RECONFIGURABLE CRYSTAL OSCILLATOR AND METHOD FOR RECONFIGURING CRYSTAL OSCILLATOR

Abstract

A reconfigurable crystal oscillator and a method for reconfiguring a crystal oscillator are provided. The reconfigurable crystal oscillator includes a transconductance circuit, a feedback resistor, a crystal tank, an input-end capacitor and an output-end capacitor. Both of the feedback resistor and the crystal tank are coupled between an input terminal and an output terminal of the transconductance circuit. The input-end capacitor is coupled to the input terminal of the transconductance circuit, and the output-end capacitor is coupled to the output terminal of the transconductance circuit. In particular, the transconductance circuit is configured to provide a transconductance, and when an operation mode of the reconfigurable crystal oscillator is switched, an input-end capacitance of the input-end capacitor and an output-end capacitance of the output-end capacitor are switched, respectively.

Claims

1. A reconfigurable crystal oscillator, comprising: a transconductance circuit, configured to provide a transconductance; a feedback resistor, coupled between an input terminal and an output terminal of the transconductance circuit; a crystal tank, coupled between the input terminal and the output terminal of the transconductance circuit; an input-end capacitor, coupled to the input terminal of the transconductance circuit; and an output-end capacitor, coupled to the output terminal of the transconductance circuit; wherein when an operation mode of the reconfigurable crystal oscillator is switched, an input-end capacitance of the input-end capacitor and an output-end capacitance of the output-end capacitor are switched, respectively.

2. The reconfigurable crystal oscillator of claim 1, wherein when the operation mode of the reconfigurable crystal oscillator is switched from a first mode to a second mode, the input-end capacitance of the input-end capacitor is switched from a first input-end capacitance to a second input-end capacitance and the output-end capacitance of the output-end capacitor is switched from a first output-end capacitance to a second output-end capacitance.

3. The reconfigurable crystal oscillator of claim 2, wherein a first ratio of the input-end capacitance and the output-end capacitance is set to make the reconfigurable crystal oscillator have a first voltage swing in the first mode, and a second ratio of the input-end capacitor and the output-end capacitor is set to make the reconfigurable crystal oscillator have a second voltage swing different from the first voltage swing in the second mode.

4. The reconfigurable crystal oscillator of claim 3, wherein the input-end capacitance and the output-end capacitance are trimmed under a condition where the first ratio is fixed, to derive the first input-end capacitance and the first output-end capacitance which make a first frequency error between a first oscillation frequency of the reconfigurable crystal and a target oscillation frequency be minimized or less than a first predetermined value, and the input-end capacitance and the output-end capacitance are trimmed under a condition where the second ratio is fixed, to derive the second input-end capacitance and the second output-end capacitance which make a second frequency error between a second oscillation frequency of the reconfigurable crystal and the target oscillation frequency be minimized or less than a second predetermined value.

5. The reconfigurable crystal oscillator of claim 4, wherein after trimming of the first input-end capacitance, the first output-end capacitance, the second input-end capacitance and the second output-end capacitance are finished, values corresponding to the first input-end capacitance, the first output-end capacitance, the second input-end capacitance and the second output-end capacitor are written into a storage device.

6. The reconfigurable crystal oscillator of claim 2, wherein the input-end capacitance is stepwise switched from the first input-end capacitance to the second input-end capacitance through at least one intermediate input-end capacitance, and the output-end capacitance is stepwise switched from the first output-end capacitance to the second output-end capacitance through at least one intermediate output-end capacitance.

7. The reconfigurable crystal oscillator of claim 6, wherein the at least one intermediate input-end capacitance is between the first input-end capacitance and the second input-end capacitance, and the at least one intermediate output-end capacitance is between the first output-end capacitance and the second output-end capacitance.

8. A method for reconfiguring a crystal oscillator, comprising: utilizing a transconductance circuit of the crystal oscillator to provide a transconductance, wherein a feedback resistor and a crystal tank are coupled between an input terminal and an output terminal of the transconductance circuit; and switching an input-end capacitance of an input-end capacitor and an output-end capacitance of an output-end capacitor in response to switching of an operation mode of the crystal oscillator, respectively, wherein the input-end capacitor is coupled to the input terminal of the transconductance circuit, and the output-end capacitor is coupled to the output terminal of the transconductance circuit.

