Opportunistic Write for Fast and Reliable STT-MRAM

Nour Sayeda, Mojtaba Ebrahimib, Rajendra Bishnoic and Mehdi B. Tahoorid
Karlsruhe Institute of Technology (KIT) Karlsruhe, Germany.


Due to the stochastic switching behavior of the bitcell in Spin Transfer Torque Magnetic Random Access Memory (STT-MRAM), an excessive write margin is required to guarantee an acceptable level of reliability and yield. This prevents the usage of STT-MRAM in fast memories such as L1 or L2 caches. The excessive write margin of STT-MRAM can be reduced to a large extent by an opportunistic write (i.e., terminating the write process before all bit switchings are completed) and by reducing thermal stability factor. The bits with unfinished writes have to be processed by robust Error Correction Codes (ECCs). However, such coding schemes have relatively large decoding latencies, which increases the overall read latency significantly. Moreover, thermally induced retention failures can limit the applicability of such schemes. In this paper, we exploit the fact that error detection is much faster than correction. Therefore, the errors can be detected quickly and all erroneous data can be reverted before they arrive critical parts of the system (e.g., commit stage or memory ports). We also provide an adaptive approach to manage temperature-dependent retention failures at runtime. Hence, our proposed approach enables the use of STT-MRAM technology for fast cache applications.

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