# The Eye-Opening Arbiter-PUF FPGA Implementation with Auto Error Detection
## 5 PUF Stability Evaluation and Automatic Detection of Unstable Bits
### Intrinsic Autodetection of Unstable Bits in EoA-PUF  
Blanking of unstable bits is a commonly seen technique in PUF design in order to improve PUF robustness [5; 33; 34]. This usually requires many repetitive measurements of all PUF instances and challenges and subsequently, after the statistical detection of (too) unstable responses, an exclusion of unstable bits. Detection of unstable bits is often conducted externally, requiring the responses to be shifted off-chip, which can leak the secret information through an unprotected port. Since repetitive measurements are time-consuming and detection based on statistical analysis complex, an onboard method that requires only a limited number of measurements without prior statistical knowledge would be advantageous. To the authors' knowledge, both requirements are met by the unique features of the EoA-PUF as it allows auto-detection of unstable PUF-bits, which is explained in the following.  
In the presented EoA-PUF architecture, RO pairs with small frequency differences must oscillate more frequently than RO pairs with large frequency differences to generate a large enough phase difference to leave the dead zone and thus cause the arbiter to make a decision. This explains the auto-detection of unstable bits; i.e., if a decision takes too long, it is rather considered unstable.  
To evaluate this, we examined the number of required RO oscillations until the arbiter makes a decision. This is plotted over adjusted arbiter deadzones in Figure 14; thereby, we separate prior found unstable responses from stable responses. It is seen that the number of required oscillations provides a very clear indication if a response was rather stable or not. This is an intrinsic property and advantage of the eye-opening effect of the arbiter and does not rely on the applied arbiter tuning.  
By defining a threshold \(O_{\text{max}}\), all challenges that oscillated longer than \(O_{\text{max}}\) until a final decision was made can be blanked in an initializing step. This can be repeated at every system start-up, making non-volatile memory to permanently store a blanking matrix obsolete, which is an improvement compared to the SotA. Further, it is not necessary to move the secret information, i.e., the PUF-bit responses, off the device for stability assessment. Although not evaluated in this work, these findings are also true for strong  
Figure 14: Number of oscillations required to exceed the deadzone and trigger a decision by the arbiter, plotted over PDL-tuned arbiter, based on H-SDE; the average delay per additional deadzone configuration bit is \(2\cdot 7\) ps = 14 ps. Shown for “stable” and “unstable” bits.  
PUF implementations. Challenges leading to reasonable phase difference will lead to fast and stable results, while the remaining will be detected as unstable, without the need to classify every challenge in advance.  
One could argue that such an auto-detection of unstable bits should also be possible in other oscillating PUFs like the TERO-PUF. The TERO-PUF is enabled for a fixed amount of time. Stable bits collapse into a stable state during this evaluation, while unstable ones do not; this should theoretically be detectable. The difference to the proposed EoA-PUF is that the latter has the XOR-gate output as an intrinsic signal, indicating if a decision was made. For the TERO-PUF, an external analysis of the outputs must be performed. Just adding an XOR-gate to the TERO-PUF would also not solve the issue as it would already recognize small noise-induced differences. Additional noise-suppressing hardware would be necessary, which results in the idea of the eye-opening arbiter, as presented in this work.