Design and Simulation of 8 Bit Arithmetic Logic Unit

DESIGN AND SIMULATION OF 8 BIT ARITHMETIC LOGIC UNIT USING VHDL

A MINOR PROJECT REPORT

Submitted by:

1. SHIVANTH KUMAR(0282092807)

2. BHUVNESH KUMAR(0282092807)

3. MOHAMMAD IHTESHAM(0752092807)

In partial fulfillment for the award of the degree

of the degree of

BACHELOR OF TECHNOLOGY

IN

ELECTRONICS AND COMMUNICATION ENGINEERING

AT

DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING

G B PANT ENGINEERING COLLEGE

NEW DELHI

DECEMBER 2010

DECLARATION

We hereby declare that the project entitled

“Design and Simulation of 8 Bit Arithmetic Logic Unit”

submitted for the Bachelor of Technology degree is our original work

and the project has not formed for the basis for the award of any degree,

associate ship, fellowship for any other similar titles.

Name Signature

Mohammad Ihtesham

Shivnath Kumar

Bhuvnesh Kumar

PLACE: New Delhi DATE:

CERTIFICATE

This is to certify that the project entitled “Design and Simulation of 8 Bit Arithmetic Logic Unit” is the bona fide work carried out by:

Mohammad Ihtesham (0752092807) Shivnath Kumar (0292092807) Bhuvnesh Kumar (0282092807)

Students of B.Tech ‘G.B. PANT ENGINEERING COLLEGE’,DELHI during the

year 2010, in partial fulfillment of the requirements for the award of the degree of

BACHELOR IN ELECTRONICS AND COMMUNICATION ENGINEERING and

that the project has not formed the basis for the award previously of any degree,

diploma, associate ship, fellowship or any other similar title.

SIGNATURE OF THE MENTOR:

Mr. Arun Prakash

(Lecturer)

E.C.E.

PLACE:

DATE: (SEAL OF THE COLLEGE)

ACKNOWLEDGEMENT

First and foremost, we would like to express our sincere gratitude to our project mentor Mr. ARUN PRAKASH. We were privileged to experience a sustained enthusiastic and involved interest from her side. This fueled our enthusiasm even further and encouraged us to boldly step into what was a totally dark and unexplored expanse before us.

We would also like to thank our Friends who were ready with positive comments all the time, whether it was an off-hand comment to encourage us or a constructive piece of criticism and a special thanks to Mrs. Mamta Jain (HOD ECE) who arranged the needed requirements for all of us.

Last but not least, we would like to thank the G.B Pant staff members and Mr.Dhnanjay Kimothi, in particular, for extending a helping hand at every juncture of need.

Date: 10th December Mr. Mohammad Ihtesham

Place: New Delhi Mr. Shivnath Kumar

Mr. Bhuvnesh Kumar

ABSTRACT

This project includes the designing of 8 –Bit Arithmetic Logic Unit and simulating its components using VHSIC HDL. The implementation strategies have been borrowed from most popular Xilinx ISE software. The method applied in this project is to design and test all the wat through starting with discrete component design and later VHDL based design on the FPGA.

An arithmetic logic unit (ALU) is the part of a computer processor (CPU) that carries out arithmetic and logic operations on the operands in computer instruction words. ALU performs operations such as addition, subtraction and multiplication of integers and bitwise AND, OR, NOT, XOR and other Boolean operations. The CPU's instruction decode logic determines which particular operation the ALU should perform, the source of the operands and the destination of the result.

VHDL is a hardware description language for delineating digital electronic systems. It arose out of the US government’s Very High Speed Integrated Circuits (VHSIC) program.

We have presented our work in the use of the VHDL environment. The work included the development of a number of VHDL models and sequences of input stimuli and the gathering of performance data from their execution. The instruction set adopted here is extremely simple that gives an insight into the kind of hardware, which should be able to execute the set of instructions properly. For synthesis purpose the targeted FPGA device used is SPARTAN-3. The device can accept two numbers of 8 bit binary data and can perform logical/arithmetic operation depending on the 4 bit Operation code given.

8 BIT ALU DESIGN

CONTENTS

CHAPTER 1: INTRODUCTION TO VHDL

Overview of VHDL……………………………………………4

History of VHDL………………………………………………5

VHDL Application Field………………………………………7

ASIC Development ……………………………………………8

Concept of VHDL ……………………………………………9

Abstraction Levels in VHDL………………………………….10

Description of Abstraction Levels ……………………………11

Modularity and Hierarchy……………………………………..12

VHDL Language and Syntax………………………………….13

VHDL Structural Elements……………………………………14

Process…………………………………………………………20

Process Communication Model……………………………….22

CHAPTER 2 : INTRODUCTION TO SYNTHESIS

What is Logic Synthesis?.............................................................23

Impact Of HDL & Logic Synthesis…………………………….24

Design Entry & Synthesis………………………………………24

Hierarchical Design……………………………………………..24

Area Optimization………………………………………………25

Speed Optimization……………………………………………..26

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Technology Mapping…………………………………………….26

Placement…………………………………………………….......27

Routing……………………………………………………….......27

CHAPTER 3 : PROGRAMMABLE LOGIC DESIGN

History of Programmable Logic Design…………………………28

Complex Programmable Logic Device (CPLD)…………………31

Field Programmable Logic Gate Array (FPGA)…………………32

Basic Design Process……………………………………………..33

CHAPTER 4 : DESIGN OFARITHMETIC LOGIC UNIT

Introduction to Arithmetic Logic Unit……………………………35

Design Overview………………………………………………….36

Logical Unit……………………………………………………….40

Arithmetic Unit……………………………………………………44

Instruction Set…………………………………….……………….45

Design Implementation……………………………………………46

VHDL Code……………………………………………………….49

CHAPTER 5 : SYNTHESIS

Synthesis Report………………………………………………......52

Bitgen Report……………………………………………………...68

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RTL Schematic…………………………………………………….73

Simulation Waveforms…………………………………………….76

Floorplan…………………………………………………………..78

CHAPTER 6 : EPILOGUE

Conclusion………………………………………………………….79

Bibliography………………………………………………………..80

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CHAPTER 1

INTRODUCTION TO VHDL1.1 OVERVIEW OF VHDL

VHDL is an acronym which stands for VHSIC Hardware Description Language. VHSIC is another acronym which stands for Very High Speed Integrated Circuits.

Hardware description languages can be used in several ways; they can be an alternative way of representing a circuit diagram for a digital circuit or a higher level algorithmic ‘program’ that solves a particular problem. Such structural or behavioral representations are two ways of describing a model of a digital system.

VHDL can be used for documentation, verification, and synthesis of large digital designs. This is actually one of the key features of VHDL, since the same VHDL code can achieve all three of these goals, thus saving a lot of effort and reducing the introduction of errors between translating a specification into an implementation.

In addition to being used for each of these purposes, VHDL can be used to take three different approaches to describing hardware. These three different approaches are the structural, data flow, and behavioral methods of hardware description. Most of the times a mixture of the three methods are employed and a complete design will have different sections expressed in different ways. The terms algorithmic and RTL (Register Transfer Language) are sometimes used for behavioral and dataflow.

A language capable of such diverse applications has a lot of keywords and a typical user only uses a small subset of the full language. It also means that there is often more than one way of doing something in VHDL; proper software tools that compile and run (i.e simulate) the VHDL model will work with the entire language as defined in the official Language Reference Manual (LRM) but there will be some specialist tools that require certain subsets or a particular methodology to be adhered to. This is most often seen in synthesis tools that try to automate the conversion of behavioral to structural. Implementation tools that are targeted to a particular type of hardware such as PLDs may well have restrictions and some of

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the less expensive VHDL tools may not completely cover the language. The Alliance toolset for instance is a free VHDL compiler, simulator and synthesis tool that runs on PCs; it has several restrictions. In practice such tools will still allow useful work to be done, within their target environment so the restrictions are rarely too severe.

VHDL is a standard (VHDL-1076) developed by the IEEE. The language has been through a couple of revisions, the most widely used version is the 1987 (Std 1076-1987) version, sometimes referred to as VHDL’87, but also just VHDL. However, there is a newer revision of the language referred to as VHDL’93. VHDL’93 (adopted in 1994 of course) is fairly new and is still in the process of replacing VHDL’87. The appendix at the end of this document lists further reading for these standards and the library has some videotapes that describe the differences; they are unlikely to be of concern to the neophyte VHDL programmer/designer.