9. The method of claim 8, wherein switching the input-end capacitance of the input-end capacitor and the output-end capacitance of the output-end capacitor in response to switching of the operation mode of the crystal oscillator respectively further comprises: switching the input-end capacitance of the input-end capacitor from a first input-end capacitance to a second input-end capacitance and switching the output-end capacitance of the output-end capacitor from a first output-end capacitance to a second output-end capacitance in response to the operation mode of the reconfigurable crystal oscillator being switched from a first mode to a second mode.

10. The method of claim 9, wherein a first ratio of the input-end capacitance and the output-end capacitance is set to make the reconfigurable crystal oscillator have a first voltage swing in the first mode, and a second ratio of the input-end capacitor and the output-end capacitor is set to make the reconfigurable crystal oscillator have a second voltage swing different from the first voltage swing in the second mode.

11. The method of claim 10, further comprising: trimming the input-end capacitance and the output-end capacitance under a condition where the first ratio is fixed, to derive the first input-end capacitance and the first output-end capacitance which make a first frequency error between a first oscillation frequency of the reconfigurable crystal and a target oscillation frequency be minimized or less than a first predetermined value; and trimming the input-end capacitance and the output-end capacitance under a condition where the second ratio is fixed, to derive the second input-end capacitance and the second output-end capacitance which make a second frequency error between a second oscillation frequency of the reconfigurable crystal and the target oscillation frequency be minimized or less than a second predetermined value.

12. The method of claim 11, further comprising: after trimming of the first input-end capacitance, the first output-end capacitance, the second input-end capacitance and the second output-end capacitance are finished, writing values corresponding to the first input-end capacitance, the first output-end capacitance, the second input-end capacitance and the second output-end capacitor into a storage device.

13. The method of claim 9, wherein switching the input-end capacitance of the input-end capacitor from the first input-end capacitance to the second input-end capacitance and switching the output-end capacitance of the output-end capacitor from the first output-end capacitance to the second output-end capacitance in response to the operation mode of the reconfigurable crystal oscillator being switched from the first mode to the second mode further comprises: stepwise switching the input-end capacitance from the first input-end capacitance to the second input-end capacitance through at least one intermediate input-end capacitance; and stepwise switching the output-end capacitance from the first output-end capacitance to the second output-end capacitance through at least one intermediate output-end capacitance.

14. The method of claim 13, wherein the at least one intermediate input-end capacitance is between the first input-end capacitance and the second input-end capacitance, and the at least one intermediate output-end capacitance is between the first output-end capacitance and the second output-end capacitance.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] FIG. 1 is a diagram illustrating a reconfigurable crystal oscillator according to an embodiment of the present invention.

[0012] FIG. 2 is a diagram illustrating details of the reconfigurable crystal oscillator shown in FIG. 1 according to an embodiment of the present invention.

[0013] FIG. 3 is a diagram illustrating a switching process of the reconfigurable crystal oscillator shown in FIG. 2 according to an embodiment of the present invention.

[0014] FIG. 4 is a diagram illustrating a working flow of determining capacitances of switchable capacitors utilized for multiple modes according to an embodiment of the present invention.

[0015] FIG. 5 is a diagram illustrating a working flow of trimming capacitances of switchable capacitors utilized for multiple modes according to an embodiment of the present invention.

[0016] FIG. 6 is a diagram illustrating a working flow of a method for reconfiguring a crystal oscillator according to an embodiment of the present invention.

DETAILED DESCRIPTION

[0017] Certain terms are used throughout the following description and claims, which refer to particular components. As one skilled in the art will appreciate, electronic equipment manufacturers may refer to a component by different names. This document does not intend to distinguish between components that differ in name but not in function. In the following description and in the claims, the terms “include” and “comprise” are used in an open-ended fashion, and thus should be interpreted to mean “include, but not limited to . . . ”. Also, the term “couple” is intended to mean either an indirect or direct electrical connection. Accordingly, if one device is coupled to another device, that connection may be through a direct electrical connection, or through an indirect electrical connection via other devices and connections.

