Verilog and SystemVerilog Gotchas (eBook)

Dedication 5
About the Authors 6
Acknowledgments 7
Table of Contents 8
List of Gotchas 12
Foreword by Steve Golson 20
Chapter 1 Introduction, What Is A Gotcha? 22
What are Verilog and System Verilog? 22
What is a Gotcha? 23
The Verilog and System Verilog standards 24
Chapter 2 Declaration and Literal Number Gotchas 26
Gotcha 1: Case sensitivity 26
Gotcha 2: Implicit net declarations 29
Gotcba 3: Default of 1-bit internal nets 32
Gotcha 4: Single file versus multi-file compilation of $unit declarations 34
Gotcha 5: Local variable declarations 36
Gotcha 6: Escaped names in hierarchical paths 38
Gotcha 7: Hierarchical references to automatic variables 41
Gotcha 8: Hierarchical references to variables in unnamed blocks 44
Gotcha 9: Hierarchical references to imported package items 46
Gotcha 10: Importing enumerated types from packages 47
Gotcha 11: Importing from multiple packages 48
Gotcha 12: Default base of literal integers 49
Gotcha 13: Signedness of literal integers 51
Gotcha 14: Signed literal integers zero extend to their specified size 52
Gotcha 15: Literal integersize mismatch in assignments 54
Gotcha 16: Filling vectors with all ones 56
Gotcha 17: Array literals versus concatenations 57
Gotcha 18: Port connection rules 58
Gotcha 19: Back-driven ports 62
Gotcha 20: Passing real (floating point) numbers through ports 65
Chapter 3 RTL Modeling Gotchas 67
Gotcha 21: Combinational logic sensitivity lists with function calls 67
Gotcha 22: Arrays in sensitivity lists 70
Gotcha 23: Vectors in sequential logic sensitivity lists 72
Gotcha 24: Operations in sensitivity lists 74
Gotcha 25: Sequential logic blocks with begin...end groups 75
Gotcha 26: Sequential logic blocks with resets 77
Gotcha 27: Asynchronous set/reset flip-flop for simulation and synthesis 78
Gotcha 28: Blocking assignments in sequential procedural blocks 80
Gotcha 29: Sequential logic that requires blocking assignments 82
Gotcha 30: Nonblocking assignments in combinational logic 84
Gotcha 31: Combinational logic assignments in the wrong order 88
Gotcha 32: Casez/casex masks in case expressions 90
Gotcha 33: Incomplete decision statements 92
Gotcha 34: Overlapped decision statements 95
Gotcha 35: Inappropriate use of unique case statements 97
Gotcha 36: Resetting 2-state models 100
Gotcha 37: Locked state machines modeled with enumerated types 102
Gotcha 38: Hidden design problems with 4-state logic 104
Gotcha 39: Hidden design problems using 2-state types 106
Gotcha 40: Hidden problems with out-or-bounds array access 108
Gotcha 41: Out-or-bounds assignments to enumerated types 110
Gotcha 42: Undetected shared variables in modules 112
Gotcha 43: Undetected shared variables in interfaces and packages 114
Chapter 4 Operator Gotchas 116
Gotcha 44: Assignments in expressions 116
Gotcha 45: Self-determined versus context-determined operators 118
Gotcha 46: Operation size and sign extension in assignment statements 122
Gotcha 47: Signed arithmetic rules 125
Gotcha 48: Bit-select and part-select operations 128
Gotcha 49: Increment, decrement and assignment operators 129
Gotcha 50: Pre-increment versus post-increment operations 130
Gotcha 51: Modifying a variable multiple times in onestatement 132
Gotcha 52: Operator evaluation short circuiting 133
Gotcha 53: The not operator ( ! ) versus the invert operator ( ~ ) 135
Gotcha 54: Array method operations 136
Gotcha 55: Array method operations on an array subset 138
Chapter 5 General Programming Gotchas 139
Gotcha 56: Verifying asynchronous and synchronous reset at time zero 139
Gotcha 57: Nested if ...else blocks 144
Gotcha 58: Evaluation of equality with 4-state values 145
Gotcha 59: Event trigger race conditions 147
Gotcha 60: Using semaphores for synchronization 150
Gotcha 61: Using mailboxes for synchronization 153
Gotcha 62: Triggering on clocking blocks 155
Gotcha 63: Misplaced semicolons after decision statements 156
Gotcha 64: Misplaced semicolons in for loops 158
Gotcha 65: Infinite for loops 160
Gotcha 66: Locked simulation due to concurrent for loops 161
Gotcha 67: Referencing for loop control variables 163
Gotcha 68: Default function return size 164
Gotcha 69: Task/function arguments with default values 166
Gotcha 70: Continuous assignments with delays cancel glitches 167
Chapter 6 Object Oriented and Multi-Threaded Programming Gotchas 169
Gotcha 71: Programming statements in a class 169
Gotcha 72: Using interfaces with object-oriented testbenches 171
Gotcha 73: All objects in mailbox come out with the same values 173
Gotcha 74: Passing handles to methods using input versus ref arguments 174
Gotcha 75: Constructing an array of objects 175
Gotcha 76: Static tasks and functions are not re-entrant 176
Gotcha 77: Static versus automatic variable initialization 178
Gotcha 78: Forked programming threads need automatic variables 180
Gotcha 79: Disable fork kills too many threads 182
Gotcha 80: Disabling a statement block stops more than intended 184
Gotcha 81: Simulation exits prematurely, before tests complete 187
Chapter 7 Randomization, Coverage and Assertion Gotchas 189
Gotcha 82: Variables declared with rand are not getting randomized 189
Gotcha 83: Undetected randomization failures 191
Gotcha 84: $assertoff could disable randomization 193
Gotcha 85: Boolean constraints on more than two random variables 195
Gotcha 86: Unwanted negative values in random values 197
Gotcha 87: Coverage reports default to groups, not bins 198
Gotcha 88: Coverage is always reported as 0% 200
Gotcha 89: The coverage report lumps all instances together 202
Gotcha 90: Covergroup argument directions are sticky 203
Gotcha 91: Assertion pass statements execute with a vacuous success 204
Gotcha 92: Concurrent assertions in procedural blocks 206
Gotcha 93: Mismatch in assert...else statements 208
Gotcha 94: Assertions that cannot fail 209
Chapter 8 Tool Compatibility Gotchas 211
Gotcha 95: Default simulation time units and precision 211
Gotcha 96: Package chaining 214
Gotcha 97: Random number generator is not consistent across tools 216
Gotcha 98: Loading memories modeled with aIways_latch/always_ff 218
Gotcha 99: Non-standard language extensions 220
Gotcha 100: Array literals versus concatenations 222
Gotcha 101: Module ports that pass floating point values (real types) 224
Index 225

