Verilogspk1

Supriya P Kurlekar
Department of Electronics and Telecommunication
Engg.
(Sharad Institute of technology college of Engg.)

Verilog Background
 Developed by Gateway Design Automation (1980)
 Later acquired by Cadence Design(1989) who made it
public in 1990
 Became a standardized in 1995 by IEEE (Std 1364)
regulated by Open Verilog International (OVI)

VHDL Background
 VHSIC Hardware Description Language.
 VHSIC is an abbreviation for Very High Speed
Integrated Circuit.
 Developed by the department of defense (1981)
 In 1986 rights where given to IEEE
 Became a standard and published in 1987
 Revised standard we know now published in 1993
(VHDL 1076-1993) regulated by VHDL international (VI)

What is verilog?
 Verilog is a HDL- hardware description language to
design the digital system.
 VHDL is other hardware description language.
 Virtually every chip (FPGA, ASIC, etc.) is designed in
part using one of these two languages
 Verilog was introduced in 1985 by Gateway Design
System Corporation

verilog
 IEEE 1364-2001 is the latest Verilog HDL standard
 Verilog is case sensitive (Keywords are in lowercase)
 The Verilog is both a behavioral and a structure
language

Difference between VERILOG and VHDL
 Verilog is similar to c- language.
 VHDL is similar to Ada- (ada is a structured, statically
typed, imperative, wide-spectrum, and object-oriented
high-level computer programming language, extended
from Pascal )
 Many feel that Verilog is easier to learn and use than
VHDL.

7
Lexical Conventions
 Very similar to C
 Verilog is case-sensitive
 All keywords are in lowercase
 A Verilog program is a string of tokens
 Whitespace
 Comments
 Delimiters
 Numbers
 Strings
 Identifiers
 Keywords

8
Lexical Conventions (cont’d)
 Whitespace
 Blank space (b)
 Tab (t)
 Newline (n)
 Whitespace is ignored in
Verilog except
 In strings
 When separating tokens
 Comments
 Used for readability and
documentation
 Just like C:
 // single line comment
 /* multi-line
comment
*/
/* Nested comments
/* like this */ may not
be acceptable (depends
on Verilog compiler) */

Lexical Conventions (cont’d) Operators
 Unary
a = ~b;
 Binary
a = b && c;
 Ternary
a = b ? c : d; // the only ternary operator

 Number Specification
 Sized numbers
 Unsized numbers
 Unknown and high-impedance values
 Negative numbers

2 Verilog HDL 11
 Sized numbers
 General syntax:
<size>’<base><number>
 <size> number of bits (in
decimal)
 <number> is the number in
radix <base>
 <base> :
 d or D for decimal (radix 10)
 b or B for binary (radix 2)
 o or O for octal (radix 8)
 h or H for hexadecimal (radix 16)
 Examples:
 4’b1111
 12’habc
 16’d255
 Unsized numbers
 Default base is decimal
 Default size is at least 32
(depends on Verilog compiler)
 Examples
 23232
 ’habc
 ’o234

Verilog HDL 12
Lexical Conventions (cont’d) X or Z values
 Unknown value: lowercase x
 4 bits in hex, 3 bits in octal, 1 bit in binary
 High-impedance value: lowercase z
 4 bits in hex, 3 bits in octal, 1 bit in binary
 Examples
 12’h13x
 6’hx
 32’bz
 Extending the most-significant part
 Applied when <size> is bigger than the specified value
 Filled with x if the specified MSB is x
 Filled with z if the specified MSB is z
 Zero-extended otherwise
 Examples:
 6’hx

Verilog HDL 13
 Negative numbers
 Put the sign before the <size>
 Examples:
 -6’d3
 4’d-2 // illegal
 Two’s complement is used to store the value
 Underscore character and question marks
 Use ‘_’ to improve readability
 12’b1111_0000_1010
 Not allowed as the first character
 ‘?’ is the same as ‘z’ (only regarding numbers)
 4’b10?? // the same as 4’b10zz

Verilog HDL 14
 Strings
 As in C, use double-quotes
 Examples:
 “Hello world!”
 “a / b”
 “texttcolumn1bcolumn2n”
 Identifiers and keywords
 identifiers: alphanumeric characters, ‘_’, and ‘$’
 Should start with an alphabetic character or ‘_’
 Only system tasks can start with ‘$’
 Keywords: identifiers reserved by Verilog
 Examples:
 reg value;
 input clk;

