Verilog

Digital Design Crash Course
Mohamed Rayan

Overview
 Quick review

 Design Methodologies.
 Difference between C/C++ and HDL.
 Verilog HDL.

 Common rules of any digital circuit:

1- 2’s Complement for signed operations.
2- Sequential Logic Circuits must be Synchronous only not
Asynchronous.
3- Flip-Flops should be used not Latches.
4- Any Sequential Circuit should have Reset.

 Q1 : Design a combinational circuit with three inputs and

one output ?
If The output is 1 when the binary value of the inputs is less than 3.
The output is 0 otherwise.

 Q2 : Design a sequential circuit with two D flip-flops

A and B, and one input x_in ?

When x_in = 0, the state of the circuit remains the same.
When x_in = 1, the circuit goes through the state transitions
from 00 to 01, to 11, to 10, back to 00, and repeats.

 There are two basic types of digital design methodologies :

a bottom-up design methodology and a top-down design
methodology.
 Bottom-up (Traditional method of electronic design):

with the increasing complexity of new designs this approach is nearly impossible
to maintain. New systems consist of ASIC or microprocessors with a complexity
of thousands of transistors. These traditional bottom-up designs have to give way
to new structural, hierarchical design methods. Without these new practices it
would be impossible to handle the new complexity.
 Top-down :

The desired design-style of all designers is the top-down one. A real top-down
design allows early testing, easy change of different technologies, a structured
system design and offers many other advantages. But it is very difficult to follow a
pure top-down design.

 Typically, a combination of top-down and bottom-up flows is used.

Design architects define the specifications of the top-level block.

 Logic designers decide how the design should be structured by

breaking up the functionality into blocks and sub-blocks.

 At the same time, circuit designers are designing optimized circuits for

leaf-level cells. They build higher-level cells by using these leaf cells.

 The flow meets at an intermediate point where the switch-level circuit

designers have created a library of leaf cells by using switches, and the
logic level designers have designed from top-down until all modules
are defined in terms of leaf cells.

 Every time this intermediate point goes to abstracted level and this

helps to cut the design cycle and reduce time of design due to designs
complexity.

 Example : 4-bit ripple carry counter.

Difference between C/C++ and HDL

C
Program
Foo.c

Compiler

Loader

Machine
Language
Foo.exe

Assembly
Foo.asm

Linker

Assembler

Machine
language
Foo.o

Layout & Fab.

Design
Specs

HDL
Design.v

Synthesis Tool

Gate level
Description

 In C Program we have a series of instructions loaded into

memory and to be executed by microprocessor in order
(Sequentially) .
 In C program we don’t have timing control to control its
operation because the next statement will not be executed
till finishing execution of the current statement.
 Verilog Code will configure Hardware to do certain function.
Statements in Verilog may have parallel “Concurrency” or
sequential execution or both. Verilog Code must be
synthesizable i.e the compiler must be able to generate logic
that fits the description.
 In Verilog we have timing control as we have gate delays and
statements that may be executed in parallel.

Verilog HDL
(HARDWARE DESCRIPTION LANGUAGE)

 Verilog is a hardware description language is a language used to describe a

digital system: for example, a network switch, a microprocessor or a
memory or a simple flip-flop. This just means that, by using a HDL, one can
describe any (digital) hardware at any level.

 HDLs have two objectives
 Allow for testing/verification using computer simulation

» Includes syntax for timing, delays
 Allow for synthesis
» Synthesizable HDL
 It allows us to design a Digital design at Behavior Level, Register Transfer

Level (RTL), Gate level and Switch(Transistor) level.

 Verilog allows hardware designers to express their designs with behavioral

constructs, deferring the details of implementation to a later stage in the
final design.

 Verilog HDL is case sensitive
 All Verilog keywords are lower case

Module in Verilog
 Modules are the building blocks of Verilog designs.
 The following figure shows starcture of any Verilog module :
Module name,
Port list, port delectation (if ports present),
Parameters (optional)
Declare Wires and Registers of
variables used in the design.