1.2 VHDL HISTORY

1.2.1 THE REQUIREMENT

The development of VHDL was initiated in 1981 by the United States Department of Defence to address the hardware life cycle crisis. The cost of reprocuring electronic hardware as technologies became obsolete was reaching crisis point, because the function of the parts was not adequately documented, and the various components making up a system were individually verified using a wide range of different and incompatible simulation languages and tools. The requirement was for a language with a wide range of descriptive capability that would work the same on any simulator and was independent of technology or design methodology.

1.2.2 STANDARDIZATION

The standardization process for VHDL was unique in that the participation and feedback from industry was sought at an early stage. A baseline language (version 7.2) was published 2 years before the standard so that tool development could begin in earnest in advance of the standard. All rights to the language definition

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were given away by the DoD to the IEEE in order to encourage industry acceptance and investment.

1.2.3 ASIC MANDATE

DoD Mil Std 454 mandates the supply of a comprehensive VHDL description with every ASIC delivered to the DoD. The best way to provide the required level of description is to use VHDL throughout the design process.

1.2.4 VHDL 1993

As an IEEE standard, VHDL must undergo a review process every 5 years (or sooner) to ensure its ongoing relevance to the industry. The first such revision was completed in September 1993, and this is still the most widely supported version of VHDL.

1.2.5 VHDL 2000 AND VHDL 2002

One of the features that was introduced in VHDL-1993 was shared variables. Unfortunately, it wasn’t possible to use these in any meaningful way. A working group eventually resolved this by proposing the addition of protected types to VHDL. VHDL 2000 Edition is simply VHDL-1993 with protected types.

VHDL-2002 is a minor revision of VHDL 2000 Edition. There is one significant change, though: the rules on using buffer ports are relaxed, which makes these much more useful than hitherto.

1.2.6 VHPI

In 2007, an amendment to VHDL 2002 was created. This introduces the VHDL Procedural Interface (VHPI) and also makes a few minor changes to the text of VHDL 2002. Apart from the VHPI itself, no new features were added to VHDL.

The VHPI allows tools programmable access to a VHDL model before and during simulation. In other words, you can write programs in a language such as C that interact with a VHDL simulator.

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1.2.7 VHDL-2008

Since the publication of the first IEEE standard in 1987 several revised versions have appeared. The first, in 1993, had the most extensive changes. VHDL 2000 Edition introduced protected types and VHDL-2002 contains mainly minor changes. VHDL-2008 is the name of the new version of VHDL. As with the earlier revisions, this doesn’t radically alter the language, but it does provide a wider set of modifications than previously.

A draft of the proposed revision (version 4.0) was delivered by Accellera to the IEEE in 2008. The standard is now available from the IEEE and is known as IEEE Std. 1076-2008.

1.3 VHDL APPLICATION FIELD

VHDL is used mainly for the development of Application Specific Integrated Cicuits (ASICs). Tools for the automatic transformation of VHDL code into a gate-level netlist were developed already at an early point of time. This transformation is called synthesis and is an integral part of current design flows.

For the use with Field Programmable Gate Arrays (FPGAs) several problems exist. In the first step, 7ehavio equations are derived from the VHDL description, no matter, whether an ASIC or a FPGA is the target technology. But now, this 7ehavio code has to be partitioned into the configurable logic blocks (CLB) of the FPGA. This is more difficult than the mapping onto an ASIC library. Another big problem is the routing of the CLBs as the available resources for interconnections are the bottleneck of current FPGAs.

While synthesis tools cope pretty well with complex designs, they obtain usually only suboptimal results. Therefore, VHDL is hardly used for the design of low complexity Programmable Logic Devices (PLDs).

VHDL can be applied to model system 7ehavior independently from the target technology. This is either useful to provide standard solutions, e.g. for micro controllers, error correction (de-)coders, etc, or behavioural models of

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microprocessors and RAM devices are used to simulate a new device in its target environment.

An ongoing field of research is the hardware/software co design. The most interesting question is which part of the system should be implemented in software and which part in hardware. The decisive constraints are the costs and the resulting performance.

1.4 ASIC DEVELOPMENT

The development of VHDL models starts with their specification which covers functional aspects and the timing behavior. Sometimes a behavioral VHDL model is derived from there, yet synthesizable code is frequently requested right from the beginning. VHDL code can be simulated and checked for the proper functionality.

If the model shows the desired behavior, the VHDL description will be synthesized. A synthesis tool selects the appropriate gates and flip-flops from the specified ASIC library in order to reproduce the functional description. It is essential for the synthesis procedure that the sum of the resulting gate delays along the longest paths (from the output to the input of every Flip Flop) is less than the clock period.

As soon as a model built of ASIC library elements is available, a simulation on gate level can be performed. Now gate and propagation delays have to be taken into account. Delay values can be included in each VHDL model description, i.e. the designer receives the first clues about maximum clock frequency and critical paths after synthesis, already.

The propagation delay along the signal wires have to be estimated first because the real values are available after the layout is finished. The process of feeding these values back into the VHDL model is called back annotation. Once again it must be checked, whether the circuit fulfills the specified timing constraints.

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1.5 CONCEPTS OF VHDL

EXECUTION OF STATEMENTS:

SEQUENTIAL

CONCURRENT

METHODOLOGIES

ABSTRACTION

MODULARITY

HIERARCHY

VHDL distinguishes itself from other languages by the way assignments are executed because two basic types of statements are known:

Sequential statements are executed one after another, like in software programming languages. Subsequent statements can override the effects of previous statements this way. The order of the assignment must be considered when sequential statements are used.

Concurrent statements are active continuously. So the order of the statements is not relevant. Concurrent statements are especially suited model the parallelism of hardware.

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VHDL features also three important modeling techniques:

Abstraction allows for the description of different parts of a system with different amount of detail. Modules which are needed only for the simulation do not have to be described as detailed as modules that might be synthesized.

Modularity enables the designer(s) to split big functional blocks and to write a model for each part.

Hierarchy lets the designer build a design out of sub modules which may consist of several sub modules, themselves. Each level of hierarchy may contain modules of different abstraction levels. The sub modules of these models are present in the next lower hierarchical level.

1.6 ABSTRACTION LEVELS AND VHDL

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VHDL is applicable to the upper three abstraction levels. It is not suitable to describe a layout. The design entry in behavioral and RT level is usually done by text editors. Graphical tools are also available but experienced users often find it easier to write the code by hand. On the gate level, a schematic is modified as VHDL netlist descriptions tend to become too complex pretty soon.

The transition from an upper abstraction level to a lower one is supported more or less efficiently by software.

Behavioral synthesis is still a dream of many researchers as only very simple behavior models are synthesizable. A common application is the design of RAM cells for the target technology, where only the generic parameters (width, depth, number of ports, (a) synchronous ...) need to be specified.

Logic synthesis, however, has been perfected in recent years. As long as the designer confines himself to certain simple VHDL constructs that are sufficient for RT level descriptions, the synthesis tools will be able to reproduce the behavior in the logic level.

As a result of the ongoing research in efficient place and route algorithms the step from the logic level to the final layout has been widely automated for digital standard cell designs.

1.7 DESCRIPTION OF ABSTRACTION LEVELS

In the behavior level, complete systems can be modeled. Bus systems or complex algorithms are described without considering synthesizability. The stimuli for simulation of RTL models are described in the behavior level, for example. Stimuli are signal values of the input ports of the model and are described in the test bench, sometimes called validation bench.

The designer has to take great care to find a consistent set of input stimuli that do not contradict the specification. The responses of the model have to be compared with the expected values which, in the simplest case, can be done with the help of a waveform diagram that shows the simulated signal values.

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On the RT level, the system is described in terms of registers and logic that calculates the next value of the storage elements. It is possible to split the code into two blocks (cf. process statement) that contain either purely combinational logic or registers. The registers are connected to the clock signal and provide for synchronous behavior. In practice, the strict separation of Flip Flops from combinational logic is often annulated and clocked processes describe the registers and the corresponding update functions.

The gate netlist is generated from the RT description with the help of a synthesis tool. For this task, a cell library for the target technology which holds the information about all available gates and their parameters (fan-in, fan-out, delay) is needed.