Verilog HDL 15
Lexical Conventions (cont’d) Escaped identifiers
 Start with ‘’
 End with whitespace (space, tab, newline)
 Can have any printable character between start and end
 The ‘’ and whitespace are not part of the identifier
 Examples:
 a+b-c // a+b-c is the identifier
 **my_name** // **my_name** is the identifier
 Used as name of modules

Verilog HDL 17
Data Types
 Value set and strengths
 Nets and Registers
 Vectors
 Integer, Real, and Time Register Data Types
 Arrays
 Memories
 Parameters
 Strings

Verilog HDL 18
Value Set
 Verilog concepts to model hardware circuits
 Value level
 Value strength
 Used to accurately model
 Signal contention
 MOS devices
 Dynamic MOS
 Other low-level details

Verilog HDL 19
Value level HW Condition
0 Logic zero, false
1 Logic one, true
x Unknown
z High imp., floating
Strength level Type
supply Driving
strong Driving
pull Driving
large Storage
weak Driving
medium Storage
small Storage
highz High
Impedance

Elements of verilog-logic values
 0: zero, logic low, false, ground
 1: one, logic high, power
 X: unknown
 Z: high impedance, unconnected, tri-state

Elements of verilog- data type
 Nets
 Nets are physical connections between devices
 Many types of nets, but all we care about is wire.
 Declaring a net
wire [<range>] <net_name> ;
Range is specified as [MSb:LSb]. Default is one bit wide
 Registers
 Implicit storage-holds its value until a new value is assigned to it.
 Register type is denoted by reg.
 Declaring a register
reg [<range>] <reg_name>;
 Parameters are not variables, they are constants.

Verilog Primitives
 Basic logic gates only
 and
 or
 not
 buf
 xor
 nand
 nor
 xnor
 bufif1, bufif0
 notif1, notif0

Numbers in Verilog
<size>’<radix> <value>
 8’h ax = 1010xxxx
 12’o 3zx7 = 011zzzxxx111
No of
bits
Binary  b or B
Octal  o or O
Decimal  d or D
Hexadecimal  h or H
Consecutive chars
0-f, x, z

Logical Operators
 &&  logical AND
 ||  logical OR
 !  logical NOT
 Operands evaluated to ONE bit value: 0, 1 or x
 Result is ONE bit value: 0, 1 or x
A = 1; A && B  1 && 0  0
B = 0; A || !B  1 || 1  1
C = x; C || B  x || 0  x
but C&&B=0

Bitwise Operators (i)
 &  bitwise AND
 |  bitwise OR
 ~  bitwise NOT
 ^  bitwise XOR
 ~^ or ^~  bitwise XNOR
 Operation on bit by bit basis

Bitwise Operators (ii)c = ~a; c = a & b;
 a = 4’b1010;
b = 4’b1100;
 a = 4’b1010;
b = 2’b11;
c = a ^ b;

 >>  shift right
 <<  shift left
 a = 4’b1010;
d = a >> 2;// d = 0010,c = a << 1;// c =
0100
 cond_expr ? true_expr : false_expr
A
B
Y
sel
Y = (sel)? A : B;
0
1

keywords
 Note : All keywords are defined in lower case
 Examples :
 module, endmodule
 input, output, inout
 reg, integer, real, time
 not, and, nand, or, nor, xor
 parameter
 begin, end
 fork, join
 specify, endspecify

keywords
 module – fundamental building block for Verilog
designs
• Used to construct design hierarchy
• Cannot be nested
 endmodule – ends a module – not a statement
=> no “;”
 Module Declaration
• module module_name (module_port, module_port,
…);
• Example: module full_adder (A, B, c_in,
c_out, S);

Verilog keywords
 Input Declaration
• Scalar
 input list of input identifiers;
 Example: input A, B, c_in;
• Vector
 input[range] list of input identifiers;
 Example: input[15:0] A, B, data;
 Output Declaration
• Scalar Example: output c_out, OV, MINUS;
• Vector Example: output[7:0] ACC, REG_IN,
data_out;

Types verilog coding
 Behavioral
 Procedural code, similar to C programming
 Little structural detail (except module interconnect)
 Dataflow
 Specifies transfer of data between registers
 Some structural information is available (RTL)
 Sometimes similar to behavior
 Structural (gate,switch)
 Interconnection of simple components
 Purely structural

Hierarchical Design
Top Level
Module
Sub-Module
1
Sub-Module
2
Basic Module
3
Basic Module
2
Basic Module
1
Full Adder
Half Adder Half Adder
E.g.

