PURPOSE:To prevent the output of a clock signal when an oscillation is unstable, by controlling the generation of the clock signal from an oscillator by means of a detecting circuit that detects both the rise and fall of an external input signal and then driving a microprocessor. CONSTITUTION:An oscillator 2 has the oscillation with the driving of a quartz oscillator 1 and generates a clock signal 11 having a prescribed frequency necessary for a microprocessor 10. At the same time, the oscillator 2 receives the clock start signal from a rise detecting circuit 4 that detects the rise of an external input signal and then starts the generation of the signal 11. After this, a transmission control circuit 8 supplies the signal 11 given from the oscillator 2 to the processor 10 in the form of a working clock signal 9 for a period during which a clock supply signal 7 is fed from a fall detecting circuit 5 that detects the fall of the external input.
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