By the late 1970s, NMOS microprocessors had overtaken PMOS processors.[16] PMOS logic remained in use for a while due to its low cost and relatively high level of integration for applications such as simple calculators and clocks. CMOS technology promised a drastically lower power consumption than either PMOS or NMOS. Even though a CMOS circuit had been proposed already in 1963 by Frank Wanlass[17] and commercial 4000 series CMOS integrated circuits had entered production in 1968, CMOS remained complex to manufacture and allowed neither the integration level of PMOS or NMOS nor the speed of NMOS.