In this project we designed an ultra low power clock source for our BSN chip. The ultra low power clock source is designed using a 1¬µW temperature compensated on-chip digitally controlled oscillator (OscCMP) and a 100nW uncompensated oscillator (OscUCMP) with respective temperature stabilities of 5ppm/oC and 1.67%/oC. It also has a fast locking circuit that re-locks OscUCMP to OscCMP often enough to achieve high effective temperature stability. Measurements of a 130nm CMOS chip show that this combination gives a stability of 5ppm/oC from 20oC to 40oC (14ppm/oC from 20oC to 70oC) at 150nW if temperature changes by 1oC or less every second. The proposed circuit exhibits similar stability to an XTAL with ~7X less power and no off-chip components, provides a low cost ULP solution for wireless sensors and BSNs.