Crystal oscillator with frequency trim

[0002] This is a continuation of U.S. Ser. No. 08/995,485 filed Dec. 22, 1997, now U.S. Pat. No. 6,104,257.

[0003] The present invention relates to frequency generators generally and, more particularly, to a timing crystal oscillator that may be tuned after production.

[0004] Conventional approaches to implementing crystal or other resonant oscillators generally involve manufacturing a fixed capacitive load that is attached to the resonator. The fixed load is generally fabricated during the manufacturing process. As a result, the accuracy of the manufacturing process in a conventional oscillator plays an important role in determining the accuracy of the output clock, as does the manufacturing accuracy of the resonator.

[0005] Referring to FIG. 1, a circuit 10 illustrates one such conventional approach. The circuit 10 generally comprises a resistor 12, an inverter 14, a resistor 16, an inverter 18, a capacitor C1, a capacitor C2 and a crystal 20. One side of the crystal 20 is coupled between the capacitor C1 and the input of the inverter 14. A second side of the oscillator 20 is coupled to the capacitor C2 and the resistor 16. The resistor 16 is also coupled between the output of the inverter 14 and the input of the inverter 18. The output of the inverter 18 presents a signal FOUT. The signal FOUT oscillates at a particular frequency that is generally a function of the size of the capacitors C1 and C2. As a result, to change the frequency of oscillation of the signal FOUT, the size of the capacitors C1 and C2 must be adjusted during fabrication of the circuit 10.

[0006] Manufacturing processes generally have a certain amount of inherent inaccuracy. Since the circuit 10 relies on the accuracy of the manufacturing process to produce the signal FOUT, the overall accuracy of the circuit 10 may not be acceptable in certain design applications having specifications that demand extremely accurate tolerances for the frequency of oscillation of the signal FOUT. A computer motherboard is one example of such an application.

[0007] Oscillators used in watches may have configuration bits configured to trim the accuracy of the oscillator after production. The oscillator may typically be tuned to the range of parts per million. However, the oscillators used in watches are typically low speed oscillators and are generally not appropriate for high speed computer applications. Furthermore, the tuning is generally achieved with a mechanical trimming capacitor which is less desirable than electronic trimming.

[0008] The present invention concerns a timing crystal oscillator circuit that may be tuned after production. The circuit generally comprises a microprocessor configured to present one or more control signals, one or more load devices that may be activated in response to the control signals and a crystal oscillator for presenting an output signal having a frequency which is generally dependent on the number of load devices activated.

[0009] The objects, features and advantages of the present invention include providing a timing crystal oscillator that presents an output signal having a frequency that may be tuned after production.

[0010] These and other objects, features and advantages of the present invention will be apparent from the following detailed description and the appended claims and drawings in which:

[0011] FIG. 1 is a circuit diagram of a conventional crystal oscillator;

[0012] FIG. 2 is a block diagram of a preferred embodiment of the present invention;

[0013] FIG. 3 is a circuit diagram of a preferred embodiment of the present invention;

[0014] FIG. 4 is a circuit diagram of an alternate embodiment of the present invention;

[0015] FIG. 5 is a more detailed circuit diagram of a preferred embodiment of the present invention;

[0016] FIG. 6 is a chart of capacitance versus voltage for a capacitor implemented with MOS transistors; and

[0017] FIG. 7 is a circuit diagram for an inverter with a limited output voltage swing.

[0018] Referring to FIG. 2, a block diagram of a circuit 30 is shown in accordance with a preferred embodiment of the present invention. The circuit 30 generally comprises an EPROM block (or circuit) 32, a load block (or circuit) 34 and a crystal oscillator block (or circuit) 36. The EPROM block 32 generally has a number of outputs 40a-40n. The load block 34 generally has a number of inputs 42a-42n. The EPROM block 32 presents a number of control signals to the outputs 40a-40nthat may be received at the inputs 42a-42nof the load block 34. The control signals generally configure the amount of load capacitance produced by the load block 34 by enabling or disabling a number of loads internal to the load block 34. The control signals may be generated in response to a number of configuration bits internal to the EPROM block 32. The EPROM may be implemented as another type of non-volatile memory such as a flash memory, an EEPROM, etc.

