A
clock selector circuit (1) comprises a first input for receiving a first input
clock signal (CLK1) having a first frequency, and a second input for receiving a second input
clock signal (CLK2) having a second frequency, the second frequency differing from the first frequency by a
frequency offset. The clock selector circuit (1) further comprises a clock output for outputting an output
clock signal (CLK_OUT), a
phase difference detector (7) and switching circuitry (5, 6). The
phase difference detector (7) is configured to detect when a
phase difference, over time, between the first input
clock signal (CLK1) and the second input
clock signal (CLK2), determined using a predetermined type of clock edge, being either a rising edge or a falling edge, crosses zero, and to
signal this
zero crossing to the switching circuitry (5, 6). The switching circuitry (5, 6) is configured, in response to receiving a zero-crossing signal from the phase difference
detector (7), to detect an edge of opposite type to the predetermined type in the first input clock signal (CLK1) or in the second input clock signal (CLK2), and, in response to detecting said edge of opposite type, to switch the output clock signal (CLK_OUT) between the first input clock signal (CLK1) and the second input clock signal (CLK2).