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ABOUT VLSI

    The development of microelectronics spans a time which is even lesser than the average life expectancy of a human, and yet it has seen as many as four generations. Early 60’s saw the low density fabrication processes classified under Small Scale Integration (SSI) in which transistor count was limited to about 10. This rapidly gave way to Medium Scale Integration in the late 60’s when around 100 transistors could be placed on a single chip.

    It was the time when the cost of research began to decline and private firms started entering the competition in contrast to the earlier years where the main burden was borne by the military. Transistor-Transistor logic (TTL) offering higher integration densities outlasted other IC families like ECL and became the basis of the first integrated circuit revolution. It was the production of this family that gave impetus to semiconductor giants like Texas Instruments, Fairchild and National Semiconductors. Early seventies marked the growth of transistor count to about 1000 per chip called the Large Scale Integration.

Most of the students of Electronics Engineering are exposed to Integrated Circuits at a very basic level, involving SSI (small scale integration) circuits like logic gates or MSI (medium scale integration) circuits like multiplexers, parity encoders etc. But there is a lot bigger world out there involving smallness at levels so great, that a micrometer and a microsecond are literally considered huge! This is the world of VLSI - Very Large Scale Integration. The article aims at trying to introduce Electronics Engineering students to the possibilities and the work involved in this field

VLSI stands for "Very Large Scale Integration". This is the field which involves wadding more and more logic devices into smaller and smaller areas. Thanks to VLSI, circuits that would have taken boardfuls of space can now be put into a small space few millimeters across! This has opened up a big opportunity to do things that were not possible before. VLSI circuits are everywhere ... your computer, your car, your brand new state-of-the-art digital camera, the cell-phones, and what have you. All this involves a lot of expertise on many fronts within the same field, which we will look at in later sections.

VLSI has been around for a long time, there is nothing new about it ... but as a side effect of advances in the world of computers, there has been a dramatic proliferation of tools that can be used to design VLSI circuits. Alongside, obeying Moore's law, the capability of an IC has increased exponentially over the years, in terms of computation power, utilization of available area, yield. The combined effect of these two advances is that people can now put diverse functionality into the IC's, opening up new frontiers. Examples are embedded systems, where intelligent devices are put inside everyday objects, and ubiquitous computing where small computing devices proliferate to such an extent that even the shoes you wear may actually do something useful like monitoring your heartbeats! These two fields are kind of related, and getting into their description can easily lead to another article.

Digital VLSI circuits are predominantly CMOS based. The way normal blocks like latches and gates are implemented is different from what students have seen so far, but the behavior remains the same. All the miniaturization involves new things to consider. A lot of thought has to go into actual implementations as well as design. Let us look at some of the factors involved ...

1. Circuit Delays. Large complicated circuits running at very high frequencies have one big problem to tackle - the problem of delays in propagation of signals through gates and wires ... even for areas a few micrometers across! The operation speed is so large that as the delays add up, they can actually become comparable to the clock speeds.

2. Power. Another effect of high operation frequencies is increased consumption of power. This has two-fold effect - devices consume batteries faster, and heat dissipation increases. Coupled with the fact that surface areas have decreased, heat poses a major threat to the stability of the circuit itself.

3. Layout. Laying out the circuit components is task common to all branches of electronics. Whats so special in our case is that there are many possible ways to do this; there can be multiple layers of different materials on the same silicon, there can be different arrangements of the smaller parts for the same component and so on.

The power dissipation and speed in a circuit present a trade-off; if we try to optimise on one, the other is affected. The choice between the two is determined by the way we chose the layout the circuit components. Layout can also affect the fabrication of VLSI chips, making it either easy or difficult to implement the components on the silicon.

A typical digital design flow is as follows:

Specification Design In a typical VLSI flow, we start with system specifications, which is nothing but technical representation of design intent. To explain the flow, the following example will be used through this section.


Example: Specification: out1=a+b; out2=c+d; where a,b,c,d are single bit inputs and out1,out2 are two bit outputs (sum and carry).

This is the Main step where the industry work starts. With the help of the specification sheet produced by the Market The target IC’s architecture is decided and a layout is created for the same by design engineers using EDA tools. In the next step this architecture is implemented and tested with the help of programming language and tools.

RTL is an acronym for register transfer level. This implies that the VHDL/Verilog code written based on the specified architecture describes how data is transformed register to register.

This is how a 2-input MUX looks like :

Module( in1,in2,select, out);//module Declaration
Input in1, in2, select; //Input port Declaration
Output out; //Output port Declaration
always@(in1,in2,selelct)
begin
if(select)
out <= in2;
else
out <= in1;
end
endmodule

Register Transfer Level (RTL) simulation and verification ensures that the design is logically correct and without major timing errors. It gives the information on whether the our Design meets basic design Criteria or not i.e the adder is capable of performing addition, sub tractor performing subtraction, In the early stages of the design. Synopsys simulation tools may be used to perform RTL verification. A test bench file may be used here for verification.

