A Reconfigurable FeFET Content Addressable Memory for Multi-State Hamming Distance
Liu Liu, Ann Franchesca Laguna, Ramin Rajaei, Mohammad Mehdi Sharifi, Arman Kazemi, Xunzhao Yin, Michael Niemier, Xiaobo Sharon Hu
Abstract
Pattern searches, a key operation in many data analytic applications, often deal with data represented by multiple states per dimension. However, hash tables, a common software-based pattern search approach, require a large amount of additional memory, and thus, are limited by the memory wall. A hardware-based solution is to use content-addressable memories (CAMs) that support fast associative searches in parallel. Ternary CAMs (TCAMs) support bit-wise Hamming distance (HD) based searches. Detecting the HD of vectors with multiple states per dimension (i.e., multi-state Hamming distance (MSHD)) can be implemented on TCAMs with one-hot encoding, but requires one TCAM cell per state, leading to a higher area, latency, and energy overhead. We propose a Ferroelectric FET (FeFET)-based multi-state CAM design, MHCAM, which implements MSHD searches in a dense FeFET-based memory array. MHCAM only uses <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\lceil log_{2} s \rceil ~2$ </tex-math></inline-formula> FeFET CAM cells to represent <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$s$ </tex-math></inline-formula> states or symbols per dimension, and can be reconfigured to 2-bit/4-bit/6-bit/8-bit dimensions. A low-cost sensing circuit with matchline voltage scaling technique is introduced to perform both exact match and threshold match. We use DNA and protein pre-alignment filtering as application case studies to evaluate the application-level benefit of MHCAM. DNA and protein pre-alignment filtering achieve <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$3.8\times /4.7\times $ </tex-math></inline-formula> speedup and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$1.7\times /1.8\times $ </tex-math></inline-formula> energy improvement compared with the state-of-the-art 2FeFET TCAM-based implementation.