SystemVerilog Assertions and Functional Coverage: Guide to Language, Methodology and Applications

374 Pages·2013·22.542 MB·

by  Ashok B. Mehta

Ashok B. Mehta
SystemVerilog
Assertions
and Functional
Coverage
Guide to Language, Methodology and
Applications
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SystemVerilog Assertions
and Functional Coverage
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Ashok B. Mehta
SystemVerilog Assertions
and Functional Coverage
Guide to Language, Methodology
and Applications
123
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Ashok B.Mehta
Los Gatos,CA
USA
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DOI 10.1007/978-1-4614-7324-4
SpringerNewYorkHeidelbergDordrechtLondon
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To
My dear wife Ashraf Zahedi
and
My dear parents Rukshmani
and Biren Mehta
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Foreword
Louis H. Sullivan, an American architect, considered the father of the modern
skyscraper, and mentor to Frank Lloyd Wright, coined the phrase ‘‘form follows
function.’’ The actual quote is ‘‘form ever follows function’’ which is a bit more
poetic and assertive than the version that has found its way into the common
vernacular. He wrote those words in an article written for Lippincott’s Magazine
#57 published in March 1896. Here is the passage in that article that contains the
famous quote:
‘‘Whether it be the sweeping eagle in his light or the open apple-blossom, then toiling
workhorse,theblitheswan,thebranchingoak,thewindingstreamatitsbase,thedrifting
clouds—overallthecoursingsun,formeverfollowsfunction,andthisisthelaw.Where
functiondoesnotchange,formdoesnotchange.Thegraniterocks,theeverbroodinghills,
remainforages;thelightninglives,comesintoshape,anddies,inatwinkling.
It is the pervading law of all things organic and inorganic, of all things physical and
metaphysical,ofallthingshumanandallthingssuperhuman—ofalltruemanifestationsof
thehead,oftheheart,ofthesoul—thatthelifeisrecognizableinitsexpression,thatform
everfollowsfunction.Thisisthelaw.’’
Earlier in the article, Sullivan foreshadows his thought with this passage:
‘‘Allthingsinnaturehaveashape,thatistosay,aform,anoutwardsemblance,thattells
uswhattheyare,thatdistinguishesthemfromourselvesandfromeachother.’’
The precise meaning of this pithy phrase has been debated in art and archi-
tecture circles since Sullivan’s article was first published. However, it is widely
accepted to mean that the form of something—its shape, color, size, etc.—is
related to what it does. Water flows, rocks sit, and birds fly.
In his book ‘‘The Design of Everyday Things,’’(Basic Books 1988) Don Nor-
man discusses a similar concept, the notion of affordances. Norman defines the
term as ‘‘… the perceived and actual properties of the thing, primarily those
fundamentalpropertiesthatdeterminejusthowthethingcouldpossiblybeused.’’
Hecitessomeexamples:‘‘Achairaffords(‘‘isfor’’)supportand,therefore,affords
sitting. A chair can also be carried. Glass is for seeing through, and for breaking.
Wood is normally used for solidity, opacity, support or carving.’’
vii
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viii Foreword
Norman’s idea turns Sullivan’s upside down. He is saying function follows
form. The shape, color, size, etc., of an object affects what it does. Nonetheless,
both men would likely agree that form and function, whichever drives the other,
are inextricably linked.
Software designers have the luxury of choosing the form to fit the function.
They are not as constrained by the laws of physics as say, a cabinetmaker. The
cabinetmakermustchoosematerialsthatwillnotonlylooknice,butwillwithstand
theweightofbooksordishesorwhateveristobeplacedontheshelves.Software
designershavesomeconstraintswithregardtomemoryspaceandprocessingtime,
but beyond that they have a lot offreedom to build whatever comes to mind.
Sullivanreferredto‘‘allthingsphysicalandmetaphysical.’’Withoutmuchofa
stretch we can interpret that to include software, a most abstract human creation.
Theformofapieceofsoftwareislinkedtoitsfunction.Thecomplexsoftwarethat
verification engineers build, called a testbench, must be designed before it can be
built. The verification engineer, like an architect, must determine the form of his
creation.
Thearchitecturespaceiswideopen.Computercode,whilemuchmoreabstract
thansay,astaircaseoradoorhandleonacar,hasaformandafunction.Theform
ofcomputercodeisthesetofsyntacticelementsstrungtogetherinaprogram.The
function is what the program does when executed, often referred to as its
semantics.
A verification engineer is typically presented a set of requirements, often as a
designspecification,andaskedtobuildatestbenchthatmeetstheserequirements.
Because of the tremendous flexibility afforded by the software medium he must
choose the form carefully to ensure that not only meets the requirements, but is
easy to use, reusable, and robust. He must choose a form that fits the function.
Oftenanassertionisjusttherightthingtocapturetheessenceofsomepartofa
design.Theformofanassertionisshortsequenceoftextthatcanbeinsertedeasily
without disrupting the design. With their compact syntax and concise semantics
assertions can be used to check low-level invariants, protocols, or end-to-end
behavior.
Thefunctionofanassertion,inasimulationcontext,istoassertthatsomething
is always (or never) the case. It ensures that invariants are indeed invariant.
Assertions can operate as checkers or as coverpoints. The fact that they can be
includedin-lineinRTLcodeorinseparatecheckers,theycanbeshortorlongfor
simple or complex checking makes them invaluable in any testbench.
The wise verification engineer uses all the tools as his disposal to create an
effective andeasytousetestbench.Hewillconsider thefunctionofthetestbench
anddeviseaformthatsuitstherequiredfunction.Assertionsareanimportantpart
of any testbench.
AshokMehtahaswrittenabookthatmakesassertionsaccessible.Hisapproach
is very pragmatic, choosing to show you how to build and use assertions rather
than engage in a lot of theoretical discussion. Not that theoretical discussion is
