The use of observability and external don't cares for the simplification of multi-level networks

An algorithm is given for computing subsets of the observability don't cares at the nodes of a multilevel Boolean network. These subsets are based on an extension of the methods introduced by S. Muroga et al. (IEEE Trans. on Computers, Oct. 1989) for computing compatible sets of permissible functions (CSPFs) at the nodes of networks composed of NOR gates. The extensions presented are in four directions: an arbitrary logic function is allowed at any node, the don't cares are expressed in terms of both primary inputs and intermediate variables, a new ordering scheme is used. and maximal CSPFs are computed. These ideas are incorporated in an algorithm designed to take full advantage of the power of two-level minimization in multilevel logic synthesis systems. This has been implemented in MIS-II, and results are presented that demonstrate the effectiveness of these techniques.<>