"Foreword by Steve Golson (p. 1-2)

Some people collectbaseball cards, old car magazines, or maybe rubber duckies. I collectVerilog books. It started back in 1989 with a looseleaf copy of "Gateway VERILOG-XL Reference Manual Version I.5a" in a three-ring binder. Verilog was a bit simpler back then-its hard to believe we actually designed chipsusing onlyone typeof procedural assignment (nonblocking assigns were not part of the language yet). Andwe ran our simulations on a VAX, or maybe a fancy Apollo workstation.

Sincethen Ive bought prettymuchevery Verilog bookthatcamealong. Ive got a few synthesis books, and Ill pick up an occasional VHDL reference or maybe a text on the history of hardware description languages, but mostly its Verilog. Dozens and dozens of booksabout Verilog. Theresa funny thingaboutmostof these books though.

AfterI leaf through them a few times, they sit on the shelf. I admitthat it looks pretty impressive onceyou have an entire bookcase filled with Verilog books, but the discerning visitor will notice howfresh andnewthey all are. Unused. Unread. Useless. Im often disappointed to find very little information which is useful for the practicing engineer. What Im looking for is a book I can use every day, a book that will helpme getmy chip out the door, on timeand working. StuandDonhavewritten sucha book.

Ive known these guysformanyyears, and they have probably forgotten more Verilog than Ive ever known. They have distilled their collective knowledge into this helpful and extremely useful book. Readit and you wontbe disappointed. If you are an old hand at Verilog try to pick out all the Gotchas that you have found the hard way. Smile and say to yourself "Oh yeah, I remember getting caught by that one!"

Those of you who are new to Verilog and SystemVerilog, welcome aboard! Heres your chance to learn from two of the leading experts in the field. And if you ever have a chance to take a training class from either of these gentlemen, donthesitate to signup. I guarantee you wont regretit. Ohby the way, myfavorite Gotcha is "Gotcha 65: Infinite for loops". Why? Well, I builta chipwiththatbug in it. Believe me,when a modeling errorcauses youto have broken silicon, you never forget why it happened. Back then I didnt have this book to help me, but you do! Keep this book close at hand, refer to it often, andmayall yourmodels compile andall yourloops terminate.

Steve Golson
Trilobyte Systems
http://www.trilobyte.com"