORT( d, clk : IN STD_LOGIC;
q : OUT STD_LOGIC);
END COMPONENT dff;
FOR d1: dff USE ENTITY WORK.dff(behave)
GENERIC MAP (clk_to_q => 5.2 NS)
PORT MAP( d => din, clk => clock, q =>
open );
SIGNAL
Appendix D: VHDL93 Updates
459
BEGIN
-- ..
-- ..
d1 : dff PORT MAP( z, clock, qout);
END ARCHITECTURE struct;
CONFIGURATION topcon OF top IS
FOR struct
FOR d1 : dff GENERIC MAP( clk_to_q => 8.1 NS) PORT
MAP( q => dout );
END FOR;
END FOR;
END CONFIGURATION topcon;
In this example, a dff component is instantiated in entity top. A
configuration specification in the architecture declaration section specifies a value for the clk_to_q generic of 5.2 NS, and maps ports d and
clk. Port q is not mapped but is left open. After the end of the architecture a configuration declaration specifies a new value for the clk_to_q
generic of 8.1 NS, and maps port q to dout. The new clk_to_q generic
value will override the previous value specified in the configuration specification. The mapping of port q mapped to open in the configuration
specification is also overriden with the new value dout as specified
in the configuration declaration.
Postponed Process
In VHDL93 a new type of process has been added, the postponed
process. A postponed process is executed after all of the delta cycles
have been processed so that each signal receives the final value of a simulation time. A typical use for such a process is to perform timing checks.
There are cases in performing timing checks where the input signals need
to stabilize before the timing checks are performed. The postoned
process will allow all of the input signals to stabilize, and finally the
postponed process will be executed.
A postponed process is specified.
p1: POSTPONED PROCESS( clk, reset) IS
-- postponed process declaration section
BEGIN
IF reset = ‘1’ THEN
-END IF;
END PROCESS p1;
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Appendix D: VHDL93 Updates
The keyword POSTPONED is specified before the PROCESS keyword to
specify a postponed process.
Pure and Impure Functions
Functions in VHDL87 were very restrictively defined. The input mode of
all input arguments were constant, and only input arguments were
allowed. The function could have no side effects such as modifying a value
outside the function. The only information returned from the function was
through the return value. In VHDL93 this type of function is known as a
pure function. VHDL93 also contains impure functions which can modify data outside their own scope. These functions must be explicitly
declared as being impure as shown here:
FILE bit_file : TEXT OPEN READ_OPEN IS “ram_data”;
IMPURE FUNCTION get_val RETURN BIT IS
VARIABLE myline : LINE;
VARIABLE result : BIT;
BEGIN
READLINE( bit_file, myline );
READ ( myline, result );
RETURN result;
END get_val;
This function is used to read a set of bits from a file. Function get_val
is declared impure so that it has access to data outside the function. The file
bit_file is opened externally to function get_val but since the function
is impure, access to file bit_file is possible.
Functions in VHDL87 do not have access to data outside of the function
so this function would not work. In VHDL87 the file would have to be
declared within the function declaration section, and implicitly opened and
closed from within the function. In VHDL93 the file can be opened external
to the function and an impure function can access the file.
Pulse Reject
In VHDL87 there were two types of delay categories, inertial and
transport. Chapter 2, “Behavioral Modeling,” talks about the differences
between them. The VHDL87 inertial delay will reject pulses smaller than
Appendix D: VHDL93 Updates
461
the inertial delay specified. In some cases this is too pessimistic. In
VHDL93 the modeler has the ability to specify a pulse reject limit which
can be less than or equal to the inertial delay through the device.
s1 <= REJECT 5 NS INERTIAL newval AFTER 15 NS;
In this example the inertial delay through the device is 15 NS, but the
reject limit is 5 NS. Any pulses of 5 NS or less will be rejected but pulses
greater than 5 NS will be passed through with a 15 NS delay.
