A mixed-precision memristor and SRAM compute-in-memory AI processor

Abstract

Artificial intelligence (AI) edge devices^{1,2,3,4,5,6,7,8,9,10,11,12} demand high-precision energy-efficient computations, large on-chip model storage, rapid wakeup-to-response time and cost-effective foundry-ready solutions. Floating point (FP) computation provides precision exceeding that of integer (INT) formats at the cost of higher power and storage overhead. Multi-level-cell (MLC) memristor compute-in-memory (CIM)^13,14,15 provides compact non-volatile storage and energy-efficient computation but is prone to accuracy loss owing to process variation. Digital static random-access memory (SRAM)-CIM^{16,17,18,19,20,21,22} enables lossless computation; however, storage is low as a result of large bit-cell area and model loading is required during inference. Thus, conventional approaches using homogeneous CIM architectures and computation formats impose a trade-off between efficiency, storage, wakeup latency and inference accuracy. Here we present a mixed-precision heterogeneous CIM AI edge processor, which supports the layer-granular/kernel-granular partitioning of network layers among on-chip CIM architectures (that is, memristor-CIM, SRAM-CIM and tiny-digital units) and computation number formats (INT and FP) based on sensitivity to error. This layer-granular/kernel-granular flexibility allows simultaneous optimization within the two-dimensional design space at the hardware level. The proposed hardware achieved high energy efficiency (40.91 TFLOPS W⁻¹ for ResNet-20 with CIFAR-100 and 28.63 TFLOPS W⁻¹ for MobileNet-v2 with ImageNet), low accuracy degradation (<0.45% for ResNet-20 with CIFAR-100 and for MobilNet-v2 with ImageNet) and rapid wakeup-to-response time (373.52 μs).