© 2006 Hewlett-Packard Development Company, L.P.The information contained herein is subject to change without notice

What’s with all this talk about coverage?David Lacey and Rob PorterHewlett Packard CompanyJune 20, 2006

2 April 12, 2023

“You have this awesome generation that pseudo-randomly creates all sorts of good scenarios. You also have created equally awesome scoreboard and temporal checker infrastructure that will catch all the bugs. Next, you run it like mad, with all sorts of seeds to hit as much of the verification space as possible.”Peet James

Verification Plans

3 April 12, 2023

Given all that, what really happened?• Where did all those transaction go?• Which lines of RTL were exercised?• Which sections of the specification were tested?• Which corner cases of my implementation were hit?• What was the distribution of transaction types

issues?• Do I need to create new tests?• Can I stop running simulations?

Coverage helps provides the

answers!Coverage is a piece of the puzzle, not the final answer

4 April 12, 2023

Coverage provides…• An understanding of which portions of the

design have been exercised• Increased observability of simulation

behavior• Feedback on which tests are effective• Feedback to direct future verification

efforts

5 April 12, 2023

Agenda• Coverage terms and tools• How to get started• Coverage planning• Coverage execution• Coverage analysis• Coverage results

6 April 12, 2023

Coverage terms and tools

7 April 12, 2023

Coverage terms• Coverage strategy

− Approach defined to utilize coverage technology − Generate, gather, and analyze coverage data

• Coverage model− Collection of coverage spaces− Definition of one or more coverage spaces of interest

• Coverage space− Set of coverage points associated with a single aspect of the design and

a single coverage technology• Coverage technology

− Specific mechanism such as code, functional, assertion, transaction• Coverage point

− A specific named aspect of the design behavior− FCP, line of code, state transition, transaction or sequence of transactions

• Coverage data− Raw data collected from all coverage points and coverage spaces

• Coverage results− Interpretation of coverage data in context of coverage model

8 April 12, 2023

Coverage model vs. coverage tools

•WHAT−Coverage model High Level

ArchitectureSpecification

Design detailLow Level

TransactionBug rates

Code

Coverage

ModelFunctional

Assertion

Coverage tools

Sim cycles

•HOWCoverage tools

9 April 12, 2023

Code coverage• Line/block, branch, path, expression, state• Measures controllability aspect of our stimulus

−i.e. What lines of code have we exercised

• Does not connect us to the actual functionality of the chip −No insight into functional correctness

• Takes a blind approach to coverage (low observability)−Activating an erroneous statement does not mean the error will

propagate to an observable point during the course of a simulation

• Generates a lot of data−Difficult to interpret what is significant and what is not

10 April 12, 2023

Assertion coverage Assertions monitor and report undesirable

behavior • Ensures that preconditions of an assertion

check have been met

// SVA: if asserting stop or flush, no new requestassert property (@(posedge clk) disable iff (rst_n)

((Flush | SMQueStop) |->SMQueNew))

else $error(“Illegal behavior”); precondition

check

11 April 12, 2023

Functional coverage• Similar in nature to assertions

Assertions monitor and report undesirable behavior

Functional coverage monitors and reports desirable behavior

• Functional coverage−Specific design details

−Corner cases of interest to engineers

−Architectural features

12 April 12, 2023

Transaction coverage• A transaction is the logging of any data structure

−A packet on a bus−Does not have to be a system packet

• Example transaction coverage points−All transaction types were seen on each interface−Transactions with specific data were seen

• Source, destination, address, address ranges• Sequences of transactions

−Have recording monitor watch for sequence−Implement advanced queries to look for sequence

• Two parts to transaction coverage−Record the right data−Correct queries

13 April 12, 2023

EDA tools• Code, FCPs and transactions are recorded

into vendor specific databases−Tools are provided to look at coverage data−Report engines provide text reports

• Debug tools for FCPs and assertions• Tools to encourage coverage-driven

methodologies• Coverage is still a young technology

−Tools still expanding set of capabilities−Development areas such as data aggregation,

multiple view extraction

14 April 12, 2023

How do I get started with this coverage stuff?

15 April 12, 2023

Coverage roadmap – getting started

PSL / SVA / OVL

Coverage tools

Code / Assertion / FCP / Txn

TxnFCP

Code4. Collect data

1. Choose specificatio

n form

2. Identify

coverage model

3. Implement coverage

model

5. Analyze

data

6. React to data

Spec, design

Adjust stimulus

Planning

Execution

Consumption

16 April 12, 2023

Coverage planning

PSL / SVA / OVL

Coverage tools

Code / Assertion / FCP / Txn

Txn

FCP

Code4. Collect

data

1. Choose specification form

2. Identify

coverage model

3. Implement coverage

model

5. Analyze

data

6. React to data

Spec, design

Adjust stimulus

Planning

Execution

Consumption

17 April 12, 2023

Coverage planning

• Identify content of the coverage model−Coverage types to be used

• Identify required tools • Coverage infrastructure• Coverage execution• Maintenance and tool enhancements• Define coverage goals and metrics• Coverage reviews

Start looking at coverage up front!Coverage results only as good as coverage

model

18 April 12, 2023

Who, what, when, where, why• Who creates coverage model?