[0018] FIG. 1 is a diagram illustrating a reconfigurable crystal oscillator 10 according to an embodiment of the present invention. As shown in FIG. 1, the reconfigurable crystal oscillator 10 may comprise a transconductance circuit such as an inverter gm, a feedback resistor RFB, a crystal tank XTAL, an input-end capacitor (e.g. a capacitor at the input end of the transconductance circuit) such as a capacitor C1, and an output-end capacitor (e.g. a capacitor atthe output end ofthe transconductance circuit) such as a capacitor C2, where the capacitance of input-end capacitor may be referred to as the input-end capacitance, and the capacitance of output-end capacitor may be referred to as the output-end capacitance. The feedback resistor RFB is coupled between an input terminal (e.g. a node X1) and an output terminal (e.g. a node X2) of the inverter gm, where the inverter gm may be configured to provide a transconductance, and the inverter gm and the feedback resistor RFB may form an inverting amplifier which generates a 180-degree phase shift. The crystal tank XTAL is coupled between the node X1 and the node X2. The capacitor C1 is coupled between the node X1 and a reference terminal such as a ground terminal, and the capacitor C2 is coupled between the node X2 and the ground terminal. In this embodiment, the crystal tank XTAL and the capacitors C1 and C2 may form a band-pass network which generate a 180-degree phase shift at an oscillation frequency of the reconfigurable crystal oscillator 10. In conjunction with the 180-degree phase shift generated by the inverting amplifier, the reconfigurable crystal oscillator 10 may have a positive loop gain, resulting in an oscillation at the aforementioned oscillation frequency.

[0019] The reconfigurable crystal oscillator 10 may be shared by a Wi-Fi circuit (e.g. a wireless communications circuit conforming to IEEE 802.11ax or IEEE 802.11ac) and a Bluetooth circuit. It should be noted that requirement of performance of the reconfigurable crystal oscillator 10 may vary according to whether the Wi-Fi circuit and the Bluetooth circuit is enabled or disabled. In order to optimize power efficiency of the reconfigurable crystal oscillator 10, configuration of the reconfigurable crystal oscillator 10 needs to vary according to whether the Wi-Fi circuit and the Bluetooth circuit is enabled or disabled. When the Wi-Fi circuit is enabled, the reconfigurable crystal oscillator 10 may have a first configuration in order to operate in a Wi-Fi mode. When the Wi-Fi circuit is disabled and the Bluetooth circuit is enabled, the reconfigurable crystal oscillator 10 may have a second configuration in order to operate in a Bluetooth mode. When both of the Wi-Fi circuit and the Bluetooth circuit are disabled, the reconfigurable crystal oscillator 10 may have a third configuration to operate in a low power mode. In this embodiment, the reconfigurable crystal oscillator 10 has the most demanding requirement of phase noise related performance in the Wi-Fi mode, followed by the Bluetooth mode, and has the least demanding requirement of phase noise related performance in the low power mode. Thus, the reconfigurable crystal oscillator 10 may be the most power consuming in the Wi-Fi mode, followed by the Bluetooth mode, and be the most power saving in the low power mode.

[0020] In some embodiments, a supply voltage VDD, which is configured to provide power for the inverter gm, may be tunable. For example, the supply voltage VDD may have different voltage level in the Wi-Fi mode, the Bluetooth mode and the low power mode, in order to switch a bias current of the inverter gm and thereby meet different requirement of phase noise related performance. There are disadvantages in this mode switching method by tuning the supply voltage VDD, however. For example, when the voltage level of the supply voltage VDD is reduced, voltage headroom of the inverter gm is reduced, and a tuning range of the supply voltage is therefore limited by threshold voltages of transistors within the inverter gm.

[0021] In some embodiments, the inverter gm may be formed by multiple inverters connected in parallel. By controlling the number of enabled inverters, an overall current of the inverter gm may be accordingly controlled in order to optimize power efficiency in different modes without changing the supply voltage VDD. There are disadvantages in this mode switching method by configuring multiple inverters connected in parallel, however. In practice, the inverter gm may have a parasitic capacitor across the input terminal and the output terminal of the inverter gm, as indicated by a capacitor CDS in FIG. 1. The capacitor CDS may degrade negative resistance of the reconfigurable crystal oscillator 10, and is therefore disadvantageous for the oscillation. When configuring multiple inverters connected in parallel in the inverter gm, an equivalent capacitance of the capacitor CDS may increase, thereby impacting the negative resistance.

[0022] In this embodiment, when an operation mode of the reconfigurable crystal oscillator 10 is switched, an input-end capacitance of the input-end capacitor (i.e. the capacitor C1) and an output-end capacitance of the output-end capacitor (i.e. the capacitor C2) can be switched without changing the supply voltage VDD and configuring multiple inverters within the inverter gm. For example, each of the capacitors C1 and C2 may be implemented by switched-capacitor, and the input-end capacitance of the capacitor C1 and the output-end capacitance of the capacitor C2 may vary in response to switching of the operation mode of the reconfigurable crystal oscillator 10, in order to optimize power efficiencies of the Wi-Fi mode, the Bluetooth mode and the low power mode, respectively.