Based upon this gate netlist the circuit layout is generated. The resulting wire lengths can be converted into propagation delays which can be fed back into the gate level model (back annotation). This allows for thorough timing simulations without the need for additional simulator software.

1.8 MODULARITY AND HIERARCHY

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Modularity allows the partitioning of big functional blocks into smaller units and to group closely related parts in self-contained sub blocks, so called modules. This way, a complex system can be divided into manageable subsystems. The guidelines for partitioning can differ from design to design. Most of the time functional aspects are considered as partitioning constraint. The existence of well defined subsystems allows several designers to work in parallel on the same project as each designer will view his part as a new, complete system.

Hierarchy allows the building of a design out of modules which themselves may be built out of (sub-) modules. One level of a hierarchical description contains one or more modules; each module can even have different degrees of abstraction. These modules can themselves contain sub modules which would be present the next lower hierarchical level.

Modularity and hierarchy help to simplify and organize a design project. Additional advantages are that different implementation alternatives can be examined for the modules, e.g. in a simulation. Only the corresponding component instantiation needs to be changed for this in the overall model. Also analogues interfaces can be modeled in VHDL and added to the system model for simulation. Sometimes, simulation models of the devices that will be connected to the new design exist and can be used for a simulation of the design under test in its real working environment.

1.9 VHDL LANGUAGE AND SYNTAX

VHDL is generally case insensitive which means that lower case and upper case letters are not distinguished. This can be exploited to define own rules for formatting the VHDL source code. VHDL keyword could for example be written in lower case letters and self defined identifiers in upper case letters. This convention is valid for the following slides.

Statements are terminated in VHDL with a semicolon. That means as many line breaks or other constructs as wanted can be inserted or left out. Only the semicolons are considered by the VHDL compiler.

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List are normally separated by commas. Signal assignments are notated with the composite assignment operator ‘<=’. Self defined identifier as defined by the VHDL 87 standard may contain letters, numbers and underscores and must begin with a letter. Further no VHDL keywords may be used. The VHDL 93 standard allows defining identifiers more flexible as the next slide will show.

-- Example VHDL Code —

signal mySignal: bit; -- an example signal

Mysignal <= ‘0’, -- start with ‘0’

‘1’ AFTER 10 ns, -- and toggle after

‘0’ after 10 ns, -- every 10 ns

‘1’ after 10 ns;

Case insensitive

Comments: ‘—‘ until end of line

Statements are terminated by ‘;’

(may span multiple lines)

List delimiter: ‘,’

Signal assignment: ‘<=’

User defined names:

letters, numbers, underscores start with a letter

1.10 VHDL Structural Elements

1.10.1 ENTITY DECLARATION:

The interface between a module and its environment is described within the entity declaration which is initiated by the keyword ' entity '. It is followed by a user-defined, (hopefully) descriptive name, in this case: HALFADDER. The interface description is placed between the keyword ' is ' and the termination of the entity statement which consists of the keyword ' end ' and the name of the entity. In the

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new VHDL'93 standard the keyword ' entity ' may be repeated after the keyword ' end ' for consistency reasons.

The input and output signal names and their data types are defined in the port statement which is initiated by the keyword ' port '. The list of ports is enclosed in a '(' ')' pair. For each list element the port name(s) is given first, followed by a ':', the port mode and the data type. Within the list, the ';' symbol is used to separate elements, not to terminate a statement. Consequently, the last list element is not followed by a ';'!

entity HALFADDER is

port(

A, B: in bit;

SUM, CARRY: out bit);

end HALFADDER;

-- VHDL'93: end entity HALFADDER ;

Interface description

No behavioral definition

entity ADDER is

port(

A, B: in integer range 0 to 3;

SUM: out integer range 0 to 3;

CARRY: out bit );

end ADDER;

Linking via port signals

data types

signal width

signal direction

Several ports with the same mode and data type can be declared by a single port statement when the port names are separated by ','. The port mode defines the data

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flow (in: input, i.e. the signal influences the module behavior; out: output, i.e. the signal value is generated by the module) while the data type determines the value range for the signals during simulation.

Entity declaration specifies the following:

The name of the entity.

A set of generic declarations specifying instance-specific parameters.

A set of port declarations defining inputs and outputs of the hardware design.

1.10.2 ARCHITECTURE

The architecture contains the implementation for an entity which may be either a behavioral description (behavioral level or, if synthesizable, RT level) or a structural netlist or a mixture of those alternatives. An architecture is strictly linked to a certain entity. An entity, however, may very well have several architectures underneath, e.g. different implementations of the same algorithm or different abstraction levels. Architectures of the same entity have to be named differently in

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order to be distinguishable. The name is placed after the keyword ‘ architecture ‘ which initiates an architecture statement. ‘RTL’ was chosen in this case.

Entity HALFADDER is

port( A, B: in bit;

SUM, CARRY: out bit);

end HALFADDER;

architecture RTL of HALFADDER is

begin

SUM <= A xor B;

CARRY <= A and B;

end RTL;

-- VHDL’93: end architecture RTL ;

Implementation of the design

Always connected with a specific entity

one entity can have several 17rchitectures

entity ports are available as signals within the architecture

Contains concurrent statements

It is followed by the keyword ‘ of ‘ and the name of entity that is used as interface (‘HALFADDER’). The architecture header is terminated by the keyword ‘ is ‘, like in entity statements. In this case, however, the keyword ‘ begin ‘ must be placed somewhere before the statement is terminated. This is done the same way as in entity statements: The keyword ‘ end ‘, followed by the architecture name. Once again, the keyword ‘ architecture ‘ may be repeated after the keyword ‘ end ‘ in VHDL’93. As the VHDL code is synthesizable, RTL was chosen as architecture name. In case of this simple function, however, there is no difference to behavioral (algorithmic) description. We will use ‘BEHAVE’, ‘RTL’, ‘GATE’, ‘STRUCT’ and ‘TEST’ to indicate the abstraction level and the implemented behavior, respectively. The name ‘EXAMPLE’ will be used whenever the architecture shows the application of new VHDL elements and is not associated with a specific entity.

1.10.3 ARCHITECTURE STRUCTURE17

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Each architecture is split into an optional declarative part and the definition part. The declarative part is located between the keywords ' is ' and ' begin '. New objects that are needed only within the architecture constants, data types, signals, subprograms, etc. can be declared here. The definition part is initiated by the keyword ' begin ' and holds concurrent statements. These can be simple signal assignments, process statements, which group together sequential statements, and component instantiations. Concurrency means that the order in which they appear in the VHDL code is not important. The signal SUM, for example, gets always the result of (3 + 7), independently of the location of the two assignments to the signals DIGIT_A and DIGIT_B.

architecture EXAMPLE of STRUCTURE is

subtype DIGIT is integer range 0 to 9;

constant BASE: integer := 10;

signal DIGIT_A, DIGIT_B: DIGIT;

signal CARRY: DIGIT;

begin

DIGIT_A <= 3;

SUM <= DIGIT_A + DIGIT_B;

DIGIT_B <= 7;

CARRY <= 0 when SUM < BASE else 1;

end EXAMPLE

Declarative part:

data types

constants

additional signals

("actual" signals)

components

...

Definition part (after 'begin'):

signal assignments

processes

component instantiations

concurrent statements:

order not important

Signal assignments are carried out by the signal assignment operator ' <= '. The symbol represents the data flow, i.e. the target signal whose value shall be updated

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is placed on the left side of the operator. The right side holds an expression that evaluates to the new signal value. The data types on the left and on the right side have to be identical. Please remember that the signals that are used in this example were defined implicitly by the port declaration of the entity.

Multiple architectures can be created for a particular entity. For example, we might wish to create several architectures for a particular entity with each architecture optimized with respect to a design goal:

Performance

Area

Power consumption

Ease of simulation

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1.11 PROCESS

Behavioral descriptions are supported with the process statement. The process statement can appear in the body of an architecture declaration just as the signal assignment statement does. The contents of the process statement can include sequential statements like those found in software programming languages. These statements are used to compute the outputs of the process from its inputs. Sequential statements are often more powerful, but sometimes have no direct correspondence to a hardware implementation. The process statement can also contain signal assignments in order to specify the outputs of the process.