Module
f
in1
in2
inN
out1
out2
outM
my_module
module my_module(out1, .., inN);
output out1, .., outM;
input in1, .., inN;
.. // declarations
.. // description of f (maybe
.. // sequential)
endmodule
Everything you write in Verilog must be inside a module
exception: compiler directives

VHDL coding for AND gate
 --The IEEE standard 1164 package, declares std_logic, etc.
 library IEEE;
 use IEEE.std_logic_1164.all;
 use IEEE.std_logic_arith.all;
 use IEEE.std_logic_unsigned.all;
 ---------------------------------- Entity Declarations -------------------------
 entity andgate is
 Port( A : in std_logic;
 B : in std_logic;
 Y : out std_logic
 );
 end andgate;
 architecture Behavioral of andgate is
 begin
 Y<= A and B ;
 end Behavioral;

VERILOG coding for all gate
 module gates(a, b, y1, y2, y3, y4, y5, y6, y7);
 input [3:0] a, b;
 output [3:0] y1, y2, y3, y4, y5, y6, y7;
 /* Seven different logic gates acting on four bit busses */
 assign y1= ~a; // NOT gate
 assign y2= a & b; // AND gate
 assign y3= a | b; // OR gate
 assign y4= ~(a & b); // NAND gate
 assign y5= ~(a | b); // NOR gate
 assign y6= a ^ b; // XOR gate
 assign y7= ~(a ^ b); // XNOR gate
 endmodule

Example: Half Adder
module half_adder(S, C, A, B);
output S, C;
input A, B;
wire S, C, A, B;
assign S = A ^ B;
assign C = A & B;
endmodule
Half
Adder
A
B
S
C
A
B
S
C

Example: Full Adder
module full_adder(sum, cout, in1, in2, cin);
output sum, cout;
input in1, in2, cin;
wire sum, cout, in1, in2, cin;
wire I1, I2, I3;
half_adder ha1(I1, I2, in1, in2);
half_adder ha2(sum, I3, I1, cin);
assign cout = I2 || I3;
endmodule
Instance
name
Module
name
Half
Adder
ha2
A
B
S
C
Half
Adder 1
ha1
A
B
S
C
in1
in2
cin
cout
sumI1
I2 I3

Example 4-bit adder
module add4 (s,c3,ci,a,b)
input [3:0] a,b ; // port declarations
input ci ;
output [3:0] s : // vector
output c3 ;
wire [2:0] co ;
add a0 (co[0], s[0], a[0], b[0], ci) ;
add a1 (co[1], s[1], a[1], b[1], co[0]) ;
add a2 (co[2], s[2], a[2], b[2], co[1]) ;
add a3 (c3, s[3], a[3], b[3], co[2]) ;
endmodule
a0a1a2a3c3 ci
Simpler than VHDL
Only Syntactical
Difference

Assignments
 Continuous assignments assign values to nets (vector and
scalar)
 They are triggered whenever simulation causes the value
of the right-hand side to change
 Keyword “assign”
e.g. assign out = in1 & in2;
 Procedural assignments drive values onto registers (vector
and scalar)
 They Occur within procedures such as always and initial
 They are triggered when the flow of execution reaches them (like in
C)
 Blocking and Non-Blocking procedural assignments

Assignments (cont.)
 Procedural Assignments
 Blocking assignment statement (= operator) acts
much like in traditional programming languages. The
whole statement is done before control passes on to the
next statement
 Nonblocking assignment statement (<= operator)
evaluates all the right-hand sides for the current time
unit and assigns the left-hand sides at the end of the
time unit

 Delay Control (#)
 Expression specifies the time duration between initially
encountering the statement and when the statement
actually executes.
 Delay in Procedural Assignments
 Inter-Statement Delay
 Intra-Statement Delay
 For example:
 Inter-Statement Delay
#10 A = A + 1;
 Intra-Statement Delay
A = #10 A + 1;
Delay based Timing Control

Example: Half Adder,
2nd Implementation
Assuming:
• XOR: 2 t.u. delay
• AND: 1 t.u. delay
module half_adder(S, C, A, B);
output S, C;
input A, B;
wire S, C, A, B;
xor #2 (S, A, B);
and #1 (C, A, B);
endmodule
A
B
S
C

