Download Verilog Tutorial I

Verilog Intro: Part 1
Hardware Design Languages
• A Hardware Description Language (HDL) is a language
used to describe a digital system, for example, a
computer or a component of a computer.
• A digital system can be described at several levels:
• Switch level: wires, resistors and transistors
• Gate level: logical gates and flip flops
• Register Transfer Level (RTL): registers and the
transfers of information between registers.
• Two Major HDLs in Industry
– VHDL
– Verilog HDL
Verilog HDL vs. VHDL
VHDL
• “V” is short for Very High Speed Integrated Circuits.
• Designed for and sponsored by US Department of Defense.
• Designed by committee (1981-1985).
• Syntax based on Ada programming language.
• Was made an IEEE Standard in 1987.
Verilog HDL (VHDL)
• Was introduced in 1985 by Gateway Design System Corporation,
now a part of Cadence Design Systems, Inc.'s Systems Division.
• Was made an IEEE Standard in 1995
• Syntax based on C programming language.
Design Methodology
Identifiers
• An identifier is composed of a space-free
sequence of uppercase and lowercase letters
from alphabet, digits (0,1,….9), underscore (_),
and the $ symbol.
• Verilog is a case sensitive language.
– c_out_bar and C_OUT_BAR are two different
identifiers
• The name of a variable may not begin with a digit
or $, and may be up to 1,024 characters long.
– e.g. clock_, state_3
Comments
• There are two kinds of comments: single line and
multiline.
• A single-line comment begins with two forward slashes
(//)
• A multiline comment begins with the pair of characters
/* and terminate with the characters */
• Example:
– // This is a single-line comments
– /* This is a multiline comments
more comments here
…………………………………. */
Numbers
• Numbers are specified using the following form
<size><base format><number>
• Size: a decimal number specifies the size of the
number in bits.
• Base format: is the character
the following characters
’ followed by one of
– b for binary,d for decimal,o(octal),h(hex).
• Number: set of digits to represent the number.
Numbers
• Example :
x = 347 // decimal number
x = 4’b101 // 4- bit binary number 0101
x = 6’o12 // 6-bit octal number
x = 16’h87f7 // 16-bit hex number h87f7
x = 2’b101010
x = 2’d83
• String in double quotes
“ this is an introduction”
Operators
• Bitwise Operators
~
NOT
&
AND
|
OR
^
XOR
~|
NOR
~&
NAND
^~ or ~^
XNOR
• Logical & Relational Operators
!, &&, | |, ==, !=, >=, <=, >, <
Operators
• Arithmetic Operators
+, -, *, / , %
• Concatenation & Replication Operators
{identifier_1, identifier_2, …}
{n{identifier}}
– Examples: {REG_IN[6:0],Serial_in},
{8 {1’b0}}
Value Logic System
• Data type for signals
• Bits (value on a wire)
– 0, 1
– x unknow value
• Vectors of bits (parallel wires or registers)
– A[3:0] is a vector of 4 bits: A[3], A[2], A[1], A[0]
– Concatenating bits/vectors into a vector
• B[7:0] = {A[3], A[3], A[3], A[3], A[3:0]};
• B[7:0] = {3{A[3]}, A[3:0]};
Data Types: Constants
• A constant is declared with the keyword
parameter in a statement assigning a name
and a value to the constant
• The value of a constant is fixed during
simulation.
• Examples:
– parameter HIGH_INDEX= 31; // integer
– parameter BYTE_SIZE = 8;
Data Types: Variables
•
•
•
•
•
•
•
•
Two basic families of data types for variables: Nets and Registers
Net variables – e.g. wire
Variable used simply to connect components together
Usually corresponds to a wire in the circuit.
Register variables – e.g. reg
Variable used to store data as part of a behavioral description
Like variables in ordinary procedural languages
Note:
– reg should only be used with always and initial blocks (to be presented
…)
– The reg variables store the last value that was procedurally assigned to
them whereas the wire variables represent physical connections
between structural entities such as gates.
Continuous Assignment
• A continuous assignment statement is declared
with the keyword assign, followed by a net(e.g.
type wire) variable, an assignment operator(=),
and an expression.
• assign corresponds to a connection.
• Target is never a reg variable.
–
–
–
–
assign A = B | (C & ~D); // not the use of ~ not !
assign B[3:0] = 4'b01XX;
assign C[15:0] = 16'h00ff; //(MSB:LSB)
Assign {Cout, S[3:0]} = A[3:0] + B[3:0] + Cin;
Procedural Assignment & String
• Procedural assignments have the form
<reg variable> = <expression>
• where the <reg variable> must be a register or
memory.