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entity AND_OR_XOR is

port (A,B : in bit;

Z_OR, Z_AND, Z_XOR : out bit);

end AND_OR_XOR;

architecture RTL of AND_OR_XOR is

begin

A_O_X: process (A, B)

Begin

Z_OR <= A or B;

Z_AND <= A and B;

Z_XOR <= A xor B;

end process A_O_X ;

end RTL;

Contains sequentially executed statements

Exist within an architecture, only

Several processes run concurrently

Execution is controlled either via

sensitivity list (contains trigger signals), or

wait-statements

The process label is optional

Because the statements within an architecture operate concurrently another VHDL construct is necessary to achieve sequential behavior. A process, as a whole, is treated concurrently like any other statement in an architecture and contains statements that are executed one after another like in conventional programming languages. In fact it is possible to use the process statement as the only concurrent VHDL statement. The execution of a process is triggered by events. Either the possible event sources are listed in the sensitivity list or explicit wait statements are used to control the flow of execution. These two options are mutually exclusive, i.e. no wait statements are allowed in a process with sensitivity list. While the

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sensitivity list is usually ignored by synthesis tools, a VHDL simulator will invoke the process code whenever the value of at least one of the listed signals changes. Consequently, all signals that are read in a purely combinational process i.e. that influence the behavior, have to be mentioned in the sensitivity list if the simulation is to produce the same results as the synthesized hardware. Of course the same is true for clocked processes, yet new register values are to be calculated with every active clock edge, only. Therefore the sensitivity list contains the clock signal and asynchronous control signals (e.g. reset).

A process statement starts with an optional label and a ':' symbol, followed by the process keyword. The sensitivity list is also optional and is enclosed in a '(' ')' pair. Similar to the architecture statement, a declarative part exists between the header code and the keyword ' begin '. The sequential statements are enclosed between “begin” and ' end process '. The keyword ' process ' has to be repeated! If a label was chosen for the process, it has to be repeated in the end statement, as well.

Important notes regarding processes:

- Processes execute in zero time.- Processes execute endlessly unless broken by:- WAIT statement or - Sensitivity list (Processes with a sensitivity list have an implied WAIT at the

end)- An Architecture can have multiple processes.- All processes execute concurrently.- Variables are local to the process in which they are declared.- Variables may only be declared within processes.

1.12 PROCESS COMMUNICATION MODEL

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Processes are concurrent statements

Several processes

run parallel

linked by signals in the sensitivity list

sequential execution of statements

Link to processes of other entity/architecture pairs via entity interface

Process statements are concurrent statements while the instructions within each process are executed sequentially, i.e. one after another. All processes of a VHDL design run in parallel, no matter in which entity or hierarchy level they are located. They communicate with each other via signals. These signals need to be ports of the entities if processes from different architectures depend from another.

A group of processes will be executed concurrently since the process is a concurrent statement, it usually contains sequential ststements. We can also group a block of concurrent statements together and specify when they are to be executed.

CHAPTER 2

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INTRODUCTION TO SYNTHESIS

2.1 WHAT IS LOGIC SYNTHESIS?

Logic synthesis is the process of converting a high-level description of design into an optimized gate-level representation. Logic synthesis uses standard cell library which have simple cells, such basic logic gates like and , or and nor, or macro cells, such as adder, muxes, memory, and flip-flops. Standard cells put together is called technology library. Normally technology library is known by the transistor size (0.18u, 90mm). A circuit description is written in Hardware Description Language (HDL) such as VHDL. The designer should first understand the architectural description. Then he should consider design constraints such as timing area, testability, and power.

The synthesis and simulation Design provides a general overview of designing Field Programmable Gate Arrays (FPGA) devices with Hardware Description Languages (HDLs).

Synthesis is a broad term often used to describe very different tools. Synthesis can include silicon compilers and function generators used by ASIC vendors to produce regular RAM and ROM type structures. Synthesis in the context of this tutorial refers to generating random logic structures from VHDL descriptions. This is best suited to gate arrays and programmable devices such as FPGAs. Sy nthesis is not an easy task. It is vital to tackle High Level Design using VHDL with realistic expectations of synthesis. Other tools which use VHDL, such as synthesis, will make their own interpretation of the VHDL language. This is an IEEE standard for VHDL synthesis (IEEE Std. 1364.1-2002) but no vendor adheres

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strictly to it. It is not sufficient that the VHDL is functionally correct, it must be written in such a way that it directs the synthesis tool to generate good hardware, and moreover, the VHDL must be matched to the idiosyncrasies of the particular synthesis tool being used. We shall tackle some of these idiosyncrasies in this VHDL tutorial.

2.2 IMPACT OF HDL AND LOGIC SYNTHESIS

High-level design is less prone to human error because designs are described at a higher level of abstraction. High-level design is done without significant concern about design constraints. Conversion from high-level design to gates is done by synthesis tools, while doing so it used various algorithms to optimize the design as a whole. This removes the problem with varied designer styles for the different blocks in the design and suboptimal designs. Logic synthesis tools allow technology independent design. Design reuse is possible for the technology-independent descriptions.

2.3 DESIGN ENTRY AND SYNTHESIS

You can enter a design with a schematic editor or a text-based tool. Design entry begin with a design concept, expressed as a drawing or functional description. From the original design, a netlist is created, then synthesized and translated into a native generic object (NGO) file. This file is fed into Xilinx software program called NGD build, which produces a logical native generic description (NGD) file. The following figure shows the design entry and synthesis process

2.4 HIERARCHICAL DESIGN

Design hierarchy is important in both schematic and HDL entry for the following reasons:

Helps you conceptualize your design

Adds structure to your design.

Promotes easier design debugging.

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8 BIT ALU DESIGN

Makes it easier to combine different design entry methods (schematic, HDL, or state directory.)

Makes it easier to design incrementally, which consists of designing, implementing, and verifying individual parts of a design in stages.

Reduces optimization time.

Facilitates concurrent design, which is the process of dividing a design among a number of people who develop different parts of the design in parallel.

2.5 AREA OPTIMIZATION

Area optimization is well studied field in digital electronics. Early technique addressed simple matrix-based manipulation for the Boolean algebra. Later

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8 BIT ALU DESIGN

developments incorporated optimizations based on do not care conditions. The optimizations may be – a general purpose one or targeted at particular structures (e.g. Programmable Logic Array). Also under lying technology strongly affects the optimization.

Where area optimization is more difficult is where a particular device technology is used. For example, the Xilinx FPGA operates a number of CLBs(). Each CLB can compute upto five inputs Boolean expressions. If the expression involves six inputs then a second CLB is needed. If four inputs are needed then the chip area is wasted.

2.6 SPEED OPTIMIZATION

Some measure of speed optimization is usually, as the area optimization remove redundant circuitry and can shorten the critical path of the circuit however, further, speed optimization can be performed.

As common approach is to identify the critical path and to work on reducing that a simple optimization might be replaced each component on the critical path with a higher rated (i.e. faster but more power intensive equivalent) but functionally equivalent component from the library. Other optimization techniques might be to strip out string inverters from a design where, e.g. AND gates are slower than Ors gates. To attempt to replace all ANDs with OR.

Speed optimization is usually applied after area optimization has taken place. It improves the performance, but only to a certain extent. If after speed optimization the circuit still does not meet the required specifications, then the designer must go back and restructure behavioral specifications with a structural or data flow.

2.7 TECHNOLOGY MAPPING

After logic optimization has produced the optimized network, technology mapping transform this network into the final circuit. This is done by selecting pieces of the network that can be implemented by one of the available circuit elements, and specifying how there are to be interconnected. The circuit is optimized to reduce a cost function that typically incorporates measures of both area and delay. Conventional approaches to technology mapping have focused on using circuit

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8 BIT ALU DESIGN

element from a set of simple gates, such as a standard cell library. The complex logic block used in FPGA presents difficulties for library-based approaches because they can implement a large number of functions.

2.8 PLACEMENT

The next step after technology mapping is placement of logic blocks. A number of efficient techniques has been developed, which can be easily adopted to use for FPGAs.

2.9 ROUTING

Routing refers to the process of inter-connecting the various logic block of the FPGA. Some of the terms used in the routing are:

Pin- a logic block input or output.

Connection- a pair of logic blocks pin that are electrically connected.

Net- a set of logic pins that are to be electrically connected. A net can be divided into one or more connection.

Wire segment- a straight segment of wire that is used to from a part of a connection.

