A 256-Mcell Phase-Change Memory Chip Operating at 2+ Bit/Cell. - Gael F. Close; Urs Frey; Jack Morrish; Richard Jordan; Scott C. Lewis; Tom Maffitt; M. J. BrightSky; Christoph Hagleitner; Chung Hon Lam; Evangelos Eleftheriou (IEEE Trans. Circuits Syst. I Regul. Pap., 2013)