VLSI Physical Design And

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VLSI Physical Design andDesign Automation

Basic VLSI design methodologies.

Algorithmic graph theory and computational complexity. Tractable and intractable problems.

Combinatorial optimization.

Layout compaction.

Placement and partitioning.

Floor planning. Routing.

Simulation.

Logic synthesis and verification.

High-level synthesis.



VLSIDesignVLSI, or Very Large Scale Integration refers to a

technology through which it is possible to

implement large circuits in silicon - circuits with

up to a million transistors. The VLSI technology

has been successfully used to build

microprocessors, signal processors, systolic

arrays, large capacity memories, memory

controllers, I/0 controllers, and interconnectionnetworks.



VLSI Design: Example

The Intel 80286 microprocessor has over

100,000 transistors, the 80386 has 275,000transistors, the 80486 has approximately

1000,000 transistors. The RISC processor

from National Semiconductor NS32SF641

has over a million transistors. The Pentium of Intel has over three million transistors



Computer Aided Design

Designing a VLSI chip with the help of computer programs isknown as CAD, or Computer Aided Design.

Design Automation (DA), on the other hand, refers to entirelycomputerized design process with no or very little humanintervention.

Some of the earliest CAD software dealt with placement of logicmodules on printed circuit boards (PCBs) and finding shortelectrical paths to wire the interconnections.

Logic minimization was also an important facet of electronicdesign, since eliminating even a handful of logic gates resulted insignificant cost savings.



Change of Design Issue

The rapidly changing technology has also radicallytransformed design issues.

For instance, in the LSI/VLSI technologies, it is notvery important to save on transistors; the costreduction through logic minimization is unlikely to besignificant when the total number of transistors is inthe order of a million.

On the other hand, it is important to save oninterconnection costs, since wires are far moreexpensive in VLSI than transistors.



Architectural design

Architectural design of a chip is carried out by expert human engineers. Decisions

made at this stage affect the cost and performance of the design significantly.

Several examples of decisions made during the architectural design of amicroprocessor are given below.

(a) What should be the instruction set of the processor? What memory addressing

modes should be supported? Should the instruction set be compatible with that of

another microprocessor available in the market?

(b) Should instruction pipelining be employed? If so, what should be the depth of

the pipeline?

(c) Should the processor be provided with an on-chip cache? How big should thecache memory be? What should be the organization of the cache? Should

instruction cache be separated from data cache?

(d) Should the arithmetic unit be designed as a bit-serial unit or as a bit-parallel

unit? If bit-serial arithmetic is used, one saves on hardware cost but loses on

performance.

(e) How will the processor interface to the external world? Are there any

international standards to be met?



Once the system architecture is defined, it is necessary to carry out twothings:

(a) Detailed logic design of individual circuit modules.(b) Derive the control signals necessary to activate and deactivate thecircuit modules

.The first step is known as data path design. The second step is called control

path design. The data path of a circuit includes the various functional blocks,

storage elements, and hardware components to allow transfer of data amongfunctional blocks and storage elements. Examples of functional blocks are adders,

multipliers, and other arithmetic/logic units. Examples of storage elements are shift

registers, random access memories, buffers, stacks, and queues. Data transfer is

achieved using tristate busses or a combination of multiplexers and

demultiplexers.

The control path of a circuit generates the various control signals necessary tooperate the circuit. Control signals are necessary to initialize the storage elements

in the circuit, to initiate data transfers among functional blocks and storage

elements, and so on. The control path may be implemented using hardwired

control (random logic) or through microprogrammed control.



hardware Synthesis & High-LevelSynthesis

Given a specification the objective is to arrive at a

design which meets all the constraints posed by the

specification, and optimizes on one or more of the

design aspects. This problem is also known as

hardware Synthesis.

Computer programs have been developed for data path

synthesis as well control path synthesis. The automaticgeneration of data path and control path is known

High-Level Synthesis



Log ic Desi g n

The data path and control path (derived automatically

or manually) will have components such asarithmetic/logic units, shift registers, multiplexers,

buffers, and so on. Further design steps depend on the

following factors.

(1) How is the circuit to be implemented, on a PCB or

as a VLSI chip?(2) Are all the components readily available as off-the-

shelf integrated circuits or as pre-designed modules?



macr o- cells

If the circuit must be implemented on a printed-circuit boardusing off-the shelf components, then the next stage in design is

to select the components so as to minimize the total cost and atthe same time maximize the performance.

Following the selection procedure, the IC chips are placed onone or more circuit boards and the necessary interconnectionsare established using one or more layers of metal deposits. Asimilar procedure may be used in case the circuit must be

implemented on a VLSI chip using pre-designed circuitcomponents from a module library . The pre-designed modulesare also known as macro-cells. The cells must be placed on thelayout surface and wired together using metal and polysilicon(poly) interconnections.



Physical Design & Automation

Physical design of an integrated circuit refers

to the process of generating the final layoutfor the circuit.

Needless to say, physical design is of vital

importance, and design automation research

has gone into developing efficient algorithmsfor automating the layout process.

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