A 32.65µm2 Spin/Area Large Scale Ising CIM with Progressive Circular Dataflow and Bi-directional eDRAM Cell Array

This paper presents a large-scale Ising computing-in-memory (CIM) for a real-world combinatorial optimization problem (COP). While the CIM approach shows promising performance improvement compared to digital-based Ising machines, it can be only used for simple COP due to limited CIM connectivity, bit precision, and graph size. To overcome limitations, the proposed processor supports reconfigurable high-bit Chimera graph topology achieving a small cell area and high energy efficiency through three key features : 1) Progressive circular dataflow reduce area by 58.8% and improved energy efficiency by 44.7% thanks to fully reuse spin and coefficient. 2) Bi-directional eDRAM cell array support bi-directional ising computation for circular dataflow with 3T-2C eDRAM cell. Due to the signed operation with compact coefficient storing, the area was reduced by 36.1%. 3) Reconfigurable spin exchange link and reconfigurable C-2C ladder support various graph sizes and various bit precision of coefficients. In conclusion, the proposed large-scale Ising CIM achieves 32.65µm² spin area and 2.09µW effective spin power which is 3.21× and 5.17× smaller than previous state-of-the art.