US20030132809A1 - Oscillator with tunable capacitor - Google Patents

Abstract

Circuitry for controlling the oscillation frequency of an oscillator by using a digitally tunable on-chip capacitor bank. The capacitor bank includes a plurality of on-chip capacitors, each of which is independently selectable by a control signal for providing a selectable amount of capacitance to the oscillator to control the oscillator's oscillation frequency.

Description

TECHNICAL FIELD
• This invention relates to oscillator circuits, and more particularly to tunable oscillator circuits.[0001]
BACKGROUND
• Oscillator circuits can be used to provide timing signals. For example, a personal computer motherboard typically has a Real Time Clock (RTC) circuit that provides an accurate 32.768 KHz oscillating signal that is further processed to obtain the second, minute, and hour values used by the computer system to keep time.[0002]
• The RTC circuit is typically part of an I/O controller hub chip (sometimes referred to as the south-bridge chipset), and is connected to an external crystal resonator that resonates within a narrow range of operating frequencies. Depending on the crystal oscillator topology, one or more discrete external load capacitors may be connected to the RTC circuit to tune the oscillating frequency. The values of the load capacitors are selected according to an initial circuit layout design so that the RTC circuit in conjunction with the external components will oscillate at a predetermined frequency.[0003]
• However, variation between different motherboard designs may result in placement of the load capacitors at slightly different locations on the motherboard, resulting in the addition of a certain amount of parasitic capacitance associated with the wiring connections. Other factors, such as tolerances in circuit components and minute routing differences, will also affect the oscillating frequency. Because a small variance in the oscillating frequency may significantly affect the accuracy of the system time signal over time, individual tuning of the capacitance value tailored to a specific motherboard design is required to obtain accurate system timing signals.[0004]
• The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.[0005]
DESCRIPTION OF DRAWINGS
• FIG. 1 is a schematic diagram of a computer chipset with external components that form a digitally tunable oscillator.[0006]
• FIG. 2 is a schematic diagram of an on-chip capacitor bank of the oscillator circuit of FIG. 1.[0007]
• FIG. 3 is a schematic diagram of a buffer circuit connected to a transmission gate switch.[0008]
• FIG. 4 is a schematic diagram of an alternative embodiment of an on-chip capacitor bank.[0009]
• FIG. 5 is a block diagram of an electronic device.[0010]
• Like reference symbols in the various drawings indicate like elements.[0011]
DETAILED DESCRIPTION
• As will be described in more detail below, the invention is directed towards circuitry for controlling oscillating frequency of an oscillator. In addition to the external load capacitors, the circuitry includes on-chip capacitors, each of which is independently selectable by a control signal, and each of which provides a controllable amount of capacitance to the oscillator to control the oscillating frequency of the oscillator. The term “on-chip capacitor” means that the capacitor is manufactured on a semiconductor chip.[0012]
• Referring to FIG. 1, a tunable clock oscillator circuit[0013]100 (enclosed in dotted lines) includes two digitally selectable on-chip capacitor banks106 and108 connected to a terminal X₁and a terminal X₂, respectively, of acrystal resonator102. Terminals X₁and X₂are connected to aninput terminal110 and anoutput terminal112, respectively, of aninverting amplifier104. A feedback resistor R_fis connected in parallel toresonator102 to biasamplifier104 into a linear mode.Capacitor banks106 and108 are used to fine-tune the oscillating frequency ofoscillator circuit100. By selecting different combinations of capacitors in the on-chip capacitor banks, a different amount of capacitance can be connected toresonator102, thereby controlling the oscillating frequency ofoscillator circuit100.
• Two external load capacitors C[0014]_L1and C_L2are connected to terminals X₁and X₂, respectively. The capacitance values of load capacitors C_L1and C_L2are selected according to specifications given by the manufacturer ofresonator102. The impedance of load capacitors, combined with the crystal's calibrated impedance, tunes the circuit to operate in a particular frequency in the “parallel or series resonance” area (depending on oscillator topology). For example,resonator102 resonates at approximately 32.77 KHz, capacitors C_L1and C_L2have capacitances of about 15 pF, and resistor R_fhas a value of about 10 MegOhms.
• [0015]Capacitor banks106 and108 each include an array of capacitors that are individually selectable by a set of externally provided control signals to provide a variable amount of capacitance. For example,capacitor banks106 and108 each provide a selectable amount of capacitance in the range of 0 to 4 pF.
• [0016]Amplifier104 andcapacitor banks106,108 are located within an RTC circuit122 (enclosed in dotted lines), which is part of achipset114 of a computer system.RTC circuit122 has a set oflatches120 that latches a set of control signals S₀to S₉that is generated byprogrammable registers118 ofchipset114. Control signals S₀to S₉are used to select the individual capacitors incapacitor banks106 and108.
