Synthesizable Memory Arrays Based on Logic Gates for Subthreshold Operation in IoT

Abstract

This paper identifies novel directions of standard-cell-based synthesizable memory design. A compact 18T-bitcell of OR-AND-Invert (OAI) and AND-OR-Invert (AOI) logic gates is presented with bit-selective write and multiplexed read accesses. It reduces the bitcell area by 11%–40% compared to the state-of-the-art clock-gating-based D-latch schemes while avoiding custom-cell design. The improved storage density comes from the tight integration on two levels–coupling the read multiplexing within each bitcell and taking advantage of structured datapath placement in bitcell arrays by using commercial CAD tools. As measured on a 40 nm SRAM of 90 kb, the OAI/AOI-gate-based synthesizable memory features 0.4 V minimum access voltage, 30 fJ minimum read energy per bit, and a leakage power of 1.4 pW per bit at 0.3 V data retention voltage. It is ideally suited for IoT-node design operating in sub- $V_{\mathrm {t}}$ for joint dynamic energy and leakage power reduction. Further, it enables fast turn-around times by IP reuse over technology nodes with minimal (re)design effort.