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Energy-efficient bitcoin mining cores have gained significant attention since the energy cost for computing dominates the mining expenses [1]. Ultra-low-voltage (ULV) digital circuits have emerged as an attractive approach to improve the energy-efficiency. However, they demand a large timing margin for the worst-case process, voltage, and temperature (PVT) variations, undermining a significant portion of energy savings. Recent works, including multi-phase latch pipeline [1], tunable replica circuits [2]–[3], in-situ error detection and correction (EDAC) [4]–[6], and dynamic timing enhancement [7], can reduce the pessimistic margin. However, it is not straightforward to adopt those techniques in mining cores due to their deeply-pipelined architecture (up to 128 stages [1]). For example, to adopt EDAC, the deep pipeline requires inserting many bulky error detectors as it has many critical paths. Our experiment with a 0.3V 28-nm mining core shows >18.9% registers need to be replaced with error detectors, considering 6σ local process variation only. Also, multiple stages can have timing errors simultaneously, making an error correction process (e.g., clock gating [5], VDD boosting [6]) complex and costly.

Emerging applications like a drone and an autonomous vehicle require system-on-a-chips (SoCs) with high reliability, e.g., the mean-time-between-failure (MTBF) needs to be over tens of thousands of hours [1]. Meanwhile, as these applications require increasingly higher performance and energy efficiency, a multi-core architecture is often desirable. Here, each core operates in an independent voltage/frequency (V/F) domain, ideally from the near-threshold voltage (NTV) to super-threshold, while communicating with one another via a network-on-chip (NoC) [2]. However, this makes it challenging to ensure robustness in clock domain crossing against metastability. Metastability becomes even more critical to NTV circuits since metastability resolution time constant T grows super-linearly with voltage scaling [3]. Conventionally, an NoC uses multi-stage (4 stages in [4]) synchronizers to improve MTBF, but they increase latency and cannot completely eliminate metastability. Recently, [5] proposed a novel NTV flip-flop, which has a lower probability of having metastability. Another recent work [6] proposed to detect the necessary condition of metastability and mitigate it by modulating the RX clock and also requesting retransmission to guarantee data correctness. However, as it detects a necessary condition, not actual metastability, it tends to overly request retransmission, hurting latency, throughput, and energy efficiency.