Evaluation and acceleration of spiking neural networks using FPGAs

Compared to conventional artificial neural networks, spiking neural networks (SNNs) are more biologically plausible and require less computation due to their event-driven nature of spiking neurons. However, the default asynchronous execution of SNNs also poses great challenges to accelerate their performance on FPGAs. In this thesis, we present a novel synchronous approach for rate encoding based Spiking Neural Networks (SNNs), which is more hardware friendly than conventional asynchronous approaches. We first quantitatively evaluate and mathematically prove that the proposed synchronous approach and asynchronous implementation alternatives of rate encoding based SNNs are the same in terms of inference accuracy and we highlight the computational performance advantage of using SyncNN over asynchronous approach. We also design and implement the SyncNN framework to accelerate SNNs on Xilinx ARM-FPGA SoCs in a synchronous fashion. To improve the computation and memory access efficiency, we first quantize the network weights to 16-bit, 8-bit, and 4-bit fixed-point values with the SNN friendly quantization techniques. Moreover, we encode only the activated neurons by recording their positions and the corresponding number of spikes to fully utilize the event-driven characteristics of SNNs, instead of using the common binary encoding (i.e., 1 for a spike and 0 for no spike). For the encoded neurons that have dynamic and irregular access patterns, we design parameterized compute engines to accelerate their performance on the FPGA, where we explore various parallelization strategies and memory access optimizations. Our experimental results on multiple Xilinx ARM-FPGA SoC boards demonstrate that our SyncNN is scalable to run multiple networks, such as LeNet, Network in Network, and VGG, on various datasets such as MNIST, SVHN, and CIFAR-10. SyncNN not only achieves competitive accuracy (99.6%) but also achieves state-of-the-art performance (13,086 frames per second) for the MNIST dataset. Finally, we compare the performance of SyncNN with conventional CNNs using the Vitis AI and find that SyncNN can achieve similar accuracy and better performance compared to Vitis AI for image classification using small networks.

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