Routing switch- a programmable switch that is used to electrically connect two wire segments.

Track- a straight section of wire that spans the entire width or length of a routing channel. A track can be composed of a number of wire segments of various lengths.

Routing channel- the rectangular area that lies between two rows or two column of logic blocks. A routing channel contains a number of tracks.

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8 BIT ALU DESIGN

CHAPTER 3

PROGRAMMABLE LOGIC DESIGN

3.1 HISTORY

By the late 1970s, standard logic devices were all the rage, and printed circuit boards were loaded with them. Then someone asked, “What if we gave designers the ability to implement different interconnections in a bigger device?” This would allow designers to integrate many standard logic devices into one part.

To offer the ultimate in design flexibility, Ron Cline from Signetics (which was later purchased by Philips and then eventually Xilinx) came up with the idea of two programmable planes. These two planes provided any combination of “AND” and “OR” gates, as well as sharing of AND terms across multiple ORs.

This architecture was very flexible, but at the time wafer geometries of 10 μm made the input-to-output delay (or propagation delay) high, which made the devices relatively slow. The features of the PLA were:

• Two programmable ground planes

• Any combination of ANDs/ORs

• Sharing of AND terms across multiple ORs

• Highest logic density available to user

• High fuse count; slower than PALs

• Programmable logic array

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8 BIT ALU DESIGN

Figure: Programmable Logic Array

MMI (later purchased by AMD™) was enlisted as a second source for the PLA array. After fabrication issues, it was modified to become the programmable array logic (PAL) architecture by fixing one of the programmable planes.

This new architecture differed from that of the PLA in that one of the programmable planes was fixed – the OR array. PAL architecture also had the added benefit of faster tPD and less complex software, but without the flexibility of the PLA structure. Other architectures followed, such as the PLD. This category of devices is often called Simple PLD.

• One programmable plane: AND/Fixed OR

• Finite combination of ANDs/ORs

• Medium logic density available to user

• Lower fuse count; faster than PLAs (at the time, fabricated on a 10 μM process)

• Programmable array logic

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8 BIT ALU DESIGN

Figure: Programmable Array Logic

The architecture had a mesh of horizontal and vertical interconnect tracks. At each junction was a fuse. With the aid of software tools, designers could select which junctions would not be connected by “blowing” all unwanted fuses. (This was done by a device programmer, but more commonly these days is achieved with ISP). Input pins were connected to the vertical interconnect. The horizontal tracks were connected to AND-OR gates, also called “product terms”. These in turn connected to dedicated flip-flops, whose outputs were connected to output pins.

PLDs provided as much as 50 times more gates in a single package than discrete logic devices! This was a huge improvement, not to mention fewer devices needed in inventory and a higher reliability over standard logic. PLD technology has moved on from the early days with companies such as Xilinx producing ultra-low-power CMOS devices based on flash memory technology.

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8 BIT ALU DESIGN

3.2 COMPLEX PROGRAMMABLE LOGIC DEVICES (CPLDS)

Complex programmable logic devices (CPLDs) extend the density of SPLDs. The concept is to have a few PLD blocks or macro-cells on a single device with a general-purpose interconnect in-between. Simple logic paths can be implemented within a single block. More sophisticated logic requires multiple blocks and uses the general-purpose interconnect in-between to make these connections. CPLDs feature:

• Central global interconnect

• Simple, deterministic timing

• Easily routed

• PLD tools add only interconnect

• Wide, fast complex gating

Figure: CPLD Architecture

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8 BIT ALU DESIGN

3.3 FIELD PROGRAMMABLE GATE ARRAYS (FPGA)

In 1985, Xilinx introduced a completely new idea: combine the user control and time to market of PLDs with the densities and cost benefits of gate arrays. Customers liked it, and the FPGA was born. Today Xilinx is the number one FPGA vendor in the world. An FPGA is a regular structure of logic cells (or modules) and interconnect, which is under your complete control. This means that you can design, program, and make changes to your circuit whenever you wish.

With FPGAs now exceeding the 10 million gate limit (the Xilinx Virtex™-4 FPGA is the current record holder), you can really dream big. FPGAs feature:

• Channel based routing

• Post layout timing

• Tools more complex than CPLDs

• Fine grained

• Fast register pipelining

Figure: FPGA Architecture

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3.4 BASIC DESIGN FLOW

Design entry or design specification can be in the form of schematic capture or hardware description language ( HDL). In schematic form, after determining the capture tool and the manufacturer’s library, designer can connect the gates from library with wires and then generates netlist, which is the textual description of the circuit. Schematic capture is not feasible for large designs because it is not scalable, not reusable, strongly vendor dependent and hard to maintain. In HDL design entry, the design is entered in high level description language emphasizing design’s function or behavior and then synthesized by the vendor independent tool and netlist is generated. The design is more maintainable, scalable and reusable than schematic design entry.

In design implementation, the first step is translation of low level and generic netlist file into device specific resources. After translation step, mapping step checks the design according to device specific rules, add further logic or make replications to meet the timing requirements using device resources. At last, in place and route step, already allocated resources are distributed along FPGA taking into account the physical constraints and routing resources. At this point physical layout is determined and timing information for design entities and interconnects (Back Annotation) is available. After routing, the device is ready to be programmed. In device programming stage, the SRAM based FPGA’s configuration, which is volatile after power on and also defining the logic and interconnect, is programmed to a Programmable Read Only Memory (PROM) device with part name xc18v02. Design specification is a parallel process to design development. Design entry in either schematic or HDL form can be simulated behaviorally, while it can be tested based on the code syntax. After synthesis phase, generated netlist format can be simulated functionally by providing test vectors and tested by checking the desired output vector. Timing simulation comes after the place and route phase using back annotation.

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CHAPT

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ER 4

DESIGN OF ARITHMETIC LOGIC UNIT

4.1 INTRODUCTION TO ALU:

An arithmetic logic unit (ALU) is the part of a computer processor (CPU) that carries out arithmetic and logic operations on the operands in computer instruction words. ALU performs operations such as addition, subtraction and multiplication of integers and bitwise AND, OR, NOT, XOR and other Boolean operations. The CPU's instruction decode logic determines which particular operation the ALU should perform, the source of the operands and the destination of the result.

The width in bits of the words which the ALU handles is usually the same as that quoted for the processor as a whole whereas its external buses may be narrower. Floating point operations are usually done by a separate "floating point unit". Some processors use the ALU for address calculations (e.g. incrementing the program counter), others have separate logic for this.

In some processors, the ALU is divided into two units, an arithmetic unit (AU) and a logic unit (LU). Some processors contain more than one AU for example, one for fixed point operations and another for floating point operations. (In personal computers floating point operations are sometimes done by a floating point unit on a separate chip called a numeric coprocessor.)

Typically, the ALU has direct input and output access to the processor controller, main memory (random access memory or RAM in a personal computer), and

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8 BIT ALU DESIGN

input/output devices. Inputs and outputs flow along an electronic path that is called a bus. The input consists of an instruction word (sometimes called a machine instruction word) that contains an operation code (sometimes called an "op code"), one or more operands, and sometimes a format code. The operation code tells the ALU what operation to perform and the operands are used in the operation. (For example, two operands might be added together or compared logically.) The format may be combined with the op code and tells, for example, whether this is a fixed point or a floating point instruction. The output consists of a result that is placed in a storage register and settings that indicate whether the operation was performed successfully. (If it isn't some sort of status will be stored in a permanent place that is sometimes called the machine status word.)

In general, the ALU includes storage places for input operands, operands that are being added, the accumulated result (stored in an accumulator), and shifted results. The flow of bits and the operations performed on them in the subunits of the ALU is controlled by gated circuits. The gates in these circuits are controlled by a sequence logic unit that uses a particular algorithm or sequence for each operation code. In the arithmetic unit, multiplication and division are done by a series of adding or subtracting and shifting operations. There are several ways to represent negative numbers. In the logic unit, one of 16 possible logic operations can be performed such as comparing two operands and identifying where bits don't match.

The design of the ALU is obviously a critical part of the processor and new approaches to speeding up instruction handling are continually being developed.

4.2 DESIGN OVERVIEW

4.2.1 INPUT SIGNALS

The Arithmetic and logic Unit has two eight-bit input signals Accumulator_in(7 -0) and Data_in(7 -0) taken and controlled from trainer switches and loaded into registers Accumulator and Data respectively.

