25 . Custom Integrated Circuits

The design of custom integrated circuits is an instance of the more general problem of the design of complex systems. Certainly if circuits are to be designed that contain between half a million and a million transistors, then techniques for the design of complex systems must be introduced that will assure quick, correct, and economical construction of the fabricated circuits. The established means of dealing with this complexity is to break up the problem into a number of subproblems, each of which can be tackled relatively easily. We also look for regularity in each of these components, so that basic circuits can be repeated many times in a regular array in order to achieve the desired functionality either in terms of arithmetic and logic function or memory. This breaking up of the overall task corresponds to the introduction of structural hierarchy, and visual inspection of contemporary large integrated circuits readily reveals the constituent blocks of this hierarchy. Use of regular structures such as memory, programmed logic arrays, register files, and iterative multiplier arrays makes the number of distinct circuits present in the overall circuit small enough so that substantial design effort can be devoted to each such circuit. There is an additional hierarchy, however, that is orthogonal to the structural hierarchy. This second hierarchy deals with the various qualitatively distinct kinds of representation that must be maintained over all levels of the design. Thus the design representations must range from a functional specification, through architectural representation to logic, circuits, devices, and a detailed geometrical layout specification which is suitable for mask making. It is striking that these different levels of representation are qualitatively different, and yet they must all be aligned to the same basic circuit since they describe in each case some limited aspect of the same overall design. The designer's job is complicated by the need to fully specify all these different levels of description, and yet keep them consistent with each other. For this reason, much of our emphasis in this project is focussed on careful choice of representations at all