[0023] FIG. 2 is a diagram illustrating details of the reconfigurable crystal oscillator shown in FIG. 1 according to an embodiment of the present invention. As show in FIG. 2, the reconfigurable crystal oscillator 10 may further comprise selectors 210 and 220 and a storage device 230. In this embodiment, when the operation mode of the reconfigurable crystal oscillator is switched from a first mode to a second mode, the input-end capacitance of the capacitor C1 may be switched from a first input-end capacitance to a second input-end capacitance and the output-end capacitance of the capacitor C2 may be switched from a first output-end capacitance to a second output-end capacitance. In this embodiment, the input-end capacitance of the capacitor C1 may be controlled by an input-end capacitance code and the output-end capacitance of the capacitor C2 may be controlled by an output-end capacitance code. In this embodiment, the selector 210 may select one of capacitance codes C1.sub.AXC, C1.sub.BTC and C1.sub.LPC to be the input-end capacitance code according to a mode selection signal SEL, wherein the capacitance codes C1.sub.AXC, C1.sub.BTC and C1.sub.LPC may make the input-end capacitance of the capacitor C1 have corresponding values in the Wi-Fi mode, the Bluetooth mode and the low power mode, respectively, for optimizing an overall power efficiency of the reconfigurable crystal oscillator 10. The selector 220 may select one of capacitance codes C2.sub.AXC, C2.sub.BTC and C2.sub.LPC to be the output-end capacitance code according to the mode selection signal SEL, wherein the capacitance codes C2.sub.AXC, C2.sub.BTC and C2.sub.LPC may make the output-end capacitance of the capacitor C1 have corresponding values in the Wi-Fi mode, the Bluetooth mode and the low power mode, respectively, for optimizing the overall power efficiency of the reconfigurable crystal oscillator 10. In this embodiment, the capacitance codes C1.sub.AXC, C1.sub.BTC, C1.sub.LPC C2.sub.AXC, C2.sub.BTC and C2.sub.LPC may be stored in the storage device 230, where the storage device 230 may be implemented by a random access memory (RAM) or a read only memory (ROM), but the present invention is not limited thereto.

[0024] When the reconfigurable crystal oscillator 10 is set to operate in the Wi-Fi mode, the mode selection signal SEL may be set to a first value, to make the selector 210 select the capacitance code C1.sub.AXC for controlling the input-end capacitance of the capacitor C1, and make the selector 220 select the capacitance code C2.sub.AXC for controlling the output-end capacitance of the capacitor C2. When the reconfigurable crystal oscillator 10 is set to operate in the Bluetooth mode, the mode selection signal SEL may be set to a second value, to make the selector 220 select the capacitance code C1.sub.BTC for controlling the input-end capacitance of the capacitor C1, and make the selector 220 select the capacitance code C2.sub.BTC for controlling the output-end capacitance of the capacitor C2. When the reconfigurable crystal oscillator 10 is set to operate in the low power mode, the mode selection signal SEL may be set to a third value, to make the selector 220 select the capacitance code C1.sub.LPC for controlling the input-end capacitance of the capacitor C1, and make the selector 220 select the capacitance code C2.sub.LPC for controlling the output-end capacitance of the capacitor C2. Thus, capacitances corresponding to any two of the capacitance codes C1.sub.AXC, C1.sub.BTC and C1.sub.LPC may be examples of the first input-end capacitance and the second input-end capacitance mentioned above, and capacitances corresponding to any two of the capacitance codes C2.sub.AXC, C2.sub.BTC and C2.sub.LPC may be examples of the first output-end capacitance and the second output-end capacitance mentioned above.

[0025] FIG. 3 is a diagram illustrating a switching process of the reconfigurable crystal oscillator 10 shown in FIG. 2 according to an embodiment of the present invention. It should be noted that this embodiment takes a condition of switching from the Wi-Fi mode to the Bluetooth mode as an example for better illustration, but the present invention is not limited thereto. As shown in FIG. 3, the reconfigurable crystal oscillator 10 operates in the Wi-Fi mode (labeled “MODE.sub.AXC” in FIG. 3 for brevity) at the beginning, where the selector 210 selects the capacitance code C1.sub.AXC for controlling the input-end capacitance of the capacitor C1, and the selector 220 selects the capacitance code C2.sub.AXC for controlling the output-end capacitance of the capacitor C2. When the operation mode of the reconfigurable crystal oscillator 10 is switched from the Wi-Fi mode to the Bluetooth mode, the input-end capacitance of the capacitor C1 will not be switched to the capacitance corresponding to the capacitance code C1.sub.BTC at once, and the output-end capacitance of the capacitor C2 will not be switched to the capacitance corresponding to the capacitance code C2.sub.BTC at once.

