The Verilog Language
COMS W4995-02
Prof. Stephen A. Edwards
Fall 2002
Columbia University
Department of Computer Science
The Verilog Language
Originally a modeling language for a very efficient
event-driven digital logic simulator
Later pushed into use as a specification language for logic
synthesis
Now, one of the two most commonly-used languages in
digital hardware design (VHDL is the other)
Virtually every chip (FPGA, ASIC, etc.) is designed in part
using one of these two languages
Combines structural and behavioral modeling styles
Multiplexer Built From Primitives
module mux(f, a, b, sel); Verilog programs
built from modules
output f;
input a, b, sel;
Each module has
an interface
and g1(f1, a, nsel),
g2(f2, b, sel);
or g3(f, f1, f2);
not g4(nsel, sel); Module may contain
structure: instances of
primitives and other
modules
endmodule
g1
g4
g2
g3
a
b
sel
f
nsel
f1
f2
Multiplexer Built with Always
module mux(f, a, b, sel);
output f;
input a, b, sel;
reg f;
always
Modules may
contain one or more
always blocks
@(a or b or sel) Sensitivity list
contains signals
whose change
makes the block
execute
if (sel) f = a;
else f = b;
endmodule
a
b
sel
f
Multiplexer Built with Always
module mux(f, a, b, sel);
output f;
input a, b, sel;
reg f;
A reg behaves like
memory: holds its value
until imperatively
assigned otherwise
always @(a or b or sel)
if (sel) f = a;
else f = b;
Body of an always block
contains traditional
imperative code
endmodule
a
b
sel
f
Mux with Continuous Assignment
module mux(f, a, b, sel);
output f;
input a, b, sel;
assign
LHS is always set to
the value on the RHS
Any change on the right
causes reevaluation
f = sel ? a : b;
endmodule
a
b
sel
f
Mux with User-Defined Primitive
primitive mux(f, a, b, sel);
output f;
input a, b, sel;
table
1?0 : 1;
Behavior defined using
a truth table that
includes “don’t cares”
0?0 : 0;
?11 : 1;
?01 : 0;
11? : 1;
This is a less pessimistic than
others: when a & b match, sel is
ignored; others produce X
00? : 0;
endtable
endprimitive
a
b
sel
f
How Are Simulators Used?
Testbench generates stimulus and checks response
Coupled to model of the system
Pair is run simultaneously
Testbench System Model
Stimulus
Response
Result checker
Structural Modeling
When Verilog was first developed (1984) most logic
simulators operated on netlists
Netlist: list of gates and how they’re connected
A natural representation of a digital logic circuit
Not the most convenient way to express test benches
Behavioral Modeling
A much easier way to write testbenches
Also good for more abstract models of circuits
• Easier to write
• Simulates faster
More flexible
Provides sequencing
Verilog succeeded in part because it allowed both the
model and the testbench to be described together
How Verilog Is Used
Virtually every ASIC is designed using either Verilog or
VHDL (a similar language)
Behavioral modeling with some structural elements
“Synthesis subset” can be translated using Synopsys’
Design Compiler or others into a netlist
Design written in Verilog
Simulated to death to check functionality
Synthesized (netlist generated)
Static timing analysis to check timing
Two Main Components of Verilog:
Behavioral
Concurrent, event-triggered processes (behavioral)
Initial and Always blocks
Imperative code that can perform standard data
manipulation tasks (assignment, if-then, case)
Processes run until they delay for a period of time or wait
for a triggering event
Two Main Components of Verilog:
Structural
Structure (Plumbing)
Verilog program build from modules with I/O interfaces
Modules may contain instances of other modules
Modules contain local signals, etc.
