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

We give an algorithm for computing subsets of observability don't cares at the nodes of a multi-level Boolean network. These subsets are based on an extension of the methods introduced in [4] for computing compatible sets of permissible functions (CSPF's) 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 CSPF's are computed. These ideas are incorporated in an algorithm designed to take full advantage of the power of two-level minimization in multi-level logic synthesis systems. This has been implemented in MIS-II and we present results that demonstrate the effectiveness of these techniques.