[0026] In order to prevent the reconfigurable crystal oscillator 10 from entering a certain transient status failing to meet a oscillation criteria (which may be referred to as “oscillation dying out”) during the switching from the Wi-Fi mode to the Bluetooth mode, the input-end capacitance of the capacitor C1 may be stepwise switched from the capacitance corresponding to the capacitance code C1.sub.AXC to the capacitance corresponding to the capacitance code C1.sub.BTC through at least one intermediate input-end capacitance (e.g. capacitances corresponding to capacitance codes C1.sub.T1C, C1.sub.T2C and C1.sub.T3C), and the output-end capacitance of the capacitor C2 may be stepwise switched from the capacitance corresponding to the capacitance code C2.sub.AXC to the capacitance corresponding to the capacitance code C2.sub.BTC through at least one intermediate output-end capacitance (e.g. capacitances corresponding to capacitance codes C2.sub.T1C, C2.sub.T2C and C2.sub.T3C), instead of switching from the capacitance codes C1.sub.AXC and C2.sub.AXC to the capacitance codes C1.sub.BTC and C2.sub.BTC at once. In detail, a switching process from the Wi-Fi mode to the Bluetooth mode may be performed with aid of three transition stages TRAN.sub.1, TRAN.sub.2 and TRAN.sub.3. When the switching process enters the transition stage TRAN.sub.1, the input-end capacitance of the capacitor C1 may be switched from the capacitance corresponding to the capacitance code C1.sub.AXC to a capacitance corresponding to the capacitance code C1.sub.T1C, and the output-end capacitance of the capacitor C2 may be switched from the capacitance corresponding to the capacitance code C2.sub.AXC to a capacitance corresponding to the capacitance code C2.sub.T1C. When the switching process enters the transition stage TRAN.sub.2, the input-end capacitance of the capacitor C1 may be switched from the capacitance corresponding to the capacitance code C1.sub.T1C to a capacitance corresponding to the capacitance code C1.sub.T2C, and the output-end capacitance of the capacitor C2 may be switched from the capacitance corresponding to the capacitance code C2.sub.T1C to a capacitance corresponding to the capacitance code C2.sub.T2C. When the switching process enters the transition stage TRAN.sub.3, the input-end capacitance of the capacitor C1 may be switched from the capacitance corresponding to the capacitance code C1.sub.T2C to a capacitance corresponding to the capacitance code C1.sub.T3C, and the output-end capacitance of the capacitor C2 may be switched from the capacitance corresponding to the capacitance code C2.sub.T2C to a capacitance corresponding to the capacitance code C2.sub.T3C. In the end of the switching process, the input-end capacitance of the capacitor C1 may be switched from the capacitance corresponding to the capacitance code C1.sub.T3C to the capacitance corresponding to the capacitance code C1.sub.BTC, and the output-end capacitance of the capacitor C2 may be switched from the capacitance corresponding to the capacitance code C2.sub.T3C to the capacitance corresponding to the capacitance code C2.sub.BTC.

[0027] It should be noted that the at least one intermediate input-end capacitance may be between the first input-end capacitance and the second input-end capacitance, and the at least one intermediate output-end capacitance may be between the first output-end capacitance and the second output-end capacitance. In particular, the capacitance corresponding to the C1.sub.T1C may be greater than the capacitance corresponding to the C1.sub.AXC, the capacitance corresponding to the C1.sub.T2C may be greater than the capacitance corresponding to the C1.sub.T1C, the capacitance corresponding to the C1.sub.T3C may be greater than the capacitance corresponding to the C1.sub.T2C, and the capacitance corresponding to the C1.sub.BTC may be greater than the capacitance corresponding to the C1.sub.T3C. In addition, the capacitance corresponding to the C2.sub.T1C may be less than the capacitance corresponding to the C2.sub.AXC, the capacitance corresponding to the C2.sub.T2C may be less than the capacitance corresponding to the C2.sub.T1C, the capacitance corresponding to the C2.sub.T3C may be less than the capacitance corresponding to the C2.sub.T2C, and the capacitance corresponding to the C2.sub.BTC may be less than the capacitance corresponding to the C2.sub.T3C. It should be noted that the number of transition stages in this embodiment is for illustrative purposes only, and is not meant to be a limitation of the present invention.