[0019] An output 44 of the load block 34 is generally coupled to an input 46 of the crystal oscillator 36. The frequency of the oscillator 36 is generally controlled by a crystal oscillator but can be controlled by other types of resonators such as ceramic resonators. The impedance (or capacitance) generated by the load block 34 is generally presented to the input 46 of the crystal oscillator 36. The crystal oscillator 36 also has an output 48 that may present a signal (e.g., OUTPUT). The signal OUTPUT may be a periodic signal that oscillates a particular frequency. The particular frequency at which the signal OUTPUT oscillates may be determined by the amount of impedance (or capacitance) presented to the input 46. As a result, the EPROM block 32, in combination with is the load block 34, may control the frequency of oscillation of the signal OUTPUT. Since the frequency of oscillation of the signal OUTPUT may be varied after production, the circuit 30 may be used in a variety of design applications including a computer motherboard. Additionally, a component manufacturer may produce a number of circuits 30 to fill a wide range of orders that previously may have been independently manufactured parts. Another important advantage is that resonators may be manufactured to relaxed tolerance standards and tuning may be used to adjust the oscillator to tighter standards.

[0020] Referring to FIG. 3, a circuit diagram of the circuit 30 is shown. The load section 34 is shown generally comprising a number of capacitors 60a, 60band 60nand a number of switches 62a, 62band 62n. The switch 62ais generally coupled between the capacitor 60aand the input 42a. The switch 62bis generally coupled between the capacitor 60band the input 42b. Similarly, the switch 62nis generally coupled between the capacitor 60nand the input 42n. The EPROM 32 generally presents a number of control signals to the inputs 42a-42nthat may turn the switches 62a-62n“ON” or “OFF” accordingly to provide the particular capacitance desired at the output 44. The switches 62a-62nmay be implemented as transistors or other devices that may turn “ON” or “OFF” in response to a control signal. For example, if the switches 62a-62nare implemented as CMOS transistors, the switches may turn “ON” if a particular control signal is received at a gate. Conversely, if the control signal is not received at the gate, the transistor making up one of the switches 62a-62nmay be turned “OFF”. Generally, when the transistor is “ON”, a particular number of the capacitors 60a-60nare connected between the output 44 and ground, which generally increases the capacitance at the output 44.

[0021] The number of capacitors 60a-60nand the number of switches 62a-62nmay be adjusted in order to meet the design criteria of a particular application. For a relatively simple load block 34, a smaller number of capacitors 60a-60nand switches 62a-62nmay be implemented. However, if greater control of the capacitance presented at the output 44 is desired, a larger number of capacitors 60a-60nand switches 62a-62nmay be implemented.

[0022] In one example, the capacitors 60a-60nmay be implemented in a one-to-one ratio with the switches 62a-62n. However, it may be desirable to provide other ratios between the capacitors 60a-60nand the switches 62a-62n. A particularly useful alternate is to use binary weighting for the capacitors 60a-60n. In this scheme a unit capacitor representing the resolution of an array (i.e., the capacitors 60a-60n) may be selected. The least significant control bit may switch one of the capacitors 60a-60nin and out. The next control bit selects one of the capacitors 60a-60nthat may have twice the capacitance of the first capacitor. The next capacitors 60a-60nmay have four times the capacitance of the first, with each subsequent control bit selecting one of the switches 62a-62nthat may control a capacitance which is twice the value of the previous capacitance. This array implementation may facilitate direct control of a tuning array with a wide range and resolution while using a minimum number of capacitors 60a-60n.

