Semiconductor device and control method thereof

1. A semiconductor device comprising:

first and second terminals respectively coupled to both ends of a crystal resonator;

an inverter circuit having an input coupled to the first terminal and an output coupled to the second terminal;

a feedback resistor which couples between the first terminal and the second terminal;

a variable capacitor coupled to at least one of the first and second terminals; and

a control circuit which controls a drive capability of the inverter circuit and a capacitance value of the variable capacitor, based on a mode signal which specifies each of first and second modes,

wherein the control circuit performs control to increase both of the drive capability of the inverter circuit and the capacitance value of the variable capacitor in the second mode rather than the first mode,

wherein upon switching from the first mode to the second mode, the control circuit increases the capacitance value of the capacitor after having increased the drive capability of the inverter circuit, and

wherein upon switching from the second mode to the first mode, the control circuit decreases the drive capability of the inverter circuit after having decreased the capacitance value of the variable capacitor.

2. The semiconductor device according to claim 1 , further including:

a comparator circuit which is coupled to the second terminal and generates a clock signal from a voltage of the second terminal; and

a frequency dividing circuit which performs a frequency division of the clock signal outputted from the comparator circuit,

wherein the control circuit changes a frequency dividing ratio of the frequency dividing circuit in response to the mode signal in such a manner that the frequency of the clock signal becomes identical between the first mode and the second mode.

3. The semiconductor device according to claim 1 ,

wherein the second mode is a mode which permits switching of a power supply voltage of the semiconductor device, and

wherein the first mode is a mode which prohibits switching of the power supply voltage.

4. The semiconductor device according to claim 3 , including:

an oscillator circuit having the first terminal, the second terminal, the inverter circuit, the feedback resistor, the variable capacitor and the control circuit;

a clock circuit which operates based on a clock signal generated by the oscillator circuit;

a power supply switching circuit which switches the power supply voltage supplied to the oscillator circuit and the clock circuit between a normal power supply voltage and a backup power supply voltage; and

a controller which controls the power supply switching circuit in such a manner that when the normal power supply voltage reaches a predetermined threshold value or less, the power supply voltage is switched from the normal power supply voltage to the backup power supply voltage and that when the normal power supply voltage reaches the predetermined threshold value or more, the power supply voltage is switched from the backup power supply voltage to the normal power supply voltage,

wherein the controller generates the mode signal, based on the switching of the power supply voltage and outputs the mode signal to the control circuit.

5. The semiconductor device according to claim 4 , wherein the controller controls the mode signal in such a manner that a mode is switched to the second mode prior to the switching of the power supply voltage and that the mode is switched to the first mode after the end of the switching of the power supply voltage.

6. The semiconductor device according to claim 1 ,

wherein the inverter circuit includes:

a transistor having a gate coupled to the first terminal, a drain coupled to the second terminal and a source coupled to a ground terminal; and

a variable current source which supplies a current to the second terminal, and

wherein the control circuit controls a current supply capacity of the variable current source in response to the mode signal to thereby control the drive capability of the inverter circuit.

7. The semiconductor device according to claim 1 , wherein the variable capacitor is coupled between the first terminal and the second terminal.

8. A semiconductor device comprising:

first and second terminals respectively coupled to both ends of a crystal resonator;

an inverter circuit having an input coupled to the first terminal and an output coupled to the second terminal;

a feedback resistor which couples between the first terminal and the second terminal;

a variable capacitor coupled to at least one of the first and second terminals; and

a control circuit which controls a drive capability of the inverter circuit and a capacitance value of the variable capacitor, based on a mode signal which specifies each of first and second modes,

wherein the control circuit performs control to increase both of the drive capability of the inverter circuit and the capacitance value of the variable capacitor in the second mode rather than the first mode,

wherein the second mode is a debug mode which performs emulation of the semiconductor device, and

wherein the first mode is a mode which does not perform the emulation.

9. The semiconductor device according to claim 8 , wherein a pin used in the emulation is adjacent to a pin coupled to the crystal resonator.

10. The semiconductor device according to claim 9 ,

wherein the variable capacitor is coupled only to one of the first and second terminals, and

wherein the pin used in the emulation is closer to the one of the first and second terminals than the other of the first and second terminals.

11. A control method of a semiconductor device comprising:

first and second terminals respectively coupled to both ends of a crystal resonator;

an inverter circuit having an input coupled to the first terminal and an output coupled to the second terminal;

a feedback resistor which couples between the first terminal and the second terminal; and

a variable capacitor coupled to at least one of the first and second terminals,

the control method comprising the steps of:

switching an operating mode between a first mode and a second mode; and

performing control in such a manner that both of a drive capability of the inverter circuit and a capacitance value of the variable capacitor become larger in the second mode than in the first mode,

wherein the control performing step includes the steps of:

increasing, upon switching from the first mode to the second mode, the capacitance value of the variable capacitor after having increased the drive capability of the inverter circuit; and

decreasing, upon switching from the second mode to the first mode, the drive capability of the inverter circuit after having decreased the capacitance value of the variable capacitor.

12. The control method according to claim 11 ,

wherein the second mode is a debug mode which performs emulation of the semiconductor device, and

wherein the first mode is a mode which does not perform the emulation.

13. The control method according to claim 11 ,

wherein the second mode is a mode which permits switching of a power supply voltage of the semiconductor device, and

wherein the first mode is a mode which prohibits the switching of the power supply voltage.