ABSTRACT

Addition is a vital arithmetic operation and is the base of other arithmetic

operations such as multiplication, subtraction and division. Adder is a digital circuit that
does addition of binary numbers. The 1-bit full adder is the basic block of an arithmetic
unit. In VLSI, there are many efficient techniques to design digital circuits. Some of the
techniques are CMOS, PTL, TG, etc. These undergoes to reduce power but each design
undergoes precise disadvantage. In this Project 8-bit Carry Increment Adder (CIA) , Carry
Look Ahead Adder (CLA) are implemented using Gate Diffusion Input (GDI) logic. These
designs are simulated and implemented using Tanner tool. The result shows thatCIA, CLA,
designs using GDI logic are more efficient compared to CMOS logic in view of power
consumption, delay, and area (transistors count) respectively. Keywords- Adder, Addition,
GDI, CMOS, Power consumption

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 LIST OF FIGUTRES
FIGURE.NO. NAME PAGE.NO
1.0 VLSI design flow 3
1.1 Physical design process 7
2.1 GDI basic cell 15
3.1 Schematic diagram of Half adder 16
3.2 schematic diagram of full adder 17
3.3 Ripple carry adder 18
3.4 carry skip adder 19
3.5 full adder- 20
3.6 Carry-Look ahead Adder 21
3.7 Parallel Carry Adder 23
3.8 16 bit carry look ahead adder 24
3.9 Carry Look ahead adder IC 26
3.10 4-Bit parallel Adder-74LS283 27
3.11 CIA_CLA 28
3.12 CIA-RCA 29
3.13 Block Diagram of CBA 30
5.1 AND gate using GDI method 40
5.2 Input and output waveform of an AND gate using GDI 41
5.3 OR GATE-GDI` 41
5.4 Input and output waveform of an OR gate using GDI 42
5.5 NOT gate using GDI method 42
5.6 Input and output waveform of an NOT gate using GDI 43
5.7 XOR gate using GDI method 44
5.8 Input and output waveform of an XOR gate using GDI 44
5.9 Graphical analysis of the average power consumption 45
6.1 AND Gate Design 46
6.2 NOT Gate Design 46
iv
6.3 OR Gate Design 47
6.3 XOR Gate Design 47
6.5 Half adder Design 48
6.6 1-bit full adder Design 48
6.7 4-bit RCA adder Design 49
6.8 8-bit CIA Adder Design 49
6.9 8-bit CBA Adder Design 50
6.10 8-bit CSKA adder Gate Design 50
6.11 8-bit CLA Adder Design 51
6.12 Output waveforms for AND Gate 51
6.13 Output waveforms for MUX 52
6.14 Output waveforms for OR Gate 52
6.15 Output waveforms for XOR Gate 53
6.16 Output waveforms for Half Adder 53
6.17 Output waveforms for 1-Bit Full Adder 54
6.18 Output waveforms for 8-Bit CIA for Adding of (1101 1011)2 and
(1110 1110) 54
6.19 Output waveforms for 8-Bit CBA for Adding of (0101 1101)2 and
(1011 1101) 55
6.20 Output waveforms for 8-Bit CKA for Adding of (1111 0111)2 and
(1100 1111) 55
6.21 Output waveforms for 8-Bit CLA for Adding of (1100 1111)2 and
(1111 1101) 56
 LIST OF TABLES

TABLE.NO. NAME PAGE.NO

2.1 GDI truth table for inverter 13

2.2 Logic functions that can be implemented with a single GDI cell.15
3.1 Truth table of half adder 16
3.2 Truth table Of Full adder (1 Bit) 17
3.3 Truth table CLA 22
3.4: CMOS vs GDI 30

v
 CONTENTS
Declaration i
Acknowledgement ii
Abstract iii
List of Figures iv
List of Tables v

CHAPTER NO DESCRIPTION PAGE NO

CHAPTER 1 INTRODUCTION 1
1.1 Introduction 1
1.2 Aim of The Project 3
1.3 VLSI Design Cycle 3
1.4 Physical Design Cycle 5
1.5 Floor planning 7
1.6 VLSI Design styles 9
1.7 Simulation 9
CHAPTER 2 LOW POWER HIGH SPEED DIFFUSION INPUT
TECHNIQUE 11
2.1 Gate Diffusion Input 11
2.2 Operational Analysis 12
2.3 Basic GDI Cell 13
2.2 Principle of Operation 14
CHAPTER 3 TYPES OF ADDERS 16
3.1 Adders architecture 16
3.2 Half Adder 16
3.3 Full Adder 17
3.4 Ripple carry adder 18
3.5 carry skip adder 18
3.6 Carry Look hear Adder 19
3.7 Carry Increment Adder 27
 3.7.1 CIA_CLA 27
3.7.1 CIA_RCA 28
3.8 Carry Bypass Adder 29
3.9 CMOS vs GDI 30
CHAPTER 4 INTRODUCTION TO TANNER TOOL 31
4.1 What is Tanner Tool 31
4.2 Schematic Edit Tool(S-Edit) 31
4.3 T-Spice pro circuit Analysis 32
4.3.1 DC Operating point analysis 33
4.3.2 DC transfer analysis 34
4.3.3 Transient Analysis 34
4.3.4 AC analysis 34
4.3.5 Noise analysis 35
4.4 Wave form edit 35
4.5 Layout Edit(L-Edit) 36
4.5.1 L- Edit: An Integrated Circuit Layout Tool 36
CHAPTER 5 GATES DESIGN USING GDI METHOD 40
5.1 AND Gate based on GDI method 40
5.2 NOT Gate based on GDI method 42
5.3 XOR gate based on GDI method 43
5.4 Performance Analysis 45
CHAPTER 6 RESULT AND ANALYSIS 46
6.1 Design Unit 46
6.2 Output wave forms 51
CHAPTER 7 CONCLUSSION AND FUTURE SCOPE 57
7.1 Conclusion 57
7.2 Future Scope 57
REFERENCE 58
 CHAPTER 1

INTRODUCTION
1.1 INTRODUCTION
Adders are extensively used circuit elements in Very Large Scale Integration
(VLSI) systems such as Digital Signal Processing (DSP) processors, microprocessors etc.
It is the nucleus of many other operations like subtraction, multiplication, division and
address calculation. In most of the digital systems, adders lie in a critical path which
influences the overall system performance. Hence, enhancing adder‟s performance is
becoming an important goal. The Pexplosive growth in portable systems like laptops has
intensified the research efforts in low power microelectronics. The reason behind is that the
battery technology does not advance at the same rate as the microelectronics technology.
There is only a limited amount of power available for the mobile systems. Therefore, low
power design has become a major design consideration . The advances in VLSI technology
allow hardware realization of most computing intensive applications such as multimedia
processing, DSP, to enhance the speed of operation. Moreover, with increasing demand
and the popularity of portable electronic products, the researchers are driven to strive for
smaller silicon area, higher speed, longer battery life and enhanced reliability. The
importance of digital computing lies in full adder design. The design criteria for full adder
are usually multifold.
Transistor count, which is one of the attributes, determines the system complexity
of arithmetic circuits like multiplier, Arithmetic Logic Unit (ALU), etc. Power
consumption and speed would be the other two important criteria when it comes to the
design of full adders. However, they have a contradictory relationship with each other.
Therefore, power delay product or energy consumption per operation has been introduced
to accomplish optimal design tradeoffs. The performance of digital circuits can be
optimized by proper selection of logic styles. Different logic styles tend to favor the
accomplishment of one performance aspect at the expense of others. The logic styles are
varied in the method of computing intermediate nodes, the number of transistor count,
though they are implementing the same function. Numerous full adder designs in the

