Frequency uniqueness in ring oscillator Physical Unclonable Functions on FPGAs

Hardware security in Field Programmable Gate Arrays (FPGAs) which use Physically Unclonable Functions (PUFs) rely on the ability to produce a large number of unique frequencies. This paper explores the frequency uniqueness as it relates to the number of stages used to build a ring oscillator. The experimental results from Xilinx Spartan 2 FPGAs show that the three stage ring oscillators have the highest standard deviation in terms of the frequencies produced. Ring oscillators with five, seven, nine and eleven stages on the other hand have a much smaller standard deviation. Based on these experimental results, it is found that the number of stages in a ring oscillator plays a critical role in determining frequency uniqueness.