A serial interface engine generates a series of digital data according to a pair of differential signals received from a high-speed Universal Serial Bus host and/or a full-speed universal serial bus host. Then, a packet identification (PID) unit identifies a packet identification of a start of each frame and a first period between two consecutive packet identifications according to the series of digital data. A count comparator is used for generating a calibration signal to calibrate an output frequency of an oscillator according to the first period.