• [0017]RTC circuit122 is powered by a system power supply, as well as a separate battery supply when the computer system is turned off. When the battery supply is first connected toRTC circuit122, default values are loaded into the latches to select a default set of capacitors. When the computer is initially booted up afterRTC circuit122 is connected to the battery supply, a predefined register setting is read from aBIOS memory116 and passed throughchipset registers118 tolatches120. The latches store the register setting for the life of the battery, or until the setting is changed bychipset registers118.
• The load capacitors C[0018]_L1and C_L2are shown as being connected outside ofchipset114. However, it is understood that the load capacitors C_L1and C_L2may also be integrated withinchipset114. It is also possible to integrate the resonator, load capacitors, resistor, and the RTC circuitry within the same package.
• Referring to FIG. 2,[0019]capacitor bank106 has acapacitor array202 that includes five capacitors, C₀to C₄. Each capacitor is selectable by one of the control signals S₀to S₄through control gate switches G₀to G₄. The capacitance values of the capacitors have a binary-weighted relationship, such that C₄has twice the amount of capacitance as C₃, C₃has twice the amount of capacitance of C₂, and so forth. For example, C₄=2 pF, C₃=1 pF, C₂=0.5 pF, C₁=0.25 pF, C₀=0.125 pF.
• [0020]Chipset registers118 provide control signals S₀to S₄, which are latched bylatches120. A control signal selects a capacitor by turning on the corresponding control gate switch so that one terminal of the capacitor is connected toterminal110 ofcrystal resonator102. To obtain a certain value of capacitance,chipset registers118 select a number of capacitors so that the sum of the capacitances of the selected capacitors most closely approximate the desired capacitance value. Capacitorbank108 operates similarly tocapacitor bank106.
• Capacitors C[0021]₀to C₄are enhancement mode P-type MOSFETs (PMOS) with the drain nodes connected to the source nodes. The gate of the MOSFET functions as one terminal of the capacitor, and the drain/source node functions as the other terminal. A VCC_FILTER signal is derived from the power supply signal VCC to bias the PMOS capacitors into saturation. Most of the high frequency noise contained in the V_ccsignal is filtered by a low pass filter composed of resistor R_biasand capacitor C_bias. The VCC_FILTER signal also provides a filtered power supply signal to a buffer circuit that drives transmission gate switches (Figure3). Use of the low pass filter also enables low power operation because capacitor C_biasblocks direct current from flowing.
• Referring to FIG. 3, a control gate switch G[0022]₀includes abuffer circuit302 and a transmission gate T₀. Buffer circuit302 is used to decouple transmission gate switch T₀from logic circuit that produces signal S₀. This is to prevent noise generated in the chipset logic circuit from reachingoscillator circuit100 through transmission gate switch T₀. Buffer circuit302 is powered by the V_cc—FILTERsignal. Use of V_cc—FILTERsignal is required to prevent unwanted noise in the RTC power supply signal from interfering with the operation ofoscillator circuit100. Additional buffer circuits (having the same configuration as buffer circuit302) are provided to decouple transmission gate switches T₁to T₉from the logic circuits that produce signals S₁to S₉.
• FIG. 4 shows an example of an on-chip capacitor bank constructed from “depletion mode” NMOS transistors. An on-[0023]chip capacitor bank402 includes an on-chip capacitor array404 and control gates G₁₀to G₁₄. Capacitor array404 includes depletion mode NMOS capacitors C₁₀to C₁₄, each of which is made of an N-type MOSFET, with one terminal of the capacitor being the gate node of the MOSFET, and the other terminal of the capacitor being the source-drain connected node of the MOSFET.
• When NMOS capacitors are used, it is not necessary to use V[0024]_cc—FILTERsignal to bias the capacitors into saturation. This is because a depletion mode transistor has a negative threshold voltage, so a channel is formed for all non-negative oscillation voltage levels, and thus provides a greater capacitance. Because V_cc—FILTERis used only to power the logic gates ofbuffer circuit302, smaller values for resistor R_biasand capacitor C_biascan be used. This allows for reduction of the size of thecapacitor bank402 because the “series effect” of the capacitor bank and the capacitor C_biasis eliminated.
• An advantage of using[0025]tunable capacitor banks106,108 is that the computer system can dynamically adjust the oscillating frequency ofoscillator circuit100 based on a reference time signal. The computer may log on to the Internet at regular time intervals, and compare the system time signal with a reference time signal, such as that provided by the NIST Internet Time Service. Based on the difference between the system time and reference time,chipset114 may change the setting ofregisters118 to select a different arrangement of capacitors incapacitor banks106 and108. By selecting a slightly higher or lower amount of capacitance to be connected tocrystal resonator102,chipset114 can fine-tune the oscillating frequency ofoscillator circuit100. After each adjustment, the chipset register setting are latched bylatches120 so thatoscillator circuit100 can provide accurate time signals even after system is powered down or off.