Report Statement
In VHDL87 the report clause could only be used within the assert statement, in VHDL93 a report clause can exist separately. In VHDL87 if a
modeler wanted to issue a message to inform the designer that a particular piece of a model was executing, the following statement would have
been required:
ASSERT FALSE REPORT “entered file procedure read”;
The report statement would have to be called from an assert statement, and the assert condition would need to return a false to trigger the
assert statement. In VHDL93 the report statement can exist separately
so the following would also work:
REPORT “entered file procedure read”;
A report statement still has the ability to specify the severity level of
the message. In the above cases the severity level defaulted to error. If
some other severity was wanted, it could be specified as follows:
REPORT “entered timing check code” SEVERITY NOTE;
The severity clause at the end of the REPORT statement allows the modeler to specify any legal level of severity.
Shared Variables
A shared variable is one that is accessible by any design unit that includes
the package where the variable is declared. In VHDL87 variables could
462
Appendix D: VHDL93 Updates
only be declared in processes and were therefore local to the process. In
VHDL93 variables are now able to be declared in packages, and therefore
become global. Any design unit that includes the package can access the
variable. In the example here PACKAGE share has shared variable
timing_checks_on declared in it.
PACKAGE share IS
VARIABLE timing_checks_on : BOOLEAN := TRUE;
END PACKAGE share;
USE WORK.share.ALL;
ENTITY dff IS
PORT( din : IN STD_LOGIC;
clk : IN STD_LOGIC;
q : OUT STD_LOGIC);
END ENTITY dff;
ARCHITECTURE behave OF dff IS
BEGIN
PROCESS(clk) IS
BEGIN
IF timing_checks_on THEN
-- timing check statements
END IF;
-- other statements
END PROCESS;
END ARCHITECTURE behave;
USE WORK.share.ALL;
ENTITY jkff IS
PORT( j, k, clk, se, clr : IN STD_LOGIC;
q, qb : OUT STD_LOGIC);
END jkff;
ARCHITECTURE behave OF jkff IS
BEGIN
PROCESS(clk, set, clr) IS
IF timing_checks_on THEN
-- timing check statements
END IF;
END PROCESS;
END ARCHITECTURE behave;
PACKAGE share is included by entities dff and jkff, making the variable
timing_checks_on globally accessible by both entities. Global vari-
ables are very useful for passing information which is not really part of
the design functionality, but affect the simulation or synthesis operation.
In this example global variable timing_checks_on allows the ability to
turn off and on timing check operation. This does not affect the actual
functionality of the behavior of the models except to disable timing check
reporting. Another use for global variables is to use them to pass input
and output file handles.
Appendix D: VHDL93 Updates
463
Shift Operators
VHDL87 did not contain operators to allow shifting or rotating. Most of
these functions were built by VHDL standard package creators. Without
the built-in operators however, overloaded shift and rotate operators were
not possible. VHDL93 includes built-in shift and rotate operators: sll
(shift left logical), srl (shift right logical), sla (shift left arithmetic), sra
(shift right arithmetic), rol (rotate left), and ror (rotate right). These operators allow shifting and rotating operations for any one-dimensional array type. These operators work as follows:
SLL—shift left logical
q <= a SLL b;
q equals a shifted left by b bits and filled on the right with the value
a’left. If b is negative then a is shifted right.
SRL—shift right logical
q <= a SRL b;
q equals a shifted right by b bits and filled on the left with the value
a’left. If b is negative then a is shifted left.
SLA—shift left arithmetic
q <= a SLA b;
q equals a shifted left by b bits and filled on the right with a(a’right).
If b is negative then a is shifted right.
SRA—shift right arithmetic
q <= a SRA b;
q equals a shifted right by b bits and filled on the left with a(a’left).
If b is negative then a is shifted left.
464
Appendix D: VHDL93 Updates
ROL—rotate left
q <= a ROL b;
q equals a rotated left by b bits. Instead of filling the right b bits with
a value, the b bits that were shifted off the left end are copied to the right
of the shifted a bits. An array that originally contained ABCD and is
rotated left one bit will become BCDA.
