DESIGN OF LOW VOLTAGE CMOS TRISTATE BUFFER

Nurul Huda binti Zulkifli

Bachelor of Engineering with Honours

(Electronic and Telecommunication Engineering)

2010

UNIVERSITI MALAYSIA SARAWAK

R13a

BORANG PENGESAHAN STATUS TESIS

Judul: DESIGN OF LOW VOLTAGE CMOS TRISTATE BUFFER

SESI PENGAJIAN: 2009/2010

Saya NURUL HUDA BINTI ZULKIFLI

(HURUF BESAR)

mengaku membenarkan tesis * ini disimpan di Pusat Khidmat Maklumat Akademik, Universiti Malaysia Sarawak

dengan syarat-syarat kegunaan seperti berikut:

1. Tesis adalah hakmilik Universiti Malaysia Sarawak.

2. Pusat Khidmat Maklumat Akademik, Universiti Malaysia Sarawak dibenarkan membuat salinan untuk

tujuan pengajian sahaja.

3. Membuat pendigitan untuk membangunkan Pangkalan Data Kandungan Tempatan.

4. Pusat Khidmat Maklumat Akademik, Universiti Malaysia Sarawak dibenarkan membuat salinan tesis ini

sebagai bahan pertukaran antara institusi pengajian tinggi.

5. ** Sila tandakan ( ) di kotak yang berkenaan

SULIT (Mengandungi maklumat yang berdarjah keselamatan atau kepentingan

Malaysia seperti yang termaktub di dalam AKTA RAHSIA RASMI 1972).

TERHAD (Mengandungi maklumat TERHAD yang telah ditentukan oleh organisasi/

badan di mana penyelidikan dijalankan).

TIDAK TERHAD

Disahkan oleh

(TANDATANGAN PENULIS) (TANDATANGAN PENYELIA)

Alamat tetap: KAMPUNG TERSUSUN AIR

KUNING,

31800 TANJONG TUALANG, MDM. SHARIFAH MASNIAH WAN

MASRA

Nama Penyelia

PERAK.

Tarikh: Tarikh:

CATATAN * Tesis dimaksudkan sebagai tesis bagi Ijazah Doktor Falsafah, Sarjana dan Sarjana Muda.

** Jika tesis ini SULIT atau TERHAD, sila lampirkan surat daripada pihak berkuasa/organisasi

berkenaan dengan menyatakan sekali sebab dan tempoh tesis ini perlu dikelaskan sebagai

SULIT dan TERHAD.

This Final Year Project attached here:

Title : Design of Low Voltage CMOS Tristate Buffer

Student Name : Nurul Huda binti Zulkifli

Matric No : 17052

has been read and approved by:

_________________________ _________________

Mdm. Sharifah Masniah Wan Masra Date

(Supervisor)

DESIGN OF LOW VOLTAGE CMOS TRISTATE BUFFER

NURUL HUDA BINTI ZULKIFLI

This project is submitted in partial fulfillment of

The requirements for the degree of Bachelor of Engineering with Honours

(Electronics and Telecommunication Engineering)

Faculty of Engineering

UNIVERSITI MALAYSIA SARAWAK

2009/2010

ii

Dedicated to my beloved family and friends.

iii

ACKNOWLEDGEMENT

First and foremost, I would like to express my utmost gratitude to God for

His abundant blessings and for the idea for me to successfully accomplished this

Final Year Project. Secondly, I am very grateful to my patience supervisor, Mdm.

