State space modeling for sub-threshold SRAM stability analysis

Continuous technology scaling has made traditional Static Noise Margin metrics for stability analysis of SRAM bitcells insufficient. Today, Dynamic Noise Margin analyses and metrics are necessary for state-of-the-art bitcell design, especially under problematic low-voltage operation. In this paper, we overview the concept of state-space modeling for dynamic stability analysis, and then develop an analytical method for evaluating SRAM bitcell operation in the sub-threshold regime. An algorithm for state-space and phase-portrait plotting is proposed and shown to correctly predict subthreshold hold and write behavior of standard bitcells in a 40nm CMOS technology. Implementation of the presented technique in mathematical CAD tools provides orders of magnitude faster evaluation than using traditional brute force approaches.