ROR—rotate right
q <= a ROR b;
q equals a rotated right by b bits. Instead of filling the left b bits with
a value, the b bits that were shifted off the right end are copied to the left
of the shifted a bits. An array that originally contained ABCD and is
rotated right one bit will become DABC.
These operators can now be overloaded to work with user-defined types.
Syntax Consistency
As part of the VHDL93 syntax update a number of the language end
clauses were modified to become more consistent. All of the clauses now
include the beginning clause identifier. For instance in VHDL87 the
entity clause was as shown here:
ENTITY test IS
--END test;
In VHDL93 the same construct can optionally look as follows:
ENTITY test IS
--END ENTITY test;
Notice that the END clause contains the starting ENTITY clause.
Including the keyword ENTITY in the END clause is optional but allowed in
VHDL93. The same holds true for the architecture, package, package
body, configuration, component, block, process, record, case, if,
procedure, and generate statement. Examples are shown here:
Appendix D: VHDL93 Updates
465
ARCHITECTURE behave OF test IS
BEGIN
--END ARCHITECTURE behave;
PACKAGE mypack IS
--END PACKAGE mypack;
PACKAGE BODY mypack IS
--END PACKAGE BODY mypack;
CONFIGURATION chip OF processor IS
--END CONFIGURATION chip;
COMPONENT memory IS -- notice addition of IS at end of
component clause
-END COMPONENT memory;
block1 : BLOCK IS
BEGIN
-END BLOCK block1;
proc1: PROCESS(clk, din) IS -- notice addition of IS at end
of process clause
BEGIN
-END PROCESS proc1;
RECORD myrec IS
-END RECORD myrec;
CASE selector IS
-END CASE selector;
lab: IF expr THEN
-END IF lab;
PROCEDURE convertval(...) IS
BEGIN
-END PROCEDURE convertval;
g1: FOR k IN 0 TO 7 GENERATE
BEGIN
-END GENERATE g1;
loop1: FOR k IN 0 TO 7 LOOP
-END LOOP loop1;
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Appendix D: VHDL93 Updates
Unaffected
In VHDL87 it was sometimes difficult to describe exactly the behavior
required with a concurrent signal assignment statement. For instance,
there are cases where the modeler wants the value to remain unchanged
if certain conditions are met. In VHDL87 this can be accomplished with
the following statement:
new_state <= state5 WHEN current_state = state1 AND input
= 7 ELSE
state6 WHEN current_state = state5 AND input
= 8 ELSE
state7 WHEN current_state = state2 AND input
= 9 ELSE
new_state;
If none of the above conditions are met the designer has to assign the
current value, new_state to new_state to ensure no value change. This
produces the correct value but has a side effect that a transaction is generated on signal new_state. Any behaviors sensitive to transactions on
new_state will be evaluated and may update their values causing further
activity when none is wanted.
VHDL93 has the new keyword UNAFFECTED that allows no change on a
signal. UNAFFECTED causes no value changes or transactions on the signal.
The same statement above rewritten to include this new feature looks
as follows:
new_state <= state5 WHEN current_state = state1 AND input
= 7 ELSE
state6 WHEN current_state = state5 AND input
= 8 ELSE
state7 WHEN current_state = state2 AND input
= 9 ELSE
UNAFFECTED;
Now if none of the conditions are met nothing will be assigned and no
transactions will be generated.
XNOR Operator
VHDL87 contained a number of operators such as OR, NOR, AND, etc. but
did not contain the XNOR operator. Therefore the XNOR operator could not
be overloaded. VHDL93 adds the XNOR operator to the list of operators
Appendix D: VHDL93 Updates
467
built into the language and therefore in VHDL93 it can be used and overloaded. The example here shows a use of the XNOR operator.