Instantiation of lower level
modules

Verilog code that describe
operations of the module :
Always and Initial blocks
(having behavioral statements
e.g if and case )

End Module

Module in Verilog
 Definition of Full adder will be as follow :

module FullAdder(A,B,Cin,Cout,S);
// Design Body
endmodule
 Ports :
Ports provide the interface by which a module can communicate with its
environment. For example, the input/output pins of an IC chip are its ports.
The environment can interact with the module only through its ports.
Verilog Keyword

Type of Port

input

Input port

output

Output port

inout

Bidirectional port

Module in Verilog
 input [range_val:range_var] list_of_identifiers;

output [range_val:range_var] list_of_identifiers;
inout [range_val:range_var] list_of_identifiers;
 Definition of 1-bit Full adder will be as follow :

input A, B, Cin;
output Cout, S;
// Design Body
endmodule

 Definition of 4-bit Full adder will be as follow :

input[3:0] A, B;
input
Cin;
output
Cout,;
output [3:0] S;
// Design Body
endmodule

Module in Verilog
:

 Instantiating a module

 A module may be consisting of logic and some lower level modules,

e.g: Full adder of 4-bit needs to be designed using 1-bit Full adder,
in the top level of 4-bit adder we will instantiate 4 1-bit adders.
 module_name instance_name ( connecting ports);
e.g : FullAdder Bit0 ( .A (A[0]) ,

.B (B[0]) ,
.Cin (Cin) ,
.S (result[0]) ,
.co (c1)
);

 A module may be instantiated multiple times within top module.

Module in Verilog
 Connecting Ports to External Signals :
 There are two methods of making connections between signals specified in the module

instantiation and the ports in a module definition. These two methods cannot be mixed:

1- Connecting by ordered list (Implicit):
all module ports is connected by order without defining the actual ports of the instantiated
modules
e.g : FullAdder Bit0 (A[0] ,
B[0] ,
Cin ,
result[0],
c1);

2- Connecting ports by name (Explicit):
all module ports is defined with its connected port
e.g : FullAdder Bit0 ( .A (A[0]) ,
.B (B[0]) ,
.Cin (Cin) ,
.S (result[0]) ,
.co (c1) );

Module in Verilog
 Port Connection Rules:
The Verilog simulator complains if any port connection rules are violated.
 Inputs : Internally, input ports must always be of the type net. Externally, the
inputs can be connected to a variable which is a reg or a net.
 Outputs : Internally, outputs ports can be of the type reg or net. Externally,
outputs must always be connected to a net. They cannot be connected to a reg.
 Inouts : Internally, inout ports must always be of the type net. Externally, inout
ports must always be connected to a net.
 Width matching: It is legal to connect internal and external items of different
sizes when making inter-module port connections. However, a warning is
typically issued that the widths do not match.
 Unconnected ports : Verilog allows ports to remain unconnected. For
example, certain output ports might be simply for debugging, and you might not
be interested in connecting them to the external signals.
e.g : fulladd4 fa0(SUM, , A, B, C_IN); // Output port c_out is unconnected

Parameterized Modules
 Parameterized modules are modules that can be defined with different

characteristics values e.g input/output width.
 Assume that we need counter of various widths, Instead of create
counter modules every module having different width, We can define
only one parameterized module and instantiate it multiple times with
needed width.
 A parameter is defined by Verilog as a constant value declared within the
module structure (Global Constants).
 module RAM ( Reset, Clock , address , data , we );

parameter DATA_WIDTH = 8 ;
parameter ADDR_WIDTH = 8 ;
parameter RAM_DEPTH = 1 << ADDR_WIDTH;
// Actual code of RAM here
endmodule

 Reusability : Different modules can use RAM module with different data

widths and address widths.

 The value can be used to define a set of attributes for the module which can

characterize its behavior as well as its physical representation.

 Parameters of RAM module may be overridden at instantiation time if they

declared global . If an overriding value is not specified, the default parameter
declaration values are used.

module RAM_Controller ();
//Insatiate RAM here
RAM#( .DATA_WIDTH(16),
.ADDR_WIDTH(8),
.RAM_DEPTH(256))
RAM_instance (Reset, Clock, address, data, we);
//continue code
endmodule
 Parameter Override using defparam
module RAM_Controller ();
//Insatiate RAM here
RAM RAM_instance (Reset, Clock, address, data, we);
//continue code
defparam RAM_instance.DATA_WIDTH = 4;
defparam RAM_instance ADDR_WIDTH = 10;
endmodule

 Local parameters (constants) that will be declared only within the

same module can defined as follow:
module FSM ();
localparam [1:0] S0 = 2’b00,
S1 = 2’b01,
S2 = 2’b10,
S3 = 2’b11;
//continue code
endmodule
 In general using Parameters/Constants will lead to easy
Reusability and easy later modifications, So it will be better if you
make your design parameterized .

