Effective Write Disturbance Mitigation Encoding Scheme for High-density PCM

Abstract

Write Disturbance (WD) is a crucial reliability concern in a high-density PCM with below 20nm scaling. WD occurs because of the inter-cell heat transfer during a RESET operation. Being dependent on the type of programming pulse and the state of the vulnerable cell, WD is significantly impacted by the data patterns. Existing encoding techniques to mitigate WD reduce the percentage of a single WD-vulnerable pattern in the data. However, it is observed that reducing the frequency of a single bit pattern may not be effective to mitigate WD for certain data patterns. This work proposes a significantly more effective encoding method which minimizes the number of vulnerable cells instead of a single bit pattern. The proposed method mitigates WD both within a word-line and across the bit-lines. In addition to WD-mitigation, the proposed method encodes the data to minimize the bit flips, thus improving the memory lifetime compared to the conventional WD-mitigation techniques. Our evaluation using SPEC CPU2006 benchmarks shows that the proposed method can reduce the aggregate (word- line+bit-line) WD errors by 42% compared to the existing state- of-the-art (SD-PCM). Compared to the state-of-the-art SD-PCM method, the proposed method improves the average write time, instructions-per-cycle (IPC) and write energy by 12%, 12% and 9%, respectively, by reducing the frequency of verify and correct operations to address WD errors. With reduction in bit flips, memory lifetime is also improved by 18% to 37% compared to SD-PCM, given an asymmetric cost of the bit flips. By integrating with the orthogonal techniques of SD-PCM, the proposed method can further enhance the performance and energy efficiency.