The Verilog Hardware Description Language
Professor Don Thomas
Carnegie Mellon University (CMU)
thomas@ece.cmu.edu
http://www.ece.cmu.edu/~thomas
This is not one cohesive presentation on Verilog. The slides contained here
are collected from different CMU classes at various academic levels.
These slides are provided as an alternate aid to learning the language. You
may find them helpful.
Send bug reports to the above address — there are some!
The Verilog Hardware Description Language, Fifth Edition is available from
Kluwer Academic Publishers,
http://www.wkap.com. Phone: 781-871-6600.
Use or reproduction of the information provided in this file for commercial
gain is strictly prohibited. Explicit permission is given for the reproduction
and use of this information in an instructional setting.
Representation: Structural Models
 Structural models

Are built from gate primitives and/or other modules

They describe the circuit using logic gates — much as you
would see in an implementation of a circuit.
 Identify:

Gate instances, wire names, delay from a or b to f.

This is a multiplexor — it selects one of n inputs (2 here) and
passes it on to the output
module mux
(output f,
input
a, b, sel);
a
f
b
sel
f=
a • sel’ + b • sel
and #5 g1 (f1, a, nsel),
g2 (f2, b, sel);
or #5 g3 (f, f1, f2);
not
g4 (nsel, sel);
endmodule
Representation: Gate-Level Models
 Need to model the gate’s:

Function

Delay
 Function

Generally, HDLs have built-in gate-level primitives
 Verilog has NAND, NOR, AND, OR, XOR, XNOR, BUF, NOT, and some
others

The gates operate on input values producing an output value
 typical Verilog gate instantiation is:
“many”
optional
and #delay instance-name (out, in1, in2, in3, …);
and #5
g1 (f1, a, nsel);
a comma here let’s you
list other instance names
and their port lists.
Four-Valued Logic
 Verilog Logic Values

The underlying data representation allows for any bit to have one of
four values

1, 0, x (unknown), z (high impedance)

x — one of: 1, 0, z, or in the state of change

z — the high impedance output of a tri-state gate.
 What basis do these have in reality?

0, 1 … no question

z … A tri-state gate drives either a zero or one on its output…and if it’s
not doing that, its output is high impedance (z). Tri-state gates are real
devices and z is a real electrical affect.

x … not a real value. There is no real gate that drives an x on to a wire.
x is used as a debugging aid. x means the simulator can’t determine
the answer and so maybe you should worry! All values in a simulation
start as x.
 BTW …

Verilog keeps track of more values than these in some situations.
Four-Valued Logic
 Logic with multi-level logic values

Logic with these four values make sense
 Nand anything with a 0, and you get a 1. This includes having an x or
z on the other input. That’s the nature of the nand gate
 Nand two x’s and you get an x — makes sense!

Note: z treated as an x on input. Their rows and columns are the
same

If you forget to connect an input … it will be seen as an z.

At the start of simulation, everything is an x.
A
B
Input A
Input B
Nand
0
1
x
z
0
1
1
1
1
1
1
0
x
x
x
1
x
x
x
z
1
x
x
x
A 4-valued truth table for a
Nand gate with two inputs
Concurrent activity
Eval g2, g3
 Do these two evaluations happen at the same time?

Yes and No!
Yes and No!
 Yes …

They happen at the same simulated (or virtual) time

After all, the time when they occur is 27
 No …

We all know the processor is only doing one thing at any given
time
 So, which is done first?

That’s undefined. We can’t assume anything except that the order
is arbitrary.
Concurrent Activity
 The point is

In the real implementation, all activity will be concurrent

Thus the simulator models the elements of the system as being
concurrent in simulated time
 The simulator stands on its head trying to do this!
 Thus,

Even though the simulator executes each element of the design
one at a time …

… we’ll call it concurrent
Delay
 Transport delay — input to output delay

“nand #35 (f1, a, b, c);”
#35 is the transport delay
 What if the input changes during that time?

i.e. how wide must an input spike be to affect the output.

Think of the gate as having inertia. — The input change must be
present long enough to get the output to change. (That “long
enough” time is called inertial delay)

in Verilog, this time is equal to the transport delay
a
b
c
a
b
c
pulse too
small, no
output change
— transport delay
a
b
c
Let’s build a wider 2-bit mux
 Build a 2-bit 2:1 mux

OK, let’s put two 1-bit 2:1 muxes in the same module with a
common select line

What would it look like?
a0
f0
b0
a
f
a1
f1
b1
b
sel
sel
Reuse!
 Reuse of smaller objects

Can we use the mux module that we already designed?

A big idea — instantiation

Modules and primitive gates can be instantiated — copied — to
many sites in a design

Previously, two ANDs, one OR, and a NOT gate were instantiated
into module mux

Now we instantiate two copies of module mux into module
wideMux
module wideMux (f1, f0, a1, a0, b1, b0, sel);
input
a1, a0, b1, b0, sel;
output f1, f0;
mux
endmodule
bit1 (f1, a1, b1, sel),
bit0 (f0, a0, b0, sel);
Instantiate two mux
modules, name them, and
specify connections (the
order is important).
Instantiation — Copies
 Modules and gate primitives are instantiated == copied

Note the word “copies”
 The copies (also called instances) share the module (or primitive)
definition
 If we ever change a module definition, the copies will all change too
 However, the internal entities (gate names, internal port names, and
other things to come) are all private, separate copies

Don’t think of module instantiations as subroutines that are called
 They are copies — there are 2 mux modules in wideMux
with a total of:
4 AND gates,
______
2 OR gates,
______
2 Not gates
______
Why is this cool?
 In Verilog

“Primitive” gates are predefined (NAND, NOR, …)

Other modules are built by instantiating these gates

Other modules are built by instantiating other modules, …
 The design hierarchy of modules is built using instantiation

Bigger modules of useful functionality are defined

You can then design with these bigger modules
 You can reuse modules that you’ve already built and tested
 You can hide the detail — why show a bunch of gates and their
interconnection when you know it’s a mux!
 Instantiation & hierarchy control complexity.

No one designs 1M+ random gates — they use hierarchy.

What are the software analogies?
How to wire modules together
 Real designs have many modules and gates
module putTogether ();
wire w1, w2, w3, w4;
bbb
aaa
lucy
ricky
(w1, w2, w3, w4);
(w3, w2, w1);
…
module bbb (i1, i2, o1, clk);
input
i1, i2, clk;
output
o1;
xor (o1, i2, …);
…
Each module has it’s own
namespace. Wires connect
elements of namespaces.
what happens when
out1 is set to 1?
module aaa (in1, out1, out2);
input
in1;
output
out1, out2;
…
nand
nand
…
#2 (out1, in1, b);
#6 (out2, in1, b);
Implicit wires
 How come there were no wires declared in some of these
modules?

Gate instantiations implicitly declare wires for their outputs.

