Extending multi‐level STT‐MRAM cell lifetime by minimising two‐step and hard state transitions in hot bits

Abstract

Shifting market trends towards mobile, Internet of things, and data-centric applications create opportunities for emerging low-power non-volatile memories. The attractive features of spin-torque-transfer magnetic-RAM (STT-MRAM) make it a promising candidate for future on-chip cache memory. Two-bit multiple-level cell (MLC) STT-MRAMs suffer from higher write energy, performance overhead, and lower cell endurance when compared with single-level counterpart. These unwanted effects are mainly due to write operations known as two-step (TT) and hard transitions (HT). Here, the authors offer a solution to tackle write energy problem in MLC STT-MRAM by minimising the number of TT and HT transitions. By analysing real applications, it was observed that specific locations within a cache block undergo much more TT and HT transitions resulting in hot locations when compared with other ones (cold locations). These hot locations are more detrimental to the lifetime and reliability of MRAM device. In this work, the authors propose a simple and intuitive dynamic encoding scheme that eliminates all TT and HT at hot locations, hence reducing energy consumption and improving MLC STT-MRAM lifetime. Results on PARSEC benchmarks demonstrate the effectiveness and scalability of the proposed approach to potentially prolong MLC STT-MRAM lifetime.