• Another advantage of using tunable capacitor banks in an oscillator circuit is that adjustment of the oscillation frequency can be performed after the hardware connections of the electronic components and circuit boards are fixed. Due to tolerances in the components and boards, the actual capacitance connected to the crystal resonator is often slightly different from the capacitance values in the original design. By adjusting the amount of capacitance provided by the tunable capacitor banks, oscillation frequency can be tuned without altering any hardware component or connection. The adjustment can be done manually or automatically through appropriate software.[0026]
• Use of tunable capacitor banks is not limited to the RTC circuit of computer systems. All circuits that require fine-tuning of an accurate amount of capacitance may use a tunable capacitor bank. All electronic devices that require an accurate oscillating signal may use tunable capacitor banks to fine-tune the oscillation frequency. The fine-tuning of the oscillation frequency may be used to compensate changes in temperature and humidity, or to compensate manufacturing tolerances. Such fine-tuning of the oscillation frequency after hardware connections are fixed allows more flexibility in the selection of electronic components and circuit board layout designs.[0027]
• Referring to FIG. 5, an[0028]electronic device500 includes achipset508 that has aRTC circuit514 that provides a stable clock signal.RTC circuit514 includes two tunable on-chip capacitor banks connected to each of the two terminals of aresonator102. TheRTC circuit514 is powered by both a main power supply and a battery supply so that it can keep the oscillation even when the main power supply is shut off.
• [0029]Chipset508 includes a set of latches that store a set of register bit values used to control the selection of on-chip capacitors in the capacitor banks. Whenchipset508 is delivered to a manufacturer ofdevice500, the latches store default values. When the manufacturer designs a circuit board using thechipset508, the manufacturer may decide to modify the values stored in the latches by writing new register bit values into aBIOS116.
• When the battery is first inserted to provide power to[0030]RTC circuit514, the register bit values are read fromBIOS116 and passed to the latches.Chipset508 includes a register that stores a “capacitor-set flag” which is used to track whether the register bit values need to be updated. Initially, the capacitor-set flag is set to “0”. Every timeelectronic device500 boots, the capacitor-set flag is checked. If the flag is set to “1”, the latch values are not changed. If the flag equals “0”, the register bit values stored inBIOS116 are read and used to overwrite the values previously stored in the latches. The capacitor-set flag is then set to “1”.
• A user can overwrite the register bit values stored in[0031]BIOS116. The user then sets the capacitor-set flag to “1” to preventBIOS116 from overwriting the user-defined settings whendevice500 boots the next time. The latch settings may also be modified by an operating system (OS) running ondevice500. For example, the OS may perform an adjustment to the clock signal. An accurate reference time signal is received from an input/output device510. The OS controlschipset508 to adjust the latch settings according to the reference time signal so thatRTC circuit514 provides an accurate time signal. The OS then sets the capacitor-set flag to “1” to preventBIOS116 from overwriting the latch settings. The register bit values defined by the user or operating system are maintained in the latches as long as the battery continues to provide power to the latches and the RTC circuit.
• [0032]Device500 further includes aprocessor502 that processes data and amemory device504 that stores data. Theelectronic device500 may be a computer, a handheld device, a consumer electronics device, or any other device that requires an accurate time signal.
• A number of embodiments of the invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. For example, the capacitor bank may incorporate a greater number of capacitors to provide a greater range of capacitance selection. The capacitance values of the capacitors in the capacitor bank may have some relationship other than a binary-weighted relationship so as to provide different capacitance combinations. The capacitors in the capacitor bank may be on-chip poly-capacitors or on-chip metal capacitors. The capacitors in the capacitor bank may even include discrete capacitors that are not made on a semiconductor chip. The external capacitors C[0033]_L1and C_L2may be connected in series toresonator102 such thatresonator102 operate in a series resonance mode. For the capacitor bank of FIG. 2, a series connection with resistor R_biasand capacitor C_biasis not required if the oscillator signals remain above the threshold voltage of the capacitors in the capacitor bank.Device500 may save the latch settings in a file in ahard drive512. The file is loaded each time afterdevice500 is booted and used to set the chipset registers to select appropriate capacitors in the capacitor banks. This prevents the loss of latch settings in the event that the battery power is lost. Accordingly, other embodiments are within the scope of the following claims.

