Accelerating FPGA debug

Abstract

FPGA technology is commonly used to prototype new digital designs before entering fabrication. Whilst these physical prototypes can operate many orders of magnitude faster than through a logic simulator, a fundamental limitation is their lack of on-chip visibility when debugging. To counter this, trace-buffer-based instrumentation can be installed into the prototype, allowing designers to capture a predetermined window of signal data during live operation for offline analysis. However, instead of requiring the designer to recompile their entire circuit every time the window is modified, this article proposes that an overlay network is constructed using only spare FPGA routing multiplexers to connect all circuit signals through to the trace instruments. Thus, during debugging, designers would only need to reconfigure this network instead of finding a new place-and-route solution. Furthermore, we describe how this network can deliver signals to both the trigger and trace units of these instruments, which are implemented simultaneously using dual-port RAMs. Our results show that new network configurations connecting any subset of signals to 80--90% of the available RAM capacity can be computed in less than 70 seconds, for a 100,000 LUT circuit, as many times as necessary. Our tool—QuickTrace—is available for download.