All other connections must be explicitly declared as wires — for
instance, connections between module ports

Output and input declarations are wires
module putTogether ();
wire
w1, w2, w3, w4;
mux
aaa
inst1 (w1, w2, w3, w4);
duh (w3, w2, w1);
…
wires explicitly
declared
module mux
(output f,
input
a, b, sel);
and #5
or #5
not
endmodule
g1 (f1, a, nsel),
g2 (f2, b, sel);
g3 (f, f1, f2);
g4 (nsel, sel);
wires implicitly
declared (f1, f2, nsel)
How to build and test a module
 Construct a “test bench” for your design

Develop your hierarchical system within a module that has input
and output ports (called “design” here)

Develop a separate module to generate tests for the module
(“test”)

Connect these together within another module (“testbench”)
module testbench ();
wire
l, m, n;
design d (l, m, n);
test
t (l, m);
initial begin
//monitor and display
…
module design (a, b, c);
input a, b;
output c;
…
module test (q, r);
output q, r;
initial begin
//drive the outputs with signals
…
Another view of this
 3 chunks of Verilog, one for each of:
TESTBENCH is the final piece of hardware which
connects DESIGN with TEST so the inputs generated
go to the thing you want to test...
Another module,
called TEST, to
generate
interesting inputs
Your hardware
called
DESIGN
An Example
Module testAdd generated inputs for module halfAdd and displayed
changes. Module halfAdd was the design
module tBench;
wire
su, co, a, b;
halfAdd
testAdd
endmodule
ad(su, co, a, b);
tb(a, b, su, co);
module halfAdd (sum, cOut, a, b);
output sum, cOut;
input
a, b;
xor #2
and #2
endmodule
(sum, a, b);
(cOut, a, b);
module testAdd(a, b, sum, cOut);
input sum, cOut;
output a, b;
reg
a, b;
initial begin
$monitor ($time,,
“a=%b, b=%b, sum=%b, cOut=%b”,
a, b, sum, cOut);
a = 0; b = 0;
#10 b = 1;
#10 a = 1;
#10 b = 0;
#10 $finish;
end
endmodule
The test module
 It’s the test generator
 $monitor

prints its string when executed.

after that, the string is printed
when one of the listed values
changes.

only one monitor can be active at
any time

prints at end of current simulation
time
 Function of this tester

at time zero, print values and set
a=b=0

after 10 time units, set b=1

after another 10, set a=1

after another 10 set b=0

then another 10 and finish
module testAdd(a, b, sum, cOut);
input sum, cOut;
output a, b;
reg
a, b;
initial begin
$monitor ($time,,
“a=%b, b=%b, sum=%b,
cOut=%b”,
a, b, sum, cOut);
a = 0; b = 0;
#10 b = 1;
#10 a = 1;
#10 b = 0;
#10 $finish;
end
endmodule
Another version of a test module
 Multi-bit “thingies”

test is a two-bit
register and output

It acts as a two-bit
number (counts 0001-10-11-00…)

Module tBench needs
to connect it correctly
— mod halfAdd has 1bit ports.
module tBench;
wire su, co;
wire [1:0] t;
halfAdd
testAdd
endmodule
ad(su, co, t[1], t[0]);
tb(t, su, co);
module testAdd(test, sum, cOut);
input
sum, cOut;
output [1:0] test;
reg
[1:0] test;
initial begin
$monitor ($time,,
"test=%b, sum=%b, cOut=%b",
test, sum, cOut);
test = 0;
#10 test = test + 1;
#10 test = test + 1;
#10 test = test + 1;
#10 $finish;
end
endmodule
Connects bit 0 or wire t to this port
(b of the module halfAdder)
Another version of testAdd
 Other procedural
statements


You can use
“for”, “while”,
“if-then-else”
and others
here.
This makes it
easier to write if
you have lots of
input bits.
module tBench;
wire su, co;
wire [1:0] t;
halfAdd
testAdd
endmodule
ad(su, co, t[1], t[0]);
tb(t, su, co);
module testAdd(test, sum, cOut);
input
sum, cOut;
output [1:0]
test;
reg
[1:0]
test;
initial begin
$monitor ($time,,
"test=%b, sum=%b, cOut=%b",
test, sum, cOut);
for (test = 0; test < 3; test = test + 1)
#10;
#10 $finish;
end
endmodule
hmm… “<3” … ?
Structural vs Behavioral Models
 Structural model

Just specifies primitive gates and wires

i.e., the structure of a logical netlist

You basically know how to do this now.
 Behavioral model

More like a procedure in a programming language

Still specify a module in Verilog with inputs and outputs...

...but inside the module you write code to tell what you want to have
happen, NOT what gates to connect to make it happen

i.e., you specify the behavior you want, not the structure to do it
 Why use behavioral models

For testbench modules to test structural designs

For high-level specs to drive logic synthesis tools
How do behavioral models fit in?
 How do they work with the
event list and scheduler?

Initial (and always) begin
executing at time 0 in
arbitrary order

They execute until they
come to a “#delay”
operator

They then suspend, putting
themselves in the event list
10 time units in the future
(for the case at the right)

At 10 time units in the
future, they resume
executing where they left
off.
 Some details omitted

...more to come
module testAdd(a, b, sum, cOut);
input
sum, cOut;
output a, b;
reg
a, b;
initial begin
$monitor ($time,,
“a=%b, b=%b,
sum=%b, cOut=%b”,
a, b, sum, cOut);
a = 0; b = 0;
#10 b = 1;
#10 a = 1;
#10 b = 0;
#10 $finish;
end
endmodule
Two initial statements?
…
initial begin
a = 0; b = 0;
#5 b = 1;
#13 a = 1;
end
…
initial begin
out = 1;
#10 out = 0;
#8 out = 1;
end
…
1
a
0
1
b
0
1
out
0
0
10
 Things to note

Which initial statement starts first?

What are the values of a, b, and out when
the simulation starts?

These appear to be executing concurrently
(at the same time). Are they?
18
arbitrary
x
Yes, in simulated time
Other things you can do
 More than modeling hardware

$monitor — give it a list of variables. When one of them changes, it prints
the information. Can only have one of these active at a time.
e.g. …
 $monitor ($time,,, “a=%b, b=%b, sum=%b, cOut=%b”,a, b, sum, cOut);
extra commas
print spaces
%b is binary (also,
%h, %d and others)
 The above will print:
2 a=0, b=0, sum=0, cOut=0<return>

$display() — sort of like printf() in C
 $display (“Hello, world — %h”, hexvalue)
display contents of data item called
“hexvalue” using hex digits (0-9,A-F)
What if what
you print has
the value x or z?
newline
automatically
included
One Modern Design Methodology
always
mumble
mumble
blah
blah
gates, gates, gates, …
Synthesis
Synthesizable Verilog
Place
and
Route
LE 1
LE 2
Logic Elements in FPGA Chip
What’s a Logic Element (LE)
A mux selects which
element of memory
to send to output
Arbitrary programmable
Boolean function of K inputs




K=4 in our particular example.
Usually see K=3, 4, 5 in real
FPGAs
0000:
0
0001:
0
0
1
0
0
0010:
It has a memory — you
download to the memory to
program the device
0011:
You also program
connections between these
Logical Elements
0111:
Synthesis tool partitions logic
into groups of 5-input
functions
1011:
0100:
0101:
0110:
1000:
1001:
1010:
1100:
1101:
1110:
1111:
0
1
0
0
0
16:1
mux
1
1
1
1
1
Really just a
1-bit memory
BD
AC
F
What do we mean by “Synthesis”?
 Logic synthesis

A program that “designs” logic from abstract descriptions of the
logic
 takes constraints (e.g. size, speed)
 uses a library (e.g. 3-input gates)
 How?

You write an “abstract” Verilog description of the logic

The synthesis tool provides alternative implementations
constraints
Verilog blah
blah blah
synthesis
library
or …
An example
 What’s cool?

You type the left, synthesis gives you the gates

It used a different library than you did. (2-input gates only)

One description suffices for a variety of alternate
implementations!