Sharifah Masniah Wan Masra for all the idea and guidance without which this study

would have been very difficult to complete. Also for Mdm. Maimun Huja Hussain

and Dr. Bakri Madon for helping me with Pspice software and circuit design. Next, I

would fell indebted to my dearest friends for their moral and material supports. Last

but not least, I would like to thank my beloved parents Encik Zulkifli bin Haji

Zakaria and Puan Noraisah binti Mohd Isa that always have trust in me. Also for my

treasured sisters and brothers who are always be my source of motivation.

iv

ABSTRAK

CMOS tristate buffer banyak digunakan dalam sistem elektronik. Dengan

meningkatnya pengggunaannya, juga menuntut bahawa buffer yang beroperasi di

kelajuan tinggi. Jadi kelajuan tinggi CMOS tristate buffer direkabentuk. Sistem

kelajuan tinggi adalah mudah untuk dilaksanakan, tetapi untuk kelajuan tinggi

dengan sistem voltan rendah yang sukar dicapai. Kerana voltan yang digunakan

adalah rendah, kuasa yang digunakan juga akan menjadi rendah. Tujuan projek ini

adalah untuk merekabentuk suatu tristate buffer voltan rendah yang boleh

menggerakkan beban kapasitif lebih besar. Masalah yang timbul adalah bagaimana

untuk mengawal throughput kerana dalam kebanyakan kes jika voltan sistem

diturunkan, kelajuan sistem akan menurun juga. Dengan melakukan pensaizan

transistor terhadap litar kelajuan tinggi itu, voltan bekalan yang digunakan dalam

litar ini diturunkan dari voltan bekalan litar kelajuan tinggi. Nilai optimum yang

dipilih untuk voltan bekalan adalah 1.6 V sementara nilai yang dipilih untuk beban

kapasitif adalah 10 pF. Disebabkan nilai voltan yang digunakan semakin rendah,

pada masa yang sama didapati PDP dan EDP juga menurun, sementara kelajuan

tinggi dikekalkan.

v

ABSTRACT

CMOS tristate buffer are widely used in electronic systems. As the increasing

of its usage, it is also demanding that the buffer is operating in high speed. So the

high speed CMOS tristate buffer is designed. High speed systems are easy to be

implemented, but for a high speed with low voltage system are difficult to achieve.

As the voltage used is low, the power consumption will be also low. The objective of

this project is to design a low voltage tristate buffer that can drive a larger capacitive

load. The problem is how to maintain the throughput while in most cases, lowering

voltage of the system will lowering the system speed too. By applying transistor

sizing to the high speed circuit, the supply voltage used in this circuit is lowered

from the high speed design. The chosen optimum value used for supply voltage is

1.6 V while chosen value for capacitive load is 10 pF. As the value of the supply

voltage is getting lower, it is shown at the same time that the Power Delay Product

and Energy Delay Product also decreasing while maintained the high speed.

vi

TABLE OF CONTENTS

CONTENT Pages

DEDICATION ii

ACKNOWLEDGEMENT iii

ABSTRAK iv

ABSTRACT v

TABLE OF CONTENTS vi

LIST OF TABLES ix

LIST OF FIGURES x

ABBREVIATIONS xii

Chapter 1 INTRODUCTION

1.1 Introduction 1

1.2 Problem Statement 3

1.3 Project Objectives 5

1.4 Project Scope 6

1.5 Report Structures 6

Chapter 2 LITERATURE REVIEW

2.1 Basic Concept of CMOS 8

2.2 Inverter 10

2.2.1 Area and Complexity 10

2.2.2 Functionality and Robustness 10

2.2.3 Noise Margins 11

vii

2.2.4 Ideal Digital Gate 12

2.2.5 Performance 13

2.2.6 Power and Energy

Consumption

15

2.3 CMOS Inverter 16

2.4 Buffer 18

2.5 Tristate Buffer 20

2.5.1 Uses of Tristate Buffer 22

2.6 CMOS Tristate Buffer 23

2.7 CMOS Power Consumption 25

2.7.1 Capacitive voltage transitions

2.7.2 Short-Circuit Component of

Power

25

28

2.7.3 Leakage Component of Power 29

2.7.4 Technology Optimization 30

2.8 Charge Pump 33

Chapter 3 METHODOLOGY

3.1 Introduction 34

3.2 Design Flow 35

3.3 Pspice Software 37

3.4 Conventional CMOS Tristate Buffer 39

3.5 High Speed CMOS Tristate Buffer 40

3.6 Proposed CMOS Tristate Buffer 42

3.7 The W/L Values 43

viii

3.8 Transistor Sizing 45

Chapter 4 RESULTS AND DISCUSSIONS

4.1 Introduction 49

4.2 During pull down (En=1, Vin= High to

Low)

