Implementation of combinational circuits

Abstract

Most of the design techniques we have encountered so far have been essentially technology-independent in that the sum-of-products or productof-sums forms represent AND/OR or OR/AND two-level circuits. In practice there are a large number of ways of implementing logic functions using commercially available components. These range from individual gates in the form of SSI components, through MSI components such as decoders and multiplexers, to semi-custom and custom VLSI circuits. In the context of semi-custom design, we may again have the choice of expressing a circuit description at ‘gate level’ or using the higher-level subcircuits provided as macrocell or standard cell library components. Moreover, by designing for a particular technology it is often possible to product a more viable and economic circuit. In this chapter we shall consider a range of implementation techniques and, where appropriate, the attendant design philosophy. We shall confine ourselves here to relatively ‘small’ designs, the additional problems of managing the complexity of large-scale designs being left to a later chapter. The use of logic simulation as a means of design verification is introduced, together with the need for logic circuit testing.