... but this assumes you know a gate level implementation —
that’s not an “abstract” Verilog description.
module gate
(output f
input
a, b, c);
and
A (a1, a, b, c),
B (a2, a, ~b, ~c),
C (a3, ~a, o1);
or
D (o1, b, c),
E (f, a1, a2, a3);
endmodule
a
b
c
f
What Do We Want Here…?
 Goal

To specify a combination ckt, inputs->outputs…

… in a form of Verilog that synthesis tools will correctly read

… and then use to make the right logic
 And...

We know the function we want, and can specify in C-like form...

… but we don’t now the exact gates (nor logic elements); we want the
tool to figure this out.
A
B
C
Combinational
Logic
F
Behavioral Modeling
 Procedural statements are used

Statements using “always” Verilog construct

Can specify both combinational and sequential circuits
 Normally don’t think of procedural stuff as “logic”

They look like C: mix of ifs, case statements, assignments …

… but there is a semantic interpretation to put on them to allow them
to be used for simulation and synthesis (giving equivalent results)
 Current technology

You can do combinational (and later, sequential) design

Sizable designs can take hours … days … to run

Companies pay $50K - 80K per copy for such software
 This ain’t shrink-wrap software!

The software we use is more like $15-20K
Behavioral Constructs
 Behavioral descriptions are introduced by initial and always
statements
Statement
Looks like
initial
initial
begin
…
end
always
always
begin
…
end
Starts
How it works
Execute once
and stop
Starts
when
simulation
Continually loop—
starts … in
while (power on)
arbitrary
do statements;
order
Use in Synthesis?
No, used as
testbench
Yes, used in
synthesis
 Points:

They all execute concurrently

They contain procedural statements like if-then-else, case, loops,
functions, …
Statements, Registers and Wires
 Registers

Define storage, can be 1-bit or more

Can only be changed by assigning value to
them on the left-hand side of a behavioral
expression.
 Wires (actually “nets”)

Electrically connect things together

Can be used on the right-hand side of an
expression
 Thus we can tie primitive gates and
behavioral blocks together!
 Statements

left-hand side = right-hand side

left-hand side must be a register

Four-valued logic
Multi-bit
registers
and wires
Logic with
registers
and wires
module silly
(input [3:0] q, r);
reg
[3:0] a, b;
always begin
…
a = (b & r) | q;
…
q = b;
…
end
endmodule
Can’t do — why?
Behavioral Statements
 if-then-else

What you would expect, except that it’s doing 4valued logic. 1 is interpreted as True; 0, x, and z are
interpreted as False
if (select == 1)
f = in1;
else f = in0;
 case

What you would expect, except that it’s doing 4valued logic

If “selector” is 2 bits, there are 42 possible caseitems to select between

There is no break statement — it is assumed.
 Sized, 4-valued constants

The first number is the number of bits, the letter is
the base of the following number that will be
converted into the bits.
8’b00x0zx10
case (selector)
2’b00: a = b + c;
2’b01: q = r + s;
2’bx1: r = 5;
default: r = 0;
endcase
assume f, a, q, and r
are registers for this
slide
Behavioral Statements
 Loops

There are some restrictions on using these for synthesis
 for this course — don’t.

They are mentioned here for use in test modules
 Two main ones — for and while

Just like in C

There is also repeat and forever — see the book
reg [4:0] testOutput, i;
…
for (i = 0; i < 15; i = i + 1) begin
testOutput = i;
#20;
end
reg [4:0] testOutput, i;
…
i = 0;
while (i < 15) begin
testOutput = i;
#20 i = i + 1;
end
Important: Loops must have a delay operator (or as we’ll see
later, an @ or wait(FALSE)). Otherwise, the simulator never stops
executing them.
Concurrent Constructs
 We already saw #delay
 Others

@ … Waiting for a change in a value — used in synthesis
 @ (var) w = 4;
 This says wait for var to change from its current value. When it does,
resume execution of the statement by setting w = 4.

Wait … Waiting for a value to be a certain level — not used in
synthesis
 wait (f == 0) q = 3;
 This says that if f is equal to zero, then continue executing and set q
= 3.
 But if f is not equal to zero, then suspend execution until it does.
When it does, this statement resumes by setting q = 3.
 Why are these concurrent?

While one model waits for something, that something happens
(concurrently) in another model
FAQs: behavioral model execution
 How does an always or initial statement start

That just happens at the start of simulation — arbitrary order
 Once executing, what stops it?

Executing either a #delay, @event, or wait(FALSE).

All always blocks need to have at least one of these. Otherwise, the
simulator will never stop running the model — (it’s an infinite loop!)
More FAQs
 How long will it stay stopped?

Until the condition that stopped it has been resolved
 #delay … until the delay time has been reached
 @(var) … until var changes
 wait(var) … until var becomes TRUE
 Does time pass when a behavioral model is executing?

No. The statements (if, case, etc) execute in zero time.

Time passes when the model stops for #, @, or wait.
 Will an always stop looping?

No. But an initial will only execute once.
A Combinational Circuit
 Using behavioral constructs

Logic for a simple MUX is specified procedurally here

This example is synthesizable
module mux
(output reg f,
input
sel, b, c);
always @ (sel or b or c)
begin
if (sel == 1)
f = b;
else
f = c;
end
endmodule
Read this as follows:
Wait for any change on sel, b, or
c, then execute the begin-end
block containing the if. Then wait
for another change.
This “if” functionally describes the MUX
c
f
b
sel
Example of Logic
Synthesized
Huh? Is it really correct?
 WWWWaaaaaiiiiiitttt a minute?

Where’s the register?
The synthesis tool figures out that this is a
combinational circuit. Therefore, it doesn’t
need a register.
The register is there as an “artifact” of the
description — things on the left-hand side
have to be registers.

How does it figure out that this is
combinational?
 The output is only a function of the inputs
(and not of previous values)
 Anytime an input changes, the output is reevaluated

From outside the module, could you tell
that there is a register in there?
 Does the loop have state?
c
f
b
?
module mux
(output reg f,
input
sel, b, c);
always @ (sel or b or c)
begin
if (sel == 1)
f = b;
else
f = c;
end
endmodule
Synthesis Template
 Using procedural statements in Verilog

Logic is specified in “always” statements (“Initial” statements are not
allowed).

Each “always” statement turns into Boolean functions
module blah
(output reg f,
input
a, b, c);
always @ (a or b or c)
begin
stuff...
stuff...
stuff...
end
endmodule
You have to declare the combinational
outputs like this, for synthesis. i.e., tool
needs to think you are putting these
computed outputs someplace.
You have to list all the block’s inputs
here in the “sensitivity list”. (*) also
works!
Actually do logic in here. There are a
bunch of subtle rules to ensure that
synthesis won’t mess this up... We’ll
see how…
How? … A Few Definitions
 There are some restrictions on specification

Input set of an “always” statement — the set of all variables that
are used on the right-hand side of procedural assignments or in
conditionals. i.e. anything “sourced”.