Module configuration is static and all run concurrently
Two Main Data Types: Nets
Nets represent connections between things
Do not hold their value
Take their value from a driver such as a gate or other
module
Cannot be assigned in an initial or always block
Two Main Data Types: Regs
Regs represent data storage
Behave exactly like memory in a computer
Hold their value until explicitly assigned in an initial or
always block
Never connected to something
Can be used to model latches, flip-flops, etc., but do not
correspond exactly
Actually shared variables with all their attendant problems
Discrete-event Simulation
Basic idea: only do work when something changes
Centered around an event queue that contains events
labeled with the simulated time at which they are to be
executed
Basic simulation paradigm
• Execute every event for the current simulated time
• Doing this changes system state and may schedule
events in the future
• When there are no events left at the current time
instance, advance simulated time soonest event in the
queue
Four-valued Data
Verilog’s nets and registers hold four-valued data
0, 1: Obvious
Z: Output of an undriven tri-state driver. Models case
where nothing is setting a wire’s value
X: Models when the simulator can’t decide the value
• Initial state of registers
• When a wire is being driven to 0 and 1 simultaneously
• Output of a gate with Z inputs
Four-valued Logic
Logical operators work on three-valued logic
0 1 X Z
0 0 0 0 0
Outputs 0 if either
input is 0
1 0 1 X X
X 0 X X X Outputs X if both
inputs are gibberishZ 0 X X X
Structural Modeling
Nets and Registers
Wires and registers can be bits, vectors, and arrays
wire a; // Simple wire
tri [15:0] dbus; // 16-bit tristate bus
tri #(5,4,8) b; // Wire with delay
reg [-1:4] vec; // Six-bit register
trireg (small) q; // Wire stores a small charge
integer imem[0:1023]; // Array of 1024 integers
reg [31:0] dcache[0:63]; // A 32-bit memory
Modules and Instances
Basic structure of a Verilog module:
module mymod(out1, out2,
Verilog convention
lists outputs first
in1, in2);
output out1;
output [3:0] out2;
input in1;
input [2:0] in2;
endmodule
Instantiating a Module
Instances of
module mymod(y, a, b);
look like
mymod mm1(y1, a1, b1); // Connect-by-position
mymod (y2, a1, b1),
(y3, a2, b2); // Instance names omitted
// Connect-by-name
mymod mm2(.a(a2), .b(b2), .y(c2));
Gate-level Primitives
Verilog provides the following:
and nand logical AND/NAND
or nor logical OR/NOR
xor xnor logical XOR/XNOR
buf not buffer/inverter
bufif0 notif0 Tristate with low enable
bifif1 notif1 Tristate with high enable
Delays on Primitive Instances
Instances of primitives may include delays
buf b1(a, b); // Zero delay
buf #3 b2(c, d); // Delay of 3
buf #(4,5) b3(e, f); // Rise=4, fall=5
buf #(3:4:5) b4(g, h); // Min-typ-max
Switch-level Primitives
Verilog also provides mechanisms for modeling CMOS
transistors that behave like switches
A more detailed modeling scheme that can catch some
additional electrical problems when transistors are used in
this way
Now, little-used because circuits generally aren’t built this
way
More seriously, model is not detailed enough to catch
many of the problems
These circuits are usually simulated using SPICE-like
simulators based on nonlinear differential equation solvers
User-Defined Primitives
Way to define gates and sequential elements using a truth
table
Often simulate faster than using expressions, collections
of primitive gates, etc.
Gives more control over behavior with X inputs
Most often used for specifying custom gate libraries
A Carry Primitive
primitive carry(out, a, b, c);
output out;
Always has exactly
one output
input a, b, c;
table
00? : 0;
0?0 : 0;
?00 : 0;
Truth table may include
don’t-care (?) entries
11? : 1;
1?1 : 1;
?11 : 1;
endtable
endprimitive
A Sequential Primitive
Primitive dff( q, clk, data);
output q; reg q;
input clk, data;
table
// clk data q new-q
(01) 0 : ? : 0; // Latch a 0
(01) 1 : ? : 1; // Latch a 1
(0x) 1 : 1 : 1; // Hold when d and q both 1
(0x) 0 : 0 : 0; // Hold when d and q both 0
(?0) ? : ? : -; // Hold when clk falls
? (??) : ? : -; // Hold when clk stable
endtable
endprimitive
Continuous Assignment
Another way to describe combinational function
Convenient for logical or datapath specifications
wire [8:0] sum; Define bus widths
wire [7:0] a, b;
wire carryin;
assign sum = a + b + carryin;
Continuous
assignment:
permanently
sets the value of
sum to be
a+b+carryin.