Data types
 Verilog Language has two primary data types:
 Nets - represent structural connections between

components.
 Registers - represent variables used to store data.
 Every signal has a data type associated with it:
 Explicitly declared with a declaration in your Verilog

code.
 Implicitly declared with no declaration when used to
connect structural building blocks in your code. Implicit
declaration is always a net type "wire" and is one bit wide.

Register Data Types
 Registers store the last value assigned to them until another assignment












statement changes their value.
Registers represent data storage constructs.
You can create regs arrays called memories.
register data types are used as variables in procedural blocks.
A register data type is required if a signal is assigned a value within a
procedural block
Procedural blocks begin with keyword initial and always.
reg [3:0] A;
// A is signal unsigned by default of width 4 bits.
reg signed[3:0] B; // B is signal signed represented in 2’s complement of width 4 bits.
integer C;
//C is signed variable of 32 bits.
Note : Of all register types, reg is the one which is most widely used.

Signal Values and Comments in Verilog
 ‘0’ represents low logic level or false condition.

 ‘1’ represents high logic level or true condition.
 ‘X’ represents unknown logic level.
 ‘Z’ represents high impedance logic level.
 Comments:

here are two forms to introduce comments.
 Single line comments begin with the token // and end with
a carriage return
 Multi line comments begin with the token /* and end with
the token */

Numbers in Verilog
 Verilog HDL allows integer numbers to be specified as
 Sized or unsized numbers (Unsized size is 32 bits)
 In a radix of binary, octal, decimal, or hexadecimal
 Radix and hex digits (a,b,c,d,e,f) are case insensitive
 Spaces are allowed between the size, radix and value
 Syntax: <size>'<radix><value>;
 Examples:

Stored as

Integer
1

00000000000000000000000000000001

8'hAA

10101010

6'b10_0011

100011

'hF

00000000000000000000000000001111

 Verilog expands <value> filling the specified <size> by working from right-to-left
 When <size> is smaller than <value>, then leftmost bits of <value> are truncated

 When <size> is larger than <value>, then leftmost bits are filled, based on the value of the

leftmost bit in <value>.

 Leftmost '0' or '1' are filled with '0'
 Leftmost 'Z' are filled with 'Z'
 Leftmost 'X' are filled with 'X'

Verilog Operators
Operator Type

Operator Symbol

Operation Performed Number of Operands

Arithmetic

*
/
+
%
**

multiply
divide
add
subtract
modulus
power (exponent)

two
two
two
two
two
two

Logical

!
&&
||

logical negation
logical and
logical or

one
two
two

Relational

>
<
>=
<=

greater than
less than
greater than or equal
less than or equal

two
two
two
two

Equality

==
!=

equality
Inequality
case equality
case inequality

two
Two
two
two

Verilog Operators Cont’
Operator Type

Operator Symbol

Operation Performed Number of Operands

Bitwise

~
&
|
^
^~ or ~^

bitwise negation
bitwise and
bitwise or
bitwise xor
bitwise xnor

one
two
two
two
two

Reduction

&
~&
|
~|
^
^~ or ~^

reduction and
reduction nand
reduction or
reduction nor
reduction xor
reduction xnor

one
one
one
one
one
one

Shift

>>
<<
>>>
<<<

Right shift
Left shift
Arithmetic right shift
Arithmetic left shift

two
two
two
two

Concatenation

{}

Concatenation

Any number

Replication

{{}}

Replication

Any number

Abstraction levels of designs using
Verilog

 In complex designs the number of gates

is very large. Thus, designers can design
more effectively if they concentrate on
implementing the function at a level of
abstraction higher than gate level.
 The higher abstraction level the lower details .
 Designs with more details may have more errors

hence less verification time.
 Verification is about 50 – 60 % of design time.

Gate Level Design
 At gate level, the circuit is described in terms of gates (e.g., and, nand
most commonly because any gate can be done using it and its transistor circuit is
easy to fabricate ).

 Hardware design at this level is about one-to-one correspondence

between the logic circuit diagram and the Verilog description. See
the following example:

input A, B, Cin;
output Cout, S;
wire w1, w2, w3, w4;
xor (w1, A, B);
and (w2, A, B);
and (w3, w1, Cin);
xor (S, w1, Cin);
or (Cout, w3, w2);
endmodule

Gate Level Design

 To define a gate in verilog :

and/nand/or/….. Instant_name (OUT, IN1, IN2,IN3,IN4,…….);
buf instant_name(OUT, IN); not n1(OUT, IN);

• Gate propagation delay can be simulated in Verilog but sure it is not

Synthesizable, Synthesis will be done with actual delays of mapped gate in
Technology library.

