presentation on verilog language with examples

1. A Presentation on
VLSI Technology
BY:UTKARSH KULSHRESTHA

2. CONTENTS
 Introduction
to VLSI
 Hardware description
language
 Verilog coding
 Vending machine
2

3. VLSI

VLSI is very large scale integration
technology.

It is related to the chip designing.

Chip designing is comprises of two areas:1. FRONT END
2. BACK END

VLSI uses CMOS technology
3

4. VLSI
BJT-PNP
BJT-NPN
MOSFET
PMOS
NMOS
CMOS
4

5. Hardware Description
Languages

Basic idea is a programming language to describe
hardware

Synthesis tools allow direct implementation from HDL
code.

Combined with modern Field Programmable Gate Array
chips large complex circuits (100000s of gates) can be
implemented.

There are many HDL’s as:1. ABEL
2. AHDL
3. VHDL
4. Verilog HDL
5. System Verilog
5

6. Verilog HDL
 Launched
in 1984 by CADENCE.
 Verilog
is Unlike VHDL is a case
sensitive language and uses
lower case.
 It
is a CONCURRENT language.
 COMPLEXITY
 Only
of Verilog is low.
used for DIGITAL DESIGNS.
6

7. Module declaration
Input
X
Module
Circuit
Y
Wire
Output
Z
O
Module name
module sample (X,Y,Z,O);
input X,Y,Z;
output O;
// Describe the circuit using logic symbols
assign O = (X^Y)&Z;
endmodule

8. Data Types
Nets and Registers
 Vectors
 Integer, Real, and Time Register Data
Types
 Arrays
 Memories
 Parameters
 Strings

Verilog HDL

9. Operators

Arithmetic:
reg [3:0] a, b, c, d;
wire[7:0] x,y,z;
parameter n =4;
*,+,-, /,%

Relational
<,<=,>,>=,==, !=

Bit-wise Operators
•
•
•
•
•

Not: ~
XOR: ^
And : &
5’b11001 & 5’b01101 ==> 5’b01001
OR: |
XNOR: ~^ or ^~
Logical Operators
Returns 1or 0, treats all nonzero as 1
! : Not
• && : AND
• || : OR
•
27 && -3 ==> 1
c = a + b;
d = a *n;
If(x==y) d = 1; else d =0;
d = a ~^ b;
if ((x>=y) && (z)) a=1;
else a = !x;
9

10. Operators

Reduction Operators:
Unary operations returns single-bit values
•
& : and
•
| :or
•
~& : nand
•
~| : nor
•
^ : xor
•
~^ :xnor

Shift Operators
Shift Left: <<
Shift right: >>

Concatenation Operator
module sample (a, b, c, d);
input [2:0] a, b;
output [2;0] c, d;
wire z,y;
assign z = ~| a;
c = a * b;
If(a==b) d = 1; else d =0;
d = a ~^ b;
if ((a>=b) && (z)) y=1;
else y = !x;
{ } (concatenation)
{ n{item} } (n fold replication of an item)
Conditional Operator
Implements if-then-else statement

(cond) ? (result if cond true) : (result if cond false)
assign d << 2; //shift left twice
assign {carry, d} = a + b;
assign c = {2{carry},2{1’b0}};
// c = {carry,carry,0,0}
assign c= (inc==2)? a+1:a-1;
10

11. Verilog Coding Styles
Gate
level modelling
Dataflow modelling
Behavioral modelling
Structural modelling

12. Gate level modelling




The gate level structure of digital design is required.
It requires the number of gates in the design.
It also requires the way the gates are connected.
The keywords defined for gates are as:AND
XOR
OR
XNOR
NOT
BUF
NAND
BUFIF1
NOR
BUFIF0
12

13. Example:Gate level modelling
Module gatelevel(A,B,C,x,y);
Input A,B.C;
Output x,y;
Wire e;
And(e,A,B);
Not(y,C);
Or(x,e,y);
Endmodule;
13

14. Dataflow modelling

The flow of data should be known from one point to
another.

It works on the Boolean expressions.

ASSIGN keyword is used to drive the values.
assign y=a&b;

Usually Boolean operators are used for dataflow
modelling and only the output equations are necessary
for designing.
14

15. Example:Dataflow modelling
4-bit Adder with instanciation

Step 1: build a 1-bit full adder as a module

S = (a) XOR (b) XOR (Cin ) ; ( S = a^b^Cin)

Cout = (a&b) |(Cin&(a+b))
module FA_1bit (S,Cout,a,b,Cin);
begin
input a,b,Cin;
Output S, Cout;
assign Sum = a^b^Cin;
assign Carry = (a&b) | (Cin&(a^b));
Module add_1bit
endmodule
15

16. 4-bit Adder
Step 2: initiate 4 instances of FA_1bit module

Cout
1-bit
Full
Adder
Cout2
1-bit
Full
Adder
S3
S2
module FA_4bits (S,Cout,A,B,Cin);
input [3:0] A, B;
input
Cin;
output [3:0]
S;
output
Cout
wire
Cout0, Cout1, Cout2
FA_1bit
FA1(S[0], Cout0,A[0],B[0],Cin);
FA_1bit
FA1(S[1], Cout1,A[1],B[1],Cout0);
FA_1bit
FA1(S[2], Cout2,A[2],B[2],Cout1);
FA_1bit
FA1(S[3], Cout,A[3],B[3],Cout2);
end
endmodule;
B0 A0
B1 A1
B2 A2
B3 A3
Cout1
1-bit
Full
Adder
S1
Cout0
1-bit
Full
Adder
Cin
S0
The inputs and the output
are 4-bits wide
we need wires to
propagate the carry from
one stage to the next
you may name the
instances with any name,
but you have to maintain
the order 16 the inputs and
of
outputs

17. Behavioral modelling

The behavior of the circuit should be known in the
terms of its inputs and outputs.

This modelling is semi-concurrent in nature.

It uses conditional statements just like in C as “if-else”
and “case” statements.

Begin….end statements are used as for sequential
execution.
17

18. Example:D Flip-flop with
Synchronous reset and Enable
always@(posedge clk)
begin
if (rst) a<=0;
else if (enable) a<=b;
end
18

19. D Flip-Flop with Asynchronous
Reset
always@(posedge clk or negedge rst)
begin
if (!rst) a<=0;
else a<=b;
end
19

20. Structural coding

It is basically not a coding style.
 It
is used to connect behavioral designs
components to design more complex
circuits.
 The
components of huge circuitry are
designed separately.
20

21. Vending machine

It is based on FSM.

FSM is the heart of any
digital design.

According to the input
state transition occurs.

The machine has four
states like
coke,mango,orange and
lime with an initial
state.
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22. Vending machine
Insert
coin
Mango
State 1
coke
State 2
lime
State 3
orange
State 4
Vending
machine
Initial
State
Enter
product
22

23. Standard Form for a
Verilog FSM
// state flip-flops
reg [2:0] state, nxt_st;
// state definitions
parameter
reset=0,S1=1,S2=2,S3=3,..
// NEXT STATE CALCULATIONS
always@(state or inputs or ...)
begin
…
next_state= ...
…
end
// REGISTER DEFINITION
always@(posedge clk)
begin
state<=next_state;
end
// OUTPUT CALCULATIONS
output= f(state, inputs)
23

24. Simulation for vending
machine
24

25. THANK YOU
25