50

4.3 During pull up (En=1, Vin= Low to High) 55

4.4 Short Circuit Current 59

4.5 Power Delay Product 61

4.6 Energy Delay Product 62

4.7 Discussion 64

Chapter 5 CONCLUSIONS AND

RECOMMENDATIONS

5.1 Conclusions 67

5.2 Recommendations 68

5.3 Problems Encountered 68

5.4 Conclusion 69

REFERENCES 70

APPENDICES

APPENDIX A 73

APPENDIX B 74

ix

LIST OF TABLES

TABLES PAGES

2.1 Enable Tristate Buffer 23

x

LIST OF FIGURES

FIGURES PAGES

2.1 Cascaded inverter gates: definition of noise margins 12

2.2 Ideal digital gate 13

2.3 The propagation delay from waveform circuit 14

2.4 Static CMOS inverter 17

2.5 Transistor Symbols 17

2.6 Inverter and buffer 18

2.7 Definition of fan-out and fan-in of a digital gate 19

2.8 Tristate buffer with active high and low control 20

2.9 Tristate buffer as a switch 22

2.10 General CMOS Tristate Buffer Circuit 23

2.11 Normalized delay vs. Vdd for a typical gate in a standard

CMOS process

30

2.12 Effect of threshold reduction on the delay for various supply

voltages

32

2.13 Compromise between dynamic and leakage power dissipation

through Vt variation

32

3.1 Tristate Buffer Design Process Flow 35

xi

3.2 Conventional CMOS Tristate Buffer Circuit 39

3.3 High Speed CMOS Tristate Buffer Circuit 40

3.4 Proposed CMOS Tristate Buffer Circuit 41

3.5 Circuit model for analyzing the effect of transistor sizing 45

3.6 Plot of Energy vs. Transistor Sizing factor for various

parasitic contributions

47

4.1 Inputs and output of conventional design during pull down 50

4.2 Output of high speed design during pull down 50

4.3 Output of proposed design during pull down 50

4.4 Inputs and output of conventional design during pull up 55

4.5 Output of high speed design during pull up 55

4.6 Output of proposed design during pull up 55

4.7 Conventional tristate buffer short circuit current through

NMOS transistor

59

4.8 High speed tristate buffer short circuit current through NMOS

transistor

60

4.9 Proposed tristate buffer short circuit current through NMOS

transistor

60

4.10 Graph of the propagation delays versus capacitive load 64

4.11 Graph of the propagation delays versus supply voltage 65

4.12 Graph of the Energy Delay Product versus supply voltage 65

4.13 Graph of the Energy Delay Product versus propagation delays 66

xii

ABBREVIATIONS

AC - Alternating current

ASIC - Application-Specific Integrated Circuit

BiCMOS - Bipolar Complementary Metal Oxide Semiconductor

BSIM - Berkeley Short-Channel IGFET Model

CMOS - Complementary Metal Oxide Semiconductor

COS-MOS - Complementary-Symmetry Metal-Oxide-Semiconductor

DC - Direct Current

EDP - Energy Delay Product

I/O - Input/Output

IC - Integrated Circuit

MOS - Metal Oxide Semiconductor

MOSFET - Metal Oxide Semiconductor Field Effect Transistor

NMOS - N-type Metal Oxide Semiconductor

PDP - Power Delay Product

PMOS - P-type Metal Oxide Semiconductor

TTL - Transistor-Transistor Logic

VLSI - Very Large Scale Integration

W/L - Width/Length

1

CHAPTER 1

INTRODUCTION

1.1 Introduction

Recently, all digital circuits are demanded to operate with low power

consumption as power dissipated by electronic circuit affect the portability and size

of electronic systems. The most important technology consideration in minimizing

power consumption for digital systems is the threshold voltage and its control which

allows the reduction of supply voltage without significant effect on logic speed.

