Introduction To Portable Devices

Published Date: 02 Nov 2017

CHAPTER 1

INTRODUCTION

Integrated circuit technology is the enabling technology for a whole host of innovative devices and systems that have changed the way we live. In the past, the major concerns of the VLSI designer were area, performance, cost and reliability. Power consideration was mostly of only secondary importance. In recent years, however this has begun to change and increasingly power is being given comparable weight to area and speed considerations. Several factors and emerging special portable devices and applications have contributed to this trend.

INTRODUCTION TO PORTABLE DEVICES

When describing hardware that is portable it means something that is small and lightweight. Portable device or machine is designed to be easily carried or moved. Any devices that can be moved around much more easily than other device are considered to be portable devices.

Devices which are small and light enough to be used while we are holding it are also part of portable devices. Portable electronic device is a device depending on the principles of electronics and using the manipulation of electron flow for its operation or a device that accomplishes its purpose electronically.

Emerging Portable Devices

For many people across the country, they cannot remember a time when they were without their portable electronic devices that have become such a large part of their lives. Imagine what it would be like to go without a cell phone, digital camera or even a laptop computer for long periods of time.

There are not a lot of us who could do so, and will go out their way to find just the right electronic accessories that can go anywhere they can. However, in comparison to a cell phone or PDA, a desktop is not a portable device. These devices would definitely be considered an example of a portable electronic device, as you can take them with you pretty much everywhere you go.

TYPES OF PORTABLE DEVICES

A wireless portable device is a device that is capable of storing, processing, or transmitting information. These devices include

Personal Digital Assistants (PDA)

Smart phones

Two-way pagers

Handheld radios

Cellular telephones

Personal Communication Services (PCS)

Scanning and Messaging devices

Handheld Game Consoles

Audio Player

Computers

Portable Applications

A portable application, sometimes also called as standalones, is a program designed to run on a compatible computer without being installed in a way that modifies the computer’s configuration. This type of application can be stored on any storage device, including internal mass storage and external storage such as USB drives and floppy disks. Like any application, the portable applications must be compatible with the computer system hardware and software.

DEMAND FOR PORTABLE DEVICES

For several generations now, we have come to love, and depend o our various portable electronic devices to get through our everyday lives. From cameras to cell phones to tablets and e-readers, there is not a lot that we do any more that does not have something created piece if electronics that is supposed to make our lives better through their presence.

We rely on cell phones and computers to communicate with our loved ones, friends, and the outside world in general, and we would feel empty without them. The key to many of our lives these days is the ability to multitask, and the best way to accomplish this is through portable electronic devices.

Emerging Technology

We go out of our way to purchase those that will allow us to do as much as we can, all within one handy little device. We have access to so much power at our fingertips that we can do anything we want, with very little, just by activating applications o the latest handheld devices.

Portable Device Selection

Not all of us can afford to have the latest portable electronic devices when they hit the market. So being able to choose what type of device we do purchase can be large issue in our lives. Some are fine with taking pictures using the tiny camera installed in our cell phone, but the results might not be what we want. To get the quality we need, we will have to find the best deals we can on digital cameras for sale.

It is based on two factors: what we need and what we can afford. The total package may not always be in sync, but what we have before us is still ten times better than any device we have used in the past, and the future holds a lot more options than might be possible today. This technology change is due to the development in very large scale integration techniques.

MAJOR PORTABLE DESIGN FACTORS

Since, all the portable devices today has got one or more microprocessor as the major component in the mother board the design criteria for any potable device includes

Area

Speed

Time delay

Power consumption

Portability

Reliability

Inexpensive

Merits of Portable devices

Portable devices possess the following merits in comparison to the normal electronic devices.

Potable – devices are small, handheld, light-weight, and can be easily carried by anyone. Inside, outside, to the library, anywhere.

Low cost – as compared to PC’s, mobile devices are less expensive.

Energy efficient – mobile devices require less power to run than PC’s.

Connected – most mobile devices come with Bluetooth or Wi-Fi as part of their standard configuration.

Rugged – all mobile devices come with a solid state disk drive, rather than moving disk drive, and can handle accidental drops and misuse.

Personal – no other technology creates such a unique, 1-to-1 feeling with the user than a mobile device. As a result, many teachers have seen greater student engagement with mobile devices for learning.

De-merits of Portable Devices

Portable devices possess the following de-merits in comparison to the normal electronic devices.

They possess small screen or display.

They have limited storage only.

Performance degradation as ageing.

Rapid change in technology switching to new products often.

Accident prone due to easy carrying capacity.