ENTITY xnor2 IS
GENERIC( delay : TIME);
PORT( a, b : IN STD_LOGIC;
q : OUT STD_LOGIC);
END ENTITY xnor2;
ARCHITECTURE behave OF xnor2 IS
BEGIN
a <= a XNOR b AFTER delay;
END ARCHITECTURE behave;
This example shows a 2-input xnor gate using the built-in XNOR
operator. The XNOR operator can be overloaded to work with any types. In
this example two STD_LOGIC type values are xnor’ed together to form the
final result.
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INDEX
a'HIGH(n), 154–156
a'LEFT(n), 154–156
a'LENGTH, 441
a'LOW(n), 154–156
a'RANGE(n), 170–171
a'REVERSE_RANGE(n), 170–171
a'RIGHT(n), 154–156
Access types, 96–102
'ACTIVE, 160
Actuals, 183
Address register, 321–322
Aggregate clause, 435
alias, 449–450
Aliases, 215
ALU, 306–308
ALU instructions, 292
AND gate symbol, 17
Apex 20KE architecture, 373
Architecture, 4–5
Architecture configurations, 201–203
Architecture local signals, 76
Architecture selection, 11
Arithmetic-logic unit (ALU),
306–308
Array types, 87–93
Arrival time, 240–241
'ASCENDING, 450
ASSERT statement, 56–59
Asynchronous reset, 259–260
At-Speed debugging, 399–412
breakpoints, 406–408
complex triggers, 410, 412
debug CPU design, 401–404
debugger, 400, 401
implement new design, 405
instrument signals, 404
instruments, 401
trigger position, 408
watchpoints, 409–410
waveform display, 408–409
write instrumented design, 405
Attribute declaration, 435
Attribute specification, 435
Attributes
foreign interface, 455–456
predefined, 143–171 (see also
Predefined attributes)
quick reference, 440–442
synthesis, 239–241
updates, 450–452
user-defined, 218–220
Bachus-Naur format (BNF), 47,
445–447
Back-annotated simulation, 397–398
'BASE, 169–170
Based integer literal, 443
Based real literal, 443
Basic VHDL building blocks, 2–3
'BEHAVIOR, 149–151
Bit string literal, 443, 452
Block configurations, 199–201
Block diagram of computer, 129
Block statements, 31–37
Blocks, 31, 34
BNF, 47, 445–447
Board-socket-chip analogy, 195–199
Boolean optimization, 244
Branch instructions, 291
Breakpoints, 406–408
Bridges2Silicon debugger, 400, 401
Buffer symbol, 20
Bus, 3
CASE statement, 48–50, 256–257
Changes to VHDL (see VHDL93
updates)
Character literals, 81, 82, 443
clear, 261–262
Clock constraints, 238–239
Comparator, 256, 309–311
Compile VHDL source dialog box, 350
Complex triggers, 410, 412
Component configurations, 176–183
Component declaration, 436
Component instantiation, 436
469
470
Index
Composite type, 86–96
Composite type resolution, 128–130
Concurrent assignment problem,
67–69
Concurrent function, 110
Concurrent procedure, 110
Concurrent process statement, 41
Concurrent signal assignment, 5
Concurrent signal assignment
statement, 16
Conditional signal assignment, 436
Conditional signal assignment
statement, 18
Configuration declaration, 436
Configuration generic table, 195
Configuration statement, 11–12
Configurations, 173–204
architecture, 201–203
block, 199–201
board-socket-chip analogy,
195–199
component, 176–183
default, 174–176
defined, 2
entity-architecture pair
configuration, 180–181
generic specifications, 190–195
generic value specification, 188–190
generics, 185–188
lower-level, 179–180
mapping library entities, 183–185
port maps, 181–183
power of, 12
Constants, 77–78
Constraints, 237–239
Control, 311–321
Conversion functions, 113–119
Copy (block copy operation), 299–302
CPU, 289–367
ALU, 306–308
block copy operation, 299–302
block diagram, 290, 304–306
