Design Methodologies

Design Methodologies

Introduction The dramatic increase of contemporary integrated circuits poses an enormous design challenges. Depending up on the applications and day by day needs, the number of logic transistors per chip are growing and leading to the IC complexity.

• The above graph shows, how IC complexity is growing faster than productivity of a design engineer, creating the design gap. • Out of all the approaches the preferred is to mapping a function onto the silicon depends largely upon the function itself.

• Example Consider a simple digital processor which composes a number of building blocks that occur in one form or another in almost every digital processor. Composition of generic digital processor are shown below . The arrows represents the possible interconnections.

INPUT/OUTPUT

MEMORY

CONTROL

DATAPATH

Custom to semicustom . Designing of microelectronics depends on a number of factors such as • Power consumption. • Cost. • Production volume.

The cost of a semiconductor device is a sum of two components : • The nonrecurring expense (NRE), which is incurred only for a design and includes the cost of designing the part. • The production cost per part, which is a function of the process complexity, design area, and process yield.

Digital Circuit Implementation Approaches

Custom

Semicustom

Cell based

Standard cells Compiled cells

Macro cells

Array based

Pre-diffuse

Pre-wired

Custom and Semicustom Circuit Design Custom design can be justified economically under the following conditions: • The custom block can be reused many times. • The cost can be amortized over a large volume. Exp: Microprocessors and semiconductor memories

With continuous progress in the designautomation arena, the share of custom design reduces from year to year. Library cell design is the only area where custom design still thrives today. virtually all portions are designed automatically using semi custom design approaches.

Cell-based Design methodology Since the custom design approach proves to be prohibitively expensive, a wide variety of design approaches have been introduced over the years to shorten and automate the design process. The idea behind cell-based is to reduce the implementation effort by reusing a limited library of cells.

Standard cell • The standard cell approach standardizes the design entry level at the logic gate. A library containing a wide range of fan-in and fan-out counts is provided. • In standard cell, cells are placed in a row which are separated by routing channels. • All the cells are of same height and the widths of the cells can vary to accommodate for the variation in complexity between the cells.

A substantial fraction of area is devoted for signal routing. With feedthrough cells we can minimize the length of the wire used to connect between cells. The only exceptions are when extreme high performance or low energy is needed.

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