On Designing ULM-Based FPGA Logic Modules

In this paper, we give a method to design FPGA logic modules, based on an extension of classical work on designing Universal Logic Modules (ULM). Specifically, we give a technique to design a class of logic modules that specialize to a large number of functions under complementations and permutations of inputs, bridging of inputs and assignment of 0/1 to inputs. Thus, a lot of functions can be implemented using a single logic module. The significance of our work lies in our ability to generate a large set of such logic modules. A choice can be made from this set based on design criteria. We demonstrate the technique by generating a set of 471 8-input functions that have a much higher coverage than the 8-input cells employed by Actel´s FPGAs. Our functions can specialize to up to 23 times the number of functions that Actel functions can. We also show that by carefully optimizing these functions one can obtain multi-level implementations of them that have delays within 10% of the delays of Actel modules. We demonstrate the effectiveness of these modules in mapping benchmark circuits. We observed a 16% reduction in area and a 21% reduction in delay using our logic modules instead of Actel´s on these circuits.