Time--to--Digital Converter (TDC)--Based Resonant Compute--in--Memory for INT8 CNNs with Layer--Optimized SRAM Mapping

Abstract

In recent years, Compute-in-memory (CiM) architectures have emerged as a promising solution for deep neural network (NN) accelerators. Multiply-accumulate~(MAC) is considered a {\textit de facto} unit operation in NNs. By leveraging the inherent parallel processing capabilities of CiM, NNs that require numerous MAC operations can be executed more efficiently. This is further facilitated by storing the weights in SRAM, reducing the need for extensive data movement and enhancing overall computational speed and efficiency. Traditional CiM architectures execute MAC operations in the analog domain, employing an Analog-to-Digital converter (ADC) to convert the analog MAC values into digital outputs. However, these ADCs introduce significant increase in area and power consumption, as well as introduce non-linearities. This work proposes a resonant time-domain compute-in-memory (TDC-CiM) architecture that eliminates the need for an ADC by using a time-to-digital converter (TDC) to digitize analog MAC results with lower power and area cost. A dedicated 8T SRAM cell enables reliable bitwise MAC operations, while the readout uses a 4-bit TDC with pulse-shrinking delay elements, achieving 1 GS/s sampling with a power consumption of only 1.25 mW. In addition, a weight stationary data mapping strategy combined with an automated SRAM macro selection algorithm enables scalable and energy-efficient deployment across CNN workloads. Evaluation across six CNN models shows that the algorithm reduces inference energy consumption by up to 8x when scaling SRAM size from 32~KB to 256~KB, while maintaining minimal accuracy loss after quantization. The feasibility of the proposed architecture is validated on an 8~KB SRAM memory array using TSMC 28~nm technology. The proposed TDC-CiM architecture demonstrates a throughput of 320~GOPS with an energy efficiency of 38.46~TOPS/W.