−Who analyses the data?

−Who owns coverage?

Add with RTLAnalyze continuously

Logic and DV engineers

Concern areas

spec, design, assertions, test plan

Because…

• What to cover in the model?

• When to add coverage points?−When to analyze coverage data?

• Where to look for ideas?

• Why mess with coverage?

19 April 12, 2023

For FCPs, ask yourself…−What should be covered?

−Where is best place to put FCPs?

−When to look for condition?

−Why have coverage point?

20 April 12, 2023

Watch out for…• Too much data

−Need information, not data−Need supporting tools to get

correct views of data• Ineffective use of coverage

−FCPs that fire every clock cycle−Duplication of coverage with

different tools• Reading too much into grading

tests−Random tests produce different

results with different seeds

21 April 12, 2023

Cost of coverage• Plan for the costs of using coverage

−Get solid infrastructure setup

−Plan for slower simulations

• Some level of cost is acceptable−Getting value back for investment

• Be smart−Architect coverage plan up front to ensure

success

22 April 12, 2023

Coverage execution

PSL / SVA / OVL

Coverage tools

Code / Assertion / FCP / Txn

Txn

FCP

Code4. Collect data

1. Choose specification form

2. Identify coverage model

3. Impleme

nt coverage

model

5. Analyze

data

6. React to data

Spec, design

Adjust stimulus

Planning

Execution

Consumption

23 April 12, 2023

Describing coverage model• Code coverage

−RTL code, pragmas

• Assertion and functional coverage−Use assertion language or library (PSL, SVA,

OVL)

• Transaction−Use hooks into Transaction Level Modeling

// PSL cover exampledefault clock = (posedge clk);sequence qFullCondition =

{reset_n ? (q_full : 1’b0);cover qFullCondition;

// SVA cover examplealways @(posedge clk) begin

if (reset_n)myQfull: cover (q_full)

$info (“queue was full”);end

24 April 12, 2023

Data collection• Collect data across volume simulation• Aggregate multiple databases• Location of coverage data repository• Manage volume of data

25 April 12, 2023

Coverage analysis PSL / SVA /

OVL

Coverage tools

Code / Assertion / FCP / Txn

Txn

FCP

Code4. Collect

data

1. Choose specification form

2. Identify

coverage model

3. Implement coverage

model

5. Analyze

data

6. React to data

Spec, design

Adjust stimulus

Planning

Execution

Consumption

26 April 12, 2023

The analysis• Easy to generate a

ton of data−Want information,

not data

Need to organize the data Can’t look at it all at

once Determine views

needed

27 April 12, 2023

Views of coverage data• Un-hit coverage• Functionality groups• Block, chip, system• Current milestone

functionality• Instance or module specific• Across environments, time,

model releases• Cross views

28 April 12, 2023

Our use of coverage• Aggregate data for each verification environment• Views: Verification effectiveness

−Verification environment• Views: TR readiness

−Major sub-blocks and chip• Filtering infrastructure

−Milestone specific functionality−Unreachable

• Aggregate coverage data across windows of time• Metrics provided for each team and full chips

29 April 12, 2023

Analysis is done… now what?• Understand all un-hit coverage• Fill coverage holes• Look for hard to hit coverage• Track coverage metrics

Don’t play games with metrics just to get coverage goals

met

Really understand the results

30 April 12, 2023

Coverage results PSL / SVA /

OVL

Coverage tools

Code / Assertion / FCP / Txn

Txn

FCP

Code4. Collect

data

1. Choose specification form

2. Identify

coverage model

3. Implement coverage

model

5. Analyze

data

6. React to data

Spec, design

Adjust stimulus

Planning

Execution

Consumption

31 April 12, 2023

Success stories• They exist!• Check them out in Assertion-Based Design

32 April 12, 2023

HP coverage data• SX1000 chipset

−6,500 FCPs• SX2000 chipset

−25,000 FCPs• Current efforts

−135,000 assertions and 650,000 FCPs−56,000 transaction points

• Coverage goals−100% coverage with understood exceptions−Team defined goals per milestone

33 April 12, 2023

Resources−J. Bergeron, Writing Testbenches: Functional Verification

of HDL Models, Second Edition, Kluwer Academic Publishers, 2003.

−H. Foster, A. Krolnick, D. Lacey, Assertion-Based Design, Second Edition, Kluwer Academic Publishers, 2004.

−P. James, Verification Plans: The Five-Day Verification Strategy for Modern Hardware Verification Languages, Kluwer Academic Publishers, 2004.

−A. Piziali, Functional Verification Coverage Measurement and Analysis, Kluwer Academic Publishers, 2004.

−B. Cohen, Using PSL/Sugar with Verilog and VHDL, Guide to Property Specification Language for ABV, VhdlCohen Publishing, 2003.

David Lacey, Hewlett Packard, david.lacey@hp.com

Rob Porter, Hewlett Packard, robert.porter@hp.com