| 摘要 |
PURPOSE:To reduce the accumulated error by updating the time using a clock for counting the frequency of an AC power supply when it is turned ON whereas updating the time using a clock for counting the frequency of an oscillation circuit while being fed with power from a backup power supply when the AC power supply is turned OFF. CONSTITUTION:When an AC power supply 9 is turned ON, a control circuit 6 counts the AC frequency signal 10 on a signal line 15, according to the preset operation of an operator, and presets the time an AC clock 5. When the power supply 9 is turned OFF, an OFF detection circuit 2 outputs a signal 12 and the circuit 6 reads out the time of the clock 5 and counts the output from an oscillation circuit thus presetting a self-oscillation clock 8. At the same time, a signal 16 is delivered to a power supply switching circuit 4 in order to switch the power supply for driving the clock 8 from a DC power supply 11 to a backup power supply 3. When the power supply 9 is turned ON, the circuits 2, 6 output the signals 12, 16 sequentially and the circuit 4 switches the power supply to the DC power supply and, at the same time, reads out the time of the clock 8 thus presetting the clock 5. |
