An Investigation of the Accuracy of the VPR and COFFE Area Models in Predicting the Layout Area of FPGA Lookup Tables

A single tile in a mesh-based FPGA includes both the routing block and the logic block. The area estimate of a tile in an FPGA is used to determine the physical length of an FPGA's routing segments. An estimate of the physical length of the routing segments is needed in order to accurately assess the performance of a proposed FPGA architecture. The VPR (Versatile Place and Route) and the COFFE (Circuit Optimization for FPGA Exploration) tools are widely used mesh-based FPGA exploration environments. These tools map, place, and route benchmark circuits on FPGA architectures. Subsequently, based on area and delay measurements, the best architectural parameters of an FPGA are decided. The area models of the VPR and COFEE tools take only transistor size as input to estimate the area of a circuit. Realistically, the layout area of a circuit depends on both the transistor size and the number of metal layers that are available to route the circuit. This work measures the effect of the number of metal layers that are available for routing on FPGA layout area through a series of carefully laid out 4-LUTs (4-input Lookup Tables). Based on measured results, a correction factor for the COFFE area equation is determined. The correction factor is a function of both the transistor drive strength and the number metal layers that are available for routing. Consequently, a new area estimation equation, that is based on the COFFE area model, is determined. The proposed area equation takes into consideration the effect of both the transistor drive strength and the number of metal layers that are available for routing on layout area. The area prediction error of the proposed area equation is significantly less than the area prediction errors of the VPR and COFFE area models.