AWARE (Asymmetric Write Architecture With REdundant Blocks): A High Write Speed STT-MRAM Cache Architecture

Spin-transfer torque magnetic RAM (STT-MRAM) is a promising memory technology for lower level caches because of its high density and nonvolatile nature. However, the high write latency is a bottleneck to its widespread adoption as the future on-chip memory. In this paper, we propose a new cache architecture-asymmetric write architecture with redundant blocks (AWARE)-that can improve the write latency by taking advantage of the asymmetric write characteristics of 1T-1MTJ STT-MRAM bit-cells. Due to the nature of the storage element in STT-MRAM, the time required for the two-state transitions ( 1→ 0 and 0→ 1) is not identical. In other words, one of the state transitions is slower than the other direction. In conventional cache architecture, the overall write latency is limited by the slower transition. However, the AWARE cache design introduces redundant blocks in each row, and they are preset to the initial state that enables the faster transition. Hence the write operations performed in these redundant blocks are much faster than the conventional write scheme. The write latency in AWARE is improved by 30% over conventional cache architecture with no area penalty in the data array. Moreover, the additional tag bits introduced in this technique result in penalty on the total cache area. In addition, the write energy increases modestly by 7% in the proposed cache design. However, this write-energy increase can be mitigated by sacrificing the cache capacity.