[0028] Briefly speaking, when the operation mode of the reconfigurable crystal oscillator 10 is switched from the Wi-Fi mode to the Bluetooth mode, the input-end capacitance of the capacitor C1 can be stepwise increased by controlling the selector 210 to sequentially select the capacitance code for the capacitor C1 in the order of C1.sub.AXC, C1.sub.T1C, C1.sub.T2C, C1.sub.T3C and C1.sub.BTC, and the output-end capacitance of the capacitor C2 can be stepwise reduced by controlling the selector 220 to sequentially select the capacitance code for capacitor C2 in the order of C2.sub.AXC, C2.sub.T1C, C2.sub.T2C, C2.sub.T3C and C1.sub.BTC. When the operation mode of the reconfigurable crystal oscillator 10 is switched from the Bluetooth mode to the Wi-Fi mode, the input-end capacitance of the capacitor C1 can be stepwise reduced by controlling the selector 210 to sequentially select the capacitance code for the capacitor C1 in the order of C1.sub.BTC, C1.sub.T3C, C1.sub.T2C, C1.sub.T1C and C1.sub.AXC, and the output-end capacitance of the capacitor C2 can be stepwise increased by controlling the selector 220 to sequentially select the capacitance code for capacitor C2 in the order of C2.sub.BTC, C2.sub.T3C, C2.sub.T2C, C2.sub.T1C and C2.sub.AXC.

[0029] In this embodiment, the capacitance codes C1.sub.AXC, C2.sub.AXC, C1.sub.BTC and C2.sub.BTC are determined to make an oscillation frequency generated in the Wi-Fi mode be substantially equal to an oscillation frequency generated in the Bluetooth mode, as indicated by FREQ.sub.0 in FIG. 3. In practice, oscillation frequencies (as indicated by FREQ.sub.1, FREQ.sub.2 and FREQ.sub.3 in FIG. 3) generated in the transition stages TRAN.sub.1, TRAN.sub.2 and TRAN.sub.3 may be different from FREQ.sub.0, but the present invention is not limited thereto.

[0030] It should be noted that this embodiment takes the condition of switching from the Wi-Fi mode to the Bluetooth mode as an example for better illustration, where the rest conditions (e.g. other switching scenarios) may be deduced by analogy, and are therefore not repeated here for brevity.

[0031] FIG. 4 is a diagram illustrating a working flow of determining capacitances of switchable capacitors (e.g. determining the capacitance codes C1.sub.AXC, C2.sub.AXC, C1.sub.BTC and C2.sub.BTC) utilized for multiple modes according to an embodiment of the present invention. It should be noted that one or more steps may be added, deleted or modified in the working flow shown in FIG. 4. For example, this embodiment merely takes the Wi-Fi mode and the Bluetooth mode for illustration, where those skilled in this art should understand how to implement a flow further including steps related to the low power mode, and related details are omitted here for brevity. In addition, if an overall result is not hindered, these steps do not have to be executed in the exact order shown in FIG. 4.

[0032] In Step S410, values of the capacitance codes C1.sub.AXC and C2.sub.AXC may be determined according to specification of the reconfigurable crystal oscillator 10 operating in the Wi-Fi mode (labeled “Determine value of C1.sub.AXC and C2.sub.AXC according to AX spec” in FIG. 4 for brevity).

[0033] In Step S420, values of the capacitance codes C1.sub.BTC and C2.sub.BTC may be determined according to specification of the reconfigurable crystal oscillator 10 operating in the Bluetooth mode (labeled “Determine value of C1.sub.BTC and C2.sub.BTC according to BT spec” in FIG. 4 for brevity).

[0034] In Step S430, values of the capacitances codes of the capacitors C1 and C2 during configuration switching, such as the capacitance codes C1.sub.T1C, C2.sub.T1C, C1.sub.T2C, C2.sub.T2C, C1.sub.T3C and C2.sub.T3C during the transition stages TRAN.sub.1, TRAN.sub.2 and TRAN.sub.3 shown in FIG. 3, may be determined (labeled “Determine value of C1 and C2 during configuration switching (TRAN.sub.I, TRAN.sub.2 and TRAN.sub.3)” in FIG. 4 for brevity). For example, the capacitance codes C1.sub.T1C, C1.sub.T2C and C1.sub.T3C may be set to be values between the capacitance codes C1.sub.AXC and C1.sub.BTC in an increasing order, and the capacitance codes C2.sub.T1C, C2.sub.T2C and C3.sub.T3C may be set to be values between the capacitance codes C1.sub.AXC and C1.sub.BTC in a decreasing order.