• There are three types of delays from the inputs to the output of a primitive gate.
 Rise delay is associated with a gate output transition to a 1 from another value.
 Fall delay is associated with a gate output transition to a 0 from another value.
 Turn-off delay is associated with a gate output transition to the high impedance

value (z) from another value.
and #(rise_delay,fall_delay,turnoff_delay) and_gate (OUT, IN1, IN2,IN3,IN4,…….);

Gate Level Design
input A, B, Cin;
output Cout, S
wire w1, w2, w3, w4;
xor #(10) (w1, A, B); // delay time of 10 units
and #(8) (w2, A, B);
and #(8) (w3, A, Cin);
and #(8) (w4, B, Cin);
xor #(10) (S, w1, Cin);
or #(10, 8)(Cout, w2, w3, w4); // (rise time of 10, fall 8)
endmodule
 If No Delay is specified then it is assumed 0 .

 Every delay value may have a range minimum, maximum and typical values.

(min_rise:typical_rise:maximum_rise, min_fall:typical_fall:maximum_fall)

 Back to Q1 : Design using Verilog a combinational circuit

with three inputs and one output ?
If The output is 1 when the binary value of the inputs is less than 3.
The output is 0 otherwise.

Register Transfer Level Design
 The modules of a digital system are best defined by a set of

registers and the operations (done by some gates) that are
performed on the binary information stored in them. Examples of
 Register operations are shift, count, clear, and load . Registers are
assumed to be the basic components of the digital system.
 A digital system is represented at the register transfer level (RTL)
when it is specified by the following three components:
1. The set of registers in the system.
2. The operations that are performed on the data stored in the registers.
3. The control that supervises the sequence of operations in the system.
R1 R1 + R2 Add contents of R2 to R1 ( R1 gets R1 + R2 )
R3  R3 + 1 Increment R3 by 1 (count upwards)
R4  shr R4 Shift right R4
R5  0 Clear R5 to 0

 The type of operations most often encountered in digital systems can be

classified into four categories :

1. Transfer operations, which transfer data from one register to another.
2. Arithmetic operations, which perform arithmetic (e.g., multiplication)
on data in registers.
3. Logic operations, which perform bit manipulation (e.g., logical OR) of
non numeric data in registers.
4. Shift operations, which shift data between registers.

 Assign Statement : is used for modeling only combinational

logic and it is executed continuously. So the assign statement is
called 'continuous assignment statement' .
 Assign statement is applied for only wires.
 EX: assign Output = (enable) ? Input : 1'bz;
 assign out = data;
 assign #10 out = in1 & in2;
 assign F = ({s1,s0} == 2’b00)? I0:

({s1,s0} == 2’b01)? I1:
({s1,s0} == 2’b10)? I2:I3;
 Use assign statement to model full adder?

 Q : Synthesize the following code and get the

corresponding circuit?

module Prob (A, B, S, E, Q);
input [1:0] A, B;
input S, E;
output [1:0] Q;
assign Q = E ? (S ? A : B) : 'bz;
endmodule

Behavioral(Algorithmic) Level Design
 Design is a Black Box

 It takes place at an algorithmic level where the designers do

not necessarily think in terms of logic gates or data flow but
in terms of the algorithm they wish to implement in
hardware.
 They are more concerned about the behavior of the
algorithm and its performance.
 Only after the high-level architecture and algorithm are
finalized, do designers start focusing on building the digital
circuit to implement the algorithm.

 Procedural Blocks
 Verilog behavioral code is inside procedure blocks, but there is an exception:

some behavioral code also exist outside procedure blocks. We can see this in
detail as we make progress.
 There are two types of procedural blocks in Verilog : initial and always blocks
 Procedural assignment statements assign values to reg, integer, real, or time

variables and can not assign values to nets (wire data types).
 Module may have more than one procedural block.

 All behavioral statements must be inside an initial or always block only.