4.2.2 INTERNALLY INITIATED SIGNALS

clk- Clock Input: This is a signal from the FPGA internal clock.

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8 BIT ALU DESIGN

4.2.3 MODULES

Figure shows the block diagram of the Arithmetic Logic Unit, which consists of various modules interconnected by an 8-bit internal data bus. Each of these

modules along with its sub components is described in this section.

REGISTER FILE

o Two 8 –bit input registers and one 8- bit output register.

o Input registers , Accumuulator_in(7-0) and Data_in(7-0)

ARITMETIC LOGIC UNIT

o 8 –Bit ALU (logical operations like AND, XOR etc.. arithmetic

operations like increment, addition etc….)

4.2.4 REGISTER FILE

Arithmetic and Logical instructions require three source registers and one destination register. Of the two source registers, three are used as input registers and the other is used as an instruction register. A total of 4 bits would be required for any ALU instruction. The actual implementation of any instruction is done with the instruction register and two 3:8 decoders. These decoder outputs can be used to drive the required output.

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8 BIT ALU DESIGN

4.2.5 ARITHMETIC LOGIC UNIT

Figure shows in detail the ALU, the 8-bit inputs A, B and the output Z. The ALU takes two operands from the A and B registers. The result register Z is used to hold the ALU output. The ALU has the capability to perform 9 operations as shown in the figure. After every ALU instruction, the output register is updated.

The various units inside the ALU are described below:

Adder/Subtractor: The 8-bit adder/subtractor in the ALU is a Reduced Full Adder built by using universal gates. A ripple carry adder is used in which the carry signal propagates from the LSB to the MSB and coming out as Cout.

Logical Unit: We provide all the possible logical operations (nand nor, exor,not,and,or,xnor) in the ALU.

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8 BIT ALU DESIGN

Figure: Arithmetic Logic Unit

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8 BIT ALU DESIGN

4.3 LOGICAL UNITS:

4.3.1 AND GATE:

(1) Instruction Format: Oprand1 AND Oprand2(2) Function: Oprand1 and Oprand2 are two register inputs. This operator performs the bitwise AND operation, and put the result in Oprand1. The truth table defines the behavior of each bit operation shown in figure 1(a). The circuit symbol is shown in figure 1(b)

4.3.2 OR GATE:

(1) Instruction Format: OR Oprand1 Oprand2(2) Function: Oprand1 and Oprand2 are two register inputs. This operator performs the bitwise OR operation, and put the result in Oprand1. The truth table defines the behavior of each bit operation shown in figure 2(a). The circuit symbol is shown in figure 2(b)

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8 BIT ALU DESIGN

4.3.3. XOR GATE:

(1)Instruction Format: Oprand1 XOR Oprand2. (2)Function: Oprand1 and Oprand2 are two register inputs. This operator performs the bitwise XOR operation, and put the result in Oprand1. The truth table defines the behavior of each bit operation shown in figure 3(a). The circuit symbol is shown in figure 3(b).

4.3.4. NOT GATE:

(1) Instruction Format: NOT Oprand1(2) Function: Oprand1 is a register input. This operator performs the bitwise NOT operation on Oprand1. The truth table defines the behavior of each bit operation shown in figure 4(a). The circuit symbol is shown in figure 4(b).

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4.3.5 NOR GATE:

(1) Instruction Format: Oprand1NOR Oprand2(2) Function: Oprand1 and Oprand2 are two register inputs. This operator performs the bitwise NOR operation, and put the result in Oprand1. The truth table defines the behavior of each bit operation shown in figure 2(a). The circuit symbol is shown in figure 2(b)

`4.3.6 NAND GATE:

(1) Instruction Format: Oprand1NAND Oprand2(2) Function: Oprand1 and Oprand2 are two register inputs. This operator performs the bitwise NAND operation, and put the result in Oprand1. The truth table defines the behavior of each bit operation shown in figure 2(a). The circuit symbol is shown in figure 2(b)

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8 BIT ALU DESIGN

4.3.7 EXCLUSIVE –NOR GATE:

(1) Instruction Format: Oprand1 XNOR Oprand2(2) Function: Oprand1 and Oprand2 are two register inputs. This operator performs the bitwise XNOR operation, and put the result in Oprand1. The truth table defines the behavior of each bit operation shown in figure 2(a). The circuit symbol is shown in figure 2(b)

4.4 Arithmetic Unit:

4.4.1 ADDER AND SUBTRACTOR CIRCUIT:

The addition and subtraction operations can be done using an Adder-Subtractor circuit. The figure shows the logic diagram of a 8-bit Adder-Subtractor circuit. The circuit has a mode control signal M which determines if the circuit is to operate as an adder or a subtractor.

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Each XOR gate receives input M and one of the inputs of B, i.e., B i. To understand the behavior of XOR gate consider its truth table given below. If one input of XOR gate is zero then the output of XOR will be same as the second input. While if one input of XOR gate is one then the output of XOR will be complement of the second input.

So when M = 0, the output of XOR gate will be Bi ⊕ 0 = Bi. If the full adders receive the value of B, and the input carry C0 is 0, the circuit performs A plus B. When M = 1, the output of XOR gate will be Bi ⊕ 1 = Bi

’. If the full adders receive the value of B’, and the input carry C0 is 1, the circuit performs A plus 1’s complement of B plus 1, which is equal to A minus B.

4.5 INSTRUCTION SET 45

8 BIT ALU DESIGN

The instructions that can be performed are shown in the table:

Opcode_in(3)

Opcode_in(2)

Opcode_in(1)

Opcode_in(0)

Function

0 0 0 0 Pass Data_in

0 0 0 1 Pass Accumulator_in

0 0 1 0 ADDITION

0 0 1 1 SUBTRACTION

0 1 0 0 AND

0 1 0 1 OR

0 1 1 0 XOR

0 1 1 1 NOT Accumulator_in

1 0 0 0 NOT Data_in

1 0 0 1 INCREMENT Accumulator_IN

1 0 1 0 DECREMENT Accumulator_in

1 0 1 1 MULTIPLICATION

1 1 0 0 NAND

1 1 0 1 NOR

1 1 1 0 XNOR

1 1 1 1 -

4.6 DESIGN IMPLEMENTATION46

8 BIT ALU DESIGN

Our basic goal was to implement the design on FPGA. We have used Xilinx Spartan 3 XC3S400 FPGA board mounted on FPGA trainer. Xilinx Spartan-3 FPGAs are ideal for low-cost, high-volume applications and are targeted as replacements for fixed-logic gate arrays and ASSP products such as bus interface chip sets. The Spartan-3 (1.2V, 90 nm) FPGA is not only available for a very low cost, but it integrates many architectural features associated with high-end programmable logic. This combination of low cost and features makes it an ideal replacement for ASICs (gate arrays) and many ASSP devices. In order to interface it with the parallel port of PC we have to setup the parallel port unidirectional settings (EEP) with IRQ 278 from the bios setup of PC. For that, we first implemented the design using the Xilinx-ISE simulator. The error free code for this has been given in coming sections.

This code was then synthesized in XILINX-PROJECT NAVIGATOR.

After synthesizing the code, we generated the bit stream file after assigning the input and output ports for the FPGA XC3S400 board using XILINX PACE.

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Then, we loaded the ALU.bit file on the FPGA using Xilinx- iMPACT. Followed by, we provided the inputs using trainer switches (via Toggle Switches) and displayed the outputs using LEDS on the trainer.

Following picture shows the whole design implemented with inputs and outputs.

