**Abstract:**
In response to the limitations of traditional Ring Oscillator Physical Unclonable Functions (RO PUFs), such as limited output bits and insufficient robustness, this paper proposes a dynamic configurable system that integrates a multi-output ring oscillator with a dynamic configuration processing module. By utilizing multiple output ring oscillators, the system enhances chip resource utilization and increases the number of available 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 robustness. 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 Introduction**
With the rapid development and widespread application of computer technology and integrated circuits, the security of chip information has become a major concern. Among various security technologies, the Ring Oscillator Physical Unclonable Function (RO PUF) has emerged as a promising solution for secure chip authentication. Traditional RO PUFs generate output bits 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 response, making it ideal for generating chip-specific identifiers. However, conventional RO PUFs are limited in terms of output bit count and robustness under varying environmental conditions.
To address these challenges, a configurable RO PUF was introduced, where the output is determined by the configuration vector that maximizes frequency differences, thus reducing the impact of process variations. However, both traditional and configurable RO PUFs still suffer from a key limitation: they can only produce one output bit per pair of oscillators. This restricts the number of available bits and limits practical applications due to resource constraints.
This paper introduces a dynamic configurable multi-output RO PUF, which not only increases the number of output bits but also allows flexibility based on actual requirements. Additionally, the system dynamically adapts its structure according to environmental changes, enhancing both robustness and security.
**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 design enables multiple signal paths depending on the configuration vector. Each switch can either allow signal crossing or parallel transmission, leading to multiple signal transmission modes. The path distributor ensures stable oscillation by preventing signal crossovers between upper and lower paths.
By adjusting the switching units and the distribution path, the system can generate more than one output bit. For example, in Figure 2, four output bits (RO1-1, RO1-2, RO2-1, RO2-2) are generated using the same configuration vector. The frequency difference between corresponding pairs is used as an output bit, ensuring uniqueness due to process variations.
Experiments conducted on a Xilinx FPGA confirmed that the output frequencies vary with different configuration vectors, validating the effectiveness of the multi-output design in generating multiple bits. Moreover, the frequency differences between different pairs remain independent, allowing them to be treated as separate output bits.
**2 Dynamic Configuration Processing Module**
The overall structure of the dynamically configurable multi-output RO PUF includes N pairs of two-output ring oscillators, generating 2(N−1) bits by comparing adjacent frequencies. A sensor, based on a five-stage ring oscillator, monitors ambient temperature and supply voltage, which directly affect oscillation frequency. When environmental conditions exceed preset thresholds, a finite state machine (FSM) controller activates, recalculating the optimal configuration vector to maintain a large frequency difference.
The "optimal configuration vector" is not just about maximizing frequency differences but is determined by the distribution of these differences. If the previous ROs show a larger positive frequency difference than the next ones, the configuration that maximizes this difference is selected. Conversely, if the difference is negative, the configuration that maximizes the negative value is chosen. This approach enhances the accuracy of frequency comparisons and improves system robustness.
**3 Experimental Results**
Experiments were conducted on 10 Xilinx Spartan XC3S1000 FPGAs, each producing a 128-bit output. Data was processed through a Fast Single Link interface and analyzed for Hamming distances. The inter-chip Hamming distance ratio was calculated to measure uniqueness among different chips, while the on-chip Hamming distance assessed system robustness.
Results showed that the dynamic configurable multi-output RO PUF achieved the highest inter-chip Hamming distance ratio, close to the ideal 50%, indicating strong uniqueness. In terms of on-chip stability, the system demonstrated minimal error and noise, especially under fixed environmental conditions. Under varying temperatures and voltages, the configurable multi-output RO PUF maintained the lowest flip rate, proving its reliability in real-world scenarios.
**4 Conclusion**
This paper presents a dynamic configurable multi-output RO PUF that addresses the limitations of traditional and configurable RO PUFs by increasing output bits and improving robustness. The system efficiently utilizes chip resources while enhancing the uniqueness and reliability of chip IDs. These improvements make the proposed RO PUF highly suitable for secure IP core protection and other critical applications in embedded systems.
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