Sensitivity list of an “always” statement — the set of all names
that appear in the event (“@”) list.
module mux
(output reg f,
input
sel, b, c);
always @ (sel or b or c)
begin
if (sel == 1)
f = b;
else
f = c;
end
endmodule
The elements in these lists are:
input: sel, b, c
sensitivity: sel, b, c
No coincidence here: a
combinational circuit is
sensitive to its inputs
More Definitions...
…

A control path of an “always” statement — a sequence of
operations performed when executing the always statement

Combinational output of an “always” statement — a variable (or
variables) assigned to in every control path
module mux
(output reg f,
input
sel, b, c);
always @ (sel or b or c)
begin
if (sel == 1)
f = b;
else
f = c;
end
endmodule
What are they here...
Control paths: through “then”
and “else” of if statement
Combinational output: f
The Basic Rules
 The rules for specifying combinational logic using procedural
statements

Every element of the input set must be in the sensitivity list

The combinational output must be assigned in every control path
module mux
(output reg f,
input
sel, b, c);
always @ (sel or b or c)
begin
if (sel == 1)
f = b;
else
f = c;
end
endmodule
So, we’re saying that if any input
changes, then the output is reevaluated. — That’s the definition
of combinational logic.
Walking this narrow line allows
you to specify and synthesize
combinational logic
What If You Mess Up?
 If you don’t follow the rules...?
… you’re dead meat

Verilog assumes you are trying to do something clever with the timing

It’s legal, but it won’t be combinational

The rules for what it does make sense — but not yet for us.
module blah
(output reg f, g;
input
a, b, c);
This says: as long as a==1, then f
follows b. (i.e. when b changes, so
does f.) But, when a==0, f remembers
the old value of b.
always @ (a or b or c)
begin
if (a == 1)
f = b;
else
g = c;
end
endmodule
Combinational circuits don’t remember
anything!
What’s wrong?
f doesn’t appear in every
control path in the always block
(neither does g).
Typical Style
 Your Verilog for combinational stuff will look like this:
module blah (<output names>, <input names>);
output <output names>;
input
<input names>;
reg
<output names>;
always @ (<names of all input vars>)
begin
< LHS = RHS assignments>
< if ... else statements>
< case statements >
end
endmodule
 Yes...it’s a pretty restricted subset of the language...
A Difficulty
 Assigning in every control path

If the function is complex, you don’t know if you assigned to the
outputs in every control path.

So, set all outputs to some known value (zero here) and write the
code to set them to other values as needed.

Synthesis tools will figure it out, but try to write clearly.
always @(coke or cola) begin
if (coke)
blah1 = 1;
else if (cola > 2’b01)
blah2 = coke;
else if ( …
…
end
always @(coke or cola) begin
blah1 = 0;
blah2 = 0;
if (coke)
blah1 = 1;
else if (cola > 2’b01)
blah2 = coke;
else if ( …
…
end
Using a case statement
 Truth table method

List each input combination

Assign to output(s) in each
case item.
 Concatenation

{a, b, c} concatenates a, b, and
c together, considering them
as a single item

Example
a = 4’b0111
b = 6’b 1x0001
c = 2’bzx
then {a, b, c} = 12’b01111x0001zx
module fred
(output reg f,
input
a, b, c);
always @ (a or b or c)
case ({a, b, c})
3’b000: f = 1’b0;
3’b001: f = 1’b1;
3’b010: f = 1’b1;
3’b011: f = 1’b1;
3’b100: f = 1’b1;
3’b101: f = 1’b0;
3’b110: f = 1’b0;
3’b111: f = 1’b1;
endcase
endmodule
Check the rules …
How about a Case Statement Ex?
module fred
(output reg f,
input
a, b, c);
always @ (a or b or c)
case ({a, b, c})
3’b000: f = 1’b0;
3’b001: f = 1’b1;
3’b010: f = 1’b1;
3’b011: f = 1’b1;
3’b100: f = 1’b1;
3’b101: f = 1’b0;
3’b110: f = 1’b0;
3’b111: f = 1’b1;
endcase
endmodule
check the rules…
Could
put a
function
here too
module fred
(output reg f,
input
a, b, c);
always @ (a or b or c)
case ({a, b, c})
3’b000: f = 1’b0;
3’b101: f = 1’b0;
3’b110: f = 1’b0;
default: f = 1’b1;
endcase
endmodule
Important: every control path is specified
x and z not considered in case enumeration!
Don’t Cares in Synthesis
 Rules

You can’t say
“if (a == 1’bx)…” — this has
meaning in simulation, but not
in synthesis.

However, an unknown x on the
right-hand side will be
interpreted as a don’t care.
a
b
f
~c
ab
00 01 11 10
c
0
x
1
0
1
1
1
1
1
x
module caseExample(
(output reg f,
input
a, b, c);
always @ (a or b or c)
case ({a, b, c})
3’b001: f = 1’b1;
3’b010: f = 1’b1;
3’b011: f = 1’b1;
3’b100: f = 1’b1;
3’b111: f = 1’b1;
3’b110: f = 1’b0;
default: f = 1’bx;
endcase
endmodule
The inverse function was implemented;
x’s taken as ones.
Alternatively…
module fred1
(output reg f,
input
a, b, c);

always @ (a or b or c)
f = ~(a & b & ~c);
endmodule
ab
These aren’t quite
equivalent to the
previous
slide…why?
module fred2
(output reg f,
input
a, b, c);
module fred3
(output reg f,
input
a, b, c);
always @ (a or b or c)
f = ~a | c | ~b;
endmodule
always @ (a or b or c)
begin
if (c ==0)
f = a~&b;
else f = 1’b1;
end
endmodule
00 01 11 10
c
0
x
1
0
1
1
1
1
1
x
Two input bits, Three output bits
reg [1:0]
newJ;
reg
out;
input
i, j;
always @(i or j)
case (j)
2’b00: begin
newJ = (i == 0) ? 2’b00 : 2’b01;
out = 0;
end
2’b01 : begin
newJ = (i == 0) ? 2’b10 : 2’b01;
out = 1;
end
2’b10 : begin
newJ = 2’b00;
out = 0;
end
default: begin
newJ = 2’b00;
out = 1'bx;
end
endcase
Works like the C
conditional operator.
(expr) ? a : b;
If the expr is true,
then the resulting
value is a, else it’s b.
What about time delay
 Could we have described the
module as shown here?


Note the delays. There is a
different delay from the b input
than from the c input.
Yes, you could write this
 But,

Synthesis tools will ignore the
time delays.

Generally, they try to minimize
the propagation from any
combinational input to any
combinational output in the
system.
module mux
(output reg f,
input
sel, b, c);
always @ (sel or b or c)
begin
if (sel == 1)
#5 f = b;
else
#88 f = c;
end
endmodule
Model Organization
 Here’s an always block for a
combinational function.

What Boolean functions can it model?

Can I have more than one of these
always blocks in a module?

Can two separate always blocks Yes
calculate function f?
always @(b1 or b2 or b3)
begin
yadda yadda
end
Only those with inputs
b1, b2, and b3
Nope!
No
module xyzzy (ports);
…
always @(b1 or b2 or b3)
begin
f = yadda;
end
always @(r1 or r2 or r3)
begin
f = yadda yadda;
end
module xyzzy (ports);
…
always @(b1 or b2 or b3)
begin
q = b1 … b2 … b3
r = b2 … b3
end
always @(r1 or b2 or b3)
begin
s = yadda yadda yadda
end
FSM — Review
 In the abstract, an FSM can be defined by:

A set of states or actions that the system will perform

The inputs to the FSM that will cause a sequence to occur

The outputs that the states (and possibly, the inputs) produce
 There are also two special inputs

A clock event, sometimes called the sequencing event, that causes the
FSM to go from one state to the next

A reset event, that causes the FSM to go to a known state
outputs
inputs
FSM
clock
reset
FSM Review
 The traditional FSM diagram

We looked at State Transition Diagrams (and Tables) last time

… and ended up with a diagram that looked like this
We’ll talk
about how to
design this
inputs
clock
reset
outputs
Combinational
Logic
Current State
Register
next
state
We’ll talk
about what
these are
D Flip Flops
 What are they?