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classes of static CMOS, dynamic circuit, transmission gate, GDI logic and Pass Transistor
Logic (PTL) are discussed in the literature .
The well known static CMOS adders with complementary pullup PMOS and pull
down NMOS network require 28 transistors for generating sum and carry outputs. PTL is
an alternative to CMOS and offers most functions implementations with fewer transistors.
This may reduce overall capacitances which in turn will increase the speed and decrease
the power dissipation. However, in the PTL based design, the output voltage is varied due
to threshold voltage drop across the input and the output. This problem can be resolved by
the adaptation of Complementary Pass Logic (CPL) and Swing Restored PTL (SRPL). But
these logics produce larger short circuit current, higher transistor count and increased
wiring complexity due to demand of complementary input signals. Building logic using
transmission gate is another choice to minimize complexity. The full adder design
implemented using transmission gate is discussed in Reference 13.
It requires 20 transistors, further reduction in transistor count is also possible using
transmission function adder which needs 16 transistors, and it is discussed in Reference 14.
GDI logic is introduced as an alternative to CMOS logic. It is a low power design
technique which offers the implementation of the logic function with fewer numbers of
transistors. GDI gates provide reduced voltage swing at their outputs, i.e. the output high
(or low) voltage is deviated from the VDD (or ground) by threshold voltage Vt. The
reduction in Voltage swing is beneficial to power consumption. On the other hand, this
may lead to slow switching in the case of cascaded operation. At low VDD operation, the
degraded output may even cause circuit malfunction. Therefore, special attention must be
needed to achieve full swing operation. In this paper, an efficient methodology for digital
circuits such as AND, OR and XOR gates with full swing is implemented. After that, three
full adders are proposed based on the full swing gates in a standard 45 nm technology. The
performance of three proposed full adder Designs are compared with other adders based
on CMOS, CPL, hybrid and GDI logic cited in the literature Wide utilization of memory
storage systems and sequential logic in modern electronics triggers a demand for high-
performance and low area implementations of basic memory components. In these circuits,
the output not only depends upon the current values of the inputs, but also upon preceding
input values. These circuits are often called cyclic logic circuits.
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 These inputs are often used to initialize the state of digital ICs at the time the power
is first applied. Normally, a set or reset input is required, but seldom both. A T flip flop
alternately sends an output signal to two different outputs when an input signal is applied.

1.2 AIM OF THE PROJECT

The aim of project is by using GDI technique the power consumption, delay, chip area and
connection and parasitic capacitors is decreased. In this project, we are implementing the
different ADDERS using GDI technique for low power and high speed in order to achieve
power delay product (PDP)

1.3 VLSI DESIGN CYCLE:

The design process of producing a package VLSI chip physically follows various steps
which is popularly known as VLSI design cycle. This design cycle is normally represented
by a flow chart shown below. The various steps involved in the design cycle are elaborated
below.

Fig1.0. VLSI design flow

(i). System specification: The specifications of the system to be designed are exactly
specified in this step. It considers performance, functionality, and the physical dimensions
of the design. The choice of fabrication technology and design techniques is also
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considered. The end results are specifications for the size, speed, power, and functionality
of the VLSI system to be designed.
(ii) Functional design: In this step, behavioral aspects of the system are considered. The
outcome is usually a timing diagram or other relationships between sub-units. This information
used to improve the overall design process and to reduce the complexity of the subsequent
phases.
(iii) Logic design: In this step, the functional design is converted into a logical design,
using the Boolean expressions. These expressions are minimized to achieve the smallest
logic design which conforms to the functional design. This logic design of the system is
simulated and tested to verify its correctness.
(iv) Circuit design: This step involves conversion of Boolean expressions into a
circuit representation by taking into consideration the speed and power requirements of the
original design. The electrical behavior of the various components is also considered in this
phase. The circuit design is usually expressed in a detailed circuit diagram.
(v) Physical design: In this step, the circuit representation of each component is
converted into a geometric representation. This representation is a set of geometric patterns
which perform the intended logic function of the corresponding component. Connections
between different components are also expressed as geometric patterns. (This geometric
representation of a circuit is called a layout). The exact details of the layout also depend on
design rules, which are guidelines based on the limitations of the fabrication process and
the electrical properties of the fabrication materials. Physical design is a very complex
process; therefore, it is usually broken down into various sub-steps in order to handle the
complexity of the problem.

(vi) Design verification: In this step, the layout is verified to ensure that the layout
meets the system specifications and the fabrication requirements. Design verification
consists of design rule checking (DRC) and circuit extraction. DRC is a process which
verifies that all geometric patterns meet the design rules imposed by the fabrication
process. After checking the layout for design rule violations and removing them, the
functionality of the layout is verified by circuit extraction. This is a reverse engineering
process and generates the circuit representation from the layout. This reverse engineered

4
circuit representation can then be compared to the original circuit representation to verify
the correctness of the layout
(vii) Fabrication: This step is followed after the design verification. The fabrication
process consists of several steps like, preparation of wafer, deposition, and diffusion of
various materials on the wafer according to the layout description. A typical wafer is 10 cm
in diameter and can be used to produce between 12 and 30 chips. Before the chip is mass
produced, a prototype is made and tested.
(viii) Packaging, testing, and debugging: In this step, the chip is fabricated and
diced in a fabrication facility. Each chip is then packaged and tested to ensure that it meets
all the design specifications and that it functions properly. Chips used in printed circuit
boards (PCBs) are packaged in a dual in-line package (DIP) or pin grid array (PGA). Chips
which are to be used in a multichip module (MCM) are not packaged because MCMs use
bare or naked chips.

1.4 PHYSICAL DESIGN CYCLE:

The Physical design cycle converts a circuit diagram into a layout. This complex
task is completed in several steps, like partitioning, floor-planning, placement, routing, and
lay-out compaction etc. The details of these steps are given below.

(a) Partitioning: The chip layout is always a complex task and hence it is divided into
several smaller tasks. A chip may contain several million transistors. Layout of the entire
circuit cannot be handled due to the limitation of memory space as well as computation
power available. Therefore, it is normally partitioned by grouping the components into
blocks. The actual partitioning process considers many factors such as size of the blocks,
number of blocks, and number of interconnections between the blocks. The output of
partitioning is a set of blocks along with the interconnections required between blocks. The
set of interconnections required is referred to as a net list. In large circuits the partitioning
rocess is hierarchical and at the topmost level a chip may have between 5 and 25 blocks.
Each module is then partitioned recursively into smaller blocks.

A disadvantage of the partitioning process is that it may degrade the performance

of the final design. During partitioning, critical components should be assigned to the same
5
partition. If such an assignment is not possible, then appropriate timing constraints must be
generated to keep the two critical components close together. Usually, several components,
forming a critical path, determine the chip performance. If each component is assigned to a
different partition, the critical path may be too long. Minimizing the length of critical paths
improves system performance.
After a chip has been partitioned, each of the sub-circuits must be placed on a fixed
plane and the nets between all the partitions must be interconnected. The placement of the
sub-circuits is done by the placement algorithms and the nets are routed by using routing
algorithms.

