Series-parallel functions and FPGA logic module design

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 functionality. 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 functions, 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 seven-input SP function that can implement all four-input SP functions. Mapping results show that, on an average, the number blocks of this function needed to map benchmark circuits are 12% less than those 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.