Combinational Circuit Design Outputs are functions of inputs
 Examples
 MUX
 decoder
 priority encoder
 adder
comb.
circuitsinputs Outputs

Procedural Statements: if
if (expr1)
true_stmt1;
else if (expr2)
true_stmt2;
..
else
def_stmt;
E.g. 4-to-1 mux:
module mux4_1(out, in, sel);
output out;
input [3:0] in;
input [1:0] sel;
reg out;
wire [3:0] in;
wire [1:0] sel;
always @(in or sel)
if (sel == 0)
out = in[0];
else if (sel == 1)
out = in[1];
else if (sel == 2)
out = in[2];
else
out = in[3];
endmodule

Procedural Statements: case
case (expr)
item_1, .., item_n: stmt1;
item_n+1, .., item_m: stmt2;
..
default: def_stmt;
endcase
E.g. 4-to-1 mux:
module mux4_1(out, in, sel);
output out;
input [3:0] in;
input [1:0] sel;
reg out;
wire [3:0] in;
wire [1:0] sel;
always @(in or sel)
case (sel)
0: out = in[0];
1: out = in[1];
2: out = in[2];
3: out = in[3];
endcase
endmodule

Decoder
 3-to 8 decoder with an
enable control
module decoder(o,enb_,sel) ;
output [7:0] o ;
input enb_ ;
input [2:0] sel ;
reg [7:0] o ;
always @ (enb_ or sel)
if(enb_)
o = 8'b1111_1111 ;
else
case(sel)
3'b000 : o = 8'b1111_1110 ;
3'b001 : o = 8'b1111_1101 ;
3'b010 : o = 8'b1111_1011 ;
3'b011 : o = 8'b1111_0111 ;
3'b100 : o = 8'b1110_1111 ;
3'b101 : o = 8'b1101_1111 ;
3'b110 : o = 8'b1011_1111 ;
3'b111 : o = 8'b0111_1111 ;
default : o = 8'bx ;
endcase
endmodule

– a feedback path
– the state of the sequential circuits
– the state transition
 synchronous circuits
 asynchronous circuits
Examples-D latch
D flip-flop
register
Memory
elements
Combinational
circuit
Inputs Outputs

d-Latch,flip-flop
 module latch (G, D, Q);
 input G, D;
 output Q;
 reg Q;
 always @(G or D)
 begin
 if (G)
 Q <= D;
 end
 endmodule
module dff(Q, D, Clk);
output Q;
input D, Clk;
reg Q;
wire D, Clk;
always @(posedge Clk)
Q = D;
endmodule

Jk flip-flopmodule jkff(J, K, clk, Q);
input J, K, clk;
output Q;
reg Q;
reg Qm;
always @(posedge clk)
if(J == 1 && K == 0)
Qm <= 1;
else if(J == 0 && K == 1)
Qm <= 0;
else if(J == 1 && K == 1)
Qm <= ~Qm;
assign Q <= Qm;
endmodule

A counter which runs through counts 0, 1, 2, 4, 9, 10, 5, 6, 8, 7, 0, …
 Sequence counter
 module CntSeq(clk, reset, state);
 parameter n = 4;
 input clk, reset;
 output [n-1:0]state;
 reg1:0]state;
 [n- integer k;
 //
 always @(posedge clk)
 if(reset)
 state = 0;
 else begin
 case (state)
 4'b0000:state = 4'b0001; //0 -> 1
 4'b0001:state = 4'b0010; //1 -> 2
 4'b0010:state = 4'b0100; //2 -> 4
 4'b0100:state = 4'b1001; //4 -> 9
 4'b1001:state = 4'b1010; //9 -> 10
 4'b1010:state = 4'b0101; //10-> 5
 4'b0101:state = 4'b0110; //5 -> 6
 4'b0110:state = 4'b1000; //6 -> 8
 4'b1000:state = 4'b0111; //8 -> 7
 default:state = 4'b0000;
 endcase
 end
 endmodule