(b) Placement: It is the process of arranging a set of modules on the layout surface.
Each module has fixed shape and fixed terminal locations. A poor placement uses larger
area and hence results in performance degradation.
The placement process determines the exact positions of the blocks on the chip, so
as to find a minimum area arrangement for the blocks that allows completion of
interconnections between the blocks. Placement is typically done in two phases. In the first
phase an initial placement is created. In the second phase the initial placement is evaluated
and iterative improvements are made until the layout has minimum area and conforms to
design specifications.
It is important to note that some space between the blocks is intentionally left
empty to allow interconnections between blocks. Placement may lead to un-routable
design, i.e., routing may not be possible in the space provided. Thus, another iteration of
placement is necessary. To limit the number of iterations of the placement algorithm, an
estimate of the required routing space is used during the placement phase. A good routing
and circuit performance heavily depend on a good placement algorithm. This is due to the
fact that once the position of each block is fixed, very little can be done to improve the
routing and the overall circuit performance. There are various types of placements.
System-level placement: Place all the PCBs together such that Area occupied is
minimum and Heat dissipation is within limits.

Board-level placement: All the chips have to be placed on a PCB. Area is fixed all
modules of rectangular shape. The objective is to, minimize the number of routing layers
and Meet system performance requirements.
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Chip-level placement: Normally, floor planning / placement carried out along with
pin assignment. It has limited number of routing layers (2 to 4). Bad placements may be
unroutable. Can be detected only later (during routing). Costly delays in design cycle.
Minimization of area.
1.5 Floor planning:
Floor-plan design is an important step in physical design of VLSI circuits to plan
the positions of a set of circuit modules on a chip in order to optimize the circuit
performance. In floor-planning, the information of a set of modules, including their areas
and interconnection is considered and the goal is to plan their positions on a chip to
minimize the total chip area and interconnect cost.
In the floor planning phase, the macro cells are positioned on the layout surface in
such a way that no blocks overlap and that there is enough space left to complete the
interconnections. The input for the floor planning is a set of modules, a list of terminals
(pins for interconnections) for each module and a net list, which describes the terminals
which have to be connected.

Fig1.1. Physical design process

(c) Routing:

The main objective in this step is to complete the interconnections

between blocks according to the specified net list. First, the space not occupied by the
blocks (called the routing space) is partitioned into rectangular regions called channels and
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switchboxes. The goal of a router is to complete all circuit connections using the shortest
possible wire length and using only the channels and switchboxes. This is usually done in
two phases, referred to as the global routing and detailed routing phases.
In global routing, connections are completed between the
proper blocks of the circuit disregarding the exact geometric details of each wire and
pin. For each wire, the global router finds a list of channels which are to be used as a
passage way for that wire. In other words, global routing specifies the „„loose route‟‟ of a
wire through different regions in the routing space. Global routing is followed by detailed
routing, which completes point-to-point connections between pins on the blocks. Loose
routing is converted into exact routing by specifying geometric information such as width
of wires and their layer assignments. Detailed routing includes channel routing and
switchbox routing.
As all problems in routing are computationally hard, the researchers have
focused on heuristic algorithms. As a result, experimental evaluation has become an
integral part of all algorithms and several benchmarks have been standardized. Due to the
nature of the routing algorithms, complete routing of all the connections cannot be
guaranteed in many cases

Compaction: The operation of layout area minimization without violating the design rules
and without altering the original functionality of layout is called as compaction. The input
of compaction is layout and output is also layout but by minimizing area.
Compaction is done by three ways:
(i) By reducing space between blocks without violating design space rule.
(ii) By reducing size of each block without violating design size rule.
(iii).By reducing shape of blocks without violating electrical characteristics of blocks.

1.6 VLSI –DESIGN STYLES

Though the partitioning of a physical design decomposes the
physical design into several conceptually easier steps, still each step is computationally
very hard. So, in order to reduce the complexity of physical design and to get high yield
certain restricted models and design styles are proposed.
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They are (i) full-custom design style (ii) standard cell design style (iii) gate array design
style
1.7 SIMULATION

The objective behind any simulation tool is to create a computer based model for the
design verification and analyzing the behavior of circuits under construction also checking
the current level of abstraction.
Types of Simulation:
Device level simulation, Circuit level simulation, Timing level & Macro level simulation,
Switch level simulation, Gate level simulation, RTL simulation, System level simulation.

Device level simulation:

This model involves with a semiconductor device like a MOS transistor used to test the
effect of fabrication parameters .Simulator techniques based on finite-element method are
used for this purpose.
Circuit level simulation:
It deals with small groups of transistors modeled in the analog domain .The variables
computed are currents and voltages and the computations are based on numerical methods.
Switch level simulation:
This simulation method, models the MOS transistors as switches, that pass signals .The
values of signals are discrete, but it also includes certain analog features to combine certain
components like resistance and capacitance.
Gate level simulation:
In this model a circuit is composed of several logic gates connected by uni-directional
memory less wires. The logic gates themselves are collections of transistors and
other_circuit elements which perform a logic function. A logic gate may be a simple
inverter or NAND gate or NOR gate or more complex functional unit like a flip-flop or
register.
Register –Transfer Level (RTL) simulation:
This model is used synchronous circuits where all registers are controlled by a system
clock signal. The registers store the state of the system, while the combinational logic

9
computes the next state and the output based on the current state and the input. Here the
important consideration is the state transitions and the precise timing of intermediate
signals in the computation of the next state is not considered.
System level Simulation:
It deals with the hardware described in terms of primitives that need not correspond with
hardware building blocks. VHDL is the most popular hardware description language used
for system level simulation. When used in the initial stages of a design, it can describe the
behavior of a circuit as a processor as a set of communicating processes.

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 CHAPTER 2
LOW POWER HIGH SPEED DIFFUSION INPUT
TECHNIQUE

2.1 GATE DIFFUSION INPUT (GDI)

Minimizing power consumption for digital systems involves optimization at all

levels of the design. This optimization includes the technology used to implement the
digital circuits, the circuit style, topology and the architecture for implementing the
circuits. This chapter demonstrates the design of Viterbi decoder using GDI logic or cell.
GDI cell has the structure similar to that of a CMOS transistor with low complexity and
low switching transition. From the literature survey, it is found that Mohammad K. Akbari
et al. (2004) dealt with the design of ACS unit with logic styles like CMOS, pseudo
nMOS, and dynamic logics. But the switching activity in these logic styles is high and thus
leads to high power dissipation.

Gate Diffusion Input (GDI) method is based on the utilization of a simple cell as
shown in Fig. 1 which can be used for low power digital circuits. This technique is
implemented in twin-well CMOS or Silicon on Insulator (SOI) technologies. In this
process, the bulks of both NMOS and PMOS transistors are hardwired to their diffusions to
reduce the bulk effect that is dependence of threshold voltage on source-to-bulk voltage .
The dependence of transistor threshold voltage on source-to-bulk voltage is as follows:

The performance of a digital circuit is judged by its speed in producing output

when an input is given to it. The most common technology for designing digital circuits is
the CMOS technology. After the development of CMOS logic, there was increasing need
to optimize circuit in terms of speed. One technique thought of was by using Pass
Transistor Technology (PTL) which makes use of lesser number of gates to realize an
operation. The Transmission Gate (TG) is one of them which are typically a combination
of NMOS and PMOS transistors connected in parallel. The GDI cell represents another
form of pass transistor technology which looks similar to CMOS but differs in the supply
provided to the input terminals. The main advantages of PTL over conventional CMOS

