Synaptic Activity and Hardware Footprint of Spiking Neural Networks in Digital Neuromorphic Systems

Spiking neural networks are expected to bring high resources, power, and energy efficiency to machine learning hardware implementations. In this regard, they could facilitate the integration of Artificial Intelligence in highly constrained embedded systems, such as image classification in drones or satellites. If their logic resource efficiency is widely accepted in the literature, their energy efficiency still remains debated. In this article, a novel high-level metric is used to characterize the expected energy efficiency gain when using Spiking Neural Networks (SNN) instead of Formal Neural Networks (FNN) for hardware implementation: Synaptic Activity Ratio (SAR). This metric is applied to a selection of classification tasks including images and 1D signals. Moreover, a high-level estimator for logic resources, power usage, execution time, and energy is introduced for neural network hardware implementations on FPGA, based on four existing accelerator architectures covering both sequential and parallel implementation paradigms for both spiking and formal coding domains. This estimator is used to evaluate the reliability of the Synaptic Activity Ratio metric to characterize spiking neural network energy efficiency gain on the proposed dataset benchmark. This study led to the conclusion that spiking domain offers significant power and energy savings in sequential implementations. This study also shows that synaptic activity is a critical factor that must be taken into account when addressing low-energy systems.