Crystal oscillator start-up circuit
View Patent ↗A crystal oscillator start-up circuit capable of reducing a start-up time of a crystal oscillator is disclosed. The crystal oscillator start-up circuit is provided with a crystal oscillation unit including a crystal oscillator, a converter and an external oscillator. The crystal oscillation unit generates an output signal corresponding to the impedance characteristic of the crystal oscillator. The converter converts the output signal of the crystal oscillation unit to a voltage signal. The external oscillator outputs to the crystal oscillation unit an oscillation signal whose frequency is adjusted by the voltage signal to approach a resonance frequency of the crystal oscillator.
1. A crystal oscillator start-up circuit, comprising:
a crystal oscillation unit configured to generate an output signal on a first node of the crystal oscillation unit according to an impedance characteristic of a crystal oscillator;
a converter configured to convert the output signal to a voltage signal corresponding to the impedance characteristic of the crystal oscillator and to output the voltage signal as a control signal, wherein the converter comprises:
a comparator coupled to the crystal oscillation unit, the comparator to compare the output signal to a voltage reference;
a transistor comprising a gate coupled to an output of the comparator, a source coupled to a power supply voltage, and a drain coupled to ground via a capacitor and a current source, respectively; and
a buffer amplifier comprising an input coupled to the drain of the transistor, the buffer amplifier to output the control signal; and
an external oscillator configured to generate an oscillation signal, a frequency of the oscillation signal being based on the control signal, and wherein the external oscillator configured to output the oscillation signal to a second node of the crystal oscillation unit, wherein the frequency of the oscillation signal is adjusted by the control signal to approach a resonance frequency of the crystal oscillator to bring a frequency of the output signal on the first node close to the resonance frequency of the crystal oscillator.
2. The crystal oscillator start-up circuit of claim 1 , further comprising a first switch configured to stop supplying the oscillation signal to the crystal oscillation unit when an amplitude of the output signal is lower than a threshold.
3. The crystal oscillator start-up circuit of claim 1 , wherein the external oscillator is configured to operate as a phase locked loop (PLL) when the crystal oscillator oscillates stably.
4. The crystal oscillator start-up circuit of claim 1 , further comprising a second switch configured to switch the control signal between the output signal and a phase difference, wherein the phase difference is between the oscillation signal and the output signal.
5. The crystal oscillator start-up circuit of claim 1 , further comprising a seamless switch configured to gradually lower an effect of the oscillation signal on the output signal.
6. A method for starting up a crystal oscillator, comprising:
generating, by a crystal oscillation unit, an output signal on a first node of the crystal oscillation unit according to an impedance characteristic of the crystal oscillator;
converting, by a converter, the output signal to a voltage signal corresponding to the impedance characteristic of the crystal oscillator, wherein the converting the output signal comprises comparing the output signal to a reference voltage by a comparator and outputting a control signal using a buffer amplifier, wherein an output of the comparator controls a gate of a transistor, the transistor comprising a source coupled to a power supply voltage and a drain coupled to ground via a capacitor and a current source, respectively; and
generating, by an external oscillator, an oscillation signal, a frequency of the oscillation signal being based on the control signal; and
outputting the oscillation signal to a second node of the crystal oscillation unit, wherein the frequency of the oscillation signal is adjusted by the control signal to approach a resonance frequency of the crystal oscillator to bring a frequency of the output signal on the first node close to the resonance frequency of the crystal oscillator.
7. The method of claim 6 , further comprising:
using a first switch to stop supplying the oscillation signal to the crystal oscillation unit when an amplitude of the output signal is lower than a threshold.
8. The method of claim 6 , wherein the external oscillator is configured to operate as a phase lock loop (PLL) when the crystal oscillator oscillates stably.
9. The method of claim 6 , further comprising:
using a second switch to switch the control signal between the output signal and a phase difference, wherein the phase difference is between the oscillation signal and the output signal.
10. The method of claim 6 , further comprising:
using a seamless switch to gradually lower an effect of the oscillation signal on the output signal.
11. A crystal oscillator start-up circuit, comprising:
a crystal oscillation unit configured to generate an output signal on a first node of the crystal oscillation unit according to an impedance characteristic of a crystal oscillator;
a converter configured to convert the output signal to a voltage signal corresponding to the impedance characteristic of the crystal oscillator and to output the voltage signal as a control signal, wherein the converter comprises:
an operational amplifier comprising a first input terminal coupled to the crystal oscillation unit, wherein an output of the operational amplifier is fed back to a second input terminal of the operational amplifier;
a transistor comprising a gate coupled to the output of the operational amplifier, a source coupled to a power supply voltage, and a drain coupled to ground via a capacitor and a current source, respectively;
a comparator comprising a first terminal coupled to the drain of the transistor and a second terminal coupled to a reference voltage, the comparator to output the control signal; and
an external oscillator configured to generate an oscillation signal, a frequency of the oscillation signal being based on the control signal, and wherein the external oscillator configured to output the oscillation signal to a second node of the crystal oscillation unit, wherein the frequency of the oscillation signal is adjusted by the control signal to approach a resonance frequency of the crystal oscillator to bring a frequency of the output signal on the first node close to the resonance frequency of the crystal oscillator.
12. The crystal oscillator start-up circuit of claim 11 , further comprising a first switch configured to stop supplying the oscillation signal to the crystal oscillation unit when an amplitude of the output signal is lower than a threshold.
13. The crystal oscillator start-up circuit of claim 11 , wherein the external oscillator is configured to operate as a phase locked loop (PLL) when the crystal oscillator oscillates stably.
14. The crystal oscillator start-up circuit of claim 11 , further comprising a second switch configured to switch the control signal between the output signal and a phase difference, wherein the phase difference is between the oscillation signal and the output signal.
15. The crystal oscillator start-up circuit of claim 11 , further comprising a seamless switch configured to gradually lower an effect of the oscillation signal on the output signal.