An enabling system that includes a controller and processing circuitry, is configured to enable an external oscillator that operates in one of single-ended, differential, and crystal modes. To enable the external oscillator, the controller is configured to detect a mode of operation of the external oscillator, and the processing circuitry is configured to operate in the detected mode. The controller detects the mode of operation of the external oscillator by sequentially initializing the processing circuitry to operate in the single-ended, differential, and crystal modes, and determining whether the current operating mode of the processing circuitry is same as the mode of operation of the external oscillator based on a clock signal outputted by the processing circuitry during the corresponding mode.

In an example, a system includes an oscillator circuit on a chip. The oscillator circuit includes a charging current generator including a current mirror and an amplifier, where the amplifier is coupled to a pin of the chip. The oscillator circuit also includes a first switch coupled to the pin, a second switch coupled to the pin and to a charging resistor, and a third switch coupled to the amplifier and an internal resistor, where the internal resistor is internal to the chip. The oscillator circuit includes a bias current source coupled to the current mirror. The system includes an external resistor coupled to the pin, where the external resistor is external to the chip. The system also includes an external capacitor coupled to the pin and coupled in parallel to the external resistor, where the external capacitor is external to the chip.

In an example, a system includes an oscillator circuit on a chip. The oscillator circuit includes a charging current generator including a current mirror and an amplifier, where the amplifier is coupled to a pin of the chip. The oscillator circuit also includes a first switch coupled to the pin, a second switch coupled to the pin and to a charging resistor, and a third switch coupled to the amplifier and an internal resistor, where the internal resistor is internal to the chip. The oscillator circuit includes a bias current source coupled to the current mirror. The system includes an external resistor coupled to the pin, where the external resistor is external to the chip. The system also includes an external capacitor coupled to the pin and coupled in parallel to the external resistor, where the external capacitor is external to the chip.

A crystal oscillator is coupled to a digital automatic gain control (AGC) having oscillation detection and amplitude control loops. The oscillation detection loop may increase the transconductance (gm) of the oscillator transistor until oscillation is detected therefrom. Then the amplitude control loop detects the amplitudes of oscillations from the crystal oscillator, compares these amplitudes to high and low voltage references and generates digital signals to find a critical transconductance (gm) for an oscillator amplifier and control this gm to maintain a constant oscillation waveform amplitude therefrom. An up/down counter defines the servo control loop bandwidth/update-rate according to an update clock rate thereto. Loop stability is achieved when the control loop bandwidth is less than the start-up time required for the oscillation envelope of the crystal oscillator to grow for oscillation. An oscillator failure detector may also be provided.

A crystal oscillator is coupled to a digital automatic gain control (AGC) having oscillation detection and amplitude control loops. The oscillation detection loop may increase the transconductance (gm) of the oscillator transistor until oscillation is detected therefrom. Then the amplitude control loop detects the amplitudes of oscillations from the crystal oscillator, compares these amplitudes to high and low voltage references and generates digital signals to find a critical transconductance (gm) for an oscillator amplifier and control this gm to maintain a constant oscillation waveform amplitude therefrom. An up/down counter defines the servo control loop bandwidth/update-rate according to an update clock rate thereto. Loop stability is achieved when the control loop bandwidth is less than the start-up time required for the oscillation envelope of the crystal oscillator to grow for oscillation. An oscillator failure detector may also be provided.

Disclosed is a temperature-compensated crystal oscillator based on analog circuit; a closed-loop compensation architecture determines the temperature compensation of a crystal oscillator. The power splitter divides the VCXO's current output signal with frequency f=f.sub.0+Δf into two signals, one signal to output of the TCXO and the other signal is sent to an analog frequency-voltage conversion circuit. According to the frequency of the VCXO's current output signal, the analog frequency-voltage conversion circuit produces a voltage signal V(T), which corresponds to current ambient temperature. The difference between V(T) and a reference voltage signal V.sub.ref is produced and amplified to obtain a compensation voltage signal ΔV through a voltage matching circuit. ΔV is smoothed by a filter, then sent to the voltage control terminal of the VCXO to make the VCXO generate a stable signal with desired frequency f.sub.0, to compensate the frequency of the VCXO's output signal when the ambient temperature is changed.

An oscillator circuit includes: a first temperature detector, detecting an internal temperature of the oscillator circuit; a current generator, generating a heater current so that the internal temperature matches a target temperature; a first and second heater, heating the resonator and the integrated circuit, respectively, based on the heater current; a second temperature detector, detecting a temperature of the integrated circuit; a first compensation voltage generation circuit, generating a first compensation voltage for compensating for a frequency variation due to a temperature change in the integrated circuit, based on a detection result of the second temperature detector; a second compensation voltage generation circuit, generating a second compensation voltage for compensating for a frequency variation due to a temperature change in the resonator, based on a detection result of the first temperature detector; and an oscillator, generating an oscillation signal based on the first and second compensation voltages.

A quartz crystal unit comprising a quartz crystal resonator having a base portion, and first and second tuning fork arms connected to the base portion, the base portion having a length less than 0.5 mm and greater than a spaced-apart distance between the first and second tuning fork arms, each of the first and second tuning fork arms having a width less than 0.1 mm and a length less than 1.56 mm, and a plurality of different widths including a first width and a second width greater than the first width, at least one groove being formed in at least one of opposite main surfaces of each of the first and second tuning fork arms so that a length of the at least one groove is within a range of 0.3 mm to 0.79 mm, the quartz crystal resonator being housed in a case, and a lid being connected to the case.

A quartz crystal unit comprising a quartz crystal resonator having a base portion, and first and second tuning fork arms connected to the base portion, the base portion having a length less than 0.5 mm and greater than a spaced-apart distance between the first and second tuning fork arms, each of the first and second tuning fork arms having a width less than 0.1 mm and a length less than 1.56 mm, and a plurality of different widths including a first width and a second width greater than the first width, at least one groove being formed in at least one of opposite main surfaces of each of the first and second tuning fork arms so that a length of the at least one groove is within a range of 0.3 mm to 0.79 mm, the quartz crystal resonator being housed in a case, and a lid being connected to the case.

A first three state driver injects a first clock signal into a crystal through an input node during a startup phase of a crystal oscillator and a second three state driver injects a second signal into the crystal through an output node during the startup phase. The first and second signals are anti-phase signals. The crystal oscillator circuit includes a first amplifier that is used during starting up and steady-state operation and includes a second amplifier. The injection through the input and output nodes is disabled after a fixed time. After injection ends, the second amplifier is turned on if voltage on the output node has reached a desired voltage and left off otherwise. If the second amplifier is turned on, the second amplifier is turned off when the voltage on the output node reaches the desired voltage.