Static noise margin (SNM)
Beginner
How much disturbance a memory cell can take before it accidentally flips its stored bit.
Novice
The largest steady voltage disturbance an SRAM cell can tolerate without losing its value. It is read off the butterfly curve as the side of the largest square that fits inside it. Hold SNM is measured with the cell idle; read SNM with the cell being read, which is worse.
Expert
Defined by Seevinck, List and Lohstroh (1987) as the side of the largest square nested between the two VTCs, which equals the largest DC series noise source the loop can absorb. Bounded by . Read SNM is the limiting case in 6T cells and shrinks with lower , lower cell ratio and mismatch.
Explained in Memory cells (Transistors).
See also: Butterfly curve, Cell ratio (β ratio), Write margin.