vhdl-notes1

7/29/2019 vhdl-notes1

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TKK S-88.133 autumn1999

Digital design with Hardware Description Languages

Autumn 1999

Mikael Platan ([email protected])

Mark Davidson ([email protected])

Vesa Koskela ([email protected])

Autumn VHDL course for TKK


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VHDL = VHSIC Hardware Description Language

This course will cover two main topics:

1.) The VHDL language and it constraints.

2.) Why one would want to use the language in digital design.

However, other questions will be answered such as

Where does it fit on the world map of hardware design?

Why is it here?

Why ever use VHDL?


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VHDL

VHSIC Hardware Description Language

VHSIC = Very High Speed Integrated Circuit


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Hardware Description Languages

There are many HDLs with different features and goals:

- VHDL: U.S. Department of Defense sponsored project (first standard 1987)Most wide spread in Europe

- VERILOG: Proprietary language first then opened to the public (late 80s)Most wide spread in the USA

- UDL/I: Unified Design Language for IC:s. Developed in Japan for commoninterchange format between CAD tools.

- SILAGE: Suited for DSP application.

VHDL and VERILOG are the most common!


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VHDL History

- Originated from the US Government VHSIC program

A need was seen for a method in which to describe a digital circuit insuch a manner that it was free from any technical constraints. Thiswould allow the Department of Defense to replace an obsoletecomponent with a new one without large amounts of re-design work.

- Further developed by Institute of Electrical and Electronics Engineers(IEEE)

- Adopted as an IEEE STANDARD1076 in 1987 Hence VHDL-87!

- All IEEE standards are subject to a review every 5 years

This is where VHDL-93 comes from!


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VHDL - language

IEEE standard hardwaredescription language 1076-1987VHDL-87

Is based on the ADA language

VHDLs worst competitor isVERILOG

New VHDL version was approvedin 1993. VHDL-93

New VHDL version to beapproved early this year. Itcontains analog signal support.

For digital design/modeling Also aAHDL (Analog HDL) is beingdeveloped. This is due to thegrowing importance of mixedsignal systems.

Most important application is inASIC (Application SpecificIntegrated Circuit) design

It is used for modeling, simulation,logic synthesis and testing

Common method of modelingbetween designers and design tools


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VHDL features

Parallelisms. Different processescan be active independently ofeach other. This is fundamentallyan accurate model for a digitalsystem.

Supports hierarchical design, top-down or bottom-up.

Supports reuse through itslibrary/package functions wherefunctions and components can becollected

Strongly typed language. This issimilar to Pascal and otherSoftware programming languages.It means that operands must be ofthe correct type for a operator to becalled

Dynamic types and their use withe.g. attributes:

FOR i IN jrange LOOP ...

where the variable js range candynamically changed.

Overloading. E.g. the same

function can be called withdifferent types of parameters. Thisis related to typing. To use thesame function (operator) ondifferent types of objects (operand)one must define it multiply to becompatible with all the requiredtypes.


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VHDL - world

Good overview from Internet:http://vhdl.org

FAQ, Frequently Asked Questions,in VHDL:http://vhdl.org/vi/comp.lang.vhdl/

VHDL International organizations

e.g. standardization Usenet news groups:

comp.lang.vhdl

Tutorials on the net, VHDLmodels, libraries, news etc

Many conferences. e.g. DAC

http://www.mkp.com

Documents: 1076-1987 StandardVHDL Language ReferenceManual, IEEE standard VHDL...(-93)

Numerous books: e.g. Perry -VHDL, Bhasker - A VHDLprimer

Numerous VHDL products, alsosome Public Domain.

The most known manufacturers:Synopsys, Mentor, Cadence,Summit, Renoir, TransEDA

Design, Lecturing, Consulting,

Subcontracting


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Modeling Digital Systems

Digital system = Any digital circuit that processes or storesinformation

These systems are complex: We need to recognize them as complexand have to find ways to deal with the complexity

The most important way is to adopt a systematic methodology ofdesign

The term MODEL will refer to our understanding of a system. Amodel should allow us to make predictions about the system.

There can be several models of one system!


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Why MODELs

1) Formal model to communicate requirements.

2) Communicate and enhance the understanding of the function of asystem

3) Models will allow testing and verification of a design usingsimulation. Thus allowing for the correction of errors before thesystem is manufactured.

