Cambricon-M: A Fibonacci-Coded Charge-Domain SRAM-Based CIM Accelerator for DNN Inference

Charge-domain SRAM-based Computing-in-memory (CIM) proves to be a promising method for DNN inference, and benefits from avoiding data movement between computing units and memory. However, the high resolution Analog-to-Digital Converters (ADCs) dominates the energy consumption (up to 64%), limiting the energy efficiency of SRAM-CIM architectures. The main reason is the wide range of input analog values, requiring high resolution ADCs to convert the high precision averaged analog voltages into high bitwidth digital data. In this paper, to reduce the ADC overhead, we propose Cambricon-M, a novel Fibonacci-coded SRAM-based charge-domain CIM accelerator for DNN inference. Cambricon-M features the Fibonacci coding, which guarantees low density of ‘1’ in operands (i.e., the adjacent two bits of each ‘1’ are both ‘0’), narrowing the output voltage range and enabling low resolution ADCs. Further, Cambricon-M exploits the high bit-level sparsity to address the extra energy and area overhead caused by the larger bitwidth in Fibonacci coding. Specifically, Cambricon-M proposes zero-skipping methods to reduce ineffectual input/output, and the bit-slice based compression method to reduce memory capacity/bandwidth pressure. Experimental results show that Cambricon-M reduces ADC energy by 68.7%, and improves the energy efficiency 3.48× and 1.62× compared to TPUv4 and an ISAAC-based charge-domain SRAM-CIM accelerator.