Claims (27)

What is claimed is:

1. Circuitry for controlling the oscillating frequency of an oscillator, the circuitry comprising:

a plurality of capacitors, each of which is independently selectable by a control signal, and each of which provides a controllable amount of capacitance to the oscillator to control the oscillating frequency of the oscillator.

2. The circuitry ofclaim 1, wherein each of the plurality of capacitors has a different capacitance than the other capacitors, and a predefined amount of capacitance is provided by a predetermined combination of capacitors.

3. The circuitry ofclaim 2, wherein the capacitors are drain-source connected MOSFETs.

4. The circuitry ofclaim 3, wherein the MOSFETs are P-type enhancement mode MOSFETs.

5. The circuitry ofclaim 3, wherein the MOSFETs are N-type depletion mode MOSFETs.

6. The circuitry ofclaim 1 wherein the capacitors are selected from the group consisting of on-chip metal capacitors, on-chip poly capacitors, and discrete capacitors.

7. The circuitry ofclaim 1, wherein each of the capacitors corresponds to a transmission gate switch.

8. The circuitry ofclaim 7, further comprising a set of memory registers to provide the control signals for selecting the individual capacitors

9. The circuitry ofclaim 8, wherein the transmission gate switches are decoupled from the set of memory registers by a set of buffer circuitry.

10. The circuitry ofclaim 9, wherein the set of buffer circuitry is powered by a filtered power signal.

11. The circuitry ofclaim 1, wherein the oscillator includes a resonator and an inverting amplifier.

12. The circuitry ofclaim 11, wherein a first subset of the plurality of capacitors is selectively electrically coupled to a first terminal of the resonator, and a second subset of the plurality of capacitors is selectively electrically coupled to a second terminal of the resonator

13 An electronic device comprising:

a real time clock for generating a system time signal, the real time clock having a digitally tunable oscillator for digitally adjusting an operating frequency of the real time clock to speed up or slow down the system time signal; and

a memory device for storing data representing a configuration of the digitally adjusted tunable oscillator.

14. The electronic device ofclaim 13, further comprising a communication port for receiving a reference time signal, wherein the digitally tunable oscillator is digitally adjusted according to the reference time signal to minimize the difference between the system time signal and the reference time signal.

15. The electronic device ofclaim 13, wherein the digitally tunable oscillator includes a capacitor bank having a set of capacitors with capacitance values in a binary-weighted relationship, the capacitors selectable through a set of control signals.

16. A method comprising:

generating a set of control signals to select a subset of capacitors from a set of capacitors;

connecting the selected subset of capacitors to an oscillator;

generating an oscillating signal using the oscillator and the selected subset of capacitors in combination; and

generating a system time signal using the oscillating signal.

17. The method ofclaim 16, further comprising receiving a reference time signal, comparing the reference time signal with the system time signal, and modifying the set of control signals in response to the difference between the reference time signal and the system time signal to select a different subset of capacitors.

18. The method ofclaim 17, further comprising saving data representing the setting of the control signals in a memory.

19. A method of generating a time signal comprising:

generating a system time signal using a real time clock circuit that has a tunable oscillator for adjusting an operation frequency of the real time clock circuit;

receiving a reference time signal over a network;

adjusting the tunable oscillator to increase or decrease the operating frequency of the real time clock circuit in response to a difference between the system time signal and the reference time signal.

20. The method ofclaim 19 wherein adjusting the tunable oscillator comprises adjusting a set of control signals to modify a selection of a set of capacitors within a capacitor bank, the selection of the set of capacitors correlating to the operating frequency of the real time clock circuit.

21. Apparatus for providing a variable level of capacitance, comprising:

a plurality of capacitors, each capacitor selectable through an independent control signal generated by a logic circuit, the selected capacitors providing an amount of capacitance that is the sum of the individual capacitances of the selected capacitors; and

buffer circuitry for decoupling the plurality of capacitors from the logic circuit to prevent noise in the logic circuit from affecting the plurality of capacitors.

22. The apparatus ofclaim 21, further comprising a filter circuit connected to a power supply to generate a filtered power supply signal that is used to power the buffer circuitry.

23. The circuit ofclaim 21, further comprising transmission gates, each of which corresponds to one of the plurality of capacitors and can be turned on by the independent control signal when the corresponding capacitor is selected.

24. Apparatus comprising:

a control unit configured to generate a set of control signals, each of which independently selects a capacitor from a plurality of capacitors, the selected capacitors being coupled to an oscillator, the selected capacitors in combination proving a controllable amount of capacitance to the oscillator to control the oscillating frequency of the oscillator.

25. The apparatus ofclaim 24 in which the control unit is disposed within a computer chipset.

26. The apparatus ofclaim 24, further comprising circuitry for generating a system time signal based on the oscillating frequency of the oscillator.

27. The apparatus ofclaim 26, further comprising a memory for storing the configuration of the set of control signals, and a data processing unit that processes data based on the system time signal.

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