PURPOSE:To permit discrimination of whether a set duty ratio is larger or smaller than a desired duty ratio by detecting the rise and fall of input pulses, up- and down-counting the clocks of different frequencies by an up-down counter and generating different decision outputs. CONSTITUTION:The rise and fall of the input pulses A are detected by a rise and fall detecting circuit 4. The up-down counter 1 up- and down-counts the 1st and 2nd clocks of the different frequency ratios according to the set duty ratios changed and outputted by a clock change over circuit 3 in the high level period and low level period of the pulses A. The 0 or this count result is detected by a circuit 2 and an RS type FF 5 is set by the detection output of a circuit 2 only when the duty ratio of the pulses A is below the set value. The FF 5 is not set when the duty ratio is above the set value and the count result is of the prescribed value. The decision output F from a D type FF 6 via the FF 5 goes to a 0 or 1 according to whether the duty ratio is above or below the set value. Decision of whether the set duty ratio is larger than the desired duty ratio or not is thus permitted by this constitution according to the selection of the clock frequencies.
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