Draws more power based on hardware component parts

MICROPROCESSOR INVENTION

The past two decades have seen the introduction of a technology that has radically changed the way in which we analyze and control the world around us. Born of parallel developments in computer architecture and integrated circuit fabrication, the microprocessor, or "computer on a chip", first became a commercial reality in 1971 with the introduction of the 4-bit 4004 by a small, unknown company by the name of Intel Corporation.

Microprocessor Definition

A microprocessor, as the term has come to be known, is a general-purpose digital computer central processing unit (CPU). Although popularly known as a "computer on a chip", the microprocessor is in no sense a complete digital computer. A microprocessor incorporates the functions of a computer’s central processing unit (CPU) on a single integrated circuit, or at most a few integrated circuits. It is a multipurpose, programmable device that accepts digital data as input, processes it according to instructions stored in its memory, and provides results as output. It is an example of sequential digital logic, as it has internal memory. Microprocessors operate on numbers and symbols represented in the binary numeral system.

Structure of Microprocessor

The internal arrangement of microprocessor varies depending on the age of the design and the intended purposes of the processor. The complexity of an integrated circuit is bounded by physical limitations of the number of transistors that can be put onto one chip. The number of package terminations that can connect the processor to other parts of the system. The number of interconnections it is possible to make on the chip. The heat the chip can dissipate.

MICROPROCESSOR BLOCK

The block diagram of a microprocessor CPU, which contains

Arithmetic and Logic Unit (ALU)

Program Counter (PC)

Stack Pointer (SP)

Registers

Clock Timing Circuits and interrupt circuits.

accumulator

Registers

Arithmetic and Logic Unit

accumulator

Registers

accumulator

Registers

Arithmetic and Logic Unit

Arithmetic and Logic Unit

accumulator

Registers

Arithmetic and Logic Unit

accumulator

Registers

Arithmetic and Logic Unit

Registers

accumulator

Arithmetic and Logic Unit

accumulator

Arithmetic and Logic Unit

Arithmetic and Logic Unit

Arithmetic and Logic Unit

Arithmetic and Logic Unit

accumulator

Registers

Stack Pointer

Program Counter

Interrupt circuits

Clock Circuit

Fig: 1.1 Block Diagram of Microprocessor

1.6.1 Block Function

A minimal hypothetical microprocessor might only include an arithmetic and logic unit (ALU) and a control logic section. The ALU performs operations such as addition, subtraction, and operations such as AND, OR. Each operation of the ALU sets one or more flags in a status register, which indicate the result of the last operation. The logic section retrieves instruction operation codes from memory, and initiates whatever sequence of operations of the ALU to carry out the instruction. A single operation code might affect many individual data paths, registers, and other elements of the processor.

1.6.2 Microprocessor Design Requirements

To make the complete microcomputer, one must add memory, usually read-only program memory (ROM) and random-access data memory (RAM), memory decoders, an oscillator, and a number of input/output (I/O) devices, such as parallel and serial data ports. Additionally, special-purpose devices, such as interrupt handlers, or counters, may be added to relieve the CPU from time-consuming counting or time chores. Equipping the microcomputer with the mass storage device, commonly floppy disk drive, and I/O peripherals, such as a keyboard and a CRT display, yields a small computer that can be applied to a range of general purpose software applications.

1.6.3 Microprocessor Design Features

The key term in describing a microprocessor is "general-purpose". The hardware design of a microprocessor CPU is arranged so that a small, or very large, system can be configured around the CPU as the application demands. The internal CPU architecture, as well as the resultant machine level code that operates that architecture, is comprehensive but flexible as possible.

DIFFERENT MICROPROCESSORS

The microprocessors are of different types based upon their configuration and area of applications as follows

General purpose

Embedded application

RISC

Special type processors

APPLICATION AREAS OF MICROPROCESSOR

An embedded product uses a microprocessor to do one task and one task only. There is only one application software that is typically burned into ROM. The areas where microprocessor is present are

Home – Appliances, intercom, telephones, security systems, garage door openers, answering machines, fax machines, home computers, TVs, cable TV tuner, VCR, camcorder, remote controls, video games, cellular phones, musical instruments, sewing machines, lighting control, paging, camera, pinball machines, toys, exercise equipment.

Office – Telephones, computers, security systems, fax machines, microwave, copier, laser printer, color printer, paging

Automotive – Trip computer, engine control, air bag, ABS, instrumentation, security system, transmission control, entertainment, climate control, cellular phones, keyless entry.

MICROPROCESSOR BLOCK ALU

The arithmetic and logic unit (ALU) concept was proposed by John Von Neumann. ALU is a digital circuit shown in Fig 1.2, that performs the arithmetic and logic operations like addition, subtraction, AND, OR, inverter, multiplexers etc. ALU is the fundamental building block of any processor unit. It compares both numbers and bit strings, and results are used by control unit to alter the sequence of operations in a program.

