A Reconfigurable LDO-Assisted Physically Unclonable Function Achieving a Zero-BER With 14% Masking

This paper presents a physically unclonable function (PUF) based on a ring oscillator (RO) collapse with an improved robustness against temperature variations. To effectively minimize the chances of having temperature-sensitive dark bits, the PUF equips a low-drop-out (LDO) regulator that enables a selection of the supply voltage for each PUF cell, so that uncertainties by temperature change can be minimized whereas those by process mismatch are kept to be large. A reconfigurable RO is also proposed for the PUF cell to effectively transform its shape to have a larger process mismatch. An implementation of the proposed PUF in a 40 nm CMOS shows a bit error rate (BER) of 0.039% in the nominal conditions (0.9 V, 25 °C). The worst-case BER with 0.9 V-to-1.4 V supply and −40 °C -to-125 °C temperature changes is 0.297%. As an indicator of the amount of stability for each PUF cell, the cycles to collapse (CTC) at the nominal conditions is used for the dark bit detection and prescreening. The prescreening reduces the worst-case BER to 0.0092% with 5% masking, to 0.00016% with 10% masking and to zero-error (BER
$< 4.88\times 10^{-8}$
in the whole 2048 cells over 10K evaluations) with 14% masking.