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Full-custom design is a methodology for designing integrated circuits by specifying the layout of each individual transistor and the interconnections between them. Alternatives to full-custom design include various forms of semi-custom design, such as the repetition of small transistor subcircuits;  one such methodology is the use of standard cell libraries standard cell libraries are themselves designed using full-custom design techniques. Full-custom design potentially maximizes the performance of the chip, and minimizes its area, but is extremely labor-intensive to implement. Full-custom design is limited to ICs that are to be fabricated in extremely high volumes, notably certain microprocessors and a small number of ASICs. The mask sets are required in order to transfer the ASIC designs onto the wafer.
Full-custom Circuit and Layout Design
As integrated circuits become more inexpensive and compact, many new types of products, such as digital cameras, digital camcorders, and digital television , 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. 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. 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.
Full-custom design is discussed in this chapter, and different approaches of semi-custom design will be discussed in the succeeding chapters. When manufacturers introduce new products or computers, it is ideal to introduce them with the highest performance in the shortest time. 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. Semi-custom design, or ASIC design, has several different approaches, as will be discussed in later chapters [3,4].
Design of logic networks with the highest performance requires deliberate design of logic networks, design of transistor circuits, layout of these transistor circuits most compactly, and manufacturing of them. Such logic networks are called random-logic gate networks and are realized by full-custom design.
Full-custom design is logic design to attain the highest performance or smallest size, utilizing the most advanced technology. Designers usually try to improve the economic aspect, that is, performance per cost, at the same time. Hence, this is the other extreme to the above quick design in terms of design time.
Every design stage is carefully done for the maximum performance, and transistor circuits are deliberately laid out on chips most compactly, spending months by many draftpeople and engineers. CAD programs are used but not extensively as in the case of semi-custom design.
When CAD programs for high performance are not available, for example, for the most compact layout of transistor circuits to which is required for high performance — manual design is used, possibly mixed with the use of some CAD programs.
Also, once mistakes sneak into some stages in the long sequence of design, designers have to repeat at least part of the long sequence of design stages to correct them. So, every stage is deliberately tested with substantial effort.
It should be noticed that the cost of a digital system highly depends on the production volume of a chip. The cost of an IC package can be approximately calculated by the following formula:.
The second term on the right-hand side of Eq. In the case of full-custom design, chips are deliberately designed by many designers spending many months. Many digital systems that use IC chips are produced in low volume and [Design expenses] must be very low.
Semi-custom design is for this purpose and CAD programs need to be used extensively for shortening design time and manpower in order to reduce [Design expenses]. In this case, [Manufacturing cost per IC chip] is higher than that in the case of full-custom design because the size of each chip is larger.
Thus, we can see the following from the formula in Eq. But chips by full-custom design are cheaper in the case of high volume production, and are expensive for low volume production. First, the architecture of a digital system is designed by a few people. The performance or cost of the entire system is predominantly determined by architectural design, which must be done based on good knowledge of all other aspects of the system, including logic design and also software to be run.
If an inappropriate architecture is chosen, the best performance or lowest cost of the system cannot be achieved, even if logic networks, or other aspects like software, are designed to yield the best results. Thus, if performance or manufacturing cost is important, realization of control logic by logic networks i. Actually, every design stage is important for the performance of the entire system.
Logic design is also one of key factors for computer performance, such as architecture design, transistor circuit design, layout design, compilers, and application programs.
Even if other factors are the same, computer speed can be significantly improved by deliberate logic design. Next, appropriate IC logic families and the corresponding transistor circuit technology are chosen for each segment of the system. Other aspects such as memories are simultaneously determined in greater detail. We do not use expensive, high-speed IC logic families where speed is not required.
Architecture and transistor circuits are outside the scope of this handbook, so they are not discussed here further. The next stage in the design sequence is the design of logic networks, considering cost reduction and the highest performance, realizing functions for different segments of the digital system. Logic design requires many engineers for a fairly long time.
Then, logic networks are converted into transistor circuits. This conversion is called technology mapping. A variety of IC logic families, such. Then, after technology mapping, these transistor circuits are laid out on a chip. Layout is also a painstaking endeavor for many draftpersons.
The above design stages are highly interactive and iterative because, if bad design is made in a certain stage, good design in other stages cannot compensate for it, thus yielding poor performance or cost increase of the entire chip. In particular, logic network design and layout design are highly interactive. In this case, it is important to notice the difference of delay time of signal propagation in logic networks from that in transistor circuits laid out on a chip.
In previous chapters, we have assumed for the sake of simplicity that signal propagation has delay time only on gates for the sake of simplicity, equal delay time is assumed on every gate but not on connections.
The longer the connection, the greater the delay time on that connection. The larger the number of connections i. Also, each logic gate realized in transistor circuit may have a different delay time. The greater the fan-in of a gate, the greater the delay time. Restrictions on maximum fan-out and maximum fan-in are very important for the performance of logic networks.
Thus, if we want to have fast transistor circuits, we need to consider these relationships in designing logic networks with AND and OR gates. Then, IC chips are manufactured and are assembled with pc-boards into a digital system.
In the case of the high-performance design discussed above, every effort is made to realize digital systems with the best performance usually speed , while simultaneously considering the reduction of cost.
When we want to develop digital systems of high performance, using the most advanced technology, much greater manpower and design time are required than that needed for semi-custom design approaches. The actual design time requirement depends on how ambitious the designers are. High-performance microprocessor chips are usually designed by full-custom design, typically taking 3 to 5 years with a large number of people, perhaps several dozen engineers.
If the digital system is not drastically different from previous models, design time can be shorter with fewer people; but if the system is based on many new ideas, it may be longer.
As we become able to pack an increasingly large number of networks in a single IC chip every year, the full-custom design of VLSI chips including microcomputers of high performance with the most advanced technology is beginning to require far greater design effort and longer time.
Thus, more extensive use of improved CAD programs is inevitable. This is because a new generation of microprocessor chips has been introduced every 4 years, having a few times as many transistors on a chip. Compared with systems of 10 years ago, contemporary systems consist of two or three order more transistors, although the physical size of these systems are far smaller.
In addition to the use of transistor circuits as logic gates, memories are becoming widely used to implement logic networks, being mixed with gates. Because of these developments, we have complex problems in designing logic networks with a mixture of gates, software, and memories. Essentially, boundaries among logic design, transistor circuits, software, and architecture have disappeared.
The number of transistors, or logic gates, used in digital systems is increasing all the time. In designing such gigantic digital systems, it is becoming extremely important to design without errors, necessitating extensive testing in every design stage.
To cope with these complex problems, CAD programs with new logic design methods have been developed in recent years. For example, recent CAD programs for logic design can synthesize far larger logic networks than manual design can, and appropriate logic expressions can be derived for functions with a large number of variables by using BDDs. Your email address will not be published. Sections About. Categories Uncategorized. Posted on September 30, by ahmed farahat Leave a comment.
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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 , 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. 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.
Full-custom Circuit and Layout Design
As integrated circuits become more inexpensive and compact, many new types of products, such as digital cameras, digital camcorders, and digital television , 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. Logic functions that are frequently used by many designers, such as a full adder, are commercially available as off-the-shelf IC packages.
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