Universal Logic Modules for Series-Parallel Functions

The need for a two-way interaction between logic synthesis and FPGA logic module design has been stressed recently. Having a logic module that can implement many functions is a good idea only if one can also give a synthesis strategy that makes efficient use of this funtionality. Traditionally technology mapping algorithms have been developed after the logic architecture has been designed. We follow a dual approach, by focusing on a specific technology mapping algorithm, namely the structural tree-based mapping algorithm, and designing a logic module that can be mapped efficiently by this algorithm. It is known that the tree-based mapping algorithm makes optimal use of a library of funtions, each of which can be represented by a tree of AND, OR and NOT gates (series-parallel or SP functions). We show how to design a SP function with a minimum number of inputs, that can implement all possible SP functions with a specified number of inputs. For instance, we demonstrate a 7-input SP function that can implement all 4-input SP functions. Mapping results show that, on an average, the number blocks of this function needed to map benchmark circuits is 12% less than that for Actel´s ACT1 logic modules. Our logic modules show a 4% improvement over ACT1, if the block count is scaled to take into account the number of transistors needed to implement different logic modules.