Synthesis can be described as follows:

Synthesis = Translation + Optimization + Mapping
This is where the design now start to get into physical. Logic synthesis is a process by which the desired circuit behavior i.e. Register Transistor Level is converted into a design in terms of logic gates which drives the circuit or architecture. This is done with the help of FPGA/CPLD/ASIC hardware tools. These target boards may be accessed using the IDE’s provided by specific vendor.

Here the final tested design after synthesis is given to the IC manufacturer This is also called as Physical_design_(electronics) .

Tape Out to Foundry to get end product….a wafer with repeated number of identical Ics

All modern digital designs start with a designer writing a hardware description of the IC (using HDL or Hardware Description Language) in Verilog/VHDL. A Verilog or VHDL program essentially describes the hardware (logic gates, Flip-Flops, counters etc) and the interconnect of the circuit blocks and the functionality. Various CAD tools are available to synthesize a circuit based on the HDL. The most widely used synthesis tools come from two CAD companies. Synposys and Cadence.


Without going into details, we can say that the VHDL, can be called as the "C" of the VLSI industry. VHDL stands for "VHSIC Hardware Definition Language", where VHSIC stands for "Very High Speed Integrated Circuit". This language is used to design the circuits at a high-level, in two ways. It can either be a behavioral description, which describes what the circuit is supposed to do, or a structural description, which describes what the circuit is made of. There are other languages for describing circuits, such as Verilog, which work in a similar fashion.


Both forms of description are then used to generate a very low-level description that actually spells out how all this is to be fabricated on the silicon chips. This will result in the manufacture of the intended IC.


VLSI or very large-scale integration is about miniaturisation to such a great extent that even a micro-metre or micro-second is considered a huge unit of measurement! With smart and intelligent devices becoming a rage everywhere in the world, career opportunities in the chip design industry have seen an exponential rise.

VLSI design has ample scope for outsourcing as circuits are designed at locations around the world and manufactured elsewhere. This outsourcing advantage of VLSI design is attracting international players in this field, like IBM, Philips, Motorola, Texas Instruments, SGS-Thompson, Intel, Mentor Graphics and Cirrus Logic, to India. VLSI design is a dynamic profession where careers can be highly satisfying as they focus on innovation in a rapidly changing environment. This field offers immense growth prospects for competent, hardworking people and keen learners.

The two main aspects in VLSI design are front end and backend. Front-end design includes digital design, design verification and design for testability. You should be not only be thorough in fundamental concepts but also be proficient with tools and design flows. Back-end design consists of Complementary metal–oxide–semiconductor library design and characterisation, physical design, packaging, test generation and fault simulation.


You can choose between various job roles such as design engineer, chip architect, production engineer, testing or process specialist, physical designer or circuit designer. On the other hand if you want to pursue Ph.D you may take up research in the hardware or software development and testing domains.

The IIT Kharagpur (cse.iitkgp.ac.in), IIT Kanpur (www.iitk.ac.in), IIT Madras (www.ee.iitm.ac.in), IIT Bombay (www.ee.iitb.ac.in) and also various NITs and other institutes offer excellent opportunities for doctoral research in the VLSI domain. The important areas of research include material research for VLSI, Circuit simulation for design of modern VLSI circuits, mixed signal VLSI design, asynchronous systems, molecular electronics using DNA porphyrins and conducting polymers.

Nowadays, we have become absolutely dependent on microchips as they are present in almost every appliance from mobiles, TV remotes, and dish washers to rockets and missiles. This has increased the demand for chip designers in every sector. Automobile sector, consumer electronics, robotics, medical electronics, space research and defence research organisations are some avenues for professionals in VLSI designing.

In the government sector, Ministry of Communications and Information technology, department of Electronics, ISRO Satellite Centre, Bharat Electronics Limited (BEL), Centre for Development of Advanced Computing (C-DAC), Centre for Development of Telematics (C-DOT), Semi-Conductor Laboratory ( SCL), Society for Integrated Circuit Technology and Applied Research (SITAR), DRDO, DLRL, ISRO and BARC are key players who recruit VLSI design professionals.

Some of the major recruitment companies in the VLSI field in the private sector are HCL Technologies, Intel, Micron Tech, Alliance Semiconductor, Texas Instruments, Analog Devices, Lucent, Motorola, Philips Semiconductor, TCS, Wipro, Synopsis and Cadenc.

Tape Out to Foundry to get end product….a wafer with repeated number of identical Ics

All modern digital designs start with a designer writing a hardware description of the IC (using HDL or Hardware Description Language) in Verilog/VHDL. A Verilog or VHDL program essentially describes the hardware (logic gates, Flip-Flops, counters etc) and the interconnect of the circuit blocks and the functionality. Various CAD tools are available to synthesize a circuit based on the HDL. The most widely used synthesis tools come from two CAD companies. Synposys and Cadence.

Without going into details, we can say that the VHDL, can be called as the "C" of the VLSI industry. VHDL stands for "VHSIC Hardware Definition Language", where VHSIC stands for "Very High Speed Integrated Circuit". This language is used to design the circuits at a high-level, in two ways. It can either be a behavioral description, which describes what the circuit is supposed to do, or a structural description, which describes what the circuit is made of. There are other languages for describing circuits, such as Verilog, which work in a similar fashion.



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