comp (comparation), 309–311
control, 311–321
debug, 401–404
gate-level timing simulation,
379–398 (see also VITAL
simulation)
CPU (Cont.):
reg (address/instruction register),
321–322
regarray (storage registers),
322–323
RTL simulation, 329–355
shift (shifting/rotation operations),
324–326
synthesis results, 357–367
testbenches, 340–348. (See also
Testbenches)
top-level design, 293–302
trireg (tristate register), 326–328
Data types, 73–108
access types, 96–102
array types, 87–93
composite types, 86–96
enumerated types, 81–84
file types, 102–105
incomplete types, 98–102
integer types, 80
overview (diagram), 79
physical types, 85–86
real types, 81
record types, 93–96
scalar types, 79–86
subtypes, 105–107
Deadlock, 64
DEALLOCATE, 96
Debugging (see At-Speed debugging)
Decimal integer literal, 443
Decimal real literal, 443
Decimal real with exponent, 443
Deferred constants, 78, 136–137
Definitions, 2–3
Delay, 20–23
Delay constraints, 237–239
DELAY_LENGTH, 452
Delay model, 242
'DELAYED, 161–164
Delta delay evaluation mechanism, 26
Design under test (DUT), 330
DFF device, 388
Direct instantiation, 452–453
Double-word instructions, 292
Drive, 240
Driver, 2, 27–29
471
Index
Driver creation, 27
'DRIVING_VALUE, 450–451
EDA Tool Settings Dialog box, 374,
376
8-bit lookahead adder, 249
8-bit ripple carry adder, 248
ELSE clause, 47, 48
ELSIF clause, 47, 48
End-of-file mark, 102
Entity, 2, 3–4
Entity-architecture pair
configuration, 180–186
Enumerated types, 81–84
'EVENT, 157–158
Event scheduling, 6
Exemplar Logic Leonardo Spectrum
synthesis tool, 231, 357
EXIT statement, 54–56
Extended identifiers, 453
Factoring, 246
Fast testbench, 345–348
Field Programmable Gate Array
(FPGA), 357
File, 102
File object declaration, 103
FILE_OPEN, 454–455
File operations, 454–455
FILE_STATUS, 454
File type declaration, 103
File types, 102–105
Flattening, 245–246
Flip-flop (VITAL model), 388–392
FOR loop, 51, 52
FOREIGN, 456
Foreign interface, 455–456
4-bit counter, 262
4-bit shifter, 264–266
Four input mux symbol and function,
43
Four state truth table, 120
FPGA, 357
FPGA vendors, 284
Full testbench, 337–340
Function array attributes, 154–156
Function kind attributes, 151–160
Function signal attributes, 156–160
Function type attributes, 151–154
Functional gate-level verification,
283–284
Functions, 110–132
composite type resolution, 128–130
conversion, 113–119
nine-value resolution, 123–128
pure/impure, 460
resolution, 119–132
resolved signals, 130–132
Gate level cells, 231
Gate level description, 236
Gate level netlist synthesis, 232
Gate level netlists, 231, 232
GENERATE statements, 220–224,
437, 456
Generic declaration, 437
Generic map, 437
Generic specifications, 190–195
Generic value specification, 188–190
Generics, 29–31
Globally static assignment, 456–457
Groups, 457–458
Guarded blocks, 35–37
Guarded signal assignment, 437
HDL debugger, 400
'HIGH, 144–147
'HIGH(n), 154–156
High-density design flow, 380
High-level design flow, 273–287
design specification process, 273
functional gate-level verification,
283–284
overview (flowchart), 274
place and route, 284–286
post layout timing simulation, 286
RTL simulation, 275–277
static timing, 287
VHDL synthesis, 277–283
Household alarm system, 253
Hybrid testbenches, 342–345
Identifier, 81
IEEE 1076-1987 standard VHDL, 2
(see also VHDL 93 updates)
IEEE 1164 standard logic package,
413–433
472
Index
IF statement, 47–48
IICE, 400