• e.g.
reg enable, d;
enable = 0;
d = 0;
Primitives
• No declaration required (predefined) ;
• Can only be instantiated
• Example: and a1 (C, A, B); //instance name
– Usually better to provide instance name for debugging.
• Example:
• Example:
or o1 (SET, ~A, C ),
o2(N, ABC,SET );
and #(10) a2(o, i1, i2); // name + delay
Program Structure: Modules
• Any digital system is a set of modules.
• Modules may run concurrently, but usually there is one top level
module to specify a closed system containing both test data and
hardware models. The top level module invokes instances of other
modules.
• A module is never called, it is instantiated.
• Modules can be specified behaviorally or structurally (or a
combination of the two).
• A behavioral specification defines the behavior of a digital system
using traditional programming language constructs (e. g.,if else,
assignment statements).
• A structural specification expresses the behavior of a digital system
(module) as a hierarchical interconnection of submodules.
The Structure of a Module
• The structure of a module is the following:
module <module name> (<port list>);
<declares>
<module items>
endmodule
• <module name> is an identifier that uniquely names the module.
• <port list> is a list of input, output and inout ports which are used to
connect to other modules.
• <declares> specifies data objects as registers, memories & wires as wells
as procedural constructs such as functions & tasks.
• <module items> may be
–
–
–
–
initial constructs,
always constructs,
continuous assignments or
instances of modules.
Taste of Verilog
Module name
Module ports
module Add_half ( sum, c_out, a, b );
input
a, b;
Declaration of port
output
sum, c_out;
modes
wire
c_out_bar;
Declaration of internal
signal
xor (sum, a, b);
nand (c_out_bar, a, b);
not (c_out, c_out_bar);
endmodule
Verilog keywords
Instantiation of primitive
gates
a
b
sum
c_out_bar
c_out
Taste of Verilog
module Add_half ( sum, c_out, a, b );
input
a, b;
output sum, c_out;
assign {c_out, sum} = a + b;
endmodule
a
b
sum
c_out_bar
c_out
Using Verilogger Pro
• An evaluation version is included in a the CD
coming with your text book.
• Start Verilogger
StartProgramSynaptiCadVerilogger
Pro..
How to Write a new code
•
•
•
•
Open any text editor.
Write in your code.
Save the file with .v extension.
Test your code using Verilogger Pro;
– Start Verilogger Pro
– Right click the project window and select add hdl file
– Click the triangular green button from the tools bar to run
the code.
– If you have any errors you will see them the report
window.
Test Bench
module <test module name> ;
// Data type declaration
// Instantiate module ( call the module that is
going to be tested)
// Apply the stimulus
// Display results
endmodule
Test Bench Example
module test_my_nand;
// Test bench to test half adder
reg A, B; wire s, cOut;
Add_half test( s, cOut, A, B ); // instantiate my_NAND.
initial
begin
end
// apply the stimulus, test data
A = 1'b0; B = 1'b0;
#100 A = 1'b1; // delay one simulation cycle, then change A=>1.
#100 B = 1'b1;
#100 A = 1'b0;
initial #500 $finish;
initial
begin
end
endmodule
// setup monitoring
//$monitor("Time=%0d a=%b b=%b out1=%b", $time, A, B, F);
User-Defined Primitives
// User defined primitive(UDP)
primitive UDP_1 (F,A,B,C);
output F; // only one output is allowed
input A,B,C;
// Truth table for F(A,B,C) = Minterms (0,2,4,6,7)
table
// A B C : F (Note that this is only a comment)
0 0 0 : 1;
0 0 1 : 0;
0 1 0 : 1;
0 1 1 : 0;
1 0 0 : 1;
1 0 1 : 0;
1 1 0 : 1;
1 1 1 : 1;
endtable
endprimitive
Procedural Blocks
• There are two types of procedural blocks in Verilog.
– initial for single-pass behavior : initial blocks execute only
once at time zero (start execution at time zero).
– always for cyclic behavior: always blocks loop to execute
over and over again, in other words as name means, it
executes always.
• Procedural assignment may only appear in initial and
always constructs.
• The initial and always constructs are used to model
sequential logic.
• Continuous statement is used to model combinational
logic.
