ABSTRACT
The logic circuit design is usually a multistep process. The realization of logical function from a word statement of the logical problem, and the subsequent minimization of the logical function isnot theendof thedesign. InChapters 2and3,wehavecome tounderstand how various visual as well as algorithmic schemesmay be used for deducing reduced logic function either in the sum-of-product (SOP) or in the product-of-sum (POS) format. These function formats canbe translatedeasily intoeither a familiarAND-ORorOR-ANDpattern of logic circuits. Unfortunately, itmaynot be possible to have an arbitrary n-input logic gate, for example, especially for large n. Digital integrated circuits (ICs) have inherent practical limitations that may directly affect the implementation of logic circuits. Some of the issues pertaining to ICs include the fan-in and fan-out limitations and the fact that ICs are more frequently available in a particular form than, for example, in the AND and OR form. The logic gates that are easier to realize with electronic components or the ones that
are available in one’s inventory eventually determine as to how a particular logic circuit may be implemented. Consequently, it is important for the designer to be familiar with the techniques for translating a reduced SOPorPOS function to an acceptable form so that other types of more frequently available logic gates may be used to implement the logic circuit. Another design consideration is speed of the logic circuit. Calculating the total propagation delay requires determining the longest delay path and then accounting for the worst-case propagation delay. This worst-case propagation delay determines the speed with which new data can be introduced to the circuit inputs. There are three basic approaches to implementing a digital logic circuit: with standard
SSI, MSI, and LSI components, with full-custom very large-scale integrated (VLSI) devices, or with the semicustom devices. Small-scale integrated (SSI) circuits contain up to an equivalent of ten 2-input logic gates. Medium-scale integrated (MSI) circuits contain up to an equivalent of 100 logic gates and large-scale ICs contain over 1000 logic gates. VLSI circuits may contain an equivalent of thousands of logic gates in it. Standard SSI andMSI functions are simple in that a circuit can be assembled quickly with readily available off-the-shelf parts. It is quite possible though that the total parts count and, thus, cost per logic gate can be too high as well as the speed may be too low for the resulting design. The alternative may be to consolidate all of the logic subfunctions into one or more custom or semicustom devices. The full-custom design, on the other hand, involves the physical electrical component layouts and carefully controlled design of the necessary interconnections. It can lend itself to highly optimized logic circuit but the involved design process is both very expensive and time-consuming.
