July 3, 1997

Software routine minimizes large logic tables

Jerzy Chrzaszcz, Warsaw University of Technology, Warsaw, Poland

You can define combinatorial logic circuitry by explicitly stating the output patterns that result from particular input patterns. Such behavioral description is sometimes more convenient than writing logic equations; virtually all logic-design systems accept table entry, among other entry formats. Unfortunately, limitations of allowable table size preclude direct minimization of large logic tables. The technique proposed here uses Espresso, a public-domain logic minimizer available from the University of California--Berkeley. You must decompose the table into fragments that fit Espresso limits. You then convert the fragments into Berkeley format. Espresso minimizes the fragments into smaller tables containing ones, zeros, and "don't cares." It then merges, reformats, and reprocesses the results.

Group minimization occurs in the first pass, and minimization of each output function takes place in the second pass. The final table contains sets of vectors corresponding to product terms, which you can easily translate into logic equations. Listing 1 shows the results obtained for a sample input table comprising 16,384 vectors of 14 inputs and eight outputs. Listing 2, a batch file, calls up the executable program heco.com for converting portions of the hex file xmpl.hex, which contains an input table (image of a 16-kbyte memory in hex format). The batch file also calls up heco.com, which converts portions of the hex file into Berkeley format, and glue.com, which reformats the merged results before the second processing pass. Listing 3 and Listing 4 show the source code for heco.com and glue.com, respectively. Click here to download the files from DI-SIG, #2042. (DI #2042)

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