IICE configuration dialog box, 407
'IMAGE, 451
Impure function, 460
IN, 133
Incomplete types, 98–102
Incremental binding, 458–459
Inertial delay, 20–21
Inertial delay buffer waveforms, 21
Inertial delay model, 22
Inferred flip-flops, 257
INOUT, 133
Input slope delay, 243
'INSTANCE_NAME, 452
Instruction, 291–293
Instruction register, 321–322
Instruction words, 292
Instruments, 401
Intelligent In-Circuit Emulator
(IICE), 400
Intrinsic delay, 242, 243
Irregular generate statement,
222–224
Krol, Paul, 195
'LAST_ACTIVE, 160
'LAST_EVENT, 158–160
'LAST_VALUE, 157–158
Late arriving signals, 241
'LEFT, 144–147
'LEFT(n), 154–156
'LEFTOF, 151–154
'LENGTH, 147–149
'LENGTH(n), 441
Leonardo menu bar, 359
Leonardo Spectrum synthesis tool,
231, 257
Leonardo Spectrum GUI, 358
Libraries, 3
Library declaration, 438
Literals, 441, 443
Load, 240
Load instructions, 291
LoadI, 352
Loading delay, 242, 243
LOOP statement, 50–54
'LOW, 144–147
'LOW(n), 154–156
Lower-level configurations, 179–180
Mapping library entities, 183–185
Mapping to gates, 247–250
Model Technology ModelSim
command language, 341, 349
ModelSim simulator, 341, 349
Multidimensional arrays, 91
Multiple WAIT conditions, 63
Multiply driven signals, 27
Mux functional table, 19
NATURAL, 106
NEW, 96
NEXT statement, 53–54
Nine-value resolution function,
123–128
Note category, 56
Object types, 74–78
Open/close files, 454–455
Operator argument type overloading,
213–215
Operators, 211, 442
Optimization process, 244
OTHERS clause, 49
OUT, 133
Overloading, 206–215
operators, 210–215
subprogram, 206–210
Overloading subprogram argument
types, 208
Package body, 138–141
Package declaration, 136
Packages, 135–141
Pass through operation, 324
Passive processes, 70–72
'PATH_NAME, 451
Physical types, 85–86
Pla structure, 245
Place and route, 284, 369–378
high-level design flow, 284–286
process, 370–372
setting up a project, 373–377
PORT clause, 438
473
Index
Port maps, 181–183
'POS, 151–154
Post layout timing simulation, 286
Postponed process, 459–460
'PRED, 151–154
Predefined attributes, 143–171
function array attributes, 154–156
function kind attributes, 151–160
function signal attributes, 156–160
function type attributes, 151–154
range kind attributes, 170–171
signal kind attributes, 160–169
type kind attributes, 169–170
value array attributes, 147–149
value block attributes, 149–151
value kind attributes, 144–151
value type attributes, 144–147
Predefined physical types, 85–86
preset, 261–262
Primary design units, 3
Procedures, 133–135
Process, 3
Process declarative part, 9, 41, 42
Process execution, 10
Process statement, 9–10, 40–42, 439
Pulse reject, 460–461
Pure function, 460
Qualified expressions, 215–218
Quartus, 373
Quartus user interface, 373
Quick reference (see VHDL reference
tables)
'QUIET, 166–168
'RANGE(n), 170–171
Range kind attributes, 170–171
Real types, 81
Record types, 93–96
Reference tables (see VHDL
reference tables)
Register and cloud diagram, 233
Register transfer level description,
232–237
Register Transfer Level (RTL) VHDL
description, 11
Registers
address, 321–322
Registers (Cont.):
instruction, 321–322
storage, 322–323
tristate, 326–328
REPORT clause, 57, 439
REPORT statement, 461
Required time constraints, 238
reset, 260
Resolution functions, 27, 119–132
Resolved signals, 130–132
RETURN statement, 111, 439
'REVERSE_RANGE(n), 170–171
'RIGHT, 144–147
'RIGHT(n), 154–156
'RIGHTOF, 151–154
ROL (rotate left), 464