Since CMOS circuits do not dissipate power if they are not switching, a

major focus of low power design is to reduce the switching activity to the minimal

level required to perform the computation. This can range from simply powering

down the complete circuit or portions of it, to more sophisticated schemes in which

2

the clocks are gated or optimized circuit architectures are used which minimize the

number of transitions [1].

An important attribute which can be used in circuit and architectural

optimization, is the correlation which can exist between values of a temporal

sequence of data, since switching should decrease if the data is slowly changing

(highly correlated) [1].

So for achieving this goal, the effect of sizing and supply voltage on the

circuit is extensively considered. The optimization to minimize area at all costs has

only been secondary to the fixation on increasing circuit speed, but this should be

examined with respect to its effect on power consumption. Energy Delay Product

(EDP) and Propagation Delay Product (PDP) have been the main objectives of this

research as it is compromise between the attempt to reduce the energy and time

consumption.

The tristate buffer are extensively used as a bus in digital systems such as

microprocessor, ASIC and memory. When one device is sending on the bus, another

sending device should be disconnected. This can be achieved by setting the output

buffers of those devices in high impedance state (Hi-Z) that effective disconnects the

gate from output wire. Such a buffer has three possible states that are 0, 1 and Hi-Z

therefore called tristate buffer. Owing to a bundle of wires connected to a bus, a

large capacitive load driving capability is one of the most importance properties of

tristate buffer [2].

3

CMOS is more attractive than BiCMOS in electronic system because of its

simplicity, low power consumption and optimization of purity in circuit compared to

BiCMOS circuit which utilizing CMOS and BJT in the same circuit [3]. For this

reason, a low voltage tristate buffer based on CMOS technology that can drive larger

capacitive load that is suitable for driving a data bus will be designed.

1.2 Problem Statement

Designs of high speed tristate buffer have been developed by some

researchers. However, only [1] have developed high speed and low voltage tristate

buffer using BiCMOS with large capacitive load driving capability. Recently

attention on BiCMOS circuit technology has increased because of their high speed

and driving capability over CMOS. However, electronic industry still chooses

CMOS over BiCMOS because BiCMOS process complexity that contributes to

higher cost and longer fabrication cycle time. Thus, a low voltage CMOS tristate

buffer that can drive larger capacitive load for driving a data bus will be designed.

Problems arise with building low voltage products to be compatible with

existing products that utilize a higher power supply potential.

One problem exists with the compatibility of conventional input/output (I/O)

buffers using CMOS low-voltage processing technology. If conventional output

buffers are used in CMOS products operating with a low voltage supply, then current

leakage problems may arise when higher voltages are driven into the output pad

4

while the buffer is in a tristate mode. Obviously, current leakage problems are

undesirable in an integrated circuit, especially in cases where the magnitude of the

leakage violates accepted I/O bus standards [4].

Other problems also exist when tristateable output buffers operating from a

low voltage supply are connected to a higher voltage bus. In addition to the leakage

problem, there are two long-term reliability issues to consider. These problems are

commonly referred to as “hot electron reliability” and “p-device stability” [4].

Hot electron reliability denotes the phenomenon wherein the operating

performance of a transistor degrades due to a build up of trapped carriers in the gate

oxide region. Because electron carriers have a higher mobility than hole carriers, this

problem is more pronounced for n-channel devices as compared to p-channel devices

[4].

The performance of a transistor degrades as the electric field of the trapped

carriers interact with the electric field applied at the gate of the transistor. Hot

electron reliability is a probabilistic model where the probability of a carrier being

trapped in the oxide is a function of the energy of the carrier as it passes through the

channel of the transistor. At the operating point of saturation devices have the

highest probability of injecting carriers into the gate oxide. Among the factors that

determine if a transistor will experience hot electron reliability problems include

frequency of switching, size of load, operating temperature, driving potential, device

channel length, device channel width, the rise/fall times of the signal on the gate, and

the expected lifetime of operation of the transistor in the field [4].

5

Another long-term reliability risk, referred to as p-device stability, involves

the undesirable shift in the threshold voltage when excessive electric fields have

been applied across the gate oxide of a p-channel transistor. Note that n-channel

devices are also susceptible to a shift in their threshold voltage as a result of

excessive electric fields being experienced across their gate oxide. However, p-

channel devices are usually more sensitive to this phenomena [4].

