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INTRODUCTION TO VLSI and VLSI Design Flow

INTRODUCTION TO VLSI 

.Very-large scale integration (VLSI) is the process of incorporating thousands of transistors into a single chip to create an integrated circuit (IC) . VLSI got its start in the 1970s, when complex semiconductor and communication technologies were being developed.  The microprocessor is a VLSI device.

Most ICs could only perform a limited set of functions prior to the introduction of VLSI technology. An electronic circuit contains a CPU, ROM, RAM, etc. IC designers can integrate all of these functions into a single chip using VLSI.

Thanks to rapid advancements in large-scale integration technologies and device design applications, the electronics industry has grown at a breakneck rate in recent decades.Since the introduction of very large scale integration (VLSI) designs, the number of integrated circuits (ICs) used in high-performance computing, controllers, telecommunications, image and video processing, and consumer electronics has been increasingly growing.

End-users benefit from cutting-edge technologies such as high-resolution, low-bit-rate video and wireless communications, which offer a wide range of applications, computing capacity, and portability. This trend is expected to escalate, with major implications for VLSI and device/system design.


VLSI Design Flow

The design method is normally evolutionary in nature at different stages. It all begins with a given set of requirements . The initial design is created and reviewed against the specifications. When specifications aren't met, the design must be tweaked. If such an upgrade is either impossible or prohibitively expensive, a condition revision and effect analysis must be considered.The Y-chart (first introduced by D. Gajski) shown in Fig. 1 illustrates a design flow for most logic chips, using design activities on three different axes (domains) which resemble the letter Y. 

                   Figure 1:Typical VLSI design flow in three domains (Y-chart representation).


The Y-chart consists of three major domains, namely:

  • behavioral domain,
  • structural domain,
  • geometrical layout domain.

The design flow begins with the algorithm that defines the target chip's actions. The processor's corresponding architecture is first described. Floorplanning is used to project it onto the chip surface. Finite state machines (FSMs), which are structurally implemented with functional modules such as registers and arithmetic logic units(ALUs) , are the next architecture development in the behavioural domain. (ALUs). These modules are then geometrically mounted onto the chip surface using CAD software for automated module positioning and routing, with the intention of reducing interconnect area and signal delays. The third evolution begins with a summary of a behavioural module. Leaf cells are then used to implement individual modules. The chip is defined in terms of logic gates (leaf cells) at this stage, which can be positioned and linked using a cell placement and routing software. A comprehensive Boolean overview of leaf cells is accompanied by a transistor level implementation of leaf cells and mask generation in the final evolution.In standard-cell based design, leaf cells are already pre-designed and stored in a library for logic design use.

                                       Figure-2: A more simplified view of VLSI design flow.

Figure 2 depicts the VLSI design flow in a more condensed manner, taking into account the various design representations, or abstractions  such as design- behavioural, logic, circuit, and mask layout. It's worth noting that concept verification is crucial at any stage of this procedure.Failure to properly verify a design in its early stages often results in substantial and costly re-design at a later level, increasing time-to-market.

Although the design process has been represented in a linear fashion for ease of understanding, there are several iterations back and forth in practise, particularly between any two neighbouring steps, and sometimes even between pairs of steps that are far apart. Although top-down design flow is effective at controlling the design process, there is no completely unidirectional top-down design flow. It is essential to incorporate top-down and bottom-up approaches. For example, if a chip designer specified an architecture without carefully estimating the corresponding chip region, the resulting chip layout is very likely to exceed the available technology's area limit.In this case, some functions may have to be removed and the design process may have to be replicated in order to fit the architecture into the available chip region. Such changes could necessitate major changes to the original specifications. As a result, it's important to get low-level data to higher levels (bottom up) as soon as possible. 

We'll look at design methodologies and systematic methods that have been built over time to deal with both complex hardware and software projects in the following sections. The fundamental concepts of formal design would increase the chances of success regardless of the project's size. Some of the classical techniques for reducing the complexity of IC design are: Hierarchy, regularity, modularity and locality.




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