What is HDL?

hardware description language describes thehardware of digital systems in textual form.

One can design any hardware at any level

Simulation of designs before fabrication

With the advent of VLSI, it is not possible to verify acomplex design with millions of gates on abreadboard, HDLs came into existence to verify thefunctionality of these circuits.

Most Commonly used HDLs Verilog

Verilog HDL is commonly used in the US industry.Major digital design companies in Pakistan use VerilogHDL as their primary choice.

most commonly used in the design, verification, andimplementation of digital logic chips

VHDL (VHSIC (Very High Speed Integrated Circuits) hardware description language)

VHDL is more popular in Europe.

commonly used as a design-entry language for field-programmable gate arrays. Field-Programmable Gate Array is a type of logic chip that can be programmed.

Verilog Simulator

There are many logic simulators used for Verilog HDL. Most common are:

Xilinx

Veriwell

Model Sim

For Beginners Veriwell is good choice and is very user friendly.

Xilinx and ModelSim are widely used.

Levels of Abstraction

There are four different levels of abstraction in verilog:

Behavioral /Algorithmic

Data flow

Gate level

Switch level.

We will cover Gate level, Data flow and Behavioral Level modeling

Getting started…

A verilog program for a particular application consists of two blocks

Design Block (Module)

Testing Block (Stimulus)

Design Block

Design Methodologies:

Two types of design methodologies Top Down Design Bottom Up Design

Design Block

inputs outputs

Top Down Design

In Top Down design methodology, we define the top level block and identify the sub-blocks necessary to build the top level block. We further divide the sub-block until we come to the leaf cells, which are the cells which cannot be divided.

Bottom Up DesignIn a Bottom Up design methodology, we first identify the building blocks , we build bigger blocks using these building blocks. These cells are then used for high level block until we build the top level block in the design

EXAMPLE

FOUR BIT ADDER (Ripple carry adder)

Module RepresentationVerilog provides the concept of module

A module is a

Basic Building block in Verilog

Basic Building block in Verilog

It can be a single element or collection of lower design blocks

A verilog code starts with module

Syntax:

module <module-name>(inputs, outputs);

//Define inputs and outputs…………

……………………

endmodule

Every verilog program starts with the

keyword module and ends with the keyword

endmodule

Input Output Definition

Once the module is defined at the start the inputs and

outputs are to be defined explicitly. e.g.

input a , b //means there are 2 inputs of one bit each

If input or output is more than 1 bit i.e. two or more bits,

then the definition will be:

input [3:0] A, B; //4 bit inputs A3-A0 and B3-B0

output [3:0] C;

Levels of Abstraction

Gate Level ModelingIn gate level modeling a circuit can be defined by use of logicgates.

These gates predefined in verilog library.

The basic gates and their syntax is as follows:

and gate_name(output, inputs);

or gate_name(output, inputs);

not gate_name (output, inputs);

xor gate_name(output, inputs);

nor gate_name(output, inputs);

nand gate_name(output, inputs);

xnor gate_name(output, inputs);

Data Flow ModelingContinuous assignment statement is used.

Keyword assign is used followed by =

Most common operator types are

Operator Types

Operator Symbol

Operation

performed

Number of

Operands

Arithmetic *

/

+

-

Multiply

Divide

Add

Subract

Two

Two

Two

two

Bitwise Logical ~

&

|

^

^~ or ~^

Bitwise negation

Bitwise and

Bitwise or

Bitwise xor

Bitwise xnor

One

Two

Two

Two

two

Shift >>

<<

Shift right

Shift left

Two

Two

Concatenation { }

Concatenation Any number

Conditional ?: Conditional three

Examples1. assign x = a + b;

2. assign y = ~ x ; // y=x’

3. assign y = a & b; // y= ab

4. assign w = a ^ b; //y= a b

5. assign y = x >> 1; //shift right x by 1

6. assign y = {b, c}; //concatenate b with c

e.g. b = 3’b101, c =3’b 111

y = 101111

assign {cout , sum} = a + b + cin; //concatenate sum and cout

7. assign y = s ? b : a // 2×1 multiplexer

when s = 1 , y = b when s = 0 , y = a

assign y = s1 ? ( s0 ? d : c ) : ( s0 ? b : a ); // 4×1 MUX

Module Instantiation

Module instantiation is a process of connecting one module to another.