Example: Initial Block
module initial_example();
reg clk,reset,enable,data;
initial begin
clk = 0;
reset = 0;
The initial block is executed at time 0.
Initial block just execute all the statements
enable = 0;
within begin and end statements.
data = 0;
end
endmodule
Control Constructs
• Control Constructs
– Can be used in the procedural sections of code.
• Selection
– if statement:
if (A == 4)
begin
B = 2;
end
else
begin
B = 4;
end
– case statements:
case (<expression>)
<value1>: <statement>
<value2>: <statement>
default: <statement>
endcase
Example: 4-1 MUX in behavioral (1)
module mux4 (sel, A, B, C, D, Y);
input [1:0] sel; // 2-bit control signal
A
input A, B, C, D;
B
output Y;
C
reg Y; // target of assignment
D
always @(sel or A or B or C or D)
If (sel == 2’b00) Y = A;
else if (sel == 2’b01) Y = B;
Sel[1:0]
else if (sel == 2’b10) Y = C;
else if (sel == 2’b11) Y = D;
endmodule
Y
Example: 4-1 MUX in behavioral (2)
// 4-1 mux using case statement
module mux4 (sel, A, B, C, D, Y);
input [1:0] sel; // 2-bit control signal
input A, B, C, D;
output Y;
reg Y; // target of assignment
always @(sel or A or B or C or D)
case (sel)
2’b00: Y = A;
2’b01: Y = B;
2’b10: Y = C;
2’b11: Y = D;
endcase
endmodule
A
B
Y
C
D
Sel[1:0]
Example: 4-1 MUX in behavioral (3)
// 4-1 mux using case statement
module mux4 (select, d, q);
input [1:0] select; // 2-bit control signal
input [3:0] d;
output q;
reg q; // target of assignment
always @(select or d)
case (select)
2’b00: q = d[0];
2’b01: q = d[1];
2’b10: q = d[2];
2’b11: q = d[3];
endcase
endmodule
Example: 4-1 MUX in structural
module mux4( select, d, q );
input[1:0] select;
input[3:0] d;
output q;
wire q, q1, q2, q3, q4, NOTselect0, NOTselect1;
wire[1:0] select;
wire[3:0] d;
not n1( NOTselect0, select[0] );
not n2( NOTselect1, select[1] );
and a1( q1, NOTselect0, NOTselect1, d[0] );
and a2( q2, select[0], NOTselect1, d[1] );
and a3( q3, NOTselect0, select[1], d[2] );
and a4( q4, select[0], select[1], d[3] );
or o1( q, q1, q2, q3, q4 );
endmodule
Another Example: 4-bit Full Adder
using 1-bit Full Adder
module FourBitAdder( sum, c_out, x, y, c_in);
output [3:0] sum;
output c_out;
input [3:0] x, y;
input c_in;
wire c1, c2, c3;
fulladder fa0( sum[0], c1, x[0], y[0], c_in );
fulladder fa1( sum[1], c2, x[1], y[1], c1 );
fulladder fa2( sum[2], c3, x[2], y[2], c2 );
fulladder fa3( sum[3], c_out, x[3], y[3], c3 );
endmodule
module fulladder( sum, c_out, x, y, c_in );
output sum, c_out;
input x, y, c_in;
wire a, b, c;
xor( a, x, y);
xor( sum, a, c_in );
and( b, x, y );
and( c, a, c_in );
or( c_out, c, b );
endmodule
Repetition
•
// for loop
for(i = 0; i < 10; i = i + 1) begin
$display(“i = %d", i); end
•
•
//while loop
i = 0;
while(i < 10)
begin
$display(“i = %d", i);
i = i + 1;
end
// repeat loop
repeat (5) //repeats the block 5 times,
begin
$display(“i = %d", i);
i = i + 1;
end
Blocking and Non-blocking Procedural
Assignments
• The blocking assignment statement (= operator)
acts much like in traditional programming
languages. Blocking statement must complete
execute before the next statement in the
behavior can execute.
• The non-blocking (<= 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. Non-blocking assignment statements
execute concurrently rather than sequentially.
References
– Cadence Design Systems, Inc., Verilog-XL
Reference Manual.
– Ciletti, Michael D., Starting Guides to Verilog 2001,
Prentice Hall 2004
– http://www.eg.bucknell.edu/~cs320/Fall2003/veri
log.html
– http://www.verilog.net/index.html
– http://www.eecs.berkeley.edu/~culler
– http://www-inst.eecs.berkeley.edu/~cs150