1. Field of the Invention
The invention relates to crystal oscillators and more particularly to start-up time associated with crystal oscillators.
2. Description of the Related Art
Crystal oscillators are well known in the art to provide timing signals for a wide variety of applications.
Accordingly, in one embodiment a method includes in response to a first condition, injecting an in-band periodic signal into a crystal oscillator driver circuit, the in-band signal having a frequency within a bandwidth of a crystal oscillator, the crystal oscillator including a crystal and the crystal oscillator driver circuit.
In another embodiment, an apparatus includes a crystal oscillator driver circuit. An in-band periodic signal generator is coupled to inject an in-band periodic signal into an input of the driver circuit, the in-band periodic signal having a frequency within a bandwidth of a crystal oscillator, the crystal oscillator including a crystal and the crystal oscillator driver circuit.
In another embodiment, an apparatus includes a crystal oscillator, which includes a crystal and a crystal oscillator driver circuit. An in-band periodic signal generator is coupled to inject an in-band periodic signal into the crystal oscillator driver circuit, the in-band periodic signal having a frequency within a bandwidth of the crystal oscillator.
The present invention may be better understood, and its numerous objects, features, and advantages made apparent to those skilled in the art by referencing the accompanying drawings.
The use of the same reference symbols in different drawings indicates similar or identical items.
In an embodiment in the power up mode, the input and output of the driver is biased to half of the supply voltage. The in-band noise in the crystal driver loop is very small because the bandpass characteristics of