Accumulator

Temporary register

register

Data Bus

Flags

Instructions

ALU

Results

Data 1

Data 2

Fig: 1.2 ALU block diagram

1.9.1 ALU Design Parts

The arithmetic and logic unit is designed with full adder units and multiplexer units.

Adder is the main component of the ALU.

Full adder unit in ALU performs all the operations of the ALU.

Adder constraints have to satisfy area, power, and consumption and speed requirement.

Multiplexer unit in ALU is used for selection of input and output signals.

1.9.2 ALU FUNCTION

The function loads data from input register with external control unit that informs operation type and results are stored in output register. Storage area is register. Data from storage is placed in registers. Manipulation is done in register and result transferred to storage. CPU has ALU, logic unit and memory. Adder core comp of ALU.

Serial adder: one digit at one time step wise addition

Parallel : all digits from one no to second no added in one step faster

Inputs : A and B can be of any number of bits

Cincarry out from previous stages

Output : in response to number of input bits

Control: 3 control signals forms logic unit S0, S1, S2.

S2 decides on arithmetic or logic operation

S0,S1 combination decides on which arithmetic or logic operation

Output of logic unit given to multiplexer

S2 decides which output either arithmetic or logic respective to selection and sends that output to multiplexer.

ALU Blocks

The arithmetic and logic consists of the full adder and the multiplexer units. The input and the output sections consist of 4x1 and 2x1 multiplexers for selection of signals and the logic is implemented using the full adder. A set of three select signals have been incorporated to determine the operation being performed and the inputs and outputs being selected.

1.10.1 Full Adder unit

An adder which can perform the addition of three binary numbers is called a Full Adder.

FA has got three inputs A, B, Cin and two outputs sum and Cout.

The third input Cin fed is obtained as the Carry out (Cout) of the previous stage in the cascade.

Based on the truth table the expression for Sum and Cout is

SUM = (A xor B) xor C

Cout = (A and B) + (Cin and (A xor B))

A

Co

Full Adder

B

SUM

Cin

Fig: 1.3 Full Adder circuit

Multiplexer Unit

Multiplication is important task in ALU. It dominates the execution time. Multiplexer unit selects binary information from one of many input lines and directs it to a single output line. Selection of input is controlled by the select lines. In general a multiplexer unit has 2n input and n select lines. Multiplexer unit is constructed using AND, OR, NOT gates.

Output

Inputs .

.

.

Selection Lines . . . .

Fig: 1.4 Multiplexer circuit

CHAPTER 2

BACKGROUND THEORY

2.1 INTEGRATED CIRCUITS

An integrated circuit or monolithic integrated circuit also referred to as an IC, a chip, or a microchip is a set of electronic circuits on one small plate of semiconductor material, normally silicon. This can be made much smaller than a discrete circuit made from independent components.

2.1.1 Integrated circuits Technology

Very Large Scale Integrated (VLSI) technology is the rapid growing technology for a wide range of innovative devices and systems that have changed the world today. Digital logic is implemented using transistors in integrated circuits containing many gates. Small-scale integrated circuits (SSI) contain 10 gates or less. Medium-scale integrated circuits (MSI) contain 10-100 gates. Large-scale integrated circuits (LSI) contain up to 104 gates. Very large-scale integrated circuits (VLSI) contain >104 gates. Improvements in manufacturing lead to ever smaller transistors allowing more chip.

2.2 VLSI TECHNOLOGY INTRODUCTION

The final step in the developing process of the IC technology, starting in the 1980s and continuing through the present, was "very large-scale integration (VLSI)". The development started with hundreds of thousands of transistors in the early 1980s, and continues beyond several transistors as of 2011.

2.2.1 VLSI Definition

Very Large Scale Integration (VLSI) is the process of creating integrated circuits by combining thousands of transistors into a single chip. VLSI began in the 1970s when complex semiconductor and communication technologies were being developed. The microprocessor is a VLSI device. Due to the fact that technology has moved far past the earlier mark, today’s microprocessors have millions of gates and billions of individual transistors.

Technology Path

In the last three decades the world of computers and especially that of microprocessors has been advanced at exponential rates in both productivity and performance. The integrated circuit industry has followed a steady path of constantly shrinking devices geometries and increased functionality that larger chips provide. The technology that enabled this exponential growth is a combination of advancements in process technology, micro architecture, architecture and design and development tools. Each new generation has approximately doubled logic circuit density and increased performance by about 40%.

MORRE’s LAW

In 1965, Gordon Moore noted that the number of transistors on a chip doubled every 18 to 24 months.

He made a prediction that semiconductor technology would double its effectiveness every 18 months.

Moore’s law continues to drive the scaling of CMOS technology. The feature size of the transistor now has been shrunk well into Nano-scale region.