4) Formal verification of the correctness of a design

5) To allow SYNTHESIS of circuits


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Why use Hardware Description Language for modeling

To model the design

To verify the model

To implement the model in hardware (synthesizing)

Using a language to do this allows for more levels of abstraction.Giving the designer more freedom to implement ideas, rather thanspecifics, results in higher quality designs in a shorter time.


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Remember!

The idea behind all of these arguments is that:

We want to achieve maximum reliability in the design process

for a

minimum cost and design time!!


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Domains and Levels of Modeling


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Functional

At the most abstract level the system may be described in terms of

ALGORITHMS

NOTE! This is often called BEHAVIOURAL MODELING

Example:

loopfor each data input loop

read the value on this input;

scale the value using the current scale factor for this input

convert and write....

end loop;wait for 10ms;

end loop;


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Structural

The system may be described as an interconnection of components likeprocessors, memories and i/o devices.

Sometimes called as the Processor Memory Switch, PMS, level.

Example:


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Geometric

Top level abstraction: How the components described in the structuralmodel are organized on the silicon die.

Pad frame


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Next level in abstraction: REGISTER TRANSFER LEVEL

Register Transfer Level: Structural model:

Example:


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Register Transfer Language (RTL)

In the functional domain a Register Transfer Language is used to specifythe operation of a system at the Register Transfer Level:

Example:

MAR


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Most designers work more and more away from the center

Tools help designers

work away from the

center


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Why VHDL as modeling language?

Includes facilities to describe structure and function at VARIOUSlevels of abstraction (above gate level)

Has an attribute mechanism to annotate information from thegeometric domain.

It can be used for specification, simulation and hardware synthesis

It is widely supported by CAD tool vendors. It is a standard being continuously reviewed.


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Graphical Entry tools - need anymore to learn VHDL?!

Today many Graphical Entry tools that automatically generate VHDLcode exist (Summit, Renoir)

They are becoming so good that they are already being used inproduction

The need for writing actual code may have been reduced due to GE

tools but:1) the need to understand what was auto generated is still there

2) we still have to manage large amounts of vhdl blocks

3) VHDL is still the descriptive part

4) Many of the GE tools are not producing correct or optimized code inall situations.

GE tools should be viewed as a complement not a replacement toVHDL.


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VHDL Modeling Concepts

We have discussed three domains of modeling:

1) Function

2) Structure

3) Geometry

We will now introduce VHDL elements to describe the basic modelingconcept in each of the above 3 domains.

NB: The objective is to GET A FEEL for VHDL as a descriptionlanguage - All these will be discussed in detail later.


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Entity


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Elements of Behavior

- Internal function of an entity is called ARCHITECTURE

- An architecture can have a number ofalternative functions

- A behavioral architecture of an entity consists ofProcess statements

- Process statements are a collection of actions executed in sequence(sequential statements)

Example:


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architecture behav of reg4 isbegin

storage : process isvariable stored_d0, stored_d1, stored_d2, stored_d3 : bit;

begin ifen = '1' and clk = '1' then

stored_d0 := d0;

stored_d1 := d1;stored_d2 := d2;stored_d3 := d3;

end if;q0


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Elements of Structure

An implementation of an entity can be viewed as being composed ofsubsystems

An architecture body that only has interconnected subsystems iscalled an structural architecture body

Example:


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Structural composition


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Subsystem description

entity d_latch isport ( d, clk : in bit; q : out bit );

end d_latch;

architecture basic ofd_latch isbegin

latch_behavior : process is

beginif clk = '1' thenq


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Structural architecture

architecture struct ofreg4 is

signal int_clk : bit;

begin

bit0 : entity work.d_latch(basic)port map (d0, int_clk, q0);




gate : entity work.and2(basic)port map (en, clk, int_clk);

end architecture struct;


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Mixed Models

In models it can be and is often useful to

mix structural and behavioral models

One of the strengths of VHDL is the amount of freedom it gives to thedesigner.

Good design is using the appropriate method of modeling.


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Test benches

It is an entirely self-contained entity, with no port list, that stimulatesthe VHDL block under test.

The block under test is instantiated as a component in the test benchand structurally connected to the test bench.

Test benches will be covered more in detail in lecture #6.


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Analysis, Elaboration and Execution

Analysis: The process of checking the syntax and semantics of thecode

Elaboration: Going through the design hierarchy and creating all of theobjects defined in the declarations.

Execution: The passage of time is simulated in discrete steps.


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Reserved Words in VHDL -93


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Extended Backus-Naur Format (EBNF)

For syntax description EBNF is used throughout the book.

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