A clock generator circuit may include a multiplex circuit and a phase-locked loop circuit. The multiplex circuit may generate a reference clock signal for the phase-locked loop circuit by selecting one of different clock signals. In response to a switch of the reference clock signal from one clock signal to another, the phase-locked loop circuit may disable phase-locking and enter into a frequency acquisition mode during which the frequency of the phase-locked loop circuit's output clock signal is adjusted based on the frequency of the newly selected reference clock signal. After a period of time has elapsed, the phase-locked loop circuit returns to phase-locking operation.