A one-bit storage devices, sometimes
called a sequential element

It “flip flops” between two states.
 What are the ports?




D is the data input — the value to be
remembered
C is the clock input. The triangle
indicates that it’s edge sensitive.
Q is the output (and Q’ is its
complement)
Reset makes Q equal to zero anytime
reset is asserted (some “preset” to
one also)
Edge triggered FF symbol
data
D
clock
Q
Q and Q’
C
reset asserted low
Edge Triggered?
How’s it work
Clock transition determines
when input is acquired
Edge triggered FF symbol
data
clock
The point in time where clock
changes value is called the
“edge”. This is a “positive edge”
since clock is rising 0->1.
The input value D at the time of
the edge is remembered and
driven on the output.
D
clock
Q
Q and Q’
C
reset asserted low
Triggering: How Inputs —> Outputs
edge triggered FF symbol
data
D
clock
Q
Q and Q’
C
Timing Diagram:
reset asserted low
Current
Next state, after
state, now clock event
D
0
0
1
1
Q
0
1
0
1
Q+
0
0
1
1
D
CLK
Q
time
Some Terminology
 The clock is a special input

It unifies all the changes in an FSM
 Two types of changes in logic signals

Synchronous — The change is synchronized to an event.
 The event is typically a repetitive one — like the clock
 In normal operation, the output Q is synchronized to the clock event
 In normal operation, Q only changes when the event occurs, not when the input
D changes

Asynchronous — The change is not synchronized to an event
 The reset is an “asynchronous reset”
 The D flip flop we’ve shown is …

synchronous with an asynchronous reset, meaning

the normal operation of the data input and output are synchronized to the
clock event, but the reset will cause the output to be set to zero
immediately (without waiting for the clock event).
Verilog for the D Flip FLop
Current Next state, after
state (now) clock event
Q+
D Q
0
0 0
0
0 1
1
1 0
1
1 1
module DFF
(output reg q,
input
d, clk, reset);
always
@(posedge clk or negedge reset)
if (~reset)
q <= 0;
else q <= d;
endmodule
Note that it doesn’t matter
what the current state (Q)
is. The new state after the
clock event will be the value
on the D input.
The change in q is
synchronized to the
clk input.
The reset is an
asynchronous reset
(q doesn’t wait for
the clk to change).
Clock Events on Other Planets
 Trigger alternatives

For flip flops, the clock event can either be a positive or negative
edge

Both have same next-state table
D
Q
C
clk
Current
Next state, after
state, now clock event
Positive edge triggered
Flip Flop
D
clk
Q
C
Negative edge triggered Flip
Flop (bubble indicates
negative edge)
D
0
0
1
1
Q
0
1
0
1
Q+
0
0
1
1
Where Are Flip Flops in FSMs?
 Current State Register

Each bit of the register is implemented with a flip flop

The clocks of all flip flops are tied together into one clock line

The reset (or preset) of all flip flops are tied together into one
reset line
We’ll talk
about how to
design this
inputs
clock
reset
outputs
Combinational
Logic
Current State
Register
next
state
Moore Machine — 111 Detector
Q2
D1
Q1
Z
reset
X
Q1
D2
Q2
Q2’
clock
reset
reset
 Note how the reset is connected

Reset will make both of the FFs zero, thus putting them into state A.

Most FFs have both reset and “preset’ inputs (preset sets the FF to one).

The reset connections (to FF reset and preset) are determined by the state
assignment of the reset state.
Verilog Organization for FSM
Q2
D1
Q1
reset
X
Q1
D2
Q2
Q2’
clock
reset
reset
 Two always blocks

One for the combinational logic — next state and output logic

One for the state register
Z
Verilog Behavioral Specification
module FSM (x, z, clk, reset);
input
clk, reset, x;
output
z;
reg
[1:2] q, d;
reg
z;
always
@(posedge clk or negedge reset)
if (~reset)
q <= 0;
else q <= d;
always @(x or q)
begin
d[1] = q[1] & x | q[2] & x;
d[2] = q[1] & x | ~q[2] & x;
z = q[1] & q[2];
end
endmodule
 Things to note

reg [1:2] — matches
numbering in state
assignment (Q2 is least
significant bit in counting)

<= vs. =
The sequential part
(the D flip flop)
The combinational
logic part
next state
output
Verilog Overview
 Verilog is a concurrent language

Aimed at modeling hardware — optimized for it!

Typical of hardware description languages (HDLs), it:
 controls time
 provides for the specification of concurrent activities
 stands on its head to make the activities look like they happened at
the same time — Why?
 allows for intricate timing specifications — Why?
 A concurrent language allows for:

Multiple concurrent “elements”

An event in one element to cause activity in another. (An event is
an output or state change at a given time)
 based on interconnection of the element’s ports

Logical concurrency — software

True physical concurrency — e.g., “<=” in Verilog
Discrete Time Simulation
 Discrete Time Simulation

Models evaluated and state updated only at time intervals — n
 Even if there is no change on an input
 Even if there is no state to be changed
 Need to execute at finest time granularity
 Might think of this as cycle accurate — things only happen
@(posedge clock)

You could do logic circuits this way, but either:
 Lots of gate detail lost — as with cycle accurate above (no gates!)
 Lots of simulation where nothing happens — every gate is executed
whether an input changes or not.
 Discrete Event Simulation…

picks up simulation efficiency due to its selective evaluation

only execute models when inputs change
Discrete Event (DE) Simulation
 Discrete Event Simulation

Events — changes in state at discrete times. These cause other
events to occur.

Only execute something when an event has occurred at its input

Events are maintained in time order

Time advances in discrete steps when all events for a given time
have been processed
 Quick example

Gate A changes its output.

Only then will B and C execute
 Observations
B
A
C

The elements in the diagram don’t need to be logic gates

DE simulation works because there is a sparseness to gate
execution — maybe only 12% of gates change at any one time.
 The overhead of the event list pays off then.
Observations
 Hmm…

Implicit model execution of fanout elements
B
 Implicit?

Concurrency — is it guaranteed? How?

Time — a fundamental thingie

Can’t you represent this all in C? After all,
the simulator is written in it!
 Or assembly language?
 What’s the issue?

