Accounting for Floorplan Irregularity and Configuration Dependence in FPGA Routing Delay Models

Abstract

Accurate delay estimates for a user application implemented in a Field-Programmable Gate Array (FPGA) are essential for a quality FPGA timing flow and to avoid leaving performance on the table. FPGA inter-cluster routing consists of wire segments of a limited number of types which repeat in a somewhat regular pattern, interconnected by configurable muxes. The delay at any fanout of a segment can be significantly impacted by configuration-dependent capacitive loading related to other fanouts. Also, the insertion of RAM and math blocks into the FPGA floorplan introduces irregular stretching of the wire segments, altering their delays. We explain why and how commercial FPGA software typically employs a parameterized model for the delay at each fanout of a segment, based on the configuration and the irregularities present, with the parameters determined by fitting SPICE simulation data for a representative sample of cases. We propose incorporating readily-computed common path resistance values into the model. This enables high accuracy with fewer parameters and without the large amounts of SPICE data that would otherwise be required to explore interactions between floorplan irregularities and the set of active fanouts. In combination with other features of our models, errors in segment delay are reduced by almost half.