Practical VHDL optimization for timing critical FPGA applications

Abstract

This paper gives a hands-on example of how low-level optimization of the VHSIC Hardware Description Language (VHDL) code is extremely difficult within a contemporary Field Programmable Gate Array (FPGA) design flow. However, low-level optimization can be accomplished, and by changing the VHDL coding style synthesis results can be improved. The design flow is considered from high-level descriptions (bubble diagrams), through logic synthesis to the point where hand optimization is required. For performance benchmarking a state machine from a contemporary computer bus, PCI, implemented in a Xilinx FPGA, is used. Practical design issues applied to time-critical implementations using FPGAs, especially the trade-offs of high-level versus low-level synthesis, are analyzed. Performance evaluation results of several PCI target state machines, coded using different styles and design methods are given in terms of time and area efficiency. Based on these findings improvements to the FPGA design methodology are proposed.