Abstract: Systems, methods, and apparatuses including computer program products for oscillator calibration. In one aspect, a calibration module includes a monitor submodule that evaluates operation of a system to determine if the system is operating in a first mode or a second different mode, where the second clock source is not operating as expected in the second different mode; a calibration engine that determines a calibration value using the first clock source and the second clock source in response to the system operating in the first mode; and a register that stores the calibration value; where the calibration engine calibrates the first clock source using the calibration value and provides to the system the calibrated first clock source as a timing reference instead of the second clock source as the timing reference, in response to the system operating in the second different mode.