//Initialize clock at time zero
initial clock = 1'b0;

//Toggle clock every half-cycle (time period = 20)
always #10 clock = ~clock;

 Procedural Blocks
 initial : initial blocks execute only once at time zero (start execution at time zero)

used in test benches.
 always : always blocks loop to execute over and over again; in other words, as the

name suggests, it executes always.
module initial_example();
reg clk,reset,enable,data;
initial begin
clk = 0;
reset = 0;
enable = 0;
data = 0;
end
endmodule

module always_example();
reg clk,reset,enable,q_in,data;
always @ (posedge clk) begin
if (reset) begin
data <= 0;
end else if (enable) begin
data <= q_in;
end
endmodule

 Always Block
 Is the heart of Hardware modeling using Verilog HDL

 always @(…..sensitivity list…..)

begin
//behavioral constructs e.g if/else, case that describe needed fuctionality
end
 Sensitivity list : Without any signals in it the statements inside always block
i.e between begin .. end will be executed forever. If we have signals in this list
then statements inside always block will be executed if and only if these
signals chagned .
 Modeling Combinational Circuits e.g(Muxes, Decoders, Encoders, ……).
always @ (A,B,Cin) begin
{Cout,S} = A+B+Cin;
end
All signals in Right hand side must appear in the sensitivity list .

 Always Block
 Modeling Sequential Circuits e.g(Registers, Counters, ……).

always @ (posedge Clock, negedeg Reset) begin
if( !Reset)
Q <= 0;
else
Q <= D;
end
Clock and Asynchronous signals must appear in the sensitivity list .
 Design may have any number of always block e.g we can model every register in an

independent always block.
always @(posedge clk)
R2 = R1;

R4 = R3;

different always blocks are working independent on each other .
always @(posedge clk) blocks run in some undefined sequence.

 Q : Modify the following design of Flip-Flop to have

synchronous reset instead asynchronous one?
always @ (posedge Clock, negedeg Reset) begin
if( !Reset)
Q <= 0;
else
Q <= D;
end

 Blocking and Nonblocking assignment
Blocking assignments(Modeling Combinational Circuits)
are executed in the order they are coded, hence they are sequential. Since
they block the execution of next statement, till the current statement is
executed, they are called blocking assignments. Assignment are made with
"=" symbol. Example a = b;
 Example :

reg x, y, z;
reg [15:0] reg_a, reg_b;
integer count;
initial begin
x = 0; y = 1; z = 1;
reg_a = 16'b0; reg_b = reg_a;
#15 reg_a[2] = 1'b1;
#10 reg_b[15:13] = {x, y, z};
count = count + 1;
end

Nonblocking assignments(Modeling Sequential Circuits)

are executed in parallel i.e (at the same time e.g all registers and flip-flops must
update their outputs at the same moment). Since the execution of next statement
is not blocked due to execution of current statement, they are called nonblocking
statement. Assignments are made with "<=" symbol. Example a <= b;
 To generate Concurrent Statements.
 Example :

reg x, y, z;
reg [15:0] reg_a, reg_b;
integer count;
initial begin
x = 0; y = 1; z = 1;
reg_a = 16'b0; reg_b = reg_a; 11
#15 reg_a[2] <= 1'b1;
#10 reg_b[15:13] <= {x, y, z};
count <= count + 1;
end

initial begin
a = #1 1; // assignment at time 1
b = #3 0; // assignment at time 4 (3+1)
c = #6 1; // assignment at time 10 (6+3+1)
End
initial begin
#1 a <= 1; // assignment at time 1
#3 b <= 0; // assignment at time 3
#6 c <= 1; // assignment at time 6
end

 Question: What the difference between following codes?

a = 1;
b = a;
c=b;

a<= 1;
b <= a;
c <= b

 Why we should use Non blocking assignment for sequential

instead of blocking?
 To Overcome Race condition, The following example will
clarify why?
reg R1, R2, R3, R4;
R2 = R1;
R2 <= R1;
R3 = R2;
R3 <= R2;
R4 = R3;
R4 <= R3;
These run in some order,
but you don’t know which.

RHS evaluated when assignment runs
LHS updated only after all events
for the current instant have run.

 Timing Controls and Delays
 Timing controls provide a way to specify the simulation time at

which procedural statements will execute.

 If there are no timing control statements, the simulation time

does not advance, and the simulator can’t track signals in the
design.

 There are three methods of timing control: delay-based timing

control, event-based timing control, and level-sensitive timing
control.

 Delay-Based Timing Control
 Delay-based timing control in an expression specifies the time

duration between when the statement is encountered and when it
is executed.
 Delay-based timing control can be specified by a number,

identifier,
or a (minimum, typical, maximum) expression.
 There are three types of delay control for procedural

assignments: regular delay control, intra-assignment delay
control, and zero delay control.