A large single VLSl chip can contain over one billion transistor.

The ever-increasing level of integration has enabled higher performance and richer feature sets on a single chip.

As the geometry of the transistor is getting smaller and the number of transistors on a single chip grows exponentially, the power management for a state-of-the-art VLSI design has become increasingly important.

To maintain the performance trend of the VLSI system as the technology scaling continues, many advanced design techniques, especially in power management, have to be employed in order to achieve a balanced design to meet platform and end-user needs.

DESIGN LEVELS IN VLSI

The design levels in VLSI includes various levels of implementation like system level, algorithm level, architecture level, circuit level, process device level etc. The tools available support one or more levels of implementation. The detailed description of VLSI design levels is as follows in next sub-section.

2.4.1 System Level

Electronic system level (ESL) design and verification is an emerging electronic design methodology that focuses on the higher abstraction level concerns first and foremost. The term Electronic System Level or ESL Design was first defined by Gartner Dataquest. It states that, the utilization of appropriate abstractions in order to increase comprehension about a system, and to enhance the probability of a successful implementation of functionality in a cost effective manner.

The basic premise is to model the behavior of the entire system using a high-level language such as C, C++, LabVIEW, or MATLAB or using graphical "model- based" design tools like SystemVue or Simulink.

Algorithm Level

An algorithm-level behavioral description, annotated with probabilistic bounds on data dependent loop indices, and with transition probabilities on alternative control paths. A set of timing constraints and a set of optimization goals and/or power are included. The algorithmic model is constructed by compiling the input specification into a set of hierarchical data flow graphs, where nodes represent computations and edges represent data dependencies.

The resulting model is hierarchical since basic blocks, defined by alternative control paths, are first represented as single complex nodes. Such complex nodes are recursively decomposed into sub-graphs, containing less complex nodes, until the lowest level of granularity is reached. These last nodes are designated atomic nodes.

Higher Impact

System Level

Algorithm Level

Architectural Level

Circuit Level

Process Device Level

Fig: 2.1 VLSI Design Hierarchies

Architectural level

The design decisions made during architectural-level exploration are compiled in a broad architectural specification, which comprises: a first cut partitioning of the algorithmic model into a set of algorithmic segments, each of which is to be implemented by an individual architectural component and a set of fundamental design decisions on the implementation of these architectural component and their interfaces.

The basic elements of the proposed architectural model are: architectural components, modules, and physical resources. In order to efficiently support architectural exploration, three additional elements were added to the algorithmic model: algorithmic segments, clusters of nodes, and pipe- line stages.

Circuit Level

The circuit level design includes the implementation of design architecture in two ways namely register-transfer level and transistor level. The RTL is usually in the logic phase of the integrated circuit design. An RTL description is usually converted to gate-level description of the circuit by a logic synthesis tool. The synthesis results are then used by the placement and routing tools to create a physical layout.

Much logic is designed at the gate-level or higher, using well-tested standard cells. But custom logic, arrays are designed at the transistor level, not the gate level. Spice-based analog simulators are used to resolve tricky timing issues, characterize gates, and check power usage. Transistor level is faster and can handle much larger designs and longer simulations.

VLSI DESIGN PROCESS

Move from higher to lower levels of abstraction. Use CAD tools to automate parts of the process. Use hierarchy to manage complexity. Different design styles trade off are design time, non-recurring engineering (NRE) cost, unit cost, performance, power consumption.

2.5.1 VLSI Design Trade-Offs

Non –recurring Engineering (NRE) costs includes the design costs and the mask tooling costs. Unit –cost is related to the chip size, amount of logic and current technology. Performance is based on clock speed and implementation procedures involved. Power- consumption is a relatively new concern which is based on power supply voltage and clock speed.

VLSI DESIGN STYLES

The design styles in VLSI includes four styles namely

Full Custom

Application Specific Integrated Circuit (ASIC)

Programmable Logic (PLD, FPGA)

System-on-a-Chip

2.6.1 Full Custom Style

Each circuit element is carefully "handcrafted". Huge design effort is accompanied. High Design and NRE Costs or Low Unit Cost is obtained. High performance is achieved. Typically used for high-volume applications.

ASIC

Constrained design is carried using pre-designed and sometimes pre-manufactured components. ASIC is also called as semi-custom design. CAD tools greatly reduce design effort. Low Design Cost or High NRE Cost or Medium Unit Cost. Medium performance is achieved.

PLD & FPGA

Pre-manufactured components with programmable interconnect. CAD tools greatly reduce design effort. Low Design Cost or Low NRE Cost or High Unit Cost. Lower performance is achieved

SOC

Combine several large blocks. Predesigned custom cores (e.g., microcontroller) – "intellectual property" (IP). ASIC logic for special-purpose hardware is used. Programmable Logic (PLD, FPGA) in analog environment is done.