Recomputed
when a, b, or
carryin changes
Behavioral Modeling
Initial and Always Blocks
initial
begin
// imperative statements
end
Runs when simulation starts
Terminates when control
reaches the end
Good for providing stimulus
always
begin
// imperative statements
end
Runs when simulation starts
Restarts when control
reaches the end
Good for modeling or
specifying hardware
Initial and Always
Run until they encounter a delay
initial begin
#10 a = 1; b = 0;
#10 a = 0; b = 1;
end
or a wait for an event
always @(posedge clk) q = d;
always begin
wait(i);
a = 0;
wait(˜i);
a = 1;
end
Procedural Assignment
Inside an initial or always block:
sum = a + b + cin;
Just like in C: RHS evaluated and assigned to LHS before
next statement executes
RHS may contain wires and/or regs
LHS must be a reg
(only primitives or continuous assignment may set wire
values)
Imperative Statements
if (select == 1) y = a;
else y = b;
case (op)
2’b00: y = a + b;
2’b01: y = a - b;
2’b10: y = a ˆ b;
default: y = ’hxxxx;
endcase
For Loops
Example generates an increasing sequence of values on
an output
reg [3:0] i, output;
for ( i = 0 ; i <= 15 ; i = i + 1 ) begin
output = i;
#10;
end
While Loops
A increasing sequence of values on an output
reg [3:0] i, output;
i = 0;
while (i <= 15) begin
output = i;
#10 i = i + 1;
end
Modeling A Flip-Flop With Always
Very basic: an edge-sensitive flip-flop
reg q;
always @(posedge clk)
q = d;
q = d assignment runs when clock rises: exactly the
behavior you expect
Blocking vs. Nonblocking
Verilog has two types of procedural assignment
Fundamental problem:
• In a synchronous system, all flip-flops sample
simultaneously
• In Verilog, always @(posedge clk) blocks run in
some undefined sequence
A Flawed Shift Register
This does not work as you would expect:
reg d1, d2, d3, d4;
always @(posedge clk) d2 = d1;
always @(posedge clk) d3 = d2;
always @(posedge clk) d4 = d3;
These run in some order, but you don’t know which
Non-blocking Assignments
This version does work:
reg d1, d2, d3, d4;
always @(posedge clk) d2 <= d1;
Nonblocking rule:
RHS evaluated
when assignment
runs
always @(posedge clk) d3 <= d2;
always @(posedge clk) d4
LHS updated only
after all events for
the current instant
have run
<= d3;
Nonblocking Can Behave Oddly
A sequence of nonblocking assignments don’t
communicate
a = 1;
b = a;
c = b;
Blocking assignment:
a = b = c = 1
a <= 1;
b <= a;
c <= b;
Nonblocking assignment:
a = 1
b = old value of a
c = old value of b
Nonblocking Looks Like Latches
RHS of nonblocking taken from latches
RHS of blocking taken from wires
a = 1;
b = a;
c = b;
“1 c
a b
”
a <= 1;
b <= a;
c <= b;
“
1
c
a
b
”
Building Behavioral Models
Modeling FSMs Behaviorally
There are many ways to do it:
• Define the next-state logic combinationally and define
the state-holding latches explicitly
• Define the behavior in a single always @(posedge
clk) block
• Variations on these themes
FSM with Combinational Logic
module FSM(o, a, b, reset);
output o;
reg o;
Output o is declared a reg
because it is assigned
procedurally, not because it
holds state
input a, b, reset;
reg [1:0] state, nextState;
always @(a or b or state)
case (state)
2’b00: begin
o = a & b;
nextState = a ? 2’b00 : 2’b01;
end
2’b01: begin
o = 0; nextState = 2’b10;
end
endcase
always @(posedge clk or reset)
if (reset)
state <= 2’b00;
else
state <= nextState;
endmodule
FSM with Combinational Logic
module FSM(o, a, b, reset);
output o;
reg o;
input a, b, reset;
reg [1:0] state, nextState;
always @(a or b or state)
Combinational block must be
sensitive to any change on any
of its inputs (Implies
state-holding elements
otherwise)
case (state)
2’b00: begin
o = a & b;
nextState = a ? 2’b00 : 2’b01;
end
2’b01: begin
o = 0; nextState = 2’b10;
end
endcase
always @(posedge clk or reset)
Latch implied by
sensitivity to the clock
or reset only
if (reset)
state <= 2’b00;
else
state <= nextState;
endmodule
FSM from a Single Always Block
module FSM(o, a, b);
output o; reg o;
input a, b;
reg [1:0] state;
always @(posedge clk or reset)
Expresses Moore
machine behavior:
Outputs are latched.
