A novel nonvolatile flip-flop (NVFF) using a magnetic tunnel junction (MTJ) is presented for power gating architecture. The proposed NVFF exploits spin Hall effect (SHE) for fast and low-power data backup into MTJs before the power is gated off. Owing to the high spin injection efficiency of SHE, the estimated write current for <10-ns backup operation is lower than 40 µA. Due to the low write current requirement, we do not introduce a dedicated write driver circuit. Instead, we utilize the cross-coupled inverters in the slave latch to perform the backup operation, resulting in low area overhead. The simulation results show 10× improvement in backup energy when compared with previous works on spin transfer torque-based NVFFs. Index Terms-Magnetic tunnel junction (MTJ), nonvolatile flip-flop (NVFF), power gating, spin Hall effect (SHE).
In STT-MRAM, retention-, write-and read-failures negatively impact the memory yield and density. In this paper, we jointly consider device-circuit-architecture layers to implement high-density STT-MRAM array while meeting the target yield requirement. Different types of MTJs are considered at the device level, and error correcting codes (ECCs) in conjunction with Invert-Coding are employed as an architectural solution. Through cross-layer interactions, we present a design methodology to optimize bit-cell area while satisfying the target yield and energy consumption under process variation. Furthermore, we explore the use of Invert-Coding along with ECC in order to achieve higher memory density than that obtained using ECC alone. Our proposed technique can improve memory density further by proper selection of thermal stability factor based upon two observations: 1) Invert-Coding can fix multiple write/read failures with small storage overhead and 2) as thermal stability factor increases, retention-failure probability exponentially decreases and thus simple ECC is good enough for retention failure correction.Index Terms-Cache, density, error correcting code, invert coding, magnetic tunnel junction (MTJ), process variation, spin transfer torque magnetic random access memory (STT-MRAM), thermal stability factor, yield.
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