| 摘要 |
PURPOSE:To use an external clock signal with top priority at any time by providing a holding means which holds the states of plural clock signals and a priority determining means which determines in-use priority levels of plural clock signals according to the held states. CONSTITUTION:Detectors 51-5n detect abnormality of (n) kinds of clock signals CL1-CLn supplied to (n) systems of clock signal supply sources which are not shown in a figure. The priority determining means 8 determines the in-use priority levels of said clock signals according to held contents of the 1st and the 2nd holding means 6 and 7. For example, (n) is 3, clock signals CL1 and CL2 are external clock signals, and a clock signal CL3 is a self-running clock signal. Then, if the clock CL1 becomes faulty when used, the lock CL2 is selected immediately when normal and if the clock CL2 is faulty, the clock CL3 is selected immediately when normal. |