 Regular delay control: used when a non-zero delay is specified to

the left of a procedural assignment .
initial begin
x = 0; // no delay control
#10 y = 1; // delay control with a number. Delay execution of // y = 1 by
10 units
#latency z = 0; // Delay control with identifier. Delay of 20 units
#(latency + delta) p = 1; // Delay control with expression
#y x = x + 1; // Delay control with identifier. Take value of y.
#(4:5:6) q = 0; // Minimum, typical and maximum delay values.
//Discussed in gate-level modeling chapter.
end

 Intra-assignment delay control: Instead of specifying delay control to

the left of the assignment, it is possible to assign a delay to the right of
the assignment operator.

initial begin
x = 0; z = 0;
y = #5 x + z; //Take value of x and z at the time=0, evaluate
//x + z and then wait 5 time units to assign value to y.
end
//Equivalent method with temporary variables and regular delay control
initial begin
x = 0; z = 0;
temp_xz = x + z;
#5 y = temp_xz; //Take value of x + z at the current time and
//store it in a temporary variable. Even though x and z
//might change between 0 and 5, //the value assigned to y at time 5 is unaffected.
end

end

 Zero delay control: Procedural statements in different always-initial

blocks may be evaluated at the same simulation time. The order of
execution of these statements in different always-initial blocks is
nondeterministic. Zero delay control is a method to ensure that a
statement is executed last, after all other statements in that simulation
time are executed. This is used to eliminate race conditions. However,
if there are multiple zero delay statements, the order between them is
nondeterministic.
initial begin

initial begin

x = 0;

#0 x = 1;

y = 0;

#0 y = 1;

end

end

 Event-Based Timing Control
 An event is the change in the value on a register or a net. Events can be utilized to trigger execution

of a statement or a block of statements. There are four types of event-based timing control: regular
event control, named event control, event OR control, and level-sensitive timing control.
 Regular event control:The @ symbol is used to specify an event control. Statements can be executed

on changes in signal value or at a positive or negative transition of the signal value.
@(clock) q = d;
//q = d is executed whenever signal clock changes value
@(posedge clock) q = d; //q = d is executed whenever signal clock does , a positive transition ( 0 to 1,x or z, x to 1, z to 1 )
@(negedge clock) q = d; //q = d is executed whenever signal clock does , a negative transition ( 1 to 0,x or z, x to 0, z to 0)
q = @(posedge clock) d; //d is evaluated immediately and assigned //to q at the positive edge of clock.

 Regular event control: Verilog provides the capability to declare an event and then trigger and

recognize the occurrence of that event .
event received_data;
//Define an event called received_data
always @(posedge clock) //check at each positive clock edge
begin
if(last_data_packet) //If this is the last data packet
->received_data; //trigger the event received_data
end
always @(received_data) //Await triggering of event received_data, When event is triggered, store all four , packets of received data
data_buf = {data_pkt[0], data_pkt[1], data_pkt[2], data_pkt[3]};

 Event OR Control (Sensitivity List) : Sometimes a transition on

any one of multiple signals //A level-sensitive latch with asynchronous
reset .
always @( reset or clock or d) //Wait for reset or clock or d to change
begin
if (reset) //if reset signal is high, set q to 0. q = 1'b0; else if(clock) //if clock is high, latch
input q = d;

end

 When the number of input variables to a combination logic block are very large,

sensitivity lists can become very cumbersome to write. Moreover, if an input
variable is missed from the sensitivity list, the block will not behave like a
combinational logic block. To solve this problem, Verilog HDL contains two special
symbols: @* and @(*).

//Combination logic block using the or operator
//Cumbersome to write and it is easy to miss one input to the block
always @(a or b or c or d or e or f or g or h or p or m)
begin
out1 = a ? b+c : d+e;
out2 = f ? g+h : p+m;
end
//Instead of the above method, use @(*) symbol //Alternately, the @* symbol can be used
//All input variables are automatically included in the //sensitivity list.
always @(*)
begin
out1 = a ? b+c : d+e;
out2 = f ? g+h : p+m;
end

 Level-Sensitive Timing Control : Event control discussed earlier waited for the

change of a signal value or the triggering of an event (Not Synthesizable).
always wait (count_enable) #20 count = count + 1;
In the above example, the value of count_enable is monitored continuously. If
count_enable is 0, the statement is not entered. If it is logical 1, the statement count
= count + 1 is executed after 20 time units. If count_enable stays at 1, count will be
incremented every 20 time units.
event received_event;
always wait (received_event) #20 count = count + 1;

 The Conditional Statement if-else
(appears only in a procedural block)
 The if - else statement controls the execution of other statements. In

programming language like c, if - else controls the flow of program. When more
than one statement needs to be executed for an if condition, then we need to use
begin and end as seen in earlier examples.
 Example :
 Modeling Multiplexer 4-to-1.
 What about Question 1 Now?
 Don’t write if statement for Combinational Circuit without

declaring else statement if yes then u will have latch in your design.