VLSI DESIGN TOOLS

The well known design tools used in VLSI design implementations includes the simulation of system-level, circuit-level, switch-level etc. Computer Aided Design paves way for all this tools mentioned in next sub-section.

2.7.1 ModelSim

Mentor Graphics is a US based Multinational Corporation dealing in electronic design automation (EDA) for electrical engineering and electronics. This company distributes the following simulation tool for VLSI technology, ModelSim is a popular hardware simulation and debug environment primarily targeted at smaller ASIC and FPGA design.

Xilinx

Xilinx Integrated Software Environment (ISE) is a software tool produced by Xilinx for synthesis and analysis of HDL designs, enabling the developer to synthesize their designs, perform timing analysis, examine RTL diagrams, simulate a design’s reaction to different stimuli, and configure the target device with the programmer.

Cadence

Cadence Design Systems is American electronic design automation (EDA) software and engineering services company, founded in 1988. The company produces software for designing integrated circuits also known as chips and printed circuit boards. Virtuoso Platform – Tools for designing full-custom integrated circuits; includes schematic entry, behavioral modeling (Verilog-AMS), circuit simulation, custom layout, physical verification, extraction and back-annotation. Used mainly for analog, mixed-signal, RF, and standard-cell designs, but also memory and FPGA designs.

VLSI ADVANTAGES

The following are the advantages in VLSI technology

Compactness

Mobility

Reliability

Effective use of space

Easily available productivity

Large market background

Reduces the size of the device

Reduces the cost of the device

Reduces the current consumption

Increases the speed of operation

Offers lots of employments

VLSI DISADVANTAGES

The disadvantages in VLSI technology are listed below

The production cost is high

CMOS technology has slower switching speeds

Uses low power analog front end to amplify mobile signals

Trade off between different VLSI technologies

Low design feature

High fabrication time

CHAPTER 3

METHODOLOGIES

3.1 INTRODUCTION

Implementation of power optimization on all the components present inside the processor is a choice. One of the basic and more important components inside a processor is the Arithmetic and Logic Unit (ALU). Even the simplest processor will contain one. An ALU is a digital circuit that performs the arithmetic and logic operations inside a processor as the name refers to. Full adder and multiplexer circuits comprises the ALU.

Since, the ALU being the most power consuming component inside a processor, we take the power consumption factor as the primary design criteria and have proposed a low power ALU design.

3.1.1 Objectives

Our primary objective is to design ALU satisfying low power consumption criteria.

The stringent requirement for low power consumptions has been a big issue in most embedded processor design, since power considerations have been the ultimate design criteria in special portable applications.

Hence, for large computations, efficient ALU is to be designed for minimum area and low-power without compromising high speed.

3.1.2 Design Challenges of Low Power

The electronic devices at the heart of such products need to dissipate low power, in order to conserve battery life and meet packaging reliability constraints.

Lowering power consumption is important not only for lengthening battery life in portable systems, but also for improving reliability, and reducing heat-removal cost in high-performance systems.

Consequently, power consumption is a dramatic problem for all integrated circuits designed today.

Following figure shows the relative impact on power consumption of each phase of the design process. Essentially higher – level categories have more effect on power reduction.

Low power design in terms of algorithms, architectures, and circuits has received significant attention and research input over the last decade.

3.2 POWER DISSIPATION

The rate of energy which is taken from the source and converted into heat is Power Dissipation. The types of power dissipation and the factor causing it are described below:

Static power dissipation

Due to leakage current

Dynamic Power dissipation

Due to switching activities of transistor

3.2.1 Need for Power Optimization

Due to integration of components increased the power comes in lime light

It is much important that handheld devices must possess low power devices

For better performance

For long run time (Battery time)

3.3 LOW POWER STRATEGIES

In system level – design partitioning, power down.

In algorithm level – complexity, concurrency, locality, regularity, data representation

In architecture level – voltage scaling, parallelism, instruction set, signal correlation

In circuit level – transistor sizing, logical optimization, activity driven power down, low swing logic, adiabatic switching.

Process device level – threshold reduction, multi threshold.

3.4 CIRCUIT LEVEL IMPLEMENTATION

The selection of circuit level design is made due to the reason that, circuit-level architecture dissipates less power in comparison to system-level or algorithm-level. The transistors possess three level of working namely cut-off, linear and saturation. Thus, coding has to be done for each working level of a transistor which would draw more power during simulation. The various methodologies in implementing this ALU architecture are discussed as follows.

3.4.1 CMOS Technology

Complementary metal oxide semiconductor (CMOS) is a technology for constructing integrated circuits. CMOS technology is used in microprocessors, microcontrollers, static RAM, and other digital logic circuits. CMOS technology is also used for several analog circuits such as image sensors, data converters, and highly integrated transceivers for many types of communication.

