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SRAM bitcell design margin continues to shrink due to random and systematic process variation in scaled technologies and conventional SRAM faces a challenge in realizing the power and density benefits of technology scaling. Smart and adaptive assist circuits can improve design margins while satisfying SRAM power and performance requirements in scaled technologies. This paper introduces an adaptive, dynamic SRAM word-line under-drive (ADWLUD) scheme that uses a bitcell-based sensor to dynamically optimize the strength of WLUD for each die. The ADWLUD sensor enables 130 mV reduction in SRAM Vccmin while increasing frequency yield by 9% over conventional SRAM without WLUD. The sensor area overhead is limited to 0.02% and power overhead is 2% for a 3.4 Mb SRAM array.
Date of Publication: Jan. 2011