Or how about Java? Ya-know, aren’t objects
like things you instantiate just like in
Verilog?
 Can’t A call the port-2 update method on
object B to make a change?
A
C
A Gate Level Model
 A Verilog description of an SR latch
A module
is defined
name of the
module
Draw the circuit
module nandLatch
(output
q, qBar,
input
set, reset);
nand #2
The module has ports
that are typed
type and delay of
primitive gates
g1 (q, qBar, set),
g2 (qBar, q, reset);
endmodule
primitive gates with
names and
interconnections
A Gate Level Model
 Things to note:

It doesn’t appear “executable” — no for loops, if-then-else, etc.
 it’s not in a programming language sense, rather it describes the
interconnection of elements

A new module made up of other modules (gates) has been defined
 software engineering aspect — we can hide detail
module nandLatch
(output
q, qBar,
input
set, rese)t;
nand #2
g1 (q, qBar, set),
g2 (qBar, q, reset);
endmodule
Kinds of delays
 Transport delay

input to output delay (sometimes
“propagation”)
 Zero delay models (all transport delays = 0)

functional testing

there’s no delay, not cool for circuits with
feedback!
a
b
c
 — transport delay
 Unit delay models (all transport delays = 1)

all gates have delay 1. OK for feedback
 Edge sensitive — delay is value sensitive
nand #(3, 4, 5) (c, a, b);
rising delay
delay to z
falling delay
Some more gate level examples
instance names
and delays
optional
 An adder
module adder
(output
carryOut, sum,
input
aInput, bInput, carryIn);
sum
xor
(sum, aInput, bInput, carryIn);
or
(carryOut, ab, bc, ac);
and
(ab, aInput, bInput),
ab
carryOut
bc
(bc, bInput, carryIn),
ac
(ac, aInput, carryIn);
endmodule
aInput
list of gate instances
of same function
implicit wire
declarations
carryIn
bInput
Adder with delays
 An adder with delays
module adder
(output
carryOut, sum,
input
aInput, bInput, carryIn);
xor
#(3, 5)
(sum, aInput, bInput, carryIn);
or
#2
(carryOut, ab, bc, ac);
and
#(3, 2)
(ab, aInput, bInput),
(bc, bInput, carryIn),
(ac, aInput, carryIn);
endmodule
each AND gate
instance has the
same delay
Adder, continuous assign
 Using “continuous assignment”

Continuous assignment allows you to specify combinational
logic in equation form

Anytime an input (value on the right-hand side) changes, the
simulator re-evaluates the output

No gate structure is implied — logic synthesis can design it.
 the description is more abstract

A behavioral function may be called — details later
module adder
(output
carryOut, sum,
input
aInput, bInput, carryIn);
assign
sum = aInput ^ bInput ^ carryIn,
carryOut = (aInput & bInput) | (bInput & carryIn) |
(aInput & carryIn);
endmodule
I’m sick of this adder
 Continuous assignment assigns continuously

delays can be specified (same format as for gates) on whole
equation

no instances names — nothing is being instantiated.

given the same delays in this and the gate-level model of an adder,
there is no functional difference between the models
 FYI, the gate-level model gives names to gate instances, allowing back
annotation of times.
module adder
(output
carryOut, sum,
input
aInput, bInput, carryIn);
assign
#(3, 5)
sum = aInput ^ bInput ^ carryIn;
assign
#(4, 8)
carryOut = (aInput & bInput) | (bInput & carryIn) |
(aInput & carryIn);
endmodule
Structure vs. Behavior
 Structure — Look at it from the module (adder) ports

Strong physical connotations

The internal structure of a system includes its state and state
transition mechanism as well as the state to output mapping
 Behavior — again from the module ports

Outer manifestation of a system

The external behavior of a system is the relationship it imposes
between its input time histories and output time histories
module adder
(output carryOut, sum,
input
aInput, bInput, carryIn);
xor
or
and
(sum, aInput, bInput, carryIn);
(carryOut, ab, bc, ac);
(ab, aInput, bInput),
(bc, bInput, carryIn),
(ac, aInput, carryIn);
endmodule
Structural model
module adder
(output
input
assign
carryOut, sum,
aInput, bInput, carryIn);
sum = aInput ^ bInput ^ carryIn,
carryOut = (aInput & bInput) |
(bInput & carryIn) |
(aInput & carryIn);
endmodule
Behavioral model
Where is the state in these models?
Verilog Structure vs. Behavior
 Structure

gate level — built-in models for AND, OR, …

modules and instantiations

wires
 Behavior

C-like programs or Boolean algebra (but with a few extra
operators)

assign statements

always blocks — procedural statements (next time)
 Hmm…

If a module has an assign statement in it, is it behavior or
structure?
 On the outside, it appears as structure — it’s wired in, takes up space
(it’s physical) — maybe it is an ALU slice
 On the inside, it appears as behavior — we only know the translation
of inputs to outputs, but without physical connotations
Mixing Levels
 Generally there is a mix of levels in a model

e.g. part of the system is at the gate level and another part is at
the behavioral level.

Why?
 Early in design process you might not have fully-detailed models —
you don’t actually know all the gate implementations of the
multipliers, adders, register files
 You might want to think of the design at a conceptual level before
doing all the work to obtain the gate implementations
 There might be a family of implementations planned
 Finer grain of distinction

Levels — switch, gate, functional block (e.g. ALUs), registertransfer, behavioral

for now, we’ll deal with gate and behavioral models
An execution model for gates/assigns
 Execution model

“Execution” (sometimes “timing”) model — how time is advanced,
what triggers new processing and the generation of new state in the
model

State is held on wires, gates and continuous assigns advance state
 Definition —

when an input changes, the simulator will evaluate the gate or
continuous assign, calculating a new output

if the output value is different, it is propagated to elements on the
fanout
module nandLatch
(output q, qBar,
input
set, reset);
nand #2
g1 (q, qBar, set),
g2 (qBar, q, reset);
endmodule
Gate level timing model
 For gates and continuous assigns…

What’s an input?

What’s an output?

What’s state?
Gate inputs and RHS of assign equation
Gate outputs and LHS of assign equation
Wires
 Outputs on this “side” of the language are all …
Wires

…

no registers are latched/loaded, no need to know about a clock
event
Gate level timing model
 Contrast

At the gate level, there’s nothing special about two cross-coupled
gates
R
Q
S
Q’

A register is an abstraction above this “side” of the language

The left-hand sides on the behavioral “side” of the language are
all registers
Approach to Simulating a System
 Two pieces of a simulation

The model — an executable specification including timing,
interconnect, and input vectors
 Written in a language like Verilog or VHDL
 What’s a VHDL?

The simulation scheduler —
 keeps track of when events occur,
 communicates events to appropriate parts of the model,
 executes the model of those parts, and
 as a result, possibly schedules more events for a future time.
 it maintains “simulated time” (sometimes “virtual time”) and the
event list.

Parts of the scheduler function define the language
How’s the simulator work?
 A gate level model doesn’t look like a program

No if’s or loops — what get’s executed?
 Here’s how gate-level Verilog is executed —

You specify a bunch of primitive gates that are interconnected

When an input of a gate changes, the simulator will evaluate the
gate primitive and calculate a new output

If the output value is different from the current, it is scheduled to
propagate at some time in the future (or possibly now).

After the specified time delay (possibly zero), the new value is
propagated along wires to other gate-primitive inputs
 Simulator keeps track of time

… and what has been scheduled to happen at any time
 Inputs and Outputs?