 Q : Design a 4-bit Counter with Asynchronous Reset ?

 Q : Add Feature to the counter to count up or down ?
 Q : Make it parameterized?
 Take the feature of parameterized designs to be easily reused.

 The Case Statement
(appears only in a procedural block)
 The case statement compares an expression to a series of cases and executes the

statement or statement group associated with the first matching case:
 case statement supports single or multiple statements. Group multiple statements
using begin and end keywords.
 Syntax of a case statement look as shown below from the following example :
module mux_without_default (I0,I1,I2,I3,Sel,F);
input I0, I1, I2, I3;
input [1:0] Sel; //Sel0, Sel1
output F;
reg F;
always @ (I0 or I1 or I2 or I3 or Sel) begin
case (Sel)
0 : F = I0;
1 : F = I1;
2 : F = I2;
3 : F = I3;
endcase
endmodule

 Don’t write Case statement for Combinational Circuit without

declaring default statement else u will have latch in your design.

Q : Design an 8-function ALU that takes 4-bit inputs a and b
and a 3-bit input signal select, and gives a 5-bit output
out. The ALU implements the following functions based
on a 3-bit input signal select. Inputs A, B and out should be
received in registers and final output also . (Parameterize it)
Select Signal

Function

3'b000

out = a

3'b001

out = a + b

3'b010

out = a – b

3'b011

out = a *b

3'b100

out = b

3'b101

out = a << 1

3'b101

out = a << 1

3'b110

out = a >> 1

3'b111

out = (a > b) (magnitude compare)

 Q : Design a four-input (D0, D1, D2, D3) priority

encoder with input D3 having the highest priority and
input D0 the lowest priority

 Looping Statements(appears only in a procedural block)
 Looping statements appear inside procedural blocks only; Verilog has four looping

statements like any other programming language.
 forever (The forever loop executes continually, the loop never ends. Normally we use forever statements in
initial blocks.)
reg clk;
initial begin
#1 clk = 0;
forever begin #5 clk = ! clk;
end
end

 Repeat : repeat (< number >) < statement >
 While : while (< expression >) < statement >
 For

: for (< initial assignment >; < expression >, < step assignment >) < statement >
E.g : array of registers initialization or assignements .

 Most of time are Not Synthesizable.
 Most commonly used in Test benches.

FSM Using Verilog
 State machines or FSM are the heart of any digital

design; of course a counter is a simple form of FSM.
Controller of any processor is about a group of state
machines.
 There are two types of state machines as classified by the

types of outputs generated from each. The first is the
Moore State Machine where the outputs are only a
function of the present state, the second is the Mealy
State Machine where one or more of the outputs are a
function of the present state and one or more of the
inputs.

FSM Using Verilog

 FSM code should have three sections:
 Encoding Style .

 Combinational Part (Next State Logic and output Logic).
 Sequential Part (State Registers).
 Take Q2 as an example for how to model FSM using Verilog .

FSM Using Verilog
 Encoding Style:
 Binary Encoding.
parameter [1:0] S0 = 2’b00;
 One hot Encoding.
parameter [3:0] S0 = 4’b0001;
parameter [3:0] S1 = 4’b0010;
parameter [3:0] S2 = 4’b0100;
parameter [3:0] S3 = 4’b1000;
 Gray Encoding (Only 1-bit should change from previous value)
parameter [2:0] S0 = 3’b000;
parameter [2:0] S1 = 3’b001;
parameter [2:0] S2 = 3’b011;
parameter [2:0] S3 = 3’b010;

FSM Using Verilog
 Combinational Part (Calculate next state):
always @ (*) begin
next_state = 0;
case(state)
S0 : begin
…
end
S1 : begin
…
end
S2 : begin
…
end
S3 : begin
…
end
default : next_state = S0; //Must have a default state not to refer latches
endcase
end

