Conclusion

We argued that allowing evolution to explore highly unconventional circuits can be advantageous. Analysis of the evolved circuits enhances their utility, but requires novel approaches. There are numerous tactics that can be used to piece-together answers to analysis questions even for seemingly impenetrable circuits. We applied many of these techniques to the most advanced unconventional circuit yet produced. We still do not understand fully how it works: the core of the timing mechanism is a subtle property of the VLSI medium. We have ruled out most possibilities: circuit activity (including glitch-transients and beat-frequencies), metastability [Marino 1981], and thermal time-constants from self-heating. Whatever this small effect, we understand that it is amplified by alterations in bistable and transient dynamics of oscillatory loops, and in detail how this is used to derive an orderly near-optimal output. Certain peripheral cells fine-tune particularly sensitive time delays. On the key question of long-term consistency of behaviour, we know that the entire FPGA circuitry is strongly reset to a deterministic stable logic state for every high half-cycle of the input waveform. Long-term pathologies are therefore highly unlikely, demonstrating that analysis effort can sometimes remove worries related to the use of highly unconventional circuits in practical applications.

1998-11-18