3.4.1.1 CMOS Definition

CMOS is also sometimes referred to as complementary symmetry metal oxide semiconductor. The "complementary-symmetry" refers to the fact that the typical digital design with CMOS uses complementary and symmetrical pairs of p-type and n-type metal oxide semiconductor field effect transistors for logic functions.

C:\Users\Priya\Downloads\npm.png

Fig: 3.1 CMOS Logic Circuit

3.4.1.2 CMOS Technology Characteristics

Two important characteristics of CMOS devices are high noise immunity and low static power consumption. Since one transistor of the pair is always off, the series combination draws significant power only momentarily during switching between on and off states. Consequently, CMOS devices do not produce as much waste heat as other forms of logic. CMOS also allows a high density of logic functions on a chip. It was primarily for this reason that CMOS became the most used technology to be implemented in VLSI chips.

3.4.1.3 CMOS Structure Definition

The phrase "metal oxide semiconductor" is reference to the physical structure of certain field effect transistors, having a metal gate electrode placed on top of an oxide insulator, which in turn is on top of a semiconductor material. Aluminium was once used but now the material is polysilicon. Other metal gates have made a comeback with the advent of high-k dielectric materials in the CMOS process

npmos.jpg

Fig: 3.2 CMOS Structure

3.4.1.4 Power Dissipation in CMOS

Broadly classifying, power dissipation in CMOS circuits occurs because of two components: Static dissipation due to sub-threshold condition when the transistors are off, tunneling current through gate oxide, leakage current through reverse biased diodes, contention current in ratioed circuit etc. Dynamic dissipation due to charging and discharging of load capacitances, short circuit power dissipation etc.

3.4.1.5 Merits of CMOS Technology

The CMOS technology being the wide used technology in VLSI design has the following merits in comparison to the other technologies:

Noise immunity

Low static power consumption

Do not produce much waste heat

High density of logic functions on a chip

Strong ‘0’ and Strong ‘1’ concept

3.4.1.6 Demerits of CMOS Technology

The CMOS technology being the wide used technology in VLSI design has the following demerits in comparison to the other technologies:

Intrinsic gain of single device

At high frequencies draws more power

Low frequency response

Since transistors used in pairs increases transistor count

Critical delay of the system is high

3.4.2 Transmission Gate Logic

Transmission gate logic (TGL) is one other form of CMOS technology. Transmission gate logic is used in constructing integrated circuits. Transmission gates can also be used to electronic switches and analog multiplexers etc.

3.4.2.1 Transmission Gate Definition

As transmission gates is known in electronics, especially in microelectronics, a usually integrated electronic circuit which, like the relay, by a control signal continuously flows with almost any voltage potential lead in both the directions or lock can.

3.4.2.2 Transmission Gate Structure Definition

The transmission gate structure has complementary metal oxide semiconductor connected in parallel way. Thus this transmission gate behaves more similar to CMOS technology. The transmission gate logic requires pair of metal oxide semiconductors to construct any logic circuit.

C:\Users\Priya\Downloads\tg.jpg

Fig: 3.3 Transmission Gate Logic

3.4.2.3 Merits of Transmission Gate Logic

The transmission gate logic technology has the following merits in comparison with the other technologies:

Less number of transistors compared to CMOS

No direct ground connection

Less power consumption

3.4.2.4 Demerits of Transmission Gate Logic

The transmission gate logic technology has the following demerits in comparison with the other technologies:

Degraded output, 20% of input is scaled off and given as output

Vdd != output

Output = Vdd- Vth, Vth is the threshold voltage required for the transistors to start functioning.

3.4.3 Pass Transistor Logic

In electronics, pass transistor logic (PTL) describes several logic families used in the design of integrated circuits. It reduces the count of transistors used to make different logic gates, by eliminating redundant transistors.

3.4.3.1 Pass Transistor Logic Definition

The transistors are used as switches to pass logic levels between nodes of a circuit, instead of as switches connected directly to supply voltages. This reduces the number of active devices, but has the disadvantage that the difference of the voltage between high and low logic levels decreases at each stage.

3.4.3.2 Structure Definition

Pass Transistor Logic requires only either p-MOS or n-MOS for the implementation of any logic circuit. Complementary pass transistor logic or differential pass transistor logic refers to a logic family which is designed for certain advantages. It is commonly to use this logic family for multiplexers and latches. CPL uses series transistors to select between possible inverted output values of the logic, the output of which drives an inverter to generate the non-inverted output signal.

Double pass transistor logic eliminates some of the inverter stages required for the complementary pass transistor logic by using both N and P channel transistors, with dual logic paths for every function. While it has high speed due to low input capacitance, it has only limited capacity to drive a load.

