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DIVYA SHAH24/02/09

Verilog Hardware DescriptionVerilog Hardware Description

LanguageLanguage

LECTUER-1

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WHY VERILOG

TO DESCRIBE DIGITAL CIRCUITS

GENERAL PURPOSE HARDW ARE DESCRIPTION LANGUAGE

SIMILAR IN SYNTAX TO C PROGRAMMI9NG LANGUAGE

DIFFERENT LEVEL OF ABSTRACTION MIXED IN THE SAME

MODEL(FROM BEHAVIORAL,DATAFLOW TO SWITCH LEVEL)

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DESIGN METHODOLOGIES

TOP-DOWN DESIGNDEFINE TOP LEVEL BLOCKS AND IDENTIFY THE SUB-BLOCKS

NECESSARY TO BUILD THE TOP LEVEL BLOCK.FURTER SUBDIVIDE

THE SUB-BLOCKS UNTIL GET THE LEAF-CELL BLOCKSWHICH CANT

BE DIVIDE FURTHER.

BOTTOM-UP DESIGNFIRST IDENTIFY THE BUILDING BLOCKS THAT ARE AVAILABLE .

THEN BUILD BIGGER CELL.THUS THESE CELLS USED FOR HIGHERLEVEL BLOCKS UNTILWE BUILD TOP LEVEL BLOCK IN DESIGN

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MODULE

In Verilog, the basic unit of hardware

is called a module.

Modules cannot contain definitions of 

other modules.

A module can, however, be instantiated

within another module. Allows the creation of a hi erarc hy in a

Verilog description

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MODULE DECLARATION

module module_name (portlist);

<Port declarations>;

<module internals>; endmodule

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LEVELS OF ABSTRACTION

BEHAVIORAL OR ALGORITHMIC LEVEL:

Highest level of abstraction

No need to concern for the hardware implementation detail

Designing is very similar to c programming language

DATAFLOW LEVEL:

Specify the flow of data

Designer has to concern how the data flow occur b/w hardwareregister & how the data is procecssed in the design

GATE LEVEL:Implemented in terms of logic gates and interconnection b/w thesegates

Designing is same as describe a design in terms of gate level logicdiagram

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SWITCH LEVEL:

Lowest level of abstraction

Implemented in terms of switches,storage nodes and interconnection

b/w them

IMP:

Designer can mix and match all levels of abstraction

(e.g RTL: behavioral+dataflow)

Higher level of abstraction flexible and technology independent

design

Lower level of abstraction inflexible and technology dependent

design

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INSTANCES

THE PROCESS OF CREATING OBJECTS FROM A 

MODULE TEMPLATE IS CALLED ³INSTANTIATION´

AND THE OBJECTS ARE CALLED ³INSTANCES´.

Example:

 A 4-bit full adder is shown to you .it instantiates 4 one bit full adder 

and again one bit full adder instantiates 2 xor,3 and & 2 or gates.

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Instantiating the module

Example: 4-bit full adder circuit

FA0

FA1FA2FA3

Cout Sum[3] Sum[2]

Sum[1] Sum[0]

cinIn1[0]In2[0]

c0

In1[1]In2[1]

c1

In1[2]In2[2]

c2

In1[3]In2[3]

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module fadd4(in1, in2, cin, sum,cout);input[3:0] in1,in2;

input cin;

output[3:0] sum;

output cout;wire[2:0] temp_cout;

fadd(in1[0],in2[0],cin,sum[0],temp_cout[0]);

fadd(in1[1],in2[1],temp_cout[0],sum[1],temp_cout[1]);

fadd(in1[2],in2[2],temp_cout[1],sum[2],temp_cout[2]);

fadd(in1[3],in2[3],temp_cout[2],sum[3],cout);

endmodule

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Structural model of a full adder circuit

module fadd(in1, in2, cin, sum,cout);

input in1,in2, cin;

output sum, cout;

wire x1,a1,a2,a3,o1,o2;

xor (x1,in1,in2);

xor (sum,x1,cin);

and(a1,in1,in2);

and(a2,in1,cin);

and(a3,in2,cin);or (o1, a1,a2);

or (cout,o1,a3);

endmodule

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STIMULAS BLOCK

A Verilog procedural block which

executes only once.

Used for simulation. Stimulas block also called testbench.

Testbench generates clock, reset, and

the required test vectors. Two style of stimulas application

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FIRST STYLE

HERE STIMULAS BLOCK INSTANTIATES THE DESIGN BLOCK AND

DIRECTLY DRIVE THE SIGNALS IN THE DESIGN BLOCK

CLK RESET

DESIGN BLOCK

Q

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SECOND STYLE

HERE STIMULAS AND DESIGN BLOCK BOTH ARE INSTANTIATES IN A

TOP LEVEL DUMMY MODULE.HERE STIMULAS BLOCK INTERACTS

WITH THE DESIGN BLOCK ONLY THROUGH THE INTERFACE.

STIMULASBLOCK

DESIGNBLOCK

CLK

RESET

Q

D_CLK

D_RESET

D_Q