SRAM Cell Stability: A

Abstract

SRAM cell stability assessment is traditionally based on static criteria of data stability requiring three coincident points in DC butterfly curves. This definition is based on static (DC) char- acteristics of the cell transistors. We introduce the dynamic criteria of cell data stability knowing that the cell operates in a dynamic en- vironment alternating between access and non-access conditions. The proposed definition of the dynamic data stability criteria in- troduces a new bound for the cell static noise margin (SNM). It reveals that the true noise margin of the cell can be made con- siderably higher than the conventional SNM once the cell access time is sufficiently shorter than the cell time-constant. This phe- nomena can be used to extend the noise margin in (partial) sub- threshold SRAMs. Moreover , a simulation method for verification of the dynamic data stability criteria is presented. Silicon measure- ment results in 130 nm CMOS technology confirms the concept of dynamic data stability and designer’s ability to trade timing and static parameters. Finally, it is shown that the long time constant due to the subthreshold operation of the cell can be exploited to maintain data stability with proper choice of access and recovery time.