11
design are as follows- 1) lesser number of transistors results in low power dissipation and
lesser delay. 2) Lesser number of transistors so smaller area and lesser interconnect effects.
However, PTL technologies also suffer from two main problems such as reduced circuit
speed at low power operations and greater static power dissipation.
GDI technique that can be used to design fast, low power circuits using only a few
transistors. The GDI cell is similar to a CMOS inverter structure. In a CMOS inverter the
source of the PMOS is connected to VDD and the source of NMOS is grounded. But in a
GDI cell this might not necessarily occur. There are some important differences between
the two. The three inputs in GDI are namely-
1) G- common inputs to the gate of NMOS and PMOS
2) N- input to the source/drain of NMOS
3) P- input to the source/drain of PMOS
Bulks of both NMOS and PMOS are connected to N or P (respectively), that is it can be
arbitrarily biased unlike in CMOS inverter. Moreover, the most important difference
between CMOS and GDI is that in GDI N, P and G terminals could be given a supply
„VDD‟ or can be grounded or can be supplied with input signal depending upon the circuit
to be designed and hence effectively minimizing the number of transistors used in case of
most logic circuits (eg. AND, OR, XOR, MUX, etc). As the allotment of supply and
ground to PMOS and NMOS is not fixed in case of GDI, therefore, problem of low voltage
swing arises in case of GDI which is a drawback and hence finds difficulty in case of
implementation of analog circuits.

2.2 OPERATIONAL ANALYSIS

The most common problem with PTL technique is its low voltage swing. An extra
buffer circuitry may be used additionally to eliminate the problem of low swing and
improve drivability. The problem of low swing can be understood with the help of a
random function shown in figure and table.

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Functionality Of any B Functionality Logic
Random Function
using GDI A
A B Y
0 0 pMOS Trans Gate Vtp
0 1 CMOS Inverter 1
1 0 nMOS Trans Gate 0
1 1 CMOS Inverter 0

Table 2.1 GDI truth table for inverter

The problem of low swing occurs only when A=0 and B=0 where the voltage level is VTP
instead of 0.This occurs due to the poor high to low transition characteristics of PMOS. In
the rest of the cases it provides full swing.

2.3 BASIC GDI CELL

The basic GDI cell is shown in Fig. 1. Though it resembles a conventional CMOS
inverter the source/drain diffusion input of both PMOS and NMOS transistor is different.
In conventional inverter circuit, source and drain diffusion input of PMOS and NMOS
transistors are always tied at VDD and GND potential, respectively. On the other hand, the
diffusion terminal acts as an external input in the GDI cell. It helps in the realization of
various Boolean functions such as AND, OR, MUX, INVERTER, F1 and F2, as listed in
Table 1. The main drawback of GDI gate is that it suffers due to threshold voltage drop.
This reduces current drive and affects the performance of the gate. The output voltage
reduction can be compensated by the use of swing restoration buffers at the output .
However, the presence of inverters in the buffers increases the transistor count and
also increases the static power consumption when they are connected in cascade. A
multiple Vt technique is presented in the lieu of swing restoration buffer in Reference 15].
This approach utilizes low threshold transistors in the places where a voltage drop is to
occur and also high threshold transistors for the inverters. Though this hybrid threshold

13
voltage method minimizes power consumption, it becomes a bottleneck at the transistor
fabrication process. Another method of swing restoration of GDI based, full adder output,
using an Ultra Low Power Diode (ULPD) technique is detailed in Reference 17. This
technique configures the MOS transistor to work as a diode and uses 8 additional
transistors for providing full swing. It mitigates the problem of static power dissipation as a
conventional swing restoration buffer but still the complexity issue in the fabrication of
ULPD is to be taken into account. The techniques presented so far to achieve full swing at
the full adder output either increase the number of transistors (more than half from non-full
swing design) or increase the power consumption (use of buffers). So, a general method is
required to design full swing at the gate level like AND, OR, XOR, etc. Hence, an attempt
is made to design full swing gates subsequently three adders using the proposed gates; a
detailed explanation of the same is discussed in the following section
The GDI method is based on the simple cell shown in Fig.2.2. A basic GDI
cell contains four terminals - G (the common gate input of the nMOS and pMOS
transistors), P (the outer diffusion node of the pMOS transistor), N (the outer diffusion
node of the nMOS transistor) and the D node (the common diffusion of both transistors). P,
N and D may be used as either input or output ports, depending on the circuit structure.
Table 2.3 shows how various configuration changes of the inputs P, N and G in the basic
GDI cell correspond to different Boolean functions at the output D. GDI enables simpler
gates, lower transistor count, and lower power dissipation in many implementations, as
compared with standard CMOS and Pass-transistor Logic (PTL) design techniques .

Where
K denotes device transconductance parameter,
VTH denotes threshold voltage,
W denotes channel width
14
L denotes channel length.

Figure 2.1: GDI basic cell

N P G D Function
'0' B A HB F1
B '1' A H+B F2
'1' B A A+B OR
B '0' A AB AND
C B A HB+AC MUX
'0' '1' A H NOT

Table 2.2: some logic functions that can be implemented with a single GDI cell.

From table , it can be noticed that using only 2 transistors various functions can be
performed. For instance, OR gate can be designed using a single GDI cell whereas in case
of designing of an OR gate utilizing transmission gates, it required 6 transistors. Similarly,
AND gate can be designed using only 2 transistors and even a Multiplexer (MUX) can be
devised using a single GDI cell. Thus, a simple alteration to the input configuration of the
GDI cell would yield myriad variety of Boolean functions. Multiple-input gates can be
implemented by combining several GDI cells. The main advantage of GDI cell is that a
huge number of functions can be carried out using basic GDI cell. The GDI gates are more
compact and flexible compared to static CMOS gates and have very low leakage current.
This is due to the unique structure of the GDI cell which purges both the sub-threshold as
well as gate leakage current

15
 CHAPTER-3
TYPES OF ADDERS

3.1 ADDERS ARCHITECTURE:

In electronics, an adder or summer is a digital circuit that performs addition of
numbers. In many computers and other kinds of processors, adders are used not only in the
arithmetic logic units, but also in other parts of the processor, where they are used to
calculate addresses, table indices, and similar operations.

Although adders can be constructed for many numerical representations, such as

binary-coded decimal or excess-3, the most common adders operate on binary numbers. In
cases where two‟s complement or ones complement is being used to represent negative
number.

3.2 Half adder:

The half adder is an example of a simple, functional digital circuit built from two logic
gates. The half adder adds to one-bit binary numbers (AB). The output is the sum of the
two bits (S) and the carry (C). Note how the same two inputs are directed to two different
gates.

Fig 3.1: Schematic diagram of Half adder [10.1]

Table3.1 : Truth table of half adder

16
3.3Full adder:

A full adder adds binary numbers and accounts for values carried in as well as out. A one-
bit full adder adds three one-bit numbers, often written as A, B, and Cin; A and B are the
[2]
operands, and Cin is a bit carried in from the previous less significant stage. The full
adder is usually a component in a cascade of adders, which add 8, 16, 32, etc. bit binary
numbers. The circuit produces a two-bit output, output carry and sum .

Where as the equation of the sum and carry is

S = A XOR B;------------------------------------------ ( 1)

Cout= A AND B;-------------------------------------------- (2)

Fig 3.2 : schematic diagram of full adder[10.2]

Table3.2 : Truth table Of Full adder (1 Bit)

17
3.4 Ripple carry adder:

Arithmetic operation like addition ,subtraction ,multiplication ,division are basic operation
to be implemented digital computer using basic gates among all arithmetic operation if we
can implemented addition then it is easy to perform multiplication repeated addition .Half
adders can be used to add two one bit binary numbers .it is also possible to create a logical
circuit using multiple adder to add N bit binary number .each full adder inputs carry ,which
is the output carry of the previous adder .this kind of adder is a ripple carry adder ,since
each carry bits “ripples” to the next full adder .the first full adder may be replaced by the
half adder.