Inputs only sampled
at clock edgesif (reset) state <= 2’b00;
else case (state)
2’b00: begin
state <= a ? 2’b00 : 2’b01;
o <= a & b;
end
2’b01: begin
state <= 2’b10;
o <= 0;
Nonblocking assignments
used throughout to ensure
coherency. RHS refers to
values calculated in
previous clock cycle
end
endcase
Writing Testbenches
module test;
reg a, b, sel;
Inputs to device
under test
mux m(y, a, b, sel);
Device under test
initial begin
$monitor
$monitor is a built-in even-driven “printf”
($time,,"a=%b b=%b sel=%b y=%b",
a, b, sel, y);
a = 0; b= 0; sel = 0;
#10 a = 1;
#10 sel = 1;
Stimulus generated by
sequence of
assignments and
delays
#10 b = 1;
end
Simulating Verilog
Simulation Behavior
Scheduled using an event queue
Non-preemptive, no priorities
A process must explicitly request a context switch
Events at a particular time unordered
Scheduler runs each event at the current time, possibly
scheduling more as a result
Two Types of Events
Evaluation events compute functions of inputs
Update events change outputs
Split necessary for delays, nonblocking assignments, etc.
Update event writes
new value of a and
schedules any
evaluation events
that are sensitive to
a change on a
a <= b + c
Evaluation event
reads values of b
and c, adds them,
and schedules an
update event
Simulation Behavior
Concurrent processes (initial, always) run until they stop
at one of the following
• #42
Schedule process to resume 42 time units from now
• wait(cf & of)
Resume when expression “cf & of” becomes true
• @(a or b or y)
Resume when a, b, or y changes
• @(posedge clk)
Resume when clk changes from 0 to 1
Simulation Behavior
Infinite loops are possible and the simulator does not
check for them This runs forever: no context switch
allowed, so ready can never change
while (˜ready)
count = count + 1;
Instead, use
wait(ready);
Simulation Behavior
Race conditions abound in Verilog
These can execute in either order: final value of a
undefined:
always @(posedge clk) a = 0;
always @(posedge clk) a = 1;
Simulation Behavior
Semantics of the language closely tied to simulator
implementation
Context switching behavior convenient for simulation, not
always best way to model
Undefined execution order convenient for implementing
event queue
Compiled-Code Discrete-Event Sim.
Most modern simulators use this approach
Verilog program compiled into C
Each concurrent process (e.g., continuous assignment,
always block) becomes one or more C functions
Initial and always blocks split into multiple functions, one
per segment of code between a delay, a wait, or event
control (@)
Central, dynamic event queue invokes these functions and
advances simulation time
Verilog and Logic Synthesis
Logic Synthesis
Verilog is used in two ways
Model for discrete-event simulation
Specification for a logic synthesis system
Logic synthesis converts a subset of the Verilog language
into an efficient netlist
One of the major breakthroughs in designing logic chips in
the last 20 years
Most chips are designed using at least some logic
synthesis
Logic Synthesis Tools
Mostly commercial tools
• Very difficult, complicated programs to write well
• Limited market
• Commercial products in $10k – $100k price range
Major vendors
• Synopsys Design Compiler, FPGA Express
• Cadence BuildGates
• Synplicity (FPGAs)
• Exemplar (FPGAs)
Academic tools
• SIS (UC Berkeley)
Logic Synthesis
Takes place in two stages:
1. Translation of Verilog (or VHDL) source to a netlist
Register inference performed here
2. Optimization of the resulting netlist to improve speed
and area
Most critical part of the process
Algorithms very complicated and beyond the scope of
this class: Take Prof. Nowick’s class for details
Logic Optimization
Netlist optimization the critical enabling technology
Takes a slow or large netlist and transforms it into one that
implements the same function more cheaply
Typical operations:
• Constant propagation
• Common subexpression elimination
• Function factoring
Time-consuming operation. Can take hours for large chips
Translating Verilog into Gates
Parts of the language easy to translate
Structural descriptions with primitives is already a netlist
Continuous assignment expressions turn into little
datapaths
Behavioral statements the bigger challenge
What Can Be Translated
Every structural definition
Behavioral blocks
• Depends on sensitivity list
• Only when they have reasonable interpretation as
combinational logic, edge, or level-sensitive latches
• Blocks sensitive to both edges of the clock, changes on
unrelated signals, changing sensitivity lists, etc. cannot be
synthesized
User-defined primitives
• Primitives defined with truth tables
• Some sequential UDPs can’t be translated (not latches or
flip-flops)
What Is Not Translated
