This application is directed to frequency controlling in an electronic device that includes a selector, a clock generated, and a controller. The selector selects one of a first reference signal and a second reference signal as an input signal having an input phase. The clock generator receives the input signal and generates a periodic signal, and the periodic signal has an output phase that matches the input phase of the input signal. While the first reference signal is selected as the input signal, the controller identifies a temporal range including a peak instant at which the second reference signal reaches a peak frequency, select a switching instant within the temporal range based on a known temporal position of the peak instant with respect to the temporal range, and control the selector to select the second reference signal as the input signal at the switching instant.