Fig 3.3 : Ripple carry adder[10.3]

3.5 Carry skip adder:

A carry-skip adder (also known as a carry-bypass adder) is an adder implementation that

improves on the delay of a ripple carry adder with little effort compared to other adders.
The improvement of the worst-case delay is achieved by using several carry-skip adders to
form a block-carry-skip adder.
The block diagram of CSA is shown in figure. A CSKA consists of a RCA with a
skip chain used to speed up the carry chain. This chain explains the sharing of ripple carry
blocks, which comprise the skip adder. The worst case for a RCA occurs, when the

18
transmit state is true for each pair (ai,bi). For each operand input bit pair (ai,bi) the transmit
conditions are determined using an XOR gate. When all transmit conditions are true, then
the carry-in bit determines the carry-out bit. The n-bit CSKA consists of mux, AND gate
with n-inputs and n-bit carry ripple chain. The carry ripple chain provides transmit bit,
which is connected to the AND gate. The mux uses the resultant bit

Figure3.4 : carry skip adder

3.6 Carry-Look ahead Adder

In case of parallel adders, the binary addition of two numbers is initiated when all the bits
of the augend and the addend must be available at the same time to perform the
computation. In a parallel adder circuit, the carry output of each full adder stage is
connected to the carry input of the next higher-order stage, hence it is also called as ripple
carry type adder.

In such adder circuits, it is not possible to produce the sum and carry outputs of any
stage until the input carry occurs. So there will be a considerable time delay in the addition
process , which is known as , carry propagation delay. In any combinational circuit , signal
must propagate through the gates before the correct output sum is available in the output
terminals.

19
 fig3.5 : full adder

Consider the above figure, in which the sum S4 is produced by the corresponding full
adder as soon as the input signals are applied to it. But the carry input C4 is not available
on its final steady state value until carry c3 is available at its steady state value. Similarly
C3 depends on C2 and C2 on C1. Therefore, carry must propagate to all the stages in order
that output S4 and carry C5 settle their final steady-state value.

The propagation time is equal to the propagation delay of the typical gate times the number
of gate levels in the circuit. For example, if each full adder stage has a propagation delay of
20n seconds, then S4 will reach its final correct value after 80n (20 × 4) seconds. If we
extend the number of stages for adding more number of bits then this situation becomes
much worse.So the speed at which the number of bits added in the parallel adder depends
on the carry propagation time. However, signals must be propagated through the gates at a
given enough time to produce the correct or desired output.

The following are the methods to get the high speed in the parallel adder to produce the
binary addition.

1. By employing faster gates with reduced delays, we can reduce the propagation
delay. But there will be a capability limit for every physical logic gate.
2. Another way is to increase the circuit complexity in order to reduce the carry delay
time. There are several methods available to speeding up the parallel adder, one commonly

20
used method employs the principle of look ahead-carry addition by eliminating inter stage
carry logic.

Carry-Lookahead Adder

A carry-Lookahead adder is a fast parallel adder as it reduces the propagation delay by

more complex hardware, hence it is costlier. In this design, the carry logic over fixed
groups of bits of the adder is reduced to two-level logic, which is nothing but a
transformation of the ripple carry design.

This method makes use of logic gates so as to look at the lower order bits of the augend
and addend to see whether a higher order carry is to be generated or not. Let us discuss in
detail.

Fig3.6: Carry-Lookahead Adder

21
 Table 3.3: Truth table CLA

Consider the full adder circuit shown above with corresponding truth table. If we define
two variables as carry generate Gi and carry propagate Pi then,

Pi = Ai ⊕ Bi

Gi = Ai Bi

The sum output and carry output can be expressed as

Si = Pi ⊕ Ci

C i +1 = Gi + Pi Ci

Where Gi is a carry generate which produces the carry when both Ai, Bi are one regardless
of the input carry. Pi is a carry propagate and it is associate with the propagation of carry
from Ci to Ci +1.

The carry output Boolean function of each stage in a 4 stage carry-Lookahead adder can be
expressed as

22
C1 = G0 + P0 Cin

C2 = G1 + P1 C1

= G1 + P1 G0 + P1 P0 Cin

C3 = G2 + P2 C2

= G2 + P2 G1+ P2 P1 G0 + P2 P1 P0 Cin

C4 = G3 + P3 C3

= G3 + P3 G2+ P3 P2 G1 + P3 P2 P1 G0 + P3 P2 P1 P0 Cin

From the above Boolean equations we can observe that C4 does not have to wait for C3
and C2 to propagate but actually C4 is propagated at the same time as C3 and C2. Since
the Boolean expression for each carry output is the sum of products so these can be
implemented with one level of AND gates followed by an OR gate.

The implementation of three Boolean functions for each carry output (C2, C3 and C4) for a
carry-Look ahead carry generator shown in below figure.

23
 Fig: 3.7 bit parallel-carry look head adder

Therefore, a 4 bit parallel adder can be implemented with the carry-Lookahead scheme to
increase the speed of binary addition as shown in below figure. In this, two Ex-OR gates
are required by each sum output. The first Ex-OR gate generates Pi variable output and the
AND gate generates Gi variable.

Hence, in two gates levels all these P‟s and G‟s are generated. The carry-Lookahead
generators allows all these P and G signals to propagate after they settle into their steady
state values and produces the output carriers at a delay of two levels of gates. Therefore,
the sum outputs S2 to S4 have equal propagation delay times.

Fig 3.8 : 16 bit carry-Look ahead adder

It is also possible to construct 16 bit and 32 bit parallel adders by cascading the number of
4 bit adders with carry logic. A 16 bit carry-Lookahead adder is constructed by cascading

24
the four 4 bit adders with two more gate delays, whereas the 32 bit carry-Lookahead adder
is formed by cascading of two 16 bit adders.

In a 16 bit carry-Lookahead adder, 5 and 8 gate delays are required to get C16 and S15
respectively, which are less as compared to the 9 and 10 gate delay for C16 and S15
respectively in cascaded four bit carry-Lookahead adder blocks. Similarly, in 32 bit adder,
7 and 10 gate delays are required by C32 and S31 which are less compared to 18 and 17
gate delays for the same outputs if the 32 bit adder is implemented by eight 4 bit adders.

Carry-Lookahead Adder ICs

The high speed carry-Lookahead adders are integrated on integrated circuits in different bit
configurations by several manufacturers. There are several individual carry generator ICs
are available so that we have to make connection with logic gates to perform the addition
operation.

A typical carry-Lookahead generator IC is 74182 which accept four pairs of active low
carry propagate (as P0, P1, P2 and P3) and carry generate (Go, G1, G2 and G3) signals and
an active high input (Cn).

It provides active high carriers (Cn+x, Cn+y, Cn+z) across the four groups of binary
adders. This IC also facilitates the other levels of look ahead by active low propagate and
carry generate outputs.

The logic expressions provided by the IC 74182 are

25
 Fig 3.9: Carry-Look ahead Adder ICs

On the other hand, there are many high speed adder ICs which combine a set of full adders
with carry-Look ahead circuitry. The most popular form of such IC is 74LS83/74S283
which is a 4 bit parallel adder high speed IC that contains four interconnected full adders
with a carry-Lookahead circuitry.