FSM Using Verilog
 Combinational Part (Calculate next state):
always @ (*) begin
next_state = 0; //default value will be overwritten
case(state)
S0 : begin
…
end
S1 : begin
…
end
S2 : begin
…
end
S3 : begin
…
end
default : next_state = S0; //Must have a default state not to refer latches
endcase
end

FSM Using Verilog
 Sequential Part (State Register and Output):

always @ (posedge Clock,negedge Reset) begin
if (!Reset) begin
current_state <= S0;
output_reg <= 0;
end
else begin
current_state <= next_state;
output_reg <= fn (current_state);
//Moore output
output_reg <= fn (current_state,inputs); //Mealy output
end
end

Memories
 Memory is a collection of storage cells together with associated circuits needed
to transfer information into and out of these cells.
 Memory Interfaces for Accessing Data
 Asynchronous (un-clocked):
A change in the address results in data appearing.
 Synchronous (clocked):
A change in address, followed by an edge on CLK results in data appearing or
write operation occurring.
A common arrangement is to have synchronous write operations and asynchronous
read operations.
 Volatile:
Looses its state when the power goes off.
 Nonvolatile:
Retains it state when power goes off.

Memories
 Modeling Memory using verilog :

Parameter WIDTH = 10;
Parameter DEPTH = 1024
reg [WIDTH -1:0] my_memory [0: DEPTH-1];
 Storing Values
my_memory[address] = data_in;
 Reading Values
data_out = my_memory[address] ;
 Bit Read
data_out_0 = data_out[0];
 Initialization Memories :
we can use system task (command for compiler) $readmemb
and $readmemh. $readmemb is used for binary representation
of memory content and $readmemh for hex representation.
e.g : $readmemh("memory.list", my_memory);

Modeling of Single Port RAM using Verilog
module (clock, data_in, address, We, data_out);
Parameter WIDTH
= 10;
Parameter Address_WIDTH = 10;
Parameter DEPTH
= 1024;
input clock,We;
input [WIDTH -1:0] data_in;
input [Address_WIDTH -1:0] address;
output [WIDTH -1:0] data_out;
reg [WIDTH -1:0] my_memory [0: DEPTH-1];
always @ (posedge clock)
begin
if(We == 1’b1)
begin
my_memory[address] <= data_in;
end
else
begin
data_out <= my_memory[address];
end
end
endmodule

We

address

data_in
clock

Single Port RAM

data_out

Modeling of Synchronous ROM using Verilog

CS

RE

address clock

ROM

data_out

Modeling of Synchronous FIFO using Verilog

Clock
Reset
Rd_en
Wr_en
data_in

FIFO

Full Empty

data_out

Block Interface
 Every block interface will be based on handshaking communication protocol.
 For input and output we will have a strobe signal .
 A strobe is a signal that is sent that validates data or other signals on adjacent parallel

lines. In memory technology, the CAS (column address strobe) and RAS ( row address
strobe ) signals are used to tell a dynamic RAM that an address is a column or row
address. To get new input we should have load/valid signal that give me an
announcement that we have a new input.
 We will have strobe_in to catch inputs and strobe_out to validate block outputs.

Strop_in
Strop_out
Input_real

Block ?

output_real

Input_imag
output_imag

Other inputs e.g design
parameter (FFT size)

…….

 Q : Design Averaging Unit. Specification :
1- design should accept 16 successive numbers and accumulate them in
an accumulator register and then calculate the average.
2-We receive this input vector (16 numbers) in 16 clock cycles.
3- After we calculate average of a vector we can receive another one.
4- Before starting in the design describe the block interface.

 Q : Design a Bubble Algorithm , Specification :

1- Memory is needed here….Why????

 Q : Design a traffic signal controller, using a finite state machine

approach. Specification :
 Consider a controller for traffic at the intersection of a main

highway and a country road.
 The traffic signal for the main highway gets highest priority
because cars are continuously present on the main highway. Thus,
the main highway signal remains green by default.
 Occasionally, cars from the country road arrive at the traffic signal.
The traffic signal for the country road must turn green only long
enough to let the cars on the country road go.
 As soon as there are no cars on the country road, the country road
traffic signal turns yellow and then red and the traffic signal on the
main highway turns green again.
 There is a sensor to detect cars waiting on the country road. The
sensor sends a signal X as input to the controller. X = 1 if there
are cars on the country road; otherwise, X= 0.
 There are delays on transitions from S1 to S2, from S2 to S3, and
from S4 to S0. The delays must be controllable.

Verilog

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