Static and dynamic types of pass transistor logic exist, with different properties with respect to speed, power and low-voltage operation.

3.4.3.3 Voltage Dissipation

As integrated circuit supply voltage decrease, the disadvantages of pass transistor logic become more significant; the threshold voltage of transistors becomes large compared to the supply voltage, severely limiting the number of sequential stages. Because complementary inputs are often required to control pass transistors, additional logic stages are required.

3.4.3.4 Merits of Pass Transistor Logic

The pass transistor logic technology has the following merits in comparison with the other technologies:

Either pMOS or nMOS only required

Reduced transistor count

Reduces the number of active devices in the circuit

3.4.3.5 Demerits of Pass Transistor Logic

The pass transistor logic technology has the following demerits in comparison with the other technologies:

Difference between voltage high and low logic decreases at each stage

Each transistor less saturated at output level than input level

Less isolation between input and output

3.5 TECHOLOGY SELECTION

The existing technologies are efficient in one or other way in VLSI system design implementations. The main objective of this project work is to design ALU with low power constraint, thus pass transistor logic is chosen for the VLSI implementation of this design due to the following reasons:

One pass transistor network requires only either p-MOS or n-MOS to implement the logic circuit.

The redundant transistors are eliminated

The reduction in transistor number reduces area and in turns the power consumption of the circuit.

CHAPTER 4

POWER OPTIMIZATION IN ALU

4.1 POWER OPTIMIZATION IN ALU

Here again optimization of power can be done in all the components of the ALU, but the adder is the core component of the ALU and hence is the most power consuming part. The adder constraints have to satisfy area, power, speed, delay. Hence we choose the adder unit for applying the low power design feature.

4.2 FULL ADDER

Cout

1-bit

Full Adder

A

B

Cin

SUM

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 the inputs are written as A, B, Cin; where A and B are the operands and Cin is a bit carried in from the next less significant stage. The circuit produces a two-bit output, output carry Cout and sum typically.

Fig: 4.1 Full Adder Block

4.2.1 Full Adder Implementation

A full adder circuit can be implemented in many different ways such as with a custom transistor-level circuit or composed of other gates. One example implementation is with

SUM = (A xor B) xor C

Cout = (A and B) + (Cin and (A xor B))

A

B

SUM

CARRY

C

Fig: 4.2 Full Adder Logic Level Diagram

INPUTS

OUTPUTS

A

B

Cin

SUM

Cout

0

0

0

0

0

0

0

1

1

0

0

1

0

1

0

0

1

1

0

1

1

0

0

1

0

1

0

1

0

1

1

1

0

0

1

1

1

1

1

1

Table: 4.1 Truth Table of Full Adder

4.2.1.1 AND Gate

The AND gate is a basic digital logic gate that implements logical conjunction. It behaves according to the truth table given below. A HIGH output results only if both the inputs to the AND gate is HIGH, a LOW output results if neither or only input to the AND gate is HIGH. In another sense, the function AND effectively finds the minimum between two binary digits.

Fig: 4.3 AND Gate Symbol

Input

Output

A

B

A.B

0

0

0

0

1

0

1

0

0

1

1

1

Table: 4.2 Truth Table of AND

4.2.1.2 OR Gate

The OR gate is the digital logic that implements logical disjunction. It behaves according to the truth table given below. A HIGH output results if one or both the inputs to the gate are HIGH. If neither input is HIGH a LOW output results. In another sense, the function of OR effectively finds the maximum between two binary digits.

Fig: 4.4 OR Gate Symbol

Input

Output

A

B

A+B

0

0

0

0

1

1

1

0

1

1

1

1

Table: 4.3 Truth Table of OR Gate

4.2.1.3 XOR Gate

The XOR gate is a digital logic gate that implements an exclusive or; that is, a true output results if one, and only one, of the inputs to the gate is true. If either inputs are false or both are true, a false output results. Its behavior is summarized in the truth table shown below. A way to remember XOR is "one or the other but not both".

Fig: 4.5 XOR Gate Symbol

Input

Output

A

B

A XOR B

0

0

0

0

1

1

1

0

1

1

1

0

Table: 4.4 Truth Table of XOR Gate

4.3 OPTIMIZATION OF FULL ADDER UNIT

The circuit performance is made through transistor count minimization. The existing architecture of full adder unit has eight transistor numbers, we have proposed a full adder unit with six transistor numbers. This circuit minimization can be obtained by reducing the transistor count in full adder block components namely AND gate or OR gate or XOR gate. XOR gate forms the building block of full adder unit. Hence, we choose the XOR gate unit for optimization.

