Increasing the Robustness of TERO-TRNGs Against Process Variation

The transition effect ring oscillator is a popular design for building entropy sources because it is compact, built from digital elements only, and is very well suited for FPGAs. However, it is known to be quite sensitive to process variation. Although the latter is useful for building physical unclonable functions, it is interfering with the application as an entropy source.
In this article, we investigate an approach to increase reliability. We show that adding a third stage eliminates much of the susceptibility to process variation and how a resulting gigahertz oscillation can be evaluated on an FPGA. The design is supported by physical and stochastic modeling. The physical model is validated using an experiment with dynamically reconfigurable look-up tables.