**Abstract:**
In response to the limitations of traditional Ring Oscillator Physical Unclonable Functions (RO PUFs), such as limited output bits and low robustness, this paper proposes a dynamic configurable system that integrates a multi-output ring oscillator with a dynamic configuration processing module. By employing multiple output ring oscillators, the system enhances chip resource utilization and increases the number of output bits. The dynamic configuration processing module adjusts the oscillator structure in real-time based on environmental changes, thereby improving the system’s reliability and stability. Experimental results demonstrate that the proposed dynamic configurable multi-output RO PUF achieves a higher inter-chip Hamming distance and a lower on-chip Hamming distance compared to traditional and configurable RO PUFs, which significantly improves the accuracy of chip ID extraction.
**Keywords:** RO PUF; multi-output ring oscillator; dynamic configuration processing module; Hamming distance
**CLC number:** TP331
**Document identification code:** A
**DOI:** 10.16157/j.issn.0258-7998.170750
**Chinese citation format:** Liu Yongcong, Wang Jianye, Ding Hao. Dynamic configurable multi-output RO PUF [J]. Electronic Technology Application, 2017, 43(9): 43-45, 49.
**English Reference Format:** Liu Yongcong, Wang Jianye, Ding Hao. Dynamic configurable multi-output RO PUF [J]. Application of Electronic Technique, 2017, 43 (9): 43-45, 49.
**0 Preface**
With the rapid development of computer technology and integrated circuits, chip information security has become increasingly important. Ring Oscillator Physical Unclonable Functions (RO PUFs) have emerged as a promising solution in the field of secure authentication. Traditional RO PUFs generate a single bit by comparing the frequencies of two ring oscillators placed at different locations on the same chip. Due to process variations, each chip produces a unique and unpredictable response, making it ideal for secure identification. However, conventional RO PUFs suffer from limited output bits and poor robustness. To address these issues, configurable RO PUFs were introduced, where the output is determined by the configuration vector that maximizes frequency differences, reducing the impact of process variations.
Despite these improvements, both traditional and configurable RO PUFs still face a key limitation: they can only produce one output bit at a time. This restricts their application in systems requiring more secure and unique identifiers. To overcome this, we propose a dynamic configurable multi-output RO PUF that not only increases the number of output bits but also adapts its structure based on environmental conditions, enhancing both security and reliability.
**1 Multi-output Ring Oscillator**
The multi-output ring oscillator consists of inverters, switching units, and a path distributor, forming the foundation of the dynamically configurable multi-output RO PUF. As shown in Figure 1, the structure allows for multiple signal transmission paths depending on the configuration vector. Each switch controls whether the signal crosses or flows in parallel, resulting in different delay paths and thus varying oscillation frequencies. This flexibility enables the generation of multiple output bits from a single oscillator.
Figure 2 illustrates a one-to-multiple output ring oscillator with four output bits (RO1-1, RO1-2, RO2-1, RO2-2). The configuration vectors for RO1 and RO2 are identical, but due to physical process variations, the frequencies of RO1-1 and RO2-1 differ, allowing them to be used as a single output bit. Similarly, RO1-2 and RO2-2 provide another independent bit. This design effectively increases the number of output bits without significantly increasing chip resource usage.
Experiments conducted on a Xilinx FPGA confirmed that the output frequencies vary with different configuration vectors, validating the effectiveness of the multi-output structure. Furthermore, the experimental data showed that the outputs from different pairs are independent, indicating that the system can generate multiple reliable and distinct bits.
**2 Dynamic Configuration Process Module**
The overall structure of the dynamically configurable multi-output RO PUF includes N pairs of two-output ring oscillators, generating 2(N-1) output bits by comparing adjacent oscillators. A sensor, composed of a five-stage ring oscillator, monitors environmental conditions such as temperature and voltage. When these values exceed a threshold, the finite state machine (FSM) controller initiates a dynamic configuration process to calculate the "optimal configuration vector" for the current environment. This ensures that the frequency difference between adjacent oscillators remains large, minimizing environmental interference and improving system robustness.
The "optimal configuration vector" is not simply the one that produces the maximum frequency difference. Instead, it considers the distribution of frequency differences across all possible configurations. If the positive frequency difference of earlier oscillators is greater than that of later ones, the configuration that maximizes this difference is selected. Conversely, if the opposite is true, a configuration that maximizes the negative difference is chosen. This approach helps maintain a stable and accurate comparison, ensuring reliable chip ID extraction.
**3 Experimental Results**
Experiments were conducted on 10 Xilinx Spartan XC3S1000 FPGAs, with each producing a 128-bit output using 65 ROs. Data was processed through a Fast Single Link interface and analyzed for inter-chip and on-chip Hamming distances. The average inter-chip Hamming distance ratio, which measures uniqueness, was close to 50% for all three types of RO PUFs, indicating good discrimination. However, the dynamic configurable multi-output RO PUF exhibited the highest inter-chip Hamming distance ratio, demonstrating superior uniqueness.
On-chip Hamming distance, which reflects system robustness, was measured under varying temperature and voltage conditions. While the dynamic configurable RO PUF did not show a significant advantage in temperature variation, it demonstrated the lowest flip rate under voltage changes, leading to more accurate chip ID extraction. This makes it particularly suitable for applications where environmental stability is uncertain.
**4 Conclusion**
This paper introduces a dynamic configurable multi-output RO PUF that addresses the limitations of traditional and configurable RO PUFs. By increasing the number of output bits and adapting to environmental changes, the system improves both uniqueness and robustness. This advancement is crucial for secure chip identification and IP protection, offering a practical solution for modern electronic systems.
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