For example in a test bench or stimulus the top level design has to be instantiated

Testing Block (Stimulus)

In order to test your circuit a test bench code isto be written which is commonly called Stimulus.

The design block has to be instantiated/called

It displays the output of the design based on theinputs.

Example

2- Input AND Gate

The Design and Stimulus blocks will be as follows:

Design Block

1)Gate Level Modeling

module practice (y, a, b); //module definition

input a, b; // inputs(by default it takes 1 bit input

output y; // one bit output

and gate_1(y, a, b) ;

endmodule

2) Data Flow Modeling

module practice (y, a, b); //module definition

input a, b; // by default it takes 1 bit input

output y; // one bit output

assign y = a & b;

endmodule

Stimulus Blockmodule stimulus;

reg a, b;

wire y;

//Instantiate the practice module

practice p0(y, a, b);

initial

begin

a=0; b=0;

#5 a=1; b=1;

#5 a=0; b=1;

#5 a=1; b=0;

#5 a=1; b=1;

#5 $stop; // stop the simulation

#5 $finish; // terminate the simulation

end

initial

begin

$display("|%b| and |%b| = ", a, b);

$monitor ($time, "|%b |" , y);

end

//initial

//$vw_dumpvars; // display the simulation in the form of timing diagram

endmodule

Example #2:4 bit ripple carry adder

Full Adder

Bottom Level module//Define a full adder

module fulladder (sum, c_out, a, b, c_in);

//I/O Port declaration

output sum, c_out;

input a, b, c_in;

//Internal nets

wire s1, c1, c2;

//full adder logic configuration

xor ( s1,a,b);

and (c1,a,b);

xor (sum,s1,c_in);

and (c2,s1,c_in);

or (c_out,c2,c1);

endmodule

TOP LEVEL MODULE//Define a 4 bit 4 adder

module toplevel_fa(sum,c_out,a,b,c_in);

//I/O port declaration

output [3:0] sum;

output c_out;

input [3:0] a, b;

input c_in;

//internal nets

wire c1,c2,c3;

//Instantiate four 1-bit full adder

fulladder fa0(sum[0],c1,a[0],b[0],c_in);

fulladder fa1(sum[1],c2,a[1],b[1],c1);

fulladder fa2(sum[2],c3,a[2],b[2],c2);

fulladder fa3(sum[3],c_out,a[3],b[3],c3);

endmodule

Test Bench (stimulus)

//define stimulus toplevel module

module stimulus;

reg [3:0]a,b; //set up variables

reg c_in;

wire [3:0] sum;

wire c_out;

//Instantiate the toplevelmodule(ripple carry adder) call it tl

toplevel_fa tl(sum,c_out,a,b,c_in);

//stimulate inputs

initial

begin

a = 4'b0000; b = 4'b0010; c_in = 1'b0;

#1 $display (“ a = %b, b = %b, c_in = %b, sum = %b", a, b, c_in, sum);

a = 4'd1; b = 4'd2; c_in = 1'b1;

#2$display (“ a = %b, b = %b, c_in = %b, sum = %b", a, b, c_in, sum);

a = 4'hf; b = 4'ha; c_in = 1'b0;

#2$display (“ a = %b, b = %b, c_in = %b, sum = %b", a, b, c_in, sum);

end

endmodule

Verilog Keywords

Verilog uses about 100 predefined keywords. All the keywords are represented in colored font (either green, blue or red). if it is not shown in a colored font it means there must be some typing error.

All the verilog statements are terminated with a semicolon(;) except for the statements (keywords) like initial, begin, always, if, for, while etc…

Verilog is case sensitive i.e. the keywords are written in lower case.

Continued…… Most common keywords are

module, endmoduleinput, outputwire, reg$display, $print, $monitoralways, for, while, ifinitial, beginand, or, not, xor, xnor, nard, norposedge , negedge, clock, reset, case$vw_dumpvars, $stop, $finish

Single line comment is given by // ( two consecutive slash)and multi-line comment is given by /*……… */

for e.g // This is the first session of verilog/* this is the first session of verilog*/