Design Verification – An IntroductionDesign Verification – An Introduction

© Pallab Dasgupta, Dept. of Computer Sc & Engg, IIT Kharagpur 2

Main References

Hardware Design Verification: Simulation andFormal Method-Based ApproachesWilliam K LamPrentice Hall Modern Semiconductor Design Series

A Roadmap for Formal Property VerificationPallab DasguptaSpringer

Course Web: http://www.facweb.iitkgp.ernet.in/~pallab and follow link to courses

© Pallab Dasgupta, Dept. of Computer Sc & Engg, IIT Kharagpur 3

Design, Validation and Testing

Specification

Implementation

Prototyping

Manufacturing

Pre-siliconPost-silicon

Design synthesisand validation

Manufacturingand testing

© Pallab Dasgupta, Dept. of Computer Sc & Engg, IIT Kharagpur 4

Design and Verification

Specification Implementation

Design

Equivalent?

Verification

© Pallab Dasgupta, Dept. of Computer Sc & Engg, IIT Kharagpur 5

Gate LevelBoolean LogicFinite State Machines

Digital Design: Abstraction Levels

Transistor Level

Formalisms introducedat the Entry-Level

Schematic

always @( posedge clk )beginif (!rst) begin a1 <= a2;

a2 <= ~a1; end;end

Register Transfer Level

Restricted semantics ofProgramming Languages,Communicating Concurrent State Machines (CSM)

Exponential growthin circuit size(Moore’s Law)

© Pallab Dasgupta, Dept. of Computer Sc & Engg, IIT Kharagpur 6

Design Example: 2-bit Gray Counter

Gray Counter: Successive values should differ only in one bit. Reset signalresets the counter to zero.

rst

s0s1

clk

00

10 11

01

!rst

!rst

!rst

!rst

rst

rst

rst

State m/c Representation

rst

© Pallab Dasgupta, Dept. of Computer Sc & Engg, IIT Kharagpur 7

Design Example: 2-bit Gray Counter

rst

s0s1

clk

(s0s1) rst (n0n1)

00 0 01

00 1 00

01 0 11

01 1 00

10 0 00

10 1 00

11 0 10

11 1 00

00

10 11

01

!rst

!rst

!rst

!rst

rst

rst

rst

State m/c Representation

rst

State Transition Table

© Pallab Dasgupta, Dept. of Computer Sc & Engg, IIT Kharagpur 8

Design Example: 2-bit Gray Counter

(s0s1) rst (n0n1)

00 0 01

00 1 00

01 0 11

01 1 00

10 0 00

10 1 00

11 0 10

11 1 00

State Transition Table State Transition Functions:

n0 = s0s1r + s0s1rn1 = s0s1r + s0s1r

After Logic Minimization:

n0 = s1rn1 = s0r

© Pallab Dasgupta, Dept. of Computer Sc & Engg, IIT Kharagpur 9

module GrayCounter(s0, s1, rst)input rst;reg s0, s1;

always @ (posedge clk)begin

s0 <= s1 & ~rst;s1 <= ~s0 & ~rst;

endendmodule

Verilog Code (RTL):

Design Example: 2-bit Gray Counter

rst s0

s1

clk

State Transition Functions:

n0 = s1rn1 = s0r

Synthesis

© Pallab Dasgupta, Dept. of Computer Sc & Engg, IIT Kharagpur 10

Abstractions in Design Flow

Functional Specification

Algorithmic Description

RTL

Gate Netlist

Transistor Netlist

Physical Layout

higher

lower

abstraction

less

more

details

© Pallab Dasgupta, Dept. of Computer Sc & Engg, IIT Kharagpur 11

Design and Verification

specifications

micro-architecture

RTL

gate netlist

layout

design

does it meet the specs?

does it implement the -arch?

are they equivalent?

are they equivalent?

propertychecking

equivalencechecking

verification

© Pallab Dasgupta, Dept. of Computer Sc & Engg, IIT Kharagpur 12

Design Flows: Digital versus Analog

Design Concept

Design Entry

Behavioral Simulation

Synthesis

Place & Route

Post-Layout Simulation

Full Chip Assembly

Full Chip DRC

Full Chip Simulation

Tape Out

Schematic Entry

Spice Simulation

Custom Layout

DRC

Post-Layout Simulation

Extract netlist

Verilog / VHDL SDL

DRC: Design Rule CheckingSDL: Schematic Driven Layout

© Pallab Dasgupta, Dept. of Computer Sc & Engg, IIT Kharagpur 13

Design Cycle: Implementation

Specs Document

RTL implementation

Gate Level Netlist

Verilog, VHDL

English documents

Transistor Level(Schematic)

