Introduction to Formal Equivalence Verification (FEV)

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# Introduction to Formal Equivalence Verification (FEV) - PowerPoint PPT Presentation

Introduction to Formal Equivalence Verification (FEV). Erik Seligman CS 510, Lecture 4, January 2009. Goals. Introduce basic concepts of FEV Enable you to try FEV using Cadence Conformal Examine some corner cases. FEV: The Basic Concepts. What Is FEV?. Best-established form of FV

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## Introduction to Formal Equivalence Verification (FEV)

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### Introduction to Formal Equivalence Verification (FEV)

Erik Seligman

CS 510, Lecture 4, January 2009

Goals
• Introduce basic concepts of FEV
• Enable you to try FEV using Cadence Conformal
• Examine some corner cases

### FEV: The Basic Concepts

What Is FEV?
• Best-established form of FV
• Other names: Equivalence Checking
• Answers: Are two models equivalent?
Main Uses of FEV
• RTL-Netlist equivalence
• Essential part of design flows
• Also leveraged for late hand edits (ECOs)
• Verifying quick changes to a model
• Fast & easy if model almost the same
Types of FEV
• Combinatorial / Synchronous
• Models must be (mostly) state-matching
• Very efficient due to no time calculations
• Works very well for synthesized netlists
• Most synthesis tools expect this
• Others: Synopsys Formality, Magma Quartz
• Sequential
• Allows more abstract RTL, or HLM-RTL FEV
• More flexibility for late netlist timing edits
• Much more risk/expense
• Few commercial tools (Calypto, NEC)

### State-Matching FEV

Are these equivalent?

a

f1

f2

b

out

ck

a

f3

f4

b

out

ck

Step 1: Map key points

a

f1

f2

b

out

f4

f3

ck

a

f3

f4

b

out

ck

Inputs?- Match. Outputs? Match.

States? f1->f3, f2->f4

Step 2: Build Equations

a

f1

f2

b

out

f4

f3

ck

a

f3

f4

b

out

ck

f3 = b, f4 = f3, out = !(a&f4)

f3=b, f4 = !(!f3), out = !a | !f4

Step 3: Compare Equations

a

f1

f2

b

out

f4

f3

ck

a

f3

f4

b

out

ck

f3 = b bEQUAL

f4 = f3 !(!f3) EQUAL

out =!(a&f4) !a | !f4 EQUAL

What if there was an error?

a

f1

f2

b

out

f4

f3

ck

a

f3

f4

b

out

ck

f3 = b bEQUAL

f4 = f3 !f3DIFFER

out =!(a&f4) !a | !f4 EQUAL

Debugging: Where To Look
• Fanin cones (“support set”)
• Different fanin  major issue
• Set of counterexample values
• If only specific values cause cex, provides hint of root cause
• “Intelligent” hints from tools
• Is an overall inversion suspected?
• Identify similar areas of logic within cone?
• Isolate error
Debug Schematic View

1

1

f1

f2

f4

f3

ck

1

0

1

f3

f4

ck

• Combinational  other logic irrelevant
• Good tools provide annotated cex value

### Introduction To Conformal

Conformal Terminology
• Gold = golden model (often RTL)
• Rev = revised model (often netlist)
• Many commands have –gold/-rev option
• Key Point = points to map
• Basic ones: primary inputs/outputs, states
• Others: blackboxes, dangling (Z) nodes, …
• Can refer to by name or integer ID
• Support Set = fanin cone
Conformal Modes
• Setup Mode: initial state
• Can load models, assign renaming rules
• Can set various global options
• Return to this mode: “set sys mode setup”
• LEC Mode: checking state
• Transition with “set sys mode lec”
• Automatically tries to map key points
• Models have been loaded, can compare
Conformal Usage Model
• Based on command console
• Startup with “LEC –nogui”
• Capable of taking general tcl scripts
• “help” available for any command
• Example: “help read design”
• Full manuals in /pkgs/cadence6/CONFRML71/doc
• “set log file <filename>” to start logging
• Always do this for homework!
• “set gui on” / “set gui off” can be done any time
• “dofile <filename>.do” to execute script
• Script = any set of console commands
Mapping Key Points
• LEC has good automapper
• Can guess many mappings
• But sometimes fails
• View mapping as “renaming”
• Temporarily rename RTL sig to match netlist
• “add renaming rule” to specify mappings
• Or “add mapped point” in LEC mode
Skeleton LEC Dofile

