Abstract: A circuit for detecting and selecting one of clock signals of at least two clock sources, is provided. The circuit is coupled to the available clock sources so as to receive their clock signals. The circuit is programmable to choose one of the clock signals designated by a programmed code. Then, the circuit [1] selects the designated clock signal when it has been received and [2] automatically selects the undesignated clock signal when the designated clock signal has not been received and when the undesignated clock signal has been received. Thereafter, the circuit provides the selected clock signal to any device that requires a clock signal in order to operate.