The functional symbol for this type of IC is shown in below figure. It accepts the two 4 bit
numbers as A3A2A1A0 and B3B2B1B0 and input carry Cin0 into the LSB position. This
IC produce output sum bits as S3S2S1S0 and the carry output Cout3 into the MSB

26
 Fig3.10: 4-bit parallel adder -74LS283

By cascading two or more parallel adder ICs we can perform the addition of larger binary
numbers such as 8 bit, 24-bit and 32 bit addition.

3.7 CARRY INCREMENT ADDER (CIA)

In this section the design and working of CIA is presented. First the conventional design
of CIA which is based on RCA is elaborated. We named this design of CIA as CIA_RCA.
Secondly the modified CIA is presented and we named it as CIA_CLA.

3.7.1 CIA_CLA
CIA_CLA In this subsection we present the modified carry increment adder i.e. CIA_CLA.
We know that RCA is the basic binary adder circuit and is quite popular because of its
simple design. However it suffers from the worst propagation delay affecting the overall
performance of the system. It is proved that CLA performs better than RCA in terms of
delay at the expense of increased design complexity. We have modified CIA_RCA by
replacing the RCA with CLA block. It is quite obvious because of the property of CLA, the
overall delay performance will be improved. As similar to CIA_RCA incremental circuit
can be designed using HA‟s in ripple carry chain with a sequential order. The block
diagram representation of CIA_CLA is as shown in Figure

27
 Fig3.11: CIA_CLA

3.7.2 CIA_RCA
The standard Carry Increment Adder (CIA) consists of RCA‟s and incremental
circuitry. The incremental circuit is designed using HA‟s in ripple carry chain with a
sequential order. The addition operation is done by separating the total number of bits in to
group of 4bits and addition operation is performed by several 4-bit RCA‟s. Instead of
computing two partial sums for each group and selecting the correct one, only one partial
sum is calculated and incremented if necessary, according to the input carry. Thus the
second adder and the multiplexers in the carry-select scheme can be replaced by a much
smaller incremental circuit and the modified architecture is the Carry Increment Adder
(CIA). For example, an 8-bit CIA comprises of two 4-bit RCA. The first block of RCA
adds first 4-bits to produce 4-bit partial sum and a carry output. Thus, first 4-bit of sum of
CIA is directly obtained from first block of RCA. And the carry output of first RCA block
is given as input to the cin of incremental circuit. Incremental circuit consists of Half
Adders (HA). Hence, the partial sum obtained from the second RCA block is given to
incremental circuit

28
 Fig3.12: CIA-RCA

3.8 Carry Bypass Adder

In RCA, each full adder waits for the incoming carry to generate an outgoing carry.
This need can be removed by presenting an extra bypass to speed up the action of the
adder. An incoming carry Ci,0=1 transmits through whole adder chain and causes an
outgoing carry C0,7=1 below the conditions that all transmission signals are 1. The
operation of the adder can be speed up using this data. When P0P1P2P3P4P5P6P7 = 1, the
incoming carry is sent to the next block through the bypass and if it is not the case, the
carry is obtained via the normal route. If (P0P1 2P3P4P5P6P7 = 1) then C0,7 = Ci,0 else
either generate or delete happened. In CBA, full adders are separated into groups, each of
them is “bypassed” by a mux, when all the full adders are in propagating.

29
 Figure3.13: Block Diagram of CBA

CMOS vs GDI

Table 3.4: CMOS vs GDI

30
 CHAPTER-4
INTRODUCTION TO TANNER TOOL
4.1 What Is Tanner Tool
Tanner tool is a Spice Computer Analysis Programmed for Analogue Integrated Circuits.
Tanner tool consists of the following Engine Machines:

1. S-EDIT (Schematic Edit)

2. T-EDIT (Simulation Edit)

3. W-EDIT (Waveforms Edit)

4. L-EDIT (Layout Edit)

Using these engine tools, spice program provides facility to the use to design & simulate
new ideas in Analogue Integrated Circuits before going to the time consuming & costly
process of chip fabrication.

4.2 SCHEMATIC EDIT TOOL (S-EDIT)

S-Edit is hierarchy of files, modules & pages. It introduces symbol & schematic modes. S-
Edit provides the facility of:

1. Beginning a design.

2. Viewing, drawing & editing of objects.

3. Design connectivity.

4. Properties, net lists & simulation.

5. Instance & browse schematic & symbol mode.

Beginning a design: It explains the design process in detail in terms of file module
operation and module.

Browser: Effective schematic design requires a working knowledge of the S-Edit design
hierarchy of files & modules. S-Edit design files consist of modules. A module is a
functional unit of design such as a transistor, a gate and an amplifier.

Modules contain two components:

31
1) Primitives: Geometrical objects created with drawing tools.

2) Instances: References to other modules in file. The instanced module is the original.

S-Edit has two viewing modes:

1. Schematic Mode: to create or view a schematic, we operate in schematic mode.

2. Symbol Mode: it represents symbol of a larger functional unit such as operational

amplifier.

4.3 T-SPICE PRO CIRCUIT ANALYSIS

An introduction to the integrated components of the T- Spice Pro circuit analysis suite:

Schematic data files (.sdb): describes the circuits to be analyzed in graphical form, for
display and editing by S- Edit" Schematic Editor.

Simulation input files (.sp): describes the circuits to be analyzed in textual form, for
editing and simulation by T- Spice" Circuit Simulator.

Simulation output files (.out): containing the numerical results of the circuit analyses, for
manipulation and display by W- Edit" Waveform Viewer.

4.3 CIRCUIT SIMULATOR (T-SPICE)

T- Spice Pro‟s waveform probing feature integrates S- Edit, T- Spice, and W- Edit to allow
individual points in a circuit to be specified and analyzed. A few analysis is described
below:

The heart of T-Spice operation is the input file (also known as the circuit description, the
net list & the input deck). This is a plain text file that contains the device statement &
simulation commands, drawn from the SPICE circuit description language with which T-
Spice constructs a model of the circuit to be simulated. Input files can be created and
modified with any text editor.

T-Spice is a tool used for simulation of the circuit. It provides the facility of

1. Design Simulation

2. Simulation Commands

3. Device Statements
32
 4. User-Designed External Models

5. Small Signal & Noise Models

T-Spice uses Kirchhoff‟s Current Law (KCL) to solve circuit problems. To T-Spice, a
circuit is a set of devices attached to nodes. The voltage at all nodes represents the circuit
state. T-Spice solves for a set of node voltage that satisfied KCL (implying that sum of
currents flowing into each node is zero). In order to evaluate whether a set of node voltages
is a solution, T-Spice computers and sums all the current flowing out of each device into
nodes connected to it (its terminals). The relationship between the voltages at device
terminals and the currents through the terminal is determined by the device model for a
resistor of resistance R is

I=∆V/R

Where, ∆V represents the voltage difference across the device. A few analyses are
discussed below:

4.3.1 DC Operating Point Analysis

DC operating point analysis finds a circuit‟s steady- state condition, obtained (in principle)
after the input voltages have been applied for an infinite amount of time. The .include
command causes T- Spice to read in the contents of the model file for the evaluation of
NMOS and PMOS transistors.

The technology file assigns values to MOSFET model parameters for both n - and p -type
devices. When read by the input file, these parameters are used to evaluate MOSFET
model equations, and the results are used to construct internal tables of current and charge
values. Values read or interpolated from these tables are used in the computations called
for by the simulation. Following each transistor name are the names of its terminals. The
required order of terminal names is: drain -gate -source -bulk. Then the model name
(NMOS or PMOS in this example), and physical characteristics such as length and width,
are specified. The .op command performs a DC operating point calculation and writes the
results to the file specified in the Simulate > Start Simulation dialog. The output file lists
the DC operating point information for the circuit described by the input file.