The Spartan 3 XC3S400 kit:

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4.7 VHDL CODE ATIRHMETIC LOGIC UNIT

library IEEE;

use IEEE.STD_LOGIC_1164.ALL;

use IEEE.STD_LOGIC_ARITH.ALL;

use IEEE.STD_LOGIC_UNSIGNED.ALL;

entity minor_project is

Port ( Accumulator_in : in STD_LOGIC_VECTOR (7 downto 0);

Data_in : in STD_LOGIC_VECTOR (7 downto 0);

Opcode_in : in STD_LOGIC_VECTOR (3 downto 0);

Multiplier : in STD_LOGIC_VECTOR (3 downto 0);

Multiplicand : in STD_LOGIC_VECTOR (3 downto 0);

Result_out : out STD_LOGIC_VECTOR (7 downto 0));

end minor_project;

architecture Behavioral of minor_project is

begin

Main: process(Accumulator_in,Opcode_in, Data_in)

begin

case Opcode_in is

when "0000" => Result_out <= Data_in;

-- result = Data_in

when "0001"=> Result_out <= Accumulator_in;

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-- result = accumulator_in

when "0010"=> Result_out <= Accumulator_in + Data_in;

-- result = accumulator_in + Data_in

when "0011"=> Result_out <= Accumulator_in - Data_in;

-- result = accumulator_in - Data_in

when "0100"=> Result_out <= Accumulator_in and Data_in;

-- result = accumulator_in and Data_in

when "0101"=> Result_out <= Accumulator_in or Data_in;

-- result = accumulator_in or Data_in

when "0110"=> Result_out <= Accumulator_in xor Data_in;

-- result = accumulator_in xor Data_in

when "0111"=> Result_out <= not(accumulator_in);

-- result = not(accumulator_in)

when "1000"=> Result_out <= not(Data_in);

-- result = not(Data_in);

when "1001"=> Result_out <= Accumulator_in + 1;

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-- result Increment Accumulator_in

when "1010"=> Result_out <= Accumulator_in - 1;

-- result Decrement Accumulator_in

when "1011"=> Result_out <= Multiplier * Multiplicand;

-- result = 8 MSBs of ( Multiplier * Multiplicand)

when "1100"=> Result_out <= accumulator_in nand Data_in;

-- result = accumulator_in nand Data_in

when "1101"=> Result_out <= accumulator_in xnor Data_in;

--result=accumulator_in xnor Data_in

when "1110"=> Result_out <= accumulator_in nor Data_in;

-- result = accumulator_in nor Data_in

when "1111"=> Result_out <= "00000000";

--result=Accumulator_in

when others => Result_out <="XXXXXXXX";

end case;

end process Main;

end Behavioral;

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CHAPTER 5

SYNTHESIS

5.1 SYNTHESIS REPORT

ARITHMETIC LOGIC UNIT

Release 9.2i - xst J.36

Copyright (c) 1995-2007 Xilinx, Inc. All rights reserved.

--> Parameter TMPDIR set to ./xst/projnav.tmp

CPU : 0.00 / 0.11 s | Elapsed : 0.00 / 0.00 s

--> Parameter xsthdpdir set to ./xst

CPU : 0.00 / 0.11 s | Elapsed : 0.00 / 0.00 s

--> Reading design: minor_project.prj

TABLE OF CONTENTS

1) Synthesis Options Summary

2) HDL Compilation

3) Design Hierarchy Analysis

4) HDL Analysis

5) HDL Synthesis

5.1) HDL Synthesis Report

6) Advanced HDL Synthesis

6.1) Advanced HDL Synthesis Report

7) Low Level Synthesis

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8) Partition Report

9) Final Report

9.1) Device utilization summary

9.2) Partition Resource Summary

9.3) TIMING REPORT

===========================================================

* Synthesis Options Summary *

===========================================================---- Source Parameters

Input File Name : "minor_project.prj"

Input Format : mixed

Ignore Synthesis Constraint File : NO

---- Target Parameters

Output File Name : "minor_project"

Output Format : NGC

Target Device : xc3s400-5-pq208

---- Source Options

Top Module Name : minor_project

Automatic FSM Extraction : YES

FSM Encoding Algorithm : Auto

Safe Implementation : No

FSM Style : lut

RAM Extraction : Yes

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8 BIT ALU DESIGN

RAM Style : Auto

ROM Extraction : Yes

Mux Style : Auto

Decoder Extraction : YES

Priority Encoder Extraction : YES

Shift Register Extraction : YES

Logical Shifter Extraction : YES

XOR Collapsing : YES

ROM Style : Auto

Mux Extraction : YES

Resource Sharing : YES

Asynchronous To Synchronous : NO

Multiplier Style : auto

Automatic Register Balancing : No

---- Target Options

Add IO Buffers : YES

Global Maximum Fanout : 500

Add Generic Clock Buffer(BUFG) : 8

Register Duplication : YES

Slice Packing : YES

Optimize Instantiated Primitives : NO

Use Clock Enable : Yes

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Use Synchronous Set : Yes

Use Synchronous Reset : Yes

Pack IO Registers into IOBs : auto

Equivalent register Removal : YES

---- General Options

Optimization Goal : Speed

Optimization Effort : 1

Library Search Order : minor_project.lso

Keep Hierarchy : NO

RTL Output : Yes

Global Optimization : AllClockNets

Read Cores : YES

Write Timing Constraints : NO

Cross Clock Analysis : NO

Hierarchy Separator : /

Bus Delimiter : <>

Case Specifier : maintain

Slice Utilization Ratio : 100

BRAM Utilization Ratio : 100

Verilog 2001 : YES

Auto BRAM Packing : NO

Slice Utilization Ratio Delta : 5

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===========================================================

* HDL Compilation *

===========================================================

Compiling vhdl file "C:/Xilinx92i/shivnath/minor_project.vhd" in Library work.

Architecture behavioral of Entity minor_project is up to date.

===========================================================

* HDL Analysis *

===========================================================

Analyzing Entity <minor_project> in library <work> (Architecture <behavioral>).

INFO:Xst:1561 - "C:/Xilinx92i/shivnath/minor_project.vhd" line 91: Mux is complete : default of case is discarded

WARNING:Xst:819 - "C:/Xilinx92i/shivnath/minor_project.vhd" line 42: The following signals are missing in the process sensitivity list:

Multiplier, Multiplicand.

Entity <minor_project> analyzed. Unit <minor_project> generated.

===========================================================

* HDL Synthesis *

===========================================================

Performing bidirectional port resolution...

Synthesizing Unit <minor_project>.

Related source file is "C:/Xilinx92i/shivnath/minor_project.vhd".

WARNING:Xst:2734 - Property "use_dsp48" is not applicable for this technology.

Found 8-bit 16-to-1 multiplexer for signal <Result_out>.

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Found 8-bit addsub for signal <Result_out$addsub0000>.

Found 4x4-bit multiplier for signal <Result_out$mult0000> created at line 78.

Found 8-bit xor2 for signal <Result_out$xor0000> created at line 63.

Summary:

inferred 1 Adder/Subtractor(s).

inferred 1 Multiplier(s).

inferred 8 Multiplexer(s).

Unit <minor_project> synthesized.

INFO:Xst:1767 - HDL ADVISOR - Resource sharing has identified that some arithmetic operations in this design can share the same physical resources for reduced device utilization. For improved clock frequency you may try to disable resource sharing.

HDL Synthesis Report

8-bit xor2 : 1

Macro Statistics

8-bit addsub : 1

8-bit 16-to-1 multiplexer : 1

4x4-bit multiplier : 1

# Xors : 1

# Multipliers : 1

# Multiplexers : 1

# Adders/Subtractors : 1

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===========================================================

* Advanced HDL Synthesis *

===========================================================

Loading device for application Rf_Device from file '3s400.nph' in environment C:\Xilinx92i.

===========================================================

Advanced HDL Synthesis Report

Macro Statistics

# Multipliers : 1

4x4-bit multiplier : 1

# Adders/Subtractors : 1

8-bit addsub : 1

# Multiplexers : 1

8-bit 16-to-1 multiplexer : 1

# Xors : 1

8-bit xor2 : 1

===========================================================

* Low Level Synthesis *

===========================================================

Optimizing unit <minor_project> ...

Mapping all equations...

Building and optimizing final netlist ...

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Found area constraint ratio of 100 (+ 5) on block minor_project, actual ratio is 2.

Final Macro Processing ...

