US Patent 10250266 Oscillator calibration system

An oscillator system for an integrated circuit includes a first oscillator circuit, a second oscillator circuit, and calibration system. During a sampling routine, the calibration system is used to determine a sampled value based on a comparison of the output of the second oscillator and an external clock signal. The sampled value is stored in a memory. During a calibration routine, the calibration system determines a comparison value based on a comparison of the output of the second oscillator circuit and the output of the first oscillator circuit. The calibration circuit compares the comparison value with the sampled value to generate a tuning value to tune the frequency of the first oscillator circuit.