WireLightning: Harnessing Capacitances for In-Transit Massively Parallel Matrix Multiplication

Abstract

<p>Analog computing-in-memory accelerators promise ultra-low-power, on-device AI by reducing data transfer and energy usage. Yet inherent device variations and high energy consumption for analog-digital conversion continue to hinder their wide-scale adoption in mainstream systems. To address these issues, this paper introduces WireLightning, a novel capacitive-computing accelerator featuring a mixed-signal architecture that rethinks analog AI acceleration. Unlike conventional analog crossbars that encode weights in programmable devices, WireLightning exploits intrinsic charge dynamics in passive capacitors, encoding matrix multiplication through spike amplitude and timing. This design addresses critical limitations such as weight drift, stochasticity, and power-intensive ADC bottlenecks. Key innovations include: amplitude-temporal dual encoding that enables constant-time analog dot-products; time-based decoding scheme that significantly reduces reliance on power-intensive ADCs; row-wise parallel architecture for concurrent dot-product calculations across multiple rows to enhance throughput; and value repetition exploitation in low-bit quantized vectors to reduce multiplications to constant time complexity. A PCB pro totype achieved a range-normalized RMSE of 1.80% - 69.2% of the error in RRAM crossbar implementations - and a normalized error of 9.18%, corresponding to 77.1% of that in leading PCM crossbars. Implemented in a 40-nm CMOS technology, WireLightning macro delivers 465.47 TOPS/W at 4-bit precision, outperforming state-of-the-art analog accelerators while maintaining 0.23% range-normalized RMSE. By integrating algorithm-circuit co-design with physical computing, this work establishes capacitive computing as a promising path toward combining digital precision and analog efficiency in next-generation edge AI. <br></p>