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Introduction
Verilog is a HARDWARE DESCRIPTION LANGUAGE (HDL)
A hardware description language is a language or means used to describe or
model a digital system:
 a network switch,
 a microprocessor
 a memory
 or a simple flip-flop.
It is design representation of digital hardware describes to design software.
It is a C like programming language
Hardware Description
A digital circuit is defined as a module, like the function in C.
module device_identifier ( I/O list );
declaration of I/O;
declaration of internal wire connections;
circuit description in
either structural model
or behavioral model
endmodule
module gate1 (x1, x2, f );
input x1, x2 ;
output f ;
nand g1 (f, x1, x2);
assign f = ~ (x1 & x2);
endmodule
Structural Model / Gate Level Model
Circuit described in structural model is to define the circuit composes of:
 What type of devices / logic gates
 How they are connected
In gate level model, all common
logic gates are supported.
Specified by:
gate_type identifier (output, inputs);
not, and, nand, or, nor, xor
Structural Model
To use a predefined component, in Verilog, it is like functional call in C.
 what component is being called
 how it is connected to ( I/O lists, and its order )
Johnson Counter
module johnson_counter ( clk, q0, q1, q2, q3 ) ;
input clk ;
output q0, q1, q2, q3 ;
wire qb0, qb1, qb2, qb3 ;
dff d0 ( clk , qb3 , q0 , qb0) ;
dff d1 ( clk , q0 , q1 , qb1) ;
dff d2 ( clk , q1 , q2 , qb2) ;
dff d3 ( clk , q2 , q3 , qb3) ;
endmodule
/* D flip flop is a predefined component */
module dff (clk , D, Q, Qb);
input clk, D;
output Q, Qb;
always @(posedge clk)
Q <= D;
Qb <= ~D;
endmodule
Behavioral Model
Circuit described in behavioral model is to define the circuit by:
 the logic functionality
Specified by:
assign output = logic function (inputs);
~
: not
&
: and
|
: or