set log file lec.log –replace

read design –systemverilog –gold –f myrtl.filelist

read design –systemverilog –rev –f mynetlist.filelist

add renaming rule r1 foo bar –gold

set sys mode lec

report unmapped points

add compare points –all

compare

report compare data

Skeleton LEC Dofile

set log file lec.log –replace

read design –systemverilog –gold –f myrtl.filelist

read design –systemverilog –rev –f mynetlist.filelist

add renaming rule r1 foo bar –gold

set sys mode lec

report unmapped points

add compare points –all

compare

report compare data

Skeleton LEC Dofile

set log file lec.log –replace

read design –systemverilog –gold –f myrtl.filelist

read design –systemverilog –rev –f mynetlist.filelist

add renaming rule r1 foo bar –gold

set sys mode lec

report unmapped points

add compare points –all

compare

report compare data

Skeleton LEC Dofile

set log file lec.log –replace

read design –systemverilog –gold –f myrtl.filelist

read design –systemverilog –rev –f mynetlist.filelist

add renaming rule r1 foo bar –gold

set sys mode lec

report unmapped points

add compare points –all

compare

report compare data

Skeleton LEC Dofile

set log file lec.log –replace

read design –systemverilog –gold –f myrtl.filelist

read design –systemverilog –rev –f mynetlist.filelist

add renaming rule r1 foo bar –gold

set sys mode lec

report unmapped points

add compare points –all

compare

report compare data

Skeleton LEC Dofile

set log file lec.log –replace

read design –systemverilog –gold –f myrtl.filelist

read design –systemverilog –rev –f mynetlist.filelist

add renaming rule r1 foo bar –gold

set sys mode lec

report unmapped points

add compare points –all

compare

report compare data

Skeleton LEC Dofile

set log file lec.log –replace

read design –systemverilog –gold –f myrtl.filelist

read design –systemverilog –rev –f mynetlist.filelist

add renaming rule r1 foo bar –gold

set sys mode lec

report unmapped points

add compare points –all

compare

report compare data

Debugging Mismatches
• Debug commands available in console
• “diagnose <point>”: Display basic info
• But easier to debug in gui
• Report->Compare Data to see all points
• Red dots indicate mismatches
• Right-click at mismatch point, and “Diagnose”
• Gives support set, cex values, and LEC’s hints
• From Diagnose window can launch sch view

### Model “Flattening”

Minor exceptions to state-matching

Useful if flops/latches don’t map

Are These Equal?

rst

d

rst

d

DLAT

Are These Equal?

rst

d

rst

d

DLAT

set flatten model –dff_to_dlat_zero

Are These Equal?

rst

ck

rst

DLAT

ck

Are These Equal?

rst

ck

rst

DLAT

ck

set flatten model –dff_to_dlat_feedback

Are These Equal?

ck

set flatten model –seq_constant

Are These Equal?

d

ck

d

DLAT

DLAT

ck

Are These Equal?

d

ck

d

DLAT

DLAT

ck

set flatten model –latch_fold

Are These Equal?

DLAT

set flatten model –latch_transparent

Model Flattening
• Tool modified cases on previous slides
• Internally changes view of logic
• Only on request, not automatic
• May cause mismatches rather than curing!
• Often useful if key point imbalance
• In Conformal: “set flatten model…”
• Many options, not just ones on slides
• Can also use “remodel …” on single point

### FEV Constraints

Are these equivalent?

a

f1

f2

b

out

ck

f3

f4

b

out

ck

Are these equivalent?

a

f1

f2

b

out

ck

f3

f4

b

out

ck

• No! BUT– What if ‘a’ is always 1?
FEV: Why Constraints?
• RTL is often very general

`ifdef CHIP_VERSION_1

`define A 1

`else

`define A 2

`endif

• Design reuse: irrelevant RTL remains

assign A = 1’b1;

if (!A) …

Why Do Contraints Matter?
• Good synthesis tools take advantage
• Assume constants to reduce size/scope
• Don’t synthesize masked-out RTL
• Allow out-of-band constraint specs in control files
• FEV must recognize constraints
• Otherwise get mismatches
• No effort *if* constraints visible at FEV level
• But may be only in wrapper RTL
• Or inside analog blackbox
• Or could be due to software / outside specs
• If not visible to tool, may need to specify
• add pin constraint 0 /foo/bar
Some References
• http://en.wikipedia.org/wiki/Formal_equivalence_checking