[0023] The oscillator section 36 generally comprises a resistor 70, an inverter 72, an inverter 74, a resonator 76 (which is generally implemented as a crystal or ceramic resonator), a capacitor 78 and a capacitor 80. The capacitors 78 and 80 may be fixed capacitors tuned to produce the lowest frequency desirable at the output 48. With such a configuration, the capacitors 78 and 80 may be combined with the capacitance received at the input 46 to determine the overall frequency of oscillation of the signal OUTPUT. The capacitance received may be coupled to the output side of the oscillator inverter (node 84) instead of to the input if such a configuration is desired to meet the needs of a particular application. If none of the capacitors 60a-60nare activated by the switches 62a-62n, the oscillator section 36 will generally still function properly because capacitors 78 generally sets the minimum load capacitance. The particular configuration of the components of the oscillator block 36 illustrate one example of an oscillator. Other oscillators may be substituted in order to meet the design criteria of a particular application. Whatever the specific implementation of the oscillator block 36, the load block 34 may continue to provide the appropriate adjustment of the frequency of oscillation 48 after fabrication of the circuit 30.

[0024] Referring to FIG. 4, an alternate embodiment of a circuit 30′ is shown. The circuit 30′ further comprises a decoder block 88 coupled between the EPROM 32 and the load block 34. The decoder block 88 generally receives the signals from the outputs 40a-40nat a number of inputs 90a-90nand presents a greater number of control signals at a number of outputs 92a, 92b, 92c, 92d. . . 92n, that may be presented to inputs 42a-42n. The circuit 30′ generally increases the number of combinations of capacitance that may be presented at the output 44 while using the same number of outputs 40a-40nfrom the EPROM block 32. Specifically, if three outputs 40a-40nare implemented, 2_n(or 8) combinations may result. The implementation of the decoder 88 may be useful if the outputs 40a-40nof the EPROM 32 are limited due to other design considerations.

[0025] For low cost implementations, the oscillator load capacitors 78, 80 and 60a-60nshown in FIG. 3 may be implemented as MOS capacitors as shown in FIG. 5. MOS capacitors may be implemented with standard MOS transistors having a gate acting as one capacitor plate and the source/drain/body as the other plate. Unless special processing steps (which usually add extra manufacturing cost) such as buried implants are added to conventional MOS and CMOS processes, circuit techniques are generally required to accommodate the non-linear capacitance versus voltage characteristics of MOS capacitors as shown in FIG. 6. Since the linear region shrinks as operating voltage Vdd is reduced, the nonlinear characteristics of the capacitor may become more significant as operating voltage requirements are reduced from 5V to, for example, 3.3V, 2.5V or lower.

[0026] One circuit technique which may be employed to compensate for the non-linear MOS capacitor is to reduce the voltage swing of the oscillator so operation may be maintained in the linear portion of the MOS capacitor characteristic of FIG. 6. FIG. 7 illustrates one implementation of a circuit which limits the voltage swing of an inverter and replaces the inverter 72 in FIG. 3.

[0027] FIG. 7 illustrates a current source 100 which establishes a reference current through a transistor 101 which may be mirrored by a transistor 102aand a transistor 104a. A transistor 102bmay be connected to transistor 102ato mirror the reference current in a transistor 104b. The transistors 104aand 104bgenerally establish the current available for the conventional inverter formed by a transistor 106aand a transistor 106b. A transistor 103aand a transistor 103bmay be connected to form a voltage divider between Vdd and Gnd, which may generate a reference voltage which may normally be set to Vdd/2. A transistor 105amay form a clamp which shunts current from the current source transistor 104awhen the potential at a node 108ais less than a voltage divider bias 107 minus the threshold of voltage (Vt) of the transistor 105a. A transistor 105bmay form a clamp which shunts current from the current source transistor 104bwhen the potential at a node 108bis more than the voltage divider bias 107 plus the threshold of voltage (Vt) of the transistor 105b. The effect of the shunt transistors 105aand 105bis to limit the voltage swing of the inverter to the threshold voltage Vt of the transistor 105aplus the threshold voltage Vt of transistor 105b, or approximately 2*Vt. The swing may be centered around the divider bias voltage 107. The circuit in FIG. 7 may provide an output voltage swing centered in the linear portion of FIG. 6, which may facilitate the use of low cost MOS capacitors for the oscillator.

[0028] While the invention has been particularly shown and described with reference to the preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made without departing from the spirit and scope of the invention.