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Foreword ix
irrelevant—it is useful to understand the theoretical underpinnings of any tech-
nology. However, there are many other books on that topic. This book fills a gap
forpracticingengineerswherebeforenotextprovidedthehow-tosofbuildingand
using assertions in a real-world context.
Ashokopensuptheworldofassertionstoverificationengineerswhomayhave
thought them too opaque to consider using in a real testbench. He does an espe-
cially nice job of deconstructing assertions to show how they work and how to
write them. Through detailed examples he shows all the pieces that go into cre-
atingassertionsofdifferentkinds,andhowtheyfittogether.Ashokcompletesthe
picture by demonstrating how assertions and coverage fit together.
Part of the book is devoted to functional coverage. He deconstructs the
sometimes awkward SystemVerilog syntax of covergroups and coverpoints. Like
hehaswithassertions,hetakesthemysteryoutofbuildingahigh-qualitycoverage
model.
With the mysteries of assertions unmasked, you can now include them in your
personalvocabularyoftestbenchforms.Thiswillenableyoutocreatetestbenches
with more sophisticated function.
February 2013 Mark Glasser
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Preface
Having been an end user of EDA tools for over 20 years, I have seen that many
newtechnologiesstayonway side,becauseeither theengineersdonothave time
tolearnofnewtechnologies/languagesortheavailablematerialistoocomplexto
digest. A few years back I decided to tackle this problem by creating a very
practical,application-orienteddown-to-earthSystemVerilogAssertions(SVA)and
Functional Coverage (FC) class for professional engineers. The class was well
received and I received a lot offeedback on making the class even more useful.
That culminated in over 500 slides of class material just on SVA and FC. Many
suggestedthatIhadcollectedenoughmaterialforabook.ThatishowIendedup
on this project with the same goal that the reader should understand the concept
clearly in an easy and intuitive manner and be able to apply the concepts to real-
life applications right away.
The style of the book is such that the concepts are clarified directly in a slide
stylediagramwithtalkingpoints.Thiswillhopefullymakeiteasytousethebook
as a quick reference as well. Applications immediately following a topic will
further clarify the subject matter and my hope is that once you understand the
semantics and applications of a given topic, you are ready to apply that to your
dailydesignwork.Theseapplicationsaremodeledsuchthatyoushouldbeableto
use them in your design with minimal modifications.
This book is meant for both design and verification engineers. As a matter of
fact, I have devoted a complete section on the reasons and practicality behind
having micro-level assertions written by the design engineers and macro-level
assertions written by verification engineers. Gone are the days when designers
wouldwriteRTLandthrowitoverthewallfortheverificationengineertoquality
check.
The book covers both IEEE 1800-2005 and IEEE 1800-2009 standard SVA
language.EventhoughIhavecoveredallthefeaturesof1800-2009standardSVA,
pleasenotethatover90%ofthesefeatureswerenotsupportedbyEDAtoolsasof
thiswriting.Inotherwords,theexamplesbelongingtothislanguagesubsetarenot
simulated. I would greatly appreciate feedback on 1800-2009 SVA language
examples for any errors or omissions.
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xii Preface
Chapter 1 is Introduction to SVA and FC giving a brief history of SVA evo-
lution. It also explains how SVA and FC fall under SystemVerilog umbrella to
provide a complete assertions and functional coverage driven methodology.
Part I: System Verilog Assertions (SVA)
Chapter2goesin-depthonSVA-basedmethodologyprovidingdetailthatyoucan
right away use in your project execution. Questions like ‘‘How do I know I have
added enough assertions?’’, ‘‘What type of assertions should I add’’, etc. are
explained with clarity.
Chapter 3 describes Immediate Assertions. These are nontemporal assertions
allowed in procedural code.
Chapter4goes into the fundamentals ofConcurrentAssertions toset the stage
fortherestofthebook.Howtheconcurrentmulti-threadedsemanticswork,when
and how assertions get evaluated in a simulation time tick, formal arguments,
disabling, etc., are described here.
Chapter 5 describes the so-called sampled value functions such as $rose, $fell,
$stable, $past etc.
Chapter6isthebigone!Thischapterdescribesalltheoperatorsofferedbythe
language including Clock Delay with and without range, Consecutive repetition
with and without range, nonconsecutive repetition with and without range,
‘throughout’, ‘within’, ‘and’, ‘or’, ‘intersect’, ‘first_match’, ‘if..else’, etc. Each of
the operator description is immediately followed by examples and applications to
solidify the concept.
Chapter 7 describes the System Functions and Tasks such as $isunknown,
$onehot, etc.
Chapter 8 discusses a very important aspect of the language that being prop-
ertieswithmultipleclocks.Thereisnotasingledesignnowadaythatusesonlya
single clock. A simple asynchronous FIFO will have a Read Clock and a Write
Clock which are asynchronous. Properties need to be written such that check in
one clock domain triggers a check in another clock domain. The chapter goes in
plenty detail to demystify semantics to write assertions that cross clock domains.
The so-called CDC (Clock Domain Crossing) assertions are explained in this
chapter.
Chapter9isprobablythemostusefulonedescribingLocalVariables.Without
this multi-threaded feature many of the assertions would be impossible to write.
There are plenty of examples to help you weed through the semantics.
Chapter10isonrecursiveproperties.Thesearerarelyusedbutareveryhandy
when you want to know thata propertyholds until another becomestrue orfalse.
Chapters 11–13 describe other useful features such as ‘expect’, ‘assume’, and
detectingendpointofasequence.Theendedandmatchedend-pointsofsequences
are indeed very practical features.
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