Endpoint devices for Internet-of-Things not only need to work under extremely tight power envelope of a few milliwatts, but also need to be flexible in their computing capabilities, from a few kOPS to GOPS. Near-threshold (NT) operation can achieve higher energy efficiency, and the performance scalability can be gained through parallelism. In this paper we describe the design of an opensource RISC-V processor core specifically designed for NT operation in tightly coupled multi-core clusters. We introduce instructionextensions and microarchitectural optimizations to increase the computational density and to minimize the pressure towards the shared memory hierarchy. For typical data-intensive sensor processing workloads the proposed core is on average 3.5× faster and 3.2× more energy-efficient, thanks to a smart L0 buffer to reduce cache access contentions and support for compressed instructions. SIMD extensions, such as dot-products, and a built-in L0 storage further reduce the shared memory accesses by 8× reducing contentions by 3.2×. With four NT-optimized cores, the cluster is operational from 0.6 V to 1.2 V achieving a peak efficiency of 67 MOPS/mW in a low-cost 65 nm bulk CMOS technology. In a low power 28 nm FDSOI process a peak efficiency of 193 MOPS/mW (40 MHz, 1 mW) can be achieved.Index Terms-Internet-of-Things, Ultra-low-power, Multi-core, RISC-V, ISA-extensions.

The AES (Advanced Encryption Standard) is a new block cipher standard published by the US government in

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Near-sensor data analytics is a promising direction for IoT endpoints, as it minimizes energy spent on communication and reduces network load -but it also poses security concerns, as valuable data is stored or sent over the network at various stages of the analytics pipeline. Using encryption to protect sensitive data at the boundary of the on-chip analytics engine is a way to address data security issues. To cope with the combined workload of analytics and encryption in a tight power envelope, we propose Fulmine, a System-on-Chip based on a tightly-coupled multi-core cluster augmented with specialized blocks for compute-intensive data processing and encryption functions, supporting software programmability for regular computing tasks. The Fulmine SoC, fabricated in 65 nm technology, consumes less than 20 mW on average at 0.8 V achieving an efficiency of up to 70 pJ/B in encryption, 50 pJ/px in convolution, or up to 25 MIPS/mW in software. As a strong argument for real-life flexible application of our platform, we show experimental results for three secure analytics use cases: secure autonomous aerial surveillance with a state-of-the-art deep CNN consuming 3.16 pJ per equivalent RISC op; local CNN-based face detection with secured remote recognition in 5.74 pJ/op; and seizure detection with encrypted data collection from EEG within 12.7 pJ/op.

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