An input to a gate primitive, the output of a gate primitive
Are these two modules the same?
module muxA
(output f,
input
a, b, sel);
module muxB
(output f,
input
a, b, sel);
or #5 g3 (f, f1, f2);
not
g4 (nsel, sel);
and #5 g1 (f1, a, nsel),
g2 (f2, b, sel);
endmodule
and #5 g1 (f1, a, nsel),
g2 (f2, b, sel);
or #5 g3 (f, f1, f2);
not
g4 (nsel, sel);
endmodule
a
a
f
f
b
b
sel
Alternate drawings of a mux
sel
Inside the Simulator
 A time-ordered list of events is maintained

Event — a value-change scheduled to occur at a given time

All events for a given time are kept together
 The scheduler removes events for a given time…

…propagates values, and executes gate models, creates new events…
time-ordered
event list
remove current
events
•••
ti tj tk
all the events
for time tj
tn
updates
Gate
Outputs
schedules
new event
Scheduler
looks
at
Network Connections
(fanouts)
executes
Gate
Models
Event-Driven Simulation
while (something in time-ordered event list) {
advance simulation time to soonest event’s time
retrieve all events e for this time
e
For each event e in arbitrary order {
update the value specified
follow fanout
evaluate the model(s)
schedule resulting events
}
}
evaluate these
One traversal of the while loop is a simulation cycle.
In 1 cycle, we remove all events for a time & execute them.
New events may be scheduled for the current time —
they are put in the event list and retrieved in the next sim. cycle.
New
event
Event-Driven Simulation
the event list
initial
A=1 at values as
25
shown
1
1
A=0
g1 #2
0
Eval g1
B=0 at
27
initial
A=1 at values as
25
shown
(at 27)
g2 #3
B=1
1
g1 #2
C=0
g2 #3
B=0
A=1
1
1
g1 #2
A=1
0
D=1
g3 #5
0
(at 30)
D=1
g3 #5
1
Eval g2, g3
C=1 at B=0 at A=1 at
initial
25
30
values as
27
shown
C=0
C=1
g2 #3
B=0
g3 #5
final
D=1
How does it keep track of time?
 … Explicitly

Events are stored in an event list (actually a 2-D list) ordered by
time

Events execute at a time and possibly schedule their output to
change at a later time (a new event)

When no more events for the current time, move to the next

Events within a time are executed in arbitrary order
time a
event
event
time a + 2
event
event
event
Let’s say A
changes to 0
here. B and C
have delay 2.
1
B
A
time a+75
time a+75492
event
event
Events to
event update B and
C are added.
1
C
Two types of Events
 Update events —

Action: update state and propagate new values along a fanout.

Possibly produces new events
 Evaluation events —

Action: evaluate, or execute, a model.

Possibly produces new events
 Plan

Will deal with update events now

Evaluation events come in with behavioral models
Event-Driven Simulation
1
while something in time-ordered event list {
advance simulation time to top event’s time
B=0
#2
A= 1
0
C=0
retrieve all events for this time
update
1
#2
For each event in arbitrary order {
If it’s an update event {
1
update the value specified
B= 01
#2
A= 0
C= 01
follow fanout, evaluate gates there
If an output changes
1
#2
schedule update event for it
}
update
else // it’s an evaluation event
evaluate the model
}
}
time 
A=0
time  + 2
B=1
C=1
What about zero delay events?
while something in time-ordered event list {
advance simulation time to top event’s time
retrieve all events for this time
But it’s not
retrieved and
executed until the
next sim cycle
For each event in arbitrary order {
If it’s an update event {
update the value specified
follow fanout, evaluate gates there
A gate with #0
delay gets
scheduled for the
current time
If an output changes
schedule update event for it
}
Ain
#0
Aout
else // it’s an evaluation event
evaluate the model
}
Ain
Aout
==
01
time 
}
The simulator can spend several iterations at the same simulation time
Verilog Gate level timing model
 What if an update event is already scheduled for an primitive
gate output?

if the value being scheduled is different, the currently scheduled
value is removed from the event list; the new event is not
scheduled
Called inertial delay — oddly named, how wide must an input
spike be to be seen?
Deviation from
pure discrete
a
event simulation.
a=1
c
b

nand #5 (c, a, b);
b=1
update scheduled
c
propagation
delay = 5
update removed,
final value
a
b
alternate
c
Instantiation — Hierarchy
module above (out, …);
output [2:0] out;
wire
[2:0] h, I, j;
module r(o1,i1, i2, i3);
input
i1, i2, i3;
output o1;
assign o1 = i1 | i2 | i3;
endmodule
r
a(out[0], h[0], I[0], j[0]),
b(out[1], h[1], I[1], j[1]),
c(out[2], h[2], I[2], j[2]);
endmodule
out[2:0]
a
r
r
r
above
out[2]
b
c
o1
Not all connections shown
 Hierarchical name

o1 is really … above_inst.c.o1

Used for debugging… why just debugging?
Hierarchy
 Why?

Hides detail

Supports alternate implementations

Encapsulates — side effects understood
 Observations

Hardware resources allocated (instantiated) to perform a function
exclusively

No other function will use it

Thus, physical concurrency and structure are established
module r
(output o1,
input
i1, i2, i3);
assign o1 = i1 | i2 | i3;
endmodule
module r
(output o1,
input
i1, i2, i3);
or #(2, 5) (first, i1, i2),
(o1, first, i3);
endmodule
Summary on gate evaluation
 Simulation languages — concurrent

Maintain explicit notion of time

Describe models with physically concurrent activity

Interconnection of models allows for data-driven activity
 Timing model

timing-execution model
 how time is advanced and new state created

Any gate input or assign righthand-side change causes the model
to be evaluated during the time step
 this is not the case for behavioral models

Fanout list is static — design never changes
 What if you don’t like Verilog’s gates?

e.g., inertial delays?

use behavioral models (or user defined primitives…?)
Procedural models: what’s needed?
 Obvious things like operator set that matches hardware
functionality

Bit hacking, etc. a = { b[3], b[1], c[4] };
 Concurrent operators

Similar to what you’d find in other “threaded” languages

… plus hardware functionlity — such as:
 Edge triggering
 Concurrent/buffered state update
 Control of time
…

…minus a few — such as:
 Support for critical sections — P,V
Procedural Models
 This is the “other side” of the language

Always and initial statements are
concurrent
 They start when the simulation starts, in
arbitrary order

Assignments are made to registers
 Everything on left hand side is a register



Statements execute sequentially
Atomicity — only one element (gate,
always, initial) executing at a time. No preemption — continues executing until done.
Stuff between concurrent statements
executes in zero time
Because statements execute in zero
time and are atomic, it looks like lots of
parallel stuff is happening
always begin
@ (posedge clock)
h = f + k;
g = f * g;
@ (posedge clock)
f = g;
q = f * s;
…
Why is this
important?
At first look, it is a lot like C
 Most of the operators are the same as C

^ is XOR, etc.

makes it easy to read
 But there are major differences (quick list, we’ll get to these)

Concurrent statements like #delay, @event, wait(level)

four-valued logic (1, 0, x, z) and the operators to go with them

arbitrary bit width operations

there are a couple of procedural assignments (=, <=) with subtle
differences

a different timing model — in fact, C doesn’t have one
 It has a sequencing model — sequence being a more abstract view of
time.
 hmm, do we even know if the program sequencing holds?
Review from before
 Behavior vs. Structure

These two models are functionally interchangable — either could
have been instantiated into a register
 ports in same order
 same delay from clock to q
 one is abstract, clear
 one is structurally specific
 there are subtle differences
module d_type_FF
(output reg q,
input
clock, data);
always
@(negedge clock) q = #10 data;
endmodule
Behavioral
module d_type_FF
(output q,
input
clock, data);
nor #10
a (q, qBar, r);
nor
b (qBar, q, s),
c (s, r, clock, s1),
d (s1, s, data),
e (r, r1, clock),
f (r1, s1, r);
endmodule
Structural
Procedural Timing Model
 How does the procedural model advance
time?