ROR (rotate right), 464
Rotate operators, 464
Rotating operations, 324–326
RTL descriptions, 232–237
RTL simulation, 275–277, 329–355
CPU simulation, 349–355
testbenches, 330–348 (see also
Testbenches)
RTL VHDL description, 11
s'ACTIVE, 160
s'DELAYED, 161–164
s'EVENT, 157–158
s'LAST_ACTIVE, 160
s'LAST_EVENT, 158–160
s'LAST_VALUE, 157–158
s'QUIET, 166–168
s'STABLE, 164–166
s'TRANSACTION, 168–169
Scalar types, 79–86
Schematic Entry system, 31
SDF, 286
SDF file, 392–394
Secondary design units, 3
Selected signal assignment, 19, 439
Sensitivity list, 40, 66
Sequential behavior, 8–9
Sequential function, 110
Sequential procedure, 110
Sequential statements, 10, 46–61
ASSERT statement, 56–59
CASE statement, 48–50
474
Index
Sequential statements (Cont.):
EXIT statement, 54–56
IF statement, 47–48
LOOP statements, 50–54
NEXT statement, 53–54
VHDL synthesis, 257–259,
262–266
WAIT statements, 59–66
SEVERITY clause, 57, 439
Severity level, 56–57
Severity level error, 57
Severity level failure, 57
Shared variables, 461–462
Shift instructions, 292
Shift operators, 463
Shifting operations, 324–326
Side effects, 135
Signal assignment, 5, 72
Signal assignment statement, 5,
16–19
Signal kind attributes, 160–169
Signals, 74–76
Signals global to entities, 75–76
Simple gate - concurrent assignment,
252–253
'SIMPLE_NAME, 451
Simulation
back-annotated, 397–398
CPU, 349–355
gate-level, 379–398
post layout timing, 286
RTL, 275–277, 329–355
VITAL, 379–398 (see also VITAL
simulation)
Simulation delta circuit, 24
Simulation deltas, 23–27
Simulator-specific testbench,
340–342
Single-word instructions, 292
SLA (shift left arithmetic), 463
SLL (shift left logical), 463
SRA (shift right arithmetic), 463
SRL (shift right logical), 463
'STABLE, 164–166
Stanculescu, Alec, 196
Standard delay format (SDF), 286
Standard library, 80
Standard logic package, 413–433
Standard package, 80
State machine, 83–84
State machine example (voicemail
controller), 266–271
Statement concurrency, 6–7
Statements/clauses, 435–439
Static timing analyzer, 239, 287
Stimulus driver, 329, 330, 331
Stimulus only testbench, 333–337
Storage registers, 322–323
Store instructions, 291
String, 443
Structural architectures, 176
Structural designs, 7–8
'STRUCTURE, 149–151
Structuring, 246
Subprogram declaration, 137
Subprogram overloading, 206–210
Subprogram parameter overloading,
208–210
Subprograms, 110–135
functions, 110–132 (see also
Functions)
procedures, 133–135
Subtypes, 105–107
'SUCC, 151–154
Syntax consistency, 464–465
Synthesis, 243–250
boolean optimization, 244
factoring, 246
flattening, 245–246
mapping to gates, 247–250
overview, 244
translation, 243
VHDL, 251–272 (see also VHDL
synthesis)
Synthesis tools, 11
T'BASE, 169–170
T'HIGH, 144–147
T'LEFT, 144–147
T'LEFTOF, 151–154
T'LOW, 144–147
T'POS, 151–154
T'PRED, 151–154
T'RIGHT, 144–147
T'RIGHTOF, 151–154
T'SUCC, 151–154
475
Index
T'VAL, 151–154
Technology libraries, 241–243
Terminology (definitions), 2–3
Testbench block diagram, 331
Testbenches, 330–348
fast, 345–348
full, 337–340
hybrid, 342–345
kinds of, 331
simulator specific, 340–342
stimulus only, 333–337
TextIO, 224–229
3-input OR gate, 252–254
TIME, 85–86
Time-out clause, 64–66
Timing constraints, 238
Top-level system design, 289–302
CPU (see CPU)
instructions, 291–293
system operation, 290–291
'TRANSACTION, 168–169
Transport delay, 21–22
Transport delay buffer waveforms, 22
Transport delay model, 23
