Analysis of Ring Oscillator Structures to Develop a Design Methodology for RO-PUF Circuits

Ring Oscillators (RO) are the main primitives of Physical Unclonable Functions (PUFs) that generate chip specific signatures depending on the uncontrollable components present in the manufacturing process. RO-PUFs are one of the popular PUF types among various structures presented in the literature. However, due to the noisy nature of RO circuits, robust output generation is problematic in RO-PUFs. Maximizing the robustness of a PUF is the main design objective, and analytical solutions have not been developed yet to overcome this problem. In this work, RO structures are analyzed and the effects of RO inverter count and measurement time are examined theoretically and practically in terms of jitter and spatial variation. Next, a design methodology is presented to easily determine the measurement time and RO inverter count for best performing RO-PUFs. In addition to this, the design methodology is practically verified by comparing the jitter and spatial variation to the robustness measurements of previously built RO-PUF circuits.