A frequency variable oscillator generates a clock having a frequency according to a control signal. A reference current source generates a reference current. A path selector distributes the reference current to a first path and a second path in a time-sharing manner in synchronization with the clock. An F/V conversion circuit includes a capacitor connected to the first path, and charges or discharges the capacitor with the reference current and generates a detection voltage. The reference voltage source includes a resistor connected to the second path, and outputs a reference voltage according to a voltage across the resistor. A feedback circuit adjusts a control signal so that the detection voltage approaches the reference voltage.

A frequency variable oscillator generates a clock having a frequency according to a control signal. A reference current source generates a reference current. A path selector distributes the reference current to a first path and a second path in a time-sharing manner in synchronization with the clock. An F/V conversion circuit includes a capacitor connected to the first path, and charges or discharges the capacitor with the reference current and generates a detection voltage. The reference voltage source includes a resistor connected to the second path, and outputs a reference voltage according to a voltage across the resistor. A feedback circuit adjusts a control signal so that the detection voltage approaches the reference voltage.

A crystal oscillator includes a current source that charges a capacitor. A charge on the capacitor is periodically injected into a crystal of the crystal oscillator. A switch couples the capacitor to the crystal and a timing circuit controls the switch to cause the charge to be injected beginning at approximately a peak of a crystal output signal. The timing circuit is configurable into a self-resonant mode for calibration of a delay through the timing circuit by coupling an output of the timing circuit to an input of the timing circuit. A comparator compares a magnitude of the crystal output signal to a reference voltage and supplies compare results to a gain control circuit. The gain control circuit adjusts the current from the current source to adjust the charge being injected into the crystal from the capacitor to thereby control the magnitude of the crystal output signal.

An oscillator circuit portion 200 including a resonator 216 arranged to oscillate with a resonant frequency, a capacitor 208 arranged to provide charge to the resonator, a first switch 206 arranged to connect the capacitor to an input voltage to charge the capacitor, a second switch 210 arranged to connect the resonator to the capacitor, and a timing circuit 202 configured to generate periodically a first pulse PULSE_L and a second pulse PULSE_H. The first pulse is configured to close the first switch, the second pulse is configured to close the second switch, and the first and second switches are arranged to be open when the timing circuit is not generating the first or second pulses, to maintain oscillation of the resonator.

A crystal oscillator includes a current source that charges a capacitor. A charge on the capacitor is periodically injected into a crystal of the crystal oscillator. A switch couples the capacitor to the crystal and a timing circuit controls the switch to cause the charge to be injected beginning at approximately a peak of a crystal output signal. The timing circuit is configurable into a self-resonant mode for calibration of a delay through the timing circuit by coupling an output of the timing circuit to an input of the timing circuit. A comparator compares a magnitude of the crystal output signal to a reference voltage and supplies compare results to a gain control circuit. The gain control circuit adjusts the current from the current source to adjust the charge being injected into the crystal from the capacitor to thereby control the magnitude of the crystal output signal.