C4TERO: Configurable Cascaded Carry Chains for High Reliability TERO PUFs on FPGAs

In this paper we present a novel Transient Effect Ring Oscillator Physical Unclonable Function for FPGAs. It exploits in an original way the carry chain resources available in modern devices. The basic cell adopted in the proposed architecture can be runtime configured to implement different oscillation paths. This property enables the possibility to output more than one bit response per cell by choosing among the configurations those that exhibit the highest reliability. Such results are achieved by adopting a specific calibration process able to identify configurations of the cells showing the highest stability and the most uncorrelated responses. When implemented on several Series 7 Xilinx devices, no unstable bits were observed at 1 V and $25~^{\circ }$ C. Under voltage variation in the manufacturer recommended ranges, a worst case bit error rate of 0.046% is achieved. The circuit designed as here described consists of 64 cells, produces 128 response bits and consumes just 535 look-up-tables and 256 carry chains.