Driven ring oscillators as FPGA entropy sources

We show various degrees of complex behavior in driven ring oscillators on inexpensive FPGA student evaluation boards. Although ring oscillators typically give periodic behavior, they provide unpredictable, aperiodic behavior when driven at certain frequencies. Various non-ideal effects permit positive entropy due to the dynamics of this arrangement. The resulting topology is a driven, Boolean network consisting of long-established oscillator configurations. We implement ring oscillators driven by a variable frequency square wave generator. The resulting spectrum and autocorrelation illustrate various degrees of complex behavior. This work gives an accessible method for studying the complex dynamics found in Boolean networks through the combination of a simple, well-known network topology paired with convenient FPGA hardware.