33
4.3.2 DC Transfer Analysis

DC transfer analysis is used to study the voltage or current at one set of points in a circuit
as a function of the voltage or current at another set of points. This is done by sweeping the
source variables over specified ranges, and recording the output. A list of sources to be
swept, and the voltage ranges across which the sweeps are to take place follow the .dc
command, indicating transfer analysis. The transfer analysis will be performed as follows:
vdd will be set at 5 volts and vin will be swept over its specified range; vdd will then be
incremented and vin will be reswept over its range; and so on, until vdd reaches the upper
limit of its range. The .dc command ignores the values assigned to the voltage sources vdd
and vin in the voltage source statements, but they must still be declared in those
statements. The results for nodes in and out are reported by the .print dc command to the
specified destination.

4.3.3 Transient Analysis

Transient analysis provides information on how circuit elements vary with time. The basic
T- Spice command for transient analysis has three modes. In the default mode, the DC
operating point is computed, and T- Spice uses this as the starting point for the transient
simulation. The .tran command specifies the characteristics of the transient analysis to be
performed.

4.3.4 AC Analysis

AC analysis characterizes the circuit‟s behavior dependence on small- signal input

frequency. It involves three steps: (1) calculating the DC operating point; (2) linearizing
the circuit; and (3) solving the linearized circuit for each frequency. When ac voltage
source is to be applied, then vdiff sets the DC voltage difference between nodes the two
nodes to -0. 0007 volts; its AC magnitude is 1 volt and its AC phase is 180 degrees. The
.ac command performs an AC analysis. Following the .ac keyword is information
concerning the frequencies to be swept during the analysis. In case, the frequency is to be
swept logarithmically, by decades (DEC); 5 data points are to be included per decade is
considered to be the standard The two .print commands write the voltage magnitude (in
decibels) and phase (in degrees), respectively, for the node out to the specified file. The
.acmodel command writes the small- signal model parameters and operating point voltages

34
and currents for all circuit devices

4.3.5 Noise Analysis

Real circuits, of course, are never immune from small, random fluctuations in voltage and
current levels. In T- Spice, the influence of noise in a circuit can be simulated and reported
in conjunction with AC analysis. The purpose of noise analysis is to compute the effect of
the noise associated with various circuit devices on an output voltage or voltages as a
function of frequency. Noise analysis is performed in conjunction with AC analysis; if the
.ac command is missing, then the .noise command is ignored. With the .ac command
present, the .noise command causes noise analysis to be performed at the same frequencies.
The .noise command takes two arguments: the output at which the effects of noise are to be
computed, and the input at which the .noise can be considered to be concentrated for the
purposes of estimating the equivalent noise spectral density. The print command is used to
print results.

4.4 WAVEFORM EDIT

The ability to visualize the complex numerical data resulting from VLSI circuit simulation
is critical to testing, understanding & improving these circuits. W-Edit is a waveform
viewer that provides ease of use, power & speed in a flexible environment designed for
graphical data representation. The advantages of W-Edit include:

1. Tight Integration with T-spice, Tanner EDA_s circuit level simulator. W-Edit can chart
data generated by T-spice directly, without modification of the output text data files. The
data can also be charted dynamically as it is produced during the simulation.

2. Charts can automatically configure for the type of data being presented.

3. A data is treated by W-Edit as a unit called a trace. Multiple traces from different output
files can be viewed simultaneously in single or several windows; traces can be copied and
moved between charts & windows. Trace arithmetic can be performed on existed tracing to
create new ones.

4. Chart views can be panned back & forth and zoomed in & out, including specifying the
exact X-Y co-ordinate range.

5. Properties of axes, traces, rides, charts, text & colors can be customized.

35
Numerical data is input to W-Edit in the form of plain or binary text files. Header &
Comment information supplied by T-Spice is used for automatic chart configuration.
Runtime update of results is made possible by linking W-Edit to a running simulation in T-
Spice. W-Edit saves data with chart, trace, axis & environment settings in files with the
WDB (W-Edit Database).

4.5 LAYOUT(L-EDIT)

It is a tool that represents the masks that are used to fabricate an integrated circuit. It
describes a layout design in terms of files, cells & mask primitives. On the layout level, the
component parameters are totally different from schematic level. So it provides the facility
to the user to analyze the response of the circuit before forwarding it to the time consuming
& costly process of fabrication. There are rules for designing layout diagram of a
schematic circuit using which user can compare the output response with the expected one.

4.5.1 L- Edit: An Integrated Circuit Layout Tool

In L- Edit, layers are associated with masks used in the fabrication process. Different
layers can be conveniently represented by different colors and patterns. L- Edit describes a
layout design in terms of files, cells, instances, and mask primitives. You may load as
many files as desired into memory. A file may be composed of any number of cells. A file
may be composed of any number of cells. These cells may be hierarchically related, as in a
typical design, or they may be independent, as in a library file. Cells may contain any
number or combination of mask primitives and instances of other cells.

Cells: The Basic Building Blocks

The basic building block of the integrated circuit design in L- Edit is a cell. Design layout
occurs within cells. A cell can:

 Contain part or all of the entire design.

 Be referenced in other cells as a sub- cell, or instance.

 Be made up entirely of instances of other cells.

 Contain original drawn objects, or primitives.

 Be made up entirely of primitives or a combination of primitives and instances of

other cells.
36
Hierarchy

L- Edit supports fully hierarchical mask design. Cells may contain instances of other cells.
An instance is a reference to a cell; should you edit the instanced cell, the change is
reflected in all the instances of that cell. Instances simplify the process of updating a
design, and also reduce data storage requirements, because an instance does not need to
store all the data within the instanced cell instead, only a reference to the instanced cell is
stored, along with information on the position of the instance and on how the instance may
be rotated and mirrored.

L- Edit does not use a “separated” hierarchy: instances and primitives may coexist in the
same cell at any level in the hierarchy. Design files are self- contained. The pointer to a cell
contained in an instance always points to a cell within the same design file. When cells are
copied from one file to another, L- Edit automatically copies across any cells that are
instanced by the copied cell, to maintain the self- contained nature of the destination file.

Design Rules

Manufacturing constraints can be defined in L- Edit as design rules. Layouts can be

checked against these design rules.

Design Features

L- Edit is a full- custom mask editor. Manual layout can be accomplished more quickly
because of L Edit‟s intuitive user interface. In addition, one can construct special structures
to utilize a technology without, worrying about problems caused by automatic
transformations. Phototransistors, guard bars, vertical and horizontal bipolar transistors,
static structures, and Schottky diodes, for example, are as easy to design in CMOS- Bulk
technology as are conventional MOS transistors.

Floor plans

L- Edit is a manual floor planning tool. You have the choice of displaying instances in
outline, identified only by name, or as fully fleshed- out mask geometry. When you display
your design in outline, you can manipulate the arrangement of the cells in your design
quickly and easily to achieve the desired floor plan. One can manipulate instances at any
level in the hierarchy, with insides hidden or displayed, using the same graphical move/

37
select operations or rotation/ mirror commands that you use on primitive mask geometry.

Memory Limits

In L- Edit, one can make your design files as large as one like, given available RAM and
disk space.

Hard Copy

L- Edit provides the capability to print hard copy of the design. A multiage option allows
very large plots to be printed to a specific scale on multiple 8 1/ 2 x 11 inch pages. An L-
Edit macro is available to support large- format, high- resolution, color plotting on inkjet
plotters.