Initial blocks
• Used to set up initial state or describe finite testbench stimuli
• Don’t have obvious hardware component
Delays
• May be in the Verilog source, but are simply ignored
A variety of other obscure language features
• In general, things heavily dependent on discrete-event
simulation semantics
• Certain “disable” statements
• Pure events
Register Inference
The main trick
A reg is not always a latch or flip-flop
Rule: Combinational if outputs always depend exclusively
on sensitivity list
Sequential if outputs may also depend on previous values
Register Inference
Combinational:
reg y;
always @(a or b or sel)
Sensitive to
changes on all the
variable it reads
if (sel) y = a;
else y = b; y is always assigned
Sequential:
reg q;
always @(d or clk)
if (clk) q = d;
q only assigned
when clk is 1
Register Inference
A common mistake is not completely specifying a case
statement
This implies a latch:
always @(a or b)
case ({a, b})
2’b00 : f = 0;
2’b01 : f = 1;
2’b10 : f = 1;
endcase
f is not assigned when
{a,b}= 2’b11
Register Inference
The solution is to always have a default case
always @(a or b)
case ({a, b})
2’b00 : f = 0;
2’b01 : f = 1;
2’b10 : f = 1;
default : f = 0; f is always assigned
endcase
Inferring Latches with Reset
Latches and Flip-flops often have reset inputs
Can be synchronous or asynchronous
Asynchronous positive reset:
always @(posedge clk or posedge reset)
if (reset)
q <= 0;
else q <= d;
Simulation-synthesis Mismatches
Many possible sources of conflict
• Synthesis ignores delays (e.g., #10), but simulation
behavior can be affected by them
• Simulator models X explicitly, synthesis does not
• Behaviors resulting from shared-variable-like behavior
of regs is not synthesized:
always @(posedge clk) a = 1;
New value of a may be seen by other @(posedge clk)
statements in simulation, never in synthesis
Summary
Summary of Verilog
Systems described hierarchically
• Modules with interfaces
• Modules contain instances of primitives, other
modules
• Modules contain initial and always blocks
Based on discrete-event simulation semantics
• Concurrent processes with sensitivity lists
• Scheduler runs parts of these processes in response
to changes
Modeling Tools
Switch-level primitives: CMOS transistors as switches that
move around charge
Gate-level primitives: Boolean logic gates
User-defined primitives: Gates and sequential elements
defined with truth tables
Continuous assignment: Modeling combinational logic
with expressions
Initial and always blocks: Procedural modeling of behavior
Language Features
Nets (wires) for modeling interconnection
• Non state-holding
• Values set continuously
Regs for behavioral modeling
• Behave exactly like memory for imperative modeling
• Do not always correspond to memory elements in
synthesized netlist
Blocking vs. nonblocking assignment
• Blocking behaves like normal “C-like” assignment
• Nonblocking delays update, modeling synchronous
behavior
Language Uses
Event-driven simulation
• Event queue containing things to do at particular
simulated times
• Evaluate and update events
• Compiled-code event-driven simulation for speed
Logic synthesis
• Translating Verilog (structural and behavioral) into
netlists
• Register inference: whether output is always updated
• Logic optimization for cleaning up the result
Little-used Language Features
Switch-level modeling
• Much slower than gate or behavioral-level models
• Insufficient detail for modeling most electrical
problems
• Delicate electrical problems simulated with a
SPICE-like differential equation simulator
Little-used Language Features
Delays
• Simulating circuits with delays does not improve
confidence enough
• Hard to get timing models accurate enough
• Never sure you have simulated the worst case
• Static timing analysis has taken its place
Compared to VHDL
Verilog and VHDL are comparable languages
VHDL has a slightly wider scope
• System-level modeling
• Exposes even more discrete-event machinery
VHDL is better-behaved: Fewer sources of
nondeterminism (e.g., no shared variables)
VHDL is harder to simulate quickly
VHDL has fewer built-in facilities for hardware modeling
VHDL is a much more verbose language: Most examples
don’t fit on slides
In Conclusion
Verilog is a deeply flawed language
• Nondeterministic
• Often weird behavior due to discrete-event semantics
• Vaguely defined synthesis subset
• Many possible sources of simulation/synthesis
mismatch
In Conclusion
Verilog is widely used because it solves a problem
• Good simulation speed that continues to improve
• Designers use a well-behaved subset of the language
• Makes a reasonable specification language for logic
synthesis
• Logic synthesis one of the great design automation
success stories