===========================================================

* Final Report *

===========================================================

Final Results

RTL Top Level Output File Name : minor_project.ngr

Top Level Output File Name : minor_project

Output Format : NGC

Optimization Goal : Speed

Keep Hierarchy : NO

Design Statistics

# IOs : 36

Cell Usage :

# BELS : 153

# GND : 1

# LUT1 : 8

# LUT3 : 64

# LUT4 : 9

# MUXCY : 7

# MUXF5 : 32

# MUXF6 : 16

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8 BIT ALU DESIGN

# MUXF7 : 8

# XORCY : 8

# IO Buffers : 36

# IBUF : 28

# OBUF : 8

# MULTs : 1

# MULT18X18 : 1

===========================================================

Device utilization summary:

---------------------------

Selected Device : 3s400pq208-5

Number of Slices: 41 out of 3584 1%

Number of 4 input LUTs: 81 out of 7168 1%

Number of IOs: 36

Number of bonded IOBs: 36 out of 141 25%

Number of MULT18X18s: 1 out of 16 6%

===========================================================

TIMING REPORT

--------------------------------------------------------------------------------

Release 9.2i Trace


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8 BIT ALU DESIGN

C:\Xilinx92i\bin\nt\trce.exe -ise C:/Xilinx92i/shivnath/shivnath.ise -intstyle

ise -e 3 -s 5 -xml minor_project_preroute minor_project_map.ncd -o

minor_project_preroute.twr minor_project.pcf -ucf minor_project.ucf

Design file: minor_project_map.ncd

Physical constraint file: minor_project.pcf

Device,speed: xc3s400,-5 (PRODUCTION 1.39 2007-04-13)

Report level: error report

Environment Variable Effect

INFO:Timing:2698 - No timing constraints found, doing default enumeration.

INFO:Timing:2752 - To get complete path coverage, use the unconstrained paths

option. All paths that are not constrained will be reported in the

unconstrained paths section(s) of the report.

INFO:Timing:3284 - This timing report was generated using estimated delay

information. For accurate numbers, please refer to the post Place and Route

timing report.

INFO:Timing:3339 - The clock-to-out numbers in this timing report are based on

a 50 Ohm transmission line loading model. For the details of this model,

and for more information on accounting for different loading conditions,

please see the device datasheet.

Data Sheet report:

-----------------

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8 BIT ALU DESIGN

All values displayed in nanoseconds (ns)

Pad to Pad

-----------------+---------------+---------+-------------

Source Pad |Destination Pad| Delay |

-----------------+---------------+---------+------------

Accumulator_in<0>|Result_out<0> | 7.985|

















63

8 BIT ALU DESIGN




















Data_in<0> |Result_out<0> | 8.564|



64

8 BIT ALU DESIGN























65

8 BIT ALU DESIGN












Multiplicand<0> |Result_out<0> | 8.693|











66

8 BIT ALU DESIGN
















Multiplier<0> |Result_out<0> | 8.693|







67

8 BIT ALU DESIGN




















Opcode_in<0> |Result_out<0> | 8.945|



68

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69

8 BIT ALU DESIGN








-----------------+---------------+---------+

Analysis completed Fri Dec 10 15:44:04 2010

Peak Memory Usage: 108 MB

5.2 BITGEN REPORT

Release 9.2i - Bitgen J.36


Loading device for application Rf_Device from file '3s400.nph' in environment

C:\Xilinx92i.

"minor_project" is an NCD, version 3.1, device xc3s400, package pq208, speed

-5

Opened constraints file minor_project.pcf.

Fri Dec 10 15:57:31 2010

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8 BIT ALU DESIGN

Summary of Bitgen Options:

+----------------------+----------------------+

| Option Name | Current Setting |

+----------------------+----------------------+

| Compress | (Not Specified)* |

+----------------------+----------------------+

| Readback | (Not Specified)* |

+----------------------+----------------------+

| CRC | Enable** |

+----------------------+----------------------+

| DebugBitstream | No** |

+----------------------+----------------------+

| ConfigRate | 6** |

+----------------------+----------------------+

| StartupClk | Cclk** |

+----------------------+----------------------+

| DCMShutdown | Disable** |

+----------------------+----------------------+

| DCIUpdateMode | AsRequired** |

+----------------------+----------------------+

| CclkPin | Pullup** |

+----------------------+----------------------+

| DonePin | Pullup** |

+----------------------+----------------------+

| HswapenPin | Pullup* |

+----------------------+----------------------+

| M0Pin | Pullup** |

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8 BIT ALU DESIGN

+----------------------+----------------------+


+----------------------+----------------------+


+----------------------+----------------------+


+----------------------+----------------------+

| ProgPin | Pullup** |

+----------------------+----------------------+

| TckPin | Pullup** |

+----------------------+----------------------+

| TdiPin | Pullup** |

+----------------------+----------------------+

| TdoPin | Pullup** |

+----------------------+----------------------+

| TmsPin | Pullup** |

+----------------------+----------------------+

| UnusedPin | Pulldown** |

+----------------------+----------------------+

| GWE_cycle | 6** |

+----------------------+----------------------+

| GTS_cycle | 5** |

+----------------------+----------------------+

| LCK_cycle | NoWait** |

+----------------------+----------------------+

| Match_cycle | Auto** |

+----------------------+----------------------+

72

8 BIT ALU DESIGN

+----------------------+----------------------+


+----------------------+----------------------+

| DONE_cycle | 4** |

+----------------------+----------------------+

| Persist | No* |

+----------------------+----------------------+

| DriveDone | No** |

+----------------------+----------------------+

| DonePipe | No** |

+----------------------+----------------------+

| Security | None** |

+----------------------+----------------------+

| UserID | 0xFFFFFFFF** |

+----------------------+----------------------+

| ActivateGclk | No* |

+----------------------+----------------------+

| ActiveReconfig | No* |

+----------------------+----------------------+

| PartialMask0 | (Not Specified)* |

+----------------------+----------------------+


+----------------------+----------------------+


+----------------------+----------------------+

| PartialLeft | (Not Specified)* |

+----------------------+----------------------+

73

8 BIT ALU DESIGN

+----------------------+----------------------+


+----------------------+----------------------+

| PartialRight | (Not Specified)* |

+----------------------+----------------------+

| IEEE1532 | No* |

+----------------------+----------------------+

| Binary | No** |

+----------------------+----------------------+

* Default setting.

** The specified setting matches the default setting.

Running DRC.

DRC detected 0 errors and 0 warnings.

Creating bit map...

Saving bit stream in "minor_project.bit".Bitstream generation is complete.

74

8 BIT ALU DESIGN

5.3 RTL SCHEMATIC: ARITHMETIC LOGIC UNIT

75

8 BIT ALU DESIGN

5.3.1 DETAILED RTL SCHEMATIC: ARITHMETIC LOGIC UNIT

76

8 BIT ALU DESIGN

5.3.2 TECHNOLOGY SCHEMATIC

77

8 BIT ALU DESIGN

5.4 SIMULATION WAVEFORMSInputs: Accumulator_in : 11001011

Data_in : 00101010

Operation: ADDITION

Output: Result_out = 11110101

78

8 BIT ALU DESIGN

Inputs: Accumulator_in : 11001011

Data_in : 00101010

Operation: AND

Output: Result_out = 00001010

79

8 BIT ALU DESIGN

5.5 FLOOR PLAN

80

8 BIT ALU DESIGN

CHAPTER 6

EPILOGUE

CONCLUSION

The VHDL is a versatile language, which has great flexibility of designing

components. It can be said that VHDL fuelled modern synthesis technology and

enabled the development of ASIC semiconductor companies. From the beginning

VHDL has been a powerful language with numerous language constructs that are

capable of describing very complex behavior, this leadership of VHDL community

has assured open and internationally accredited for the electronic design

Engineering community. The legacy of team work continues to benefit the design

community today as the benchmark by which one measures openness.

With the help of which we are able to successfully implement the Arithmetic Logic Unit of a Microprocessor.

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8 BIT ALU DESIGN

BIBLIOGRAPHY

J.Bhaskar, A VHDL Primer, Pearson Education Asia, Third Edition 2006

Volnei A. Pedroni, Circuit Design with VHDL, Prentice Hall India 2005

Morris M Mano, Digital Design, Pearson Edition, 2000

XILINX INC, XILINX Foundation Series User Reference Guide

XILINX INC, XILINX Software Sampler Quick Start Guide

Charles H Roth, Principles Of Digital System Design using VHDL,Clengage Learning 2010

Enoch O. Hwang, Digital Logic and Microprocessor Design with VHDL, Thomson 2007

www.xilinx.com

www.ieee.org

82

http://www.ieee.org/

http://www.xilinx.com/

Design and Simulation of 8 Bit Arithmetic Logic Unit

Documents

logic operations

arithmetic logic unit

alu design contentschapter

alu design bitge

vhdl language

alu design speed optimization

concept of vhdl

later vhdl