Transport signal assignment, 439
Triggers, complex, 410, 412
Trigger position, 408
Tristate register, 326–328
Truth table, 120
Two-process description style, 262
Type declaration, 78
Type kind attributes, 169–170
Type_mark construct, 79
Types (see Data types)
UNAFFECTED, 466
Unconstrained array type, 92–93
Updates (see VHDL93 updates)
USE clause, 439
User-defined, 218–220
'VAL, 151–154
'VALUE, 451
Value array attributes, 147–149
Value block attributes, 149–151
Value kind attributes, 144–151
Value type attributes, 144–147
Variable assignment, 42–46
Variables, 76–77
vcom, 350–351
VHDL, 1–2
VHDL data types diagram, 79
VHDL Language Reference Manual,
211
VHDL reference tables, 435–443
attributes, 440–442
literals, 441, 443
operators (precedence), 442
statements/clauses, 435–439
VHDL synthesis, 251–272
asynchronous preset/clear,
261–262
CASE control flow statements,
256–257
high-level design flow, 277–283
IF control flow statements,
253–255
more complex sequential
statements, 262–266
simple gate–concurrent
assignment, 252–253
simple sequential statements,
257–259
state machine example (voicemail
controller), 266–271
VHDL terms, 2–3
VHDL87, 467
VHDL93 updates, 449–467
alias, 449–450
attribute changes, 450–452
bit string literal, 452
compatibility, 449
DELAY_LENGTH, 452
direct instantiation, 452–453
extended identifiers, 453
file operations, 454–455
foreign interface, 455–456
generate statement, 456
globally static assignment,
456–457
groups, 457–458
incremental binding, 458–459
open/close files, 454–455
postponed process, 459–460
pulse reject, 460–461
pure/impure functions, 460
476
Index
VHDL93 updates (Cont.):
REPORT statement, 461
rotate operators, 464
shared variables, 461–462
shift operators, 463
syntax consistency, 464–465
UNAFFECTED, 466
XNOR operator, 466–467
VITAL, 379, 381–382
VITAL AND gate, 383
VITAL architecture, 386
VITAL data flow, 381
VITAL descriptions, 382
VITAL level 0, 383
VITAL level 1, 383
VITAL library, 381–382
VITAL packages, 382–383
VITAL Primitives Package, 383
VITAL process, 380, 381
VITAL simulation, 379–398
back-annotated simulation, 397–398
flip-flop example, 388–392
high-density design flow, 380
overview, 382
running the simulation, 394–397
SDF file, 392–394
simple VITAL model, 383–386
wire delay section, 386–388
VITAL Timing Package, 382–383
vlib, 349
Voicemail controller, 266–271
vsim, 351, 397
WAIT statements, 59–66
multiple WAIT conditions, 63
sensitivity list, contrasted, 66
time-out clause, 64–66
WAIT FOR, 62–63
WAIT ON, 62
WAIT UNTIL, 62
WAIT FOR, 62–63
WAIT ON, 62
WAIT time-out, 64–66
WAIT UNTIL, 62
Warning, 56
Watchpoint expression, 409
Watchpoint Expression dialog box,
409, 411
Watchpoints, 409–410
Waveform display, 408–409
WHEN condition, 56
WHILE condition, 51
Wire delay, 242, 243
Wire delay section, 386–388
XNOR operator, 466–467
About the Author
Douglas L. Perry is Founder and VP of Customer Solutions at Bridges2Silicon
a new startup HDL hardware debugging company. Prior positions include Director of Strategic Marketing and Exemplar Logic, Inc. Mr. Perry has been active in
the CAE field for almost two decades and is also the author of the first three
editions of VHDL: Programming by Example. He earned his B.S. Degree in Electrical Engineering at South Dakota State University and also did graduate studies at the University of Santa Clara. He lives in San Ramon, California.
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