4.3.1 XOR Gate Circuit design

The early designs of XOR gates were based on either four transistors or three transistors that are conventionally used in most designs over the past decade. The previous designs of the four transistors and three transistors are used in the existing eight transistor full adder architecture. The proposed full adder with six transistors is built using this two transistor XOR gate unit designed using general logic implementation.

Y

A

V +

B

V

P

P

+

Fig: 4.6 Two Transistor XOR Gate

4.3.2 Existing Full Adder Architecture

The adder unit in the existing ALU architecture possess 8 transistor built in pass transistor logic. The function of the full adder in the ALU architecture is to implement the logic initiated by the multiplexer circuits. The full adder unit with eight transistors has the XOR gate with 3 transistors.

Cin

B

Co

sum

A

Fig: 4.7 Full Adder with 8 transistors

4.3.3 Proposed Full Adder Architecture

The proposed full adder architecture is implemented in pass transistor logic with six transistor numbers. The full adder is designed with the two transistor XOR gate. This full adder is used in the proposed six transistor full adder based ALU circuit. The reduction of number of transistor count reduces the overall power consumption of the circuit in turn.

B

Cin

SUM

Cout

A

Fig: 4.8 Proposed 6 transistors Full Adder

4.3.4 Simulation of 6T Full Adder

The simulation of proposed six transistor full adder is designed in 90nm architecture and implemented in circuit level in Cadence Design Software.

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Fig: 4.9 Simulation of 6T Full Adder

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Fig: 4.10 Full Adder Simulated Waveform

4.4 Multiplexer Design Architecture

The 4x1 MUX and 2x1 MUX were designed using transmission gates in the existing architecture. In the proposed work the multiplexers are designed using pass transistor logic. Hence reduction of transistor numbers lead to reduction of power in turn. The pass transistor logic reduces the input applied at each stage due to powering on the transistors.

C:\Users\Priya\Desktop\project pics\4x1MUX.png

Fig: 4.11 4x1 Multiplexer Architecture

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Fig: 4.12 2x1 Multiplexer Architecture

4.5 Proposed ALU Architecture

The proposed 4-bit ALU architecture is built with multiplexers and full adder units. The 1-bit ALU possess a 4x1 multiplexer and a 2x1 multiplexer in the input stage for the selection of input signals. The next phase consists of the full adder unit which implements the logic or functions proposed. The output stage again contains a pair of 4x1 multiplexer and 2x1 multiplexer for the selection of output signals to be passed out of the circuit. The replica of this architecture can lead to 2, 4, 8, 16, 32 etc bits of ALU unit. The 4-bit ALU is constructed by replicating the 1-bit ALU four times as shown below.

4:1 MUX

2:1 MUX

FULL

ADDER

4:1 MUX

2:1 MUX

4:1 MUX

2:1 MUX

FULL

ADDER

2:1 MUX

4:1 MUX

2:1 MUX

4:1 MUX

FULL

ADDER

2:1 MUX

4:1 MUX

2:1 MUX

4:1 MUX

2:1 MUX

4:1 MUX

FULL

ADDER

Fig: 4.13 ALU Architecture

4.5.1 ALU Block Function

The proposed ALU circuit has a pair of 4x1 multiplexer and a 2x1 multiplexer for the selection of input signals. The 4x1 MUX unit is given with logic1, B0, B0’, logic0 as inputs. The function of a 4x1 MUX is to select one input from the given number of inputs based on the selection of control signals S1 and S0.

Data Select inputs

Outputs

S1

S0

Y

0

0

Logic 1

0

1

B0

1

0

B0’

1

1

Logic 0

Table: 4.5 Truth Table of 4x1 Multiplexer

The selected output from the 4x1 multiplexer is passed as one input to the 2x1 multiplexer where B0 is passed as other input. The 2x1 MUX functions to select one output from two given inputs based on the select signal S2.

Data select

inputs

Output

S2

Z

0

Y

1

B0

Table: 4.6 Truth Table of 2x1 Multiplexer

The selected output from the 2x1 multiplexer is passed along with A0 to the full adder as inputs. Cin is provided as third input to the full adder. Based upon the values the select lines provide the logic is implemented by the full adder unit. The implemented logic output is passed out through the pair of 4x1 MUX and 2x1 MUX at the output side.

Select Signals

ALU

Operation

S2

S1

S0

0

0

0

DECREMENT

0

0

1

ADDITION

0

1

0

SUBTRACTION

0

1

1

INCREMENT

1

0

0

NOR

1

0

1

X-OR

1

1

0

X-NOR

1

1

1

OR

Table: 4.7 Truth Table of ALU Operation

4.5.2 Simulation of ALU Architecture

C:\Users\Priya\Desktop\ALU.png

Fig: 4.14 Simulation of ALU