Design integration

Synthesis

Technology mapping

Mask

Layout

Equivalence checking

Implementationvalidation

(Specvs RTL)

© Pallab Dasgupta, Dept. of Computer Sc & Engg, IIT Kharagpur 14

Verification Dominates Design

Simulation46%

Design27%

Structural12%

Emulation15%

• Synthesis• Timing analysis• Equivalence checking• DFT

• Behavioral modeling• Architecture level simulation• System level simulation

• High-level design• RTL coding• Block-level simulation

Source: 0-In Design Automation

© Pallab Dasgupta, Dept. of Computer Sc & Engg, IIT Kharagpur 15

Pieces of the verification puzzle

Picture source:skulladay.com

Architecturevalidation

Microcodevalidation

Timingvalidation

Powervalidation

Protocolvalidation

Full-chipvalidation

Unitvalidation

Clustervalidation

Debuggingvalidation

© Pallab Dasgupta, Dept. of Computer Sc & Engg, IIT Kharagpur 16

Design and Verification

specifications

micro-architecture

RTL

gate netlist

layout

design

does it meet the specs?

does it implement the -arch?

are they equivalent?

are they equivalent?

propertychecking

equivalencechecking

verification

© Pallab Dasgupta, Dept. of Computer Sc & Engg, IIT Kharagpur 17

Functional Verification Challenge

Is the implementation correct?

■ How do we define correct?

● Classical: Simulation result matches with golden output

● Formal: Equivalence with respect to a golden model

● Property verification: Correctness properties (assertions) expressed in a formal language Formal: Model checking Semi-formal: Assertion-based verification

■ Trade-off between computational complexity and exhaustiveness

© Pallab Dasgupta, Dept. of Computer Sc & Engg, IIT Kharagpur 18

Simulation

Design

Test Plan Test Bench

Stimulus Generation Simulation

Coverage Metrics Debug Bug Tracking

Advances:• Test bench languages are richer (such as SystemVerilog)• Coverage monitors and assertions• Layered test benches and Transaction Level Modelling

© Pallab Dasgupta, Dept. of Computer Sc & Engg, IIT Kharagpur 19

Advent of Formal Methods in EDA

always @( posedge clk )beginif (!rst) begin a1 <= a2;

a2 <= ~a1; end;end

Register Transfer Level

Gate Level

Transistor Level

Formal Properties

LogicalEquivalenceChecking

Design Intent

ModelChecking

Goal: Exhaustive verification of the design intent within feasible time limits

Philosophy: Extraction of formal models of the design intent and the implementationand comparing them using mathematical / logical methods

• Temporal Logics(Turing Award: Amir Pnueli)

• Adopted by Accelera / IEEE• Integrated into SystemVerilog• Tools:

Academia: NuSMV, VISIndustry: Magellan (Synopsys)

IFV (Cadence)• 2008: Clarke & Emerson get

Turing Award

© Pallab Dasgupta, Dept. of Computer Sc & Engg, IIT Kharagpur 20

Toy example: Priority Arbiter

r1

r2

g1

g2

• Either g1 or g2 is alwaysfalse (mutual exclusion)

G[g1 g2]

• Whenever r1 is asserted, g1 is given in the next cycleG[ r1 Xg1 ]

• When r2 is the sole request, g2 comes in the next cycleG[ (r1 r2) Xg2 ]

• When none are requesting, the arbiter parks the grant on g2 G[ (r1 r2) Xg2 ]

Violation!!

© Pallab Dasgupta, Dept. of Computer Sc & Engg, IIT Kharagpur 21

Dynamic Property Verification (DPV)

[Source: A Roadmap for Formal Property Verification, Springer, 2006]

© Pallab Dasgupta, Dept. of Computer Sc & Engg, IIT Kharagpur 22

Formal Property Verification (FPV)

always !g1 || !g2

always r2 && !r1 next g2

Formal Properties

Temporal Logics (Timed / Untimed, Linear Time / Branching Time): LTL, CTL

Early Languages: Forspec (Intel), Sugar (IBM), Open Vera Assertions (Synopsys)

Current IEEE Standards: SystemVerilog Assertions (SVA), Property Specification Language (PSL)

© Pallab Dasgupta, Dept. of Computer Sc & Engg, IIT Kharagpur 23

Assertion Based Verification Flow

Model Checker

Model +Properties

NO

YESIndeterminate Indeterminate

Results

Decompose, Abstract,

Over Constrain

YES

PASSNO

Spurious cex

YES

NO

Refine the model or assertions

Modify assumptions Stuck ? None of the

Abstractions working

Bug Hunting (Directed

Simulation assisted MC)

Closure ?

[Source: Raj Mitra, TI]