CLOCK SELECTION SYSTEM

PURPOSE:To improve the clock source by providing a circuit into a selector control device to produce the control signal which selects the transition or non-transition of the clock source according to the output state of the clock source. CONSTITUTION:A clock selecting part 2 selects a stable clock source CK1 or an ordinary clock source CK2 and the output of the part 2 is impressed to a using device 3. The outputs of both sources CK1 and CK2 are monitored by a clock monitor part 7 and a single signal, for example, is impressed to a selection controller 9 during the output of the clock. Then the clock selecting signal SEL 8 is applied to the controller 9 from outside and a circuit 9-1 is set into the controller 9 to select the output of the source CK1 or CK2 with the shift order. Then the output control signal is sent to the part 2 from the controller 9. Thus the selector 2 is actuated with the stable clock.