[0035] In practice, there are several factors that may cause frequency errors of the reconfigurable crystal oscillator 10. For example, a frequency error introduced by temperature variation (which may be referred to as a frequency stability) may be within +/−10 parts per million (ppm), a frequency error due to process variation (which may be referred to as manufacturing tolerance) may be within +/−5 ppm among different chips, a frequency error due to chip aging may be within +/−1 ppm year by year, and a frequency error due to printed circuit board (PCB) variation among different designs of different PCB manufacturers may be within +/−7 ppm. Thus, a total frequency error of the reconfigurable crystal oscillator 10 may reach +/−23 ppm. In order to make the total frequency error be within an acceptable range such as +/−20 ppm, the frequency error caused by at least one of the above factors needs to be trimmed.

[0036] FIG. 5 is a diagram illustrating a working flow of trimming capacitances of switchable capacitors utilized for multiple modes (e.g. trimming capacitance codes corresponding to the capacitances of C1 and C2 respectively utilizes for the Wi-Fi mode, the Bluetooth mode and the low power mode) according to an embodiment of the present invention. It should be noted that one or more steps may be added, deleted or modified in the working flow shown in FIG. 5. For example, this embodiment merely takes the Wi-Fi mode and the Bluetooth mode for illustration, where those skilled in this art should understand how to implement a flow further including steps related to the low power mode, and related details are omitted here for brevity. In addition, if an overall result is not hindered, these steps do not have to be executed in the exact order shown in FIG. 4.

[0037] In Step S500, power of an integrated circuit (IC) comprising the reconfigurable crystal oscillator 10 may be turned on, and a power on sequence of the reconfigurable crystal oscillator 10 may start (labeled “IC power on and run power on sequence” in FIG. 5 for brevity).

[0038] In Step S511, the IC (e.g. a calibration control circuit therein) may control the operation mode of the reconfigurable crystal oscillator 10 to enter the Wi-Fi mode for starting trimming the capacitance codes C1.sub.AXC and C2.sub.AXC (labeled “Enter AXC of XO” in FIG. 5 for brevity).

[0039] In Step S512, the IC (e.g. the calibration control circuit therein) may read a predetermined ratio α.sub.AXC, which is determined according to the specification of the reconfigurable crystal oscillator 10 operating in the Wi-Fi mode in advance, from the storage device 230 (e.g. the RAM or the ROM) and set a ratio between the capacitance of the capacitor C1 corresponding to the capacitance code C1.sub.AXC and the capacitance of the capacitor C2 corresponding to the capacitance code C2.sub.AXC to be the predetermined ratio α.sub.AXC (labeled “Set predetermined C1/C2 ratio (α.sub.AXC) from ROM or RAM” in FIG. 5 for brevity), to make the reconfigurable crystal oscillator 10 have a first voltage swing in the Wi-Fi mode.

[0040] In Step S513, the IC (e.g. the calibration control circuit therein) may trim the capacitances of the capacitors C1 and C2 by determining values of the capacitance codes C1.sub.AXC and C2.sub.AXC under a condition where the ratio between the capacitance of the capacitor C1 corresponding to the capacitance code C1.sub.AXC and the capacitance of the capacitor C2 corresponding to the capacitance code C2.sub.AXC is fixed at the predetermined ratio α.sub.AXC, in order to derive the capacitances of the capacitors C1 and C2 which make a first frequency error (e.g. a frequency error between an oscillation frequency of the Wi-Fi mode and a target frequency of the Wi-Fi mode) be minimized or less than a first predetermined threshold.

[0041] In Step S514, the IC (e.g. the calibration control circuit therein) may write the capacitance codes C1.sub.AXC and C2.sub.AXC into the storage device 230 (e.g. the RAM or the ROM).

[0042] In Step S521, the IC (e.g. the calibration control circuit therein) may control the operation mode of the reconfigurable crystal oscillator 10 to exit the Wi-Fi mode and enter the Bluetooth mode for starting trimming the capacitance codes C1.sub.BTC and C2.sub.BTC (labeled “Exit AXC and enter BTC of XO” in FIG. 5 for brevity).

