Oscillation circuit

1. An oscillation circuit, connected to an external piezoelectric resonator, comprising:

a first inverter circuit that includes an input terminal and an output terminal which are connected to both terminals of the piezoelectric resonator, respectively;

a first feedback resistor that is disposed between the input terminal and the output terminal of the first inverter circuit;

first and second variable capacitive elements that are connected to the input terminal and the output terminal of the first inverter circuit, respectively, and have capacitance values that are variably settable by a control signal;

a charging circuit that supplies the input terminal or the output terminal with a certain reference current in order to charge the first or second variable capacitive element;

a comparator that compares a charging voltage of the input terminal or the output terminal with a reference voltage; and

a control circuit that, in a calibration operation, at a first time, causes the charging circuit to start supply the reference current to the input terminal or the output terminal, and, at a second time after the first time, generates the control signal for setting a capacitance value of the first or second variable capacitive element so that the charging voltage becomes close to the reference voltage according to a comparison result of the comparator.

2. The oscillation circuit according to claim 1 , wherein at the time of calibration, the control circuit generates the control signal for increasing the capacitance value of the first or second variable capacitive element when the comparison result of the comparator represents that the charging voltage is higher than the reference voltage and generates the control signal for decreasing the capacitance value of the first or second variable capacitive element when the comparison result of the comparator represents that the charging voltage is lower than the reference voltage.

3. The oscillation circuit according to claim 1 , further comprising:

a reference current generating circuit that generates the reference current; and

a reference voltage generating circuit that generates the reference voltage by supplying the reference current to an externally connected reference resistor.

4. The oscillation circuit according to claim 1 , wherein at the time of calibration, the control circuit stops an oscillation operation by the first inverter circuit and the piezoelectric resonator.

5. The oscillation circuit according to claim 1 , further comprising a reference current generating circuit that generates the reference current, wherein

at the time of calibration, the charging circuit connects the reference current generating circuit to the input terminal or the output terminal at the first time.

6. An oscillation circuit, connected to an external piezoelectric resonator and a first feedback resistor, comprising:

a first inverter circuit that includes an input terminal and an output terminal which are connected to both terminals of the piezoelectric resonator, respectively, the first feedback resistor being connected between the input terminal and the output terminal;

first and second variable capacitive elements that are connected to the input terminal and the output terminal of the first inverter circuit, respectively, and have capacitance values that are variably settable by a control signal, respectively;

a comparator that compares a discharging voltage of the input terminal or the output terminal with a reference voltage; and

a control circuit that, in a calibration operation, applies a first voltage to the input terminal or the output terminal to charge the first or second variable capacitive element, then, at a first time, starts to discharge the first or second variable capacitive element via the first feedback resistor, and, according to a comparison result of the comparator at a second time after the first time, generates the control signal for setting a capacitance value of the first or second variable capacitive element so that a discharging voltage of the input terminal or the output terminal becomes close to the reference voltage.

7. The oscillation circuit according to claim 6 , wherein at the time of calibration, the control circuit generates the control signal for decreasing the capacitance value of the first or second variable capacitive element when the comparison result of the comparator represents that the discharging voltage is higher than the reference voltage and generates the control signal for increasing the capacitance value of the first or second variable capacitive element when the comparison result of the comparator represents that the discharging voltage is lower than the reference voltage.

8. The oscillation circuit according to claim 6 , further comprising:

a first voltage generating circuit that generates the first voltage by supplying a first resistor with a certain reference current; and

a second voltage generating circuit that generates the reference voltage that is lower than the first voltage by supplying a second resistor with the reference current.

9. The oscillation circuit according to claim 6 , wherein at the time of calibration, the control circuit stops an oscillation operation by the first inverter circuit and the piezoelectric resonator.

10. The oscillation circuit according to claim 6 , wherein at the time of calibration, the control circuit connects the input terminal or the output terminal to a reference power voltage via the feedback resistor at the first time.

11. An oscillation circuit, connected to an external piezoelectric resonator, comprising:

a first inverter circuit that includes an input terminal and an output terminal which are connected to both terminals of the piezoelectric resonator, respectively;

a first feedback resistor that is disposed between the input terminal and the output terminal of the first inverter circuit;

first and second variable capacitive elements that are connected to the input terminal and the output terminal of the first inverter circuit, respectively, and have capacitance values that are variably settable by a control signal, respectively;

a comparator that compares a monitoring voltage of the input terminal or the output terminal with a reference voltage; and

a control circuit that, in a calibration operation, short-circuits both terminals of the first or second variable capacitive element and both terminals of the piezoelectric resonator in a state in which a first voltage is applied, and then generates the control signal for setting a capacitance value of the first or second variable capacitive element so that the monitoring voltage obtained when the first or second variable capacitive element and the piezoelectric resonator are serially connected becomes close to the reference voltage according to a comparison result by the comparator between the monitoring voltage and the reference voltage.

12. The oscillation circuit according to claim 11 , wherein at the time of calibration, the control circuit generates the control signal for increasing the capacitance value of the first or second variable capacitive element when the comparison result of the comparator represents that the monitoring voltage is higher than the reference voltage and generates the control signal for decreasing the capacitance value of the first or second variable capacitive element when the comparison result of the comparator represents that the monitoring voltage is lower than the reference voltage.

13. The oscillation circuit according to claim 11 , further comprising:

a reference current generating circuit that generates the reference current;

a first voltage generating circuit that generates the first voltage by supplying a first resistor with a certain reference current; and

a second voltage generating circuit that generates the reference voltage that is lower than the first voltage by supplying a second resistor with the reference current.

14. The oscillation circuit according to claim 11 , wherein at the time of calibration, the control circuit stops an oscillation operation by the inverter circuit and the piezoelectric resonator.

15. The oscillation circuit according to claim 1 , further comprising:

a second inverter; and

a second feedback resistor that is connected between an input terminal and an output terminal of the second inverter, wherein

at the time of calibration, the control circuit connects the input terminal and the output terminal of the second inverter with the piezoelectric resonator and controls the first and second times based on timing of a clock generated by an oscillator configured by the piezoelectric resonator and the second inverter.

16. The oscillation circuit according to claim 6 , further comprising:

a second inverter; and

a second feedback resistor that is connected between an input terminal and an output terminal of the second inverter, wherein

at the time of calibration, the control circuit connects the input terminal and the output terminal of the second inverter with the piezoelectric resonator and controls the first and second times based on timing of a clock generated by an oscillator configured by the piezoelectric resonator and the second inverter.

17. The oscillation circuit according to claim 1 , further comprising

a starter circuit that supplies the control circuit with a reset signal after power is turned on and then supplies the control circuit with an enable signal for starting a calibration operation.

18. The oscillation circuit according to claim 6 , further comprising

a starter circuit that supplies the control circuit with a reset signal after power is turned on and then supplies the control circuit with an enable signal for starting a calibration operation.

19. The oscillation circuit according to claim 11 , further comprising

a starter circuit that supplies the control circuit with a reset signal after power is turned on and then supplies the control circuit with an enable signal for starting a calibration operation.

20. The oscillation circuit according to claim 17 , further comprising:

a second oscillation circuit that is externally connectable;

a detector circuit that detects a clock generated by the second oscillation circuit when the second oscillation circuit is connected, wherein

when the detector circuit has detected the clock generated by the second oscillation circuit, supply of the enable signal to the control circuit stops, and the clock generated by the second oscillation circuit is output, and

when the detector circuit has not detected the clock generated by the second oscillation circuit, the enable signal is supplied to the control circuit, and a clock generated by an oscillation circuit configured by the piezoelectric resonator and the first inverter is output.