Compare full custom, semi custom and fully programmable design style.
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As integrated circuits become more inexpensive and compact, many new types of products, such as digital cameras, digital camcorders, and digital television [2], are being introduced, based on digital systems. Consequently, logic design must be done under many different motivations. Since each case is different, we have different design problems. For example, we have to choose an appropriate IC (integrated circuit) logic family, since these cases have different performance requirements (scientific computers require high speed, but wristwatches require very low power consumption), although in recent years, CMOS has been more widely used than other IC logic families, such as ECL, which has been used for fast computers.
Logic functions that are frequently used by many designers, such as a full adder, are commercially available as off-the-shelf IC packages. (A package means an IC chip or a discrete component encased in a container.) Logic networks that realize such logic networks are often called standard (logic) networks. A single component, such as a resistor and a capacitor, is also commercially available as an off-the-shelf discrete component package. Logic networks can be assembled with these off-the-shelf packages. In many cases, not only performance requirements but also compactness and low cost are very important for products such as digital cameras. So, digital systems must accordingly be realized in IC packages that are designed, being tailored to specific objectives, rather than assembling many of these off-the-shelf packages on pc-boards, although assembling with these off-the-shelf packages has the advantage of ease of partial design changes.
Here, however, let us consider two important cases of designing an IC chip inside such an IC package, which is not off-the-shelf, that leads to two sharply contrasting logic design approaches: quick design and high-performance design. Quick design of IC chips is called semi-custom design (recently called ASIC design, abbreviating Application Specific Integrated Circuit design), whereas deliberate design for high performance is called full-custom design because full-custom design is fully customized to high performance. Full-custom design is discussed in this chapter, and different approaches of semi-custom design will be discussed in the succeeding chapters.
Semi-Custom Design
When manufacturers introduce new products or computers, it is ideal to introduce them with the highest performance in the shortest time. But it is usually very difficult to attain both, so one of them must be emphasized, based on the firm’s marketing strategy against competitors. Often, quick introduction of new computers or new merchandise with digital systems is very important for a manufacturer in terms of profits. (In some cases, introduction of a new product one year earlier than a competitor’s generates more than twice the total income that the competitor gets [1].) This is because the firm that introduces the product can capture all initial potential customers at highest prices, and latecomers are left with only the remaining fewer customers, selling at lower prices. This difference in timing often means a big difference in profits. In other words, the profits due to faster introduction of new products on a market often far exceed the profits due to careful design. The use of off-the-shelf IC packages, including off-the- shelf microprocessors, is used to be a common practice for shortening design time. But recent progress enables us to design digital systems in an IC chip more compactly with higher performance than before by curtailing time-consuming layout of transistor circuits on chips and by extensively using CAD pro- grams. Thus, in the case of small volume production, the design cost, part of the product cost, is reduced. This makes semi-custom design appropriate for debugging or prototyping of new design
Full custom, semi custom and fully programmable design style:
• Full custom design requires all the mechanisms to be designed and tested right from the transistor level.
• Semi-custom designs are used when there is a smaller amount of time for design and for fewer quantities.
• The features of the filter are made fully programmable by simply varying the ratios of the coefficient transistors.