Variable Grid

L- Edit‟s grid options support lambda- based design as well as micron- based and mil-
based design.

Error Recovery

L- Edit‟s error- trapping mechanism catches system errors and in most cases provides a
means to recover without losing or damaging data.

L- Edit Modules

 L- Edit TM: a layout editor

 L- Edit ¤ Extract TM: a layout extractor

 L- Edit ¤ DRC TM: a design rule checker

L- Edit is a full- featured, high-performance, interactive, graphical mask layout editor. L-

Edit generates layouts quickly and easily, supports fully hierarchical designs, and allows an
unlimited number of layers, cells, and levels of hierarchy. It includes all major drawing
primitives and supports 90°, 45°, and all- angle drawing modes.

L- Edit ¤ Extract creates SPICE- compatible circuit netlists from L- Edit layouts. It can
recognize active and passive devices, sub circuits, and the most common device
parameters, including resistance, capacitance, device length, width, and area, and device
source and drain area.

38
L- Edit ¤ DRC features user- programmable rules and handles minimum width, exact
width, minimum space, minimum surround, non- exist, overlap, and extension rules. It can
handle full chip and region- only DRC. DRC offers Error Browser and Object Browser
functions for quickly and easily cycling through rule- checking errors.

39
 CHAPTER-5
GATES DESIGN USING GDI METHOD

5.1 AND Gate based on GDI method

The design of an AND gate based on GDI method. It requires a single GDI cell in
which the source of the PMOS that is port P is connected to GND and A is given as an
input to port G while port N is supplied input B.

Figure5.1 : AND gate using GDI method

shows the input and output waveforms of an AND gate conceived using GDI

40
 Figure5.2 : Input and output waveform of an AND gate using GDI
OR Gate based on GDI method
The OR gate consists of a single GDI cell as shown in Fig. where port P is given an
input B, port G an input A while port N is supplied with Vdd.

Fig5.3 : OR GATE-GDI

41
Fig. shows the input and output waveforms of an OR gate constructed using GDI method

Figure5.4 : Input and output waveform of an OR gate using GDI

5.2 NOT Gate based on GDI method

The design of the NOT gate based on GDI procedure is similar to that of a
standard CMOS inverter which is quite evident from table I. Fig. shows the circuit
diagram of a NOT gate derived using GDI technique.

Figure5.5 : NOT gate using GDI method

42
Fig. shows the input and output waveforms of the NOT gate GDI approach

Figure5.6 : Input and output waveform of an NOT gate using GDI

5.3 XOR gate based on GDI method

XOR gate based on GDI procedure. It contains two GDI cells in which the first cell
acts as a basic inverter while for the second cell „x‟ is given as an input to port P of the
GDI cell whereas „y‟ is given as an input to port G and the output of the first cell is given
as an input to port N of the second cell. Fig. shows the input and output waveforms of a
XOR gate designed using GDI

43
 Figure5.7 : XOR gate using GDI method

Figure5.8 : Input and output waveform of an XOR gate using GDI

44
5.4 PERFORMANCE ANALYSIS:
displays a graphical comparative study of the average power consumption of the
XOR, AND and OR gate based on GDI method with respect to those designed using
conventional CMOS logic, transmission gates (TG)

Since the design of a NOT gate based on GDI method is equivalent to the conventional
CMOS inverter, comparison between them would be insignificant. The average power
consumption of the NOT gate by means of GDI process in 150nm technology is 3.01x10-
10 watts on application of a power supply voltage (VDD) of 1.0 volt.

Figure5.9 : Graphical analysis of the average power consumption

45
 CHAPTER-6
RESULTS AND ANALYSIS

6.1 DESIGN UNIT:

AND Gate

Fig6.1: AND Gate Design

NOT Gate

Fig6.2: NOT Gate Design

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OR Gate

Fig6.3: OR Gate Design

XOR Gate

Fig6.4: XOR Gate Design

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Half Adder

Fig6.5: Half adder Design

One Bit Full Adder

Fig6.6: 1-bit full adder Design

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4BIT RCA ADDER: (RIPPLE CARRY ADDER)

Fig6.7: 4-bit RCA adder Design

8BIT CIA ADDER: (CARRY INCREMENT ADDER)

Fig6.8: 8-bit CIA Adder Design

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BIT CBA ADDER (CARRY BYPASS ADDER):

Fig6.9:8-bit CBA Adder Design

BIT CSKA ADDER (CARRY SKIP ADDER)

Fig6.10: 8-bit CSKA adder Gate Design

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BIT CLA ADDER (CARRY LOOK AHEAD ADDER):

Fig6.11: 8-bit CLA Adder Design

6.2 OUTPUT WAVEFORMS:

AND Gate

Fig6.12: Output waveforms for AND Gate

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MUX

Fig6.13: Output waveforms for MUX

OR Gate

Fig6.14: Output waveforms for OR Gate

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XOR Gate

Fig6.15: Output waveforms for XOR Gate

Half Adder

Fig6.16: Output waveforms for Half Adder

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Full Adder

Fig6.17: Output waveforms for 1-Bit Full Adder

CIA

Fig6.18: Output waveforms for 8-Bit CIA for Adding of (1101 1011)2 and (1110 1110)2

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CBA

Fig6.19: Output waveforms for 8-Bit CBA for Adding of (0101 1101)2 and (1011 1101)2

CKA

Fig6.20: Output waveforms for 8-Bit CKA for Adding of (1111 0111)2 and (1100 1111)2

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6.21 CLA

Fig6.21: Output waveforms for 8-Bit CLA for Adding of (1100 1111)2 and (1111 1101)2

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 CHAPTER-7
CONCLUSION AND FUTURE SCOPE

7.1 CONCLUSION
A novel methodology for asynchronous circuits, based on two-transistor GDI cells, was
presented. In this project we proposed a GDI-ADDRES for low-power design was
presented. The proposed circuit has a simple structure, based power consumption
principles, and some gates to described using GDI method.
The current work proposes the design of a full subtractor using Gate Diffusion
Input (GDI) procedure which on simulation has been found to consume low power in
conjunction with lesser delay time and fewer transistors while maintaining proper output-
voltage swing. In order to establish the technology independence the present work has
been performed in 150nm technology using Tanner SPICE and the layout has been
concocted in Microwind. Comparisons with standard CMOS, transmission gate and CPL
techniques.

7.2 FUTURE SCOPE

Gate diffusion input (GDI) technique is used for designing the flops, memory devices
also the power consumption, delay, chip area and connection and parasitic capacitors are
decreased

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VLSI”, I. J. of innovative Research in Science, Eng and Technology, Vol. 4,
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 Jashanpreet kaur, et.al. “A Review on GDI”, I. J of Advance Research in
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 P.Chaitanya Kumari, “Design of 32 bit Parallel Prefix Adders”, IOSR Journal of
Electronics and Communication Engg, Vol.6, Issue 1, May 2013.
 Adilakshmi siliveru, “Design of KSA and BKA using Degenerate PTL”, I.J. of
Emerging Science and Engg, Vol. 1(4), 2013, pp.2319–6378.
 Arun Prakash Singh, “Implementation of 1-Bit Full Adder using GDI Cell”, I.J.of
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 Madhu Thakur, “ Design of Braun Multiplier with KS A & its Implementation on
FPGA”, I.J. of Scientific & Engg Research, Vol.3, Issue 10, Oct-2012.
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