//Gate-level hierarchical description of 4-bit adder
module halfadder (S,C,x,y);
input x,y;
output S,C;
//Instantiate primitive gates
xor (S,x,y);
and (C,x,y);
endmodule
module fulladder (S,C,x,y,z);
input x,y,z;
output S,C;
wire S1,D1,D2; //Outputs of first XOR and two AND gates
//Instantiate the half adders
halfadder HA1(S1,D1,x,y), HA2(S,D2,S1,z);
or g1(C,D2,D1);
endmodule
4-bit Full Adder

module _4bit_adder (S,C4,A,B,C0);
input [3:0] A,B;
input C0;
output [3:0] S;
output C4;
wire C1,C2,C3; //Intermediate carries
//Instantiate the full adder
fulladder FA0 (S[0],C1,A[0],B[0],C0),
FA1 (S[1],C2,A[1],B[1],C1),
FA2 (S[2],C3,A[2],B[2],C2),
FA3 (S[3],C4,A[3],B[3],C3);
endmodule
4-bit Full Adder

//Gate-level description of a 2-to-4-line decoder
module decoder_gl (A,B,E,D);
input A,B,E;
output[0:3]D;
wire Anot,Bnot,Enot;
not
n1 (Anot,A),
n2 (Bnot,B),
n3 (Enot,E);
nand
n4 (D[0],Anot,Bnot,Enot),
n5 (D[1],Anot,B,Enot),
n6 (D[2],A,Bnot,Enot),
n7 (D[3],A,B,Enot);
endmodule
2 to 4 Decoder

Behavioral Modeling (2)
 The procedural assignment statements inside the always block are
executed every time there is a change in any of the variable listed after
the @ symbol. (Note that there is no “;” at the end of always statement)
//Behavioral description of 2-to-1-line multiplexer
module mux2x1_bh(A,B,select,OUT);
input A,B,select;
output OUT;
reg OUT;
always @(select or A or B)
if (select == 1) OUT = A;
else OUT = B;
endmodule

4-to-1 line
multiplexer

//Behavioral description of 4-to-1 line mux
module mux4x1_bh (i0,i1,i2,i3,select,y);
input i0,i1,i2,i3;
input [1:0] select;
output y;
reg y;
always @(i0 or i1 or i2 or i3 or select)
case (select)
2'b00: y = i0;
2'b01: y = i1;
2'b10: y = i2;
2'b11: y = i3;
endcase
endmodule

Flip-Flops and Latches
module D_latch(Q,D,control);
output Q;
input D,control;
reg Q;
always @(control or D)
if(control) Q = D; //Same as: if(control=1)
endmodule

Flip-Flops and Latches
//D flip-flop
module D_FF (Q,D,CLK);
output Q;
input D,CLK;
reg Q;
always @(posedge CLK)
Q = D;
endmodule
//D flip-flop with asynchronous reset.
module DFF (Q,D,CLK,RST);
output Q;
input D,CLK,RST;
reg Q;
always @(posedge CLK or negedge RST)
if (~RST) Q = 1'b0; // Same as: if (RST = 0)
else Q = D;
endmodule

T & J-K Flip-Flops
//T flip-flop from D flip-flop and gates
module TFF (Q,T,CLK,RST);
output Q;
input T,CLK,RST;
wire DT;
assign DT = Q ^ T ;
//Instantiate the D flip-flop
DFF TF1 (Q,DT,CLK,RST);
endmodule
//JK flip-flop from D flip-flop and gates
module JKFF (Q,J,K,CLK,RST);
output Q;
input J,K,CLK,RST;
wire JK;
assign JK = (J & ~Q) | (~K & Q);
//Instantiate D flipflop
DFF JK1 (Q,JK,CLK,RST);
endmodule
flop-flipJKafor'')1(
flop-flipTafor)1(
QKJQtQ
TQtQ


Characteristic equations of the flip-flops:

Sequence Recognizer
Identify the
sequence 1101,
regardless of
where it occurs in
a longer
sequence.

Sequence Recognizer
module seq_recognizer(CLK,RST,X,Z);
input CLK,RST,X;
output Z;
reg [1:0]state, next_state;
parameter A=2’b00,B=2’b01,C=2’b10,D=2’b11;
reg Z;
always@(posedge CLK or posedge RST) begin
if(RST==1) state <= A;
else state <= next_state;
end

always@(X or state) begin
case(state)
A:if(X)next_state <= B; else next_state <= A;
B:if(X)next_state <= C; else next_state <= A;
C:if(X)next_state <= C; else next_state <= D;
D:if(X)next_state <= B; else next_state <= A;
endcase
end
always@(X or state) begin
case(state)
A:Z<=0;
B:Z<=0;
C:Z<=0;
D:Z<=X?1:0;
endcase
end
endmodule

Verilogspk1

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