[0043] In Step S522, the IC (e.g. the calibration control circuit therein) may read a predetermined ratio α.sub.BTC, which is determined according to the specification of the reconfigurable crystal oscillator 10 operating in the Bluetooth mode in advance, from the storage device 230 (e.g. the RAM or the ROM) and set a ratio between the capacitance of the capacitor C1 corresponding to the capacitance code C1.sub.BTC and the capacitance of the capacitor C2 corresponding to the capacitance code C2.sub.BTC to be the predetermined ratio α.sub.BTC (labeled “Set predetermined C1/C2 ratio (α.sub.BTC) from ROM or RAM” in FIG. 5 for brevity), to make the reconfigurable crystal oscillator 10 have a second voltage swing in the Bluetooth mode. In detail, the second voltage swing may be less than the first voltage swing, which means the phase noise related performance in the Wi-Fi mode can be better than that in the Bluetooth mode, and the power consumption of the Bluetooth mode can be less than that in the Wi-Fi mode.

[0044] In Step S523, the IC (e.g. the calibration control circuit therein) may trim the capacitances of the capacitors C1 and C2 by determining values of the capacitance codes C1.sub.BTC and C2.sub.BTC under a condition where the ratio between the capacitance of the capacitor C1 corresponding to the capacitance code C1.sub.BTC and the capacitance of the capacitor C2 corresponding to the capacitance code C2.sub.BTC is fixed at the predetermined ratio α.sub.BTC, in order to derive the capacitances of the capacitors C1 and C2 which make a second frequency error (e.g. a frequency error between an oscillation frequency of the Bluetooth mode and a target frequency of the Bluetooth mode) be minimized or less than a second predetermined threshold. In this embodiment, the target frequency of the Wi-Fi mode may be equal to the target frequency of the Bluetooth mode, and the first predetermined threshold may be equal to or different from the second predetermined threshold.

[0045] In Step S524, the IC (e.g. the calibration control circuit therein) may write the capacitance codes C1.sub.BTC and C2.sub.BTC into the storage device 230 (e.g. the RAM or the ROM).

[0046] In Step S530, the IC finishes the trimming of the capacitances of the capacitors C1 and C2 within the reconfigurable crystal oscillator 10 (labeled “XO trimming finish” in FIG. 5 for brevity).

[0047] As the capacitance code C1.sub.AXC, C2.sub.AXC, C1.sub.BTC and C2.sub.BTC corresponding to the values of the capacitances of the capacitors C1 and C2 for the Wi-Fi mode and the Bluetooth mode may be written into the storage device 230 (e.g. the RAM or the ROM) after trimming of the capacitances of the capacitors C1 and C2 for the Wi-Fi mode and the capacitances of the capacitors C1 and C2 for the Bluetooth mode are finished, the capacitance codes C1.sub.AXC, C2.sub.AXC, C1.sub.BTC and C2.sub.BTC can be directly used next time (e.g. after next power on), but the present invention is not limited thereto.

[0048] FIG. 6 is a diagram illustrating a working flow of a method for reconfiguring a crystal oscillator such as the reconfigurable crystal oscillator 10 shown in FIG. 1 according to an embodiment of the present invention. It should be noted that one or more steps may be added, deleted or modified in the working flow shown in FIG. 6. In addition, if an overall result is not hindered, these steps do not have to be executed in the exact order shown in FIG. 6.

[0049] In Step S610, the crystal oscillator may utilize a transconductance circuit thereof to provide a transconductance, wherein a feedback resistor and a crystal tank are coupled between an input terminal and an output terminal of the transconductance circuit.

[0050] In Step S620, the crystal oscillator may switch an input-end capacitance of the input-end capacitor and an output-end capacitance of the output-end capacitor in response to switching of an operation mode of the crystal oscillator, respectively, wherein the input-end capacitor is coupled to the input terminal of the transconductance circuit, and the output-end capacitor is coupled to the output terminal of the transconductance circuit.

[0051] To summarize, the reconfigurable crystal oscillator 10 and the associated method provided by the embodiments of the present invention can switch capacitances of the capacitors C1 and C2 in response to switching of the operation mode of the reconfigurable crystal oscillator 10 without changing the supply voltage VDD. In addition, parasitic capacitors introduced by switches on the capacitors C1 and C2 can be regarded as parts of the capacitors C1 and C2, and will not result in additional capacitance of the capacitor CDS. Thus, the present invention can optimize the overall power efficiency of the reconfigurable crystal oscillator 10 without introducing any side effect or in a way that is less likely to introduce side effects.

[0052] Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.