# — delaying a specific amount of time

@ — delaying until an event occurs
 “posedge”, “negedge”, or any change
 this is edge-sensitive behavior
 When the statement is encountered, the
value v is sampled. When v changes in the
specified way, execution continues.

always begin
#5 q = w;
@ (negedge v)
q = y;
wait (c == 0)
q = 3;
end
wait — possibly delaying until an event
occurs
 this is level sensitive behavior

While one model is waiting for one of the
above reasons, other models execute —
values change, time marches on
Everything executes in
zero time — time
advances when you’re
not executing!
An example of wait
 Semantics


wait (expression) statement;
—
e.g. wait (a == 35) q = q + 4;
if the expression is FALSE,
the process is stopped
 when a becomes 35, it
resumes with q = q + 4

if the expression is TRUE,
the process is not stopped
 it continues executing
module handshake (ready, dataOut, …);
(input
ready,
output reg
[7:0]
dataOut);
reg
[7:0] someValueWeCalculated;
always begin
…
wait (ready);
…
dataOut = someValueWeCalculated;
wait (~ready) …
end…
ready
No. Not if ready is already true
when the first wait is executed.
You’re not guaranteed to get
the value at the edge
Do you always get the value at
the edge when ready goes from 0
to 1? Isn’t this edge behavior?
Wait vs. While
 Are these equivalent?

No: The left example is correct, the right one isn’t — it won’t work

Wait is used to wait for an expression to become TRUE
 the expression eventually becomes TRUE because a variable in the
expression is changed by another process

While is used in the normal programming sense
 in the case shown, if the expression is TRUE, the simulator will
continuously execute the loop. Another process will never have the
chance to change “in”. Infinite loop!
 while can’t be used to wait for a change on an input to the process. Need
other variable in loop, or # or @ in loop.
module yes
(input in);
…
wait (in == 1);
…
endmodule
module no
(input in);
…
while (in != 1);
…
endmodule
Blocking assignments and #
 We’ve seen #delay

Delay for specified time
 … and blocking assignments — they use =

Options for specifying delay
Wait #10, then do the statement
#10 a = b + c;
a = #10 b + c;

Note the action of the second one:
Calculate b+c, wait 10,
then do assignment
 an intra-assignment time delay
 The event list is used for temporary storage!

The differences:
• #10 a = b + c; Values b and c are from time (now + 10)
• a = #10 b + c; Values b and c are from time (now)
Blocking — what’s it mean?
 Blocking — the always or initial block stops (blocks) for some
reason

#, @, wait(FALSE)
always begin
q = blahblah;
r = q - someInput;
It blocks (stops) here, other
things (always, gates, assigns)
execute. Finally at t+10, this
continues executing
a = #10 q + r;
t = a - someOtherInput;
…
end
Intra assignment delay –
delay within an assignment.
Events — @something
 Action

when first encountered, sample the expression

wait for expression to change in the indicated fashion

This always blocks — you never execute straight through —
guaranteed edge sensitivity
 Examples
always @(posedge ck)
q <= d;
always @(hello)
a = b;
always
a = @(hello) b;
always @(coke or cola)
a = b;
always begin
yadda = yadda;
@(posedge hello or negedge goodbye)
a = b;
…
end
Sensitivity Lists
 In the gate level timing model…

model execution was sensitive to any change on any of the inputs
at any time.

sensitivity list — a list of inputs that a model is sensitive to
 a change on any of them
will cause execution of
the model


In the gate level timing model,
the lists don’t change.
Ditto with continuous assign
 In procedural models …

the sensitivity list changes as
as function of time and
execution
module d_type_FF
(output q,
input
clock, data);
nor #10
a (q, qBar, r);
nor
b (qBar, q, s),
c (s, r, clock, s1),
d (s1, s, data),
e (r, r1, clock),
f (r1, s1, r);
endmodule
Structural
Procedural Timing Model
 What is the behavioral model sensitive to?

The behavioral statements execute in sequence

Therefore, a behavioral model is sensitive to its context
 i.e. it is only sensitive to what it is currently waiting for
 time, edge, level — (#, @, wait)

The following model is not sensitive to a change on y or w.
always begin
@ (negedge clock1)
q = y;
@ (negedge clock2)
q = w;
@ (posedge clock1)
/*nothing*/ ;
@ (posedge clock2)
q = 3;
end
Here, it is only sensitive to clock1
Here, it is only sensitive to
clock2. A posedge on
clock1 will have no effect
when waiting here.
Fanout Lists
 Outputs of things are connected to inputs of other things

No surprise

The simulator maintains a fanout list of inputs driven by each
“output”
 Why maintain a fanout list?

When the output changes, it’s easy to figure out what other
models need (to be) evaluated

Because of procedural models …

…Sensitivity lists change

Fanout lists change
 Sensitivity lists <—> Fanout lists
 What’s an “output” in a behavioral model?
List Changes
 Change in sensitivity lists in procedural models cause
fanout lists to change
clock1 fanout is A, B, D;
clock2 fanout is C.
clock1
A
B
clock2
C
always begin:D
@ (negedge clock1)
q = y;
@ (negedge clock2)
q = w;
…
end
clock1 fanout is A, B;
clock2 fanout is C, D.
Scheduling #, @, and Wait
 How are #, @, and wait tied into the event list?

# delay
 schedule the resumption of the process — put it in the event queue delay
units into the future. Essentially an evaluation event scheduled in the future

@ change
 when suspended for an @v, the behavioral model is put on the fanout list of
the variable v. i.e., the behavioral model is now sensitive to v.
 When an update event for v occurs, (e.g. posedge), then the behavioral
model resumes at the current time.

Wait (exp)
 if exp is TRUE, don’t stop
 if exp is FALSE, then the behavioral model is put on the fanout list(s) of the
variable(s) in exp. (it’s now sensitive to the variable(s))
 When there is an update event for any of the variables in exp , exp is
evaluated. If exp is TRUE, resume executing in the current time , else go
back to sleep
Procedural Model Sensitivity?
 Quick example

Gate A changes its output

What models get executed?
B
Yes
A
C
always @(A)
begin
R = ~A;
end
always @(posedge clock)
Q <= A;
No
Maybe
always begin
@(A) R = ~A;
@(D) R = ~B;
end
Order of Execution
B
 Assume A changes.


In what order do these
models execute?
Arbitrary, don’t count on
any specific order
The simulator will try to
make them look like they
all occur at the same time
— how?
By controlling virtual time.
A
C
always @(A)
begin
R = ~A;
end
always @(posedge clock)
Q <= A;
Arbitrary Order? Oops!
 Sometimes you need to
exert some control

Consider the
interconnections of this DFF

At the positive edge of c,
what models are ready to
execute?

Does it matter which one is
done first?
shiftin
Oops — The order of
execution can matter!
clock
module dff(q, d, c);
…
always @(posedge c)
q = d;
endmodule
module sreg (…);
…
dff
e (q0, shiftin, clock),
f (q1, q0, clock),
g (shiftout, q1, clock);
endmodule
D
Q
D
Q
D
Q
shiftout
Non-blocking Concurrent Asg.
module fsm
(output reg Q1, Q0,
input
clock, in);
always @(posedge clock) begin
Q1 <= in & Q0;
Q0 <= in | Q1;
end
endmodule
Q0
Q
Q1
D
Q
Q0
in
Q1
clock
Values after the clock edge (t+)
— calculated in response to
the clock edge, using values at
the clock edge
D
Values at the
clock edge.
(At t -)
 Concurrent Assignment — primary use of <=

The assignment is “guarded” by an edge

All assignments guarded by the edge happen concurrently