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Using the self-temperature sensing capability of the two C-modes of an S/C cut crystal, an all-digital oscillator has been designed and simulated. The design is called a Software Controlled Crystal Oscillator (SCXO). The major components of the proposed design are a software controller, voltage controlled oscillator (VXCO), S/C cut crystal, digital-to-analog (DAC) and analog-to-digital (ADC) converters. The software controller consists of the crystal calibration data (f vs. T), direct digital synthesizers (DDS), correlators and control logic. The simulated crystal has a 10 MHz overtone frequency and is modeled as a linear third order system. The calibration data is from an actual 10 MHz crystal and is assumed to have no errors. The VCXO is assumed to track frequency changes with zero error. The DAC and ADC are modeled as 12 bit converters. The DDS uses a 32-bit accumulator and 16 bits for the phase. The design was simulated using MATLAB. Simulation results show a steady-state rms error performance of 0.5 ppb clock error, lock-up of less than 20 millisecond and the ability to track rapid changes in temperature. In future work, the software controller will be implemented in a digital signal processor, field programmable gate array or a microprocessor. It is envisioned that the software controller function can be embedded in the user's application such as a GPS receiver or software radio.