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Full Custom Design

 While the standard-cells-based design is often referred to as full custom design , it is less so in a strict sense because the cells are pre-designed for general use and the same cells are used in a variety of chip designs. In a more comprehensive custom design, the whole mask is created from scratch, without the use of any libraries.However, the expense of developing a design style like this is becoming prohibitively expensive. As a result, the idea of design reuse is gaining attention and becoming popular  as a way to cut down on design cycle time and production costs. The design of a memory cell, whether static or dynamic, may be the most rigorous complete custom design.There will be no alternative to high density memory chip design , since the same layout design  is repeated.Using a combination of different design types on the same chip, such as regular cells, data-path cells, and PLAs, can achieve a good compromise in logic chip design. Design productivity is normally very poor in real full-custom layouts, where the designer must do the geometry, orientation, and positioning of each transistor individually, typically 10-20 transistors per day, per designer.

The production of high-volume products such as memory chips, high-performance microprocessors, and FPGA masters are exceptions to this rule.The Intel 486 microprocessor chip, which is a good example of a hybrid full-custom design, is shown in Figure . On a single chip, one can see four different design styles: Memory banks (RAM cache), bit-slice data-path modules, and regular cells and PLA blocks make up the majority of the control circuitry.


Figure- Mask layout of the Intel 486 microprocessor chip, as an example of full-custom design.

SEMI CUSTOM DESIGN:

In this style of design , all most all the basic building block are used from the standard cells liabrary. Only few cells are designed from the begning, which are not available in this standard cell liabrary or to be optimized for a specific target. The approach is faster compare  to full- custom style but slower than the standard cell base design. Performance -wise also, it is superior to the standard cell – based design but inferior to the full custom designed

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