We show an exact minimization method that is a tree search using backtracking. A considerable reduction in the search space is achieved by considering constrained implicunt sets and by eliminating some implicants altogether. Even with this improvement, the time required for exact minimization is extremely high when compared to all three heuristics.We also examine the case where only prime implicunfs are considered and show that such implicants have marginal value compared to constrained implicant sets. Our basis of comparison is the average number of product terms. We show that the heuristic methods are reasonably close to minimal and produce nearly the same average number of product terms. Interestingly though, there is surprisingly little overlap in the set of functions where the best realization is achieved. Thus, there is a benefit to applying all three heuristics to a given function and then choosing the best realization.Index Terms-Absolute minimization, heuristic minimization, implicant, multiple-valued logic, prime implicant, programmable logic arrays, sum-of-products.
I . INTRODUCTIONHE minimization of sum-of-products expressions in binary T logic has received considerable attention for over 30 years.The complexity of the problem has been known for almost as long. In 1965, Gimpel [7] showed that any instance of the set covering problem is an instance of sum-of-products extraction.In 1972, Karp [8] showed that the set covering problem is NP complete; thus, so also is sum-of-products extraction. The best known algorithm then requires exponential time. This is a real barrier; it precludes the exact minimization of functions with even a moderately low number of inputs, e.g., 20. As a result, considerable effort has been devoted to heuristic minimization methods. introduced a method which also seeks the most isolated minterm first, but chooses a product term that tends to introduce the fewest discontinuities when subtracted from the function. There has been little study of the relative merits of available heuristic algorithms even in binary logic. To the credit of Brayton, Hachtel, McMullen,, the realizations produced by ESPRESSO-IIC were compared to the realizations of MINI, PRESTO, and POP [4] over a set of 56 specifically chosen binary functions. To our knowledge, there has been no comparative analysis of minimization algorithms for multiple-valued logic. The justification of the newly introduced heuristics examined by us has rested on an intuitive notion, supported by examples. In this paper, we analyze the following synthesis methods over sets of 7000 random and 7000 random symmetric 4-valued 2-variable functions.1) Random-mintermhandom-implicant 2) Pomper and Armstrong [14] 3) Besslich [3] 4) Dueck and Miller [6] 5) Gold 6) Absolute minimization using prime implicants 7) Absolute minimization using all implicants. Our results show the Besslich heuristic is slightly inferior to the Dueck and Miller heuristic while there is superiority of each over the Pomper and Armstrong heuristic. Our analysis is...