| 摘要 |
PROBLEM TO BE SOLVED: To provide a system which is provided with a PLL circuit that surely and also stably generates a clock signal which synchronizes with a clock signal that is externally supplied. SOLUTION: This system compares a clock signal that is supplied from an external terminal with an internal clock signal through a phase comparator 1, charges it up with a charge pump circuit 2 or forms discharge current in accordance with the output and drives a filter capacity 3. It controls the oscillation frequency of a voltage controlled oscillator 4 through holding voltage of the filter capacity 3 and is provided with a PLL circuit that includes a frequency dividing circuit which forms the internal clock signal based on an oscillating output. In such cases, a voltage detection circuit which detects that the holding voltage of the filter capacity 3 is made prescribed voltage or more and a function which forcedly lowers the holding voltage of the capacity 3 to prescribed potential through a detection output of such a voltage detection circuit are added. |