Other problem is that, how to design a low voltage supply without degrade

the performance because it is extensively known that the speed degradation at low

supply voltage is the major limitation factor of both CMOS and BiCMOS in

submicron technologies [2].

1.3 Project Objectives

In this project, the following goals will be achieved:

1. To design and simulate using Pspice a low voltage tristate buffer that can

drive larger capacitive load.

2. To obtain lower EDP than previous researchers’ EDP.

3. To find the optimized tradeoff value between voltage and speed of CMOS

tristate buffer.

6

1.4 Project Scope

This project is a research on how to design a low voltage CMOS tristate

buffer while maintained or improved speed of the previous proposed design. The

new proposed design will improve recent driving capability where wasted power can

be reduced.

1.5 Report Structures

This report details overall process in developing the project starting from the

early idea of development, circuit development planning, software simulation until

report writing. This report is divided into five chapters including introduction;

literature review; methodology; results and discussion; and conclusion and

recommendation. The brief information of each chapter is described as below:

Chapter 1 provides background information, problem statement and

objectives of the project. It also provides scope of the project, project planning and

project structures.

Chapter 2 summarizes and reviews onto the overall studies and researches

work. Preliminary literature search included existing issues such as design of high

speed and low voltage tristate buffer using BiCMOS and minimizing power

consumption in digital electronic circuit are explained in detail.

7

Chapter 3 discusses the methodology and focuses on the use of Pspice

simulator to obtain the result to be achieved. The first part of this chapter

concentrates about the circuit development methodology. The second part shows the

proposed design circuit as well as the circuits of conventional design and high speed

design to be compared. The descriptions of the circuits are also have been discussed.

Chapter 4 exhibits the results and data obtained from the Pspice simulation. It

carries out analysis and discussion onto the data. It also examines whether the

simulation results fulfil the predicted or expected results as mentioned in the early

part of the project. Besides, different approaches and testing taken out to examine the

project in achieving its objectives are listed in details.

Chapter 5 summarizes the overall finding of the project. The problems faced

throughout the project are discussed too. In addition, further investigation which can

be implemented or future improvement of the project is being discussed.

Appendices and references are also included at the end of the report.

8

CHAPTER 2

LITERATURE REVIEW

2.1 Basic Concept of CMOS

Complementary metal-oxide-semiconductor (CMOS) is a technology for

making integrated circuits. CMOS technology is used in microprocessors,

microcontrollers, static RAM, and other digital logic circuits. CMOS technology is

also used for a wide variety of analog circuits such as image sensors, data converters,

and highly integrated transceivers for many types of communication. Frank Wanlass

successfully patented CMOS in 1967 (US Patent 3,356,858) [4].

CMOS is also sometimes referred to as complementary-symmetry metal-

oxide-semiconductor (or COS-MOS). The words “complementary-symmetry” refer

to the fact that the typical digital design style with CMOS uses complementary and

9

symmetrical pairs of p-type and n-type metal oxide semiconductor field effect

transistors (MOSFETs) for logic functions [4].

Two important characteristics of CMOS devices are high noise immunity and

low static power dissipation. CMOS dissipates power only when the transistors in

the CMOS device are switching between ON and OFF states. This shows that

CMOS devices do not produce as much waste heat as other logic devices such as

transistor-transistor logic (TTL) and NMOS logic, which uses all n-channel devices

without p-channel devices [4].

Other than that, CMOS is better than bipolar junction transistor (BJT) in the

aspect of power dissipation, noise margin, packing density, a good switch and also

the ability to integrate large and complex circuits and functions with high yield. This

is the reason why CMOS become the most used technology to be implemented in

VLSI chips [3].

From all of the reasons above, we can see why CMOS is preferred to be used

in most digital logic circuits rather than other logic circuits. Even though BiCMOS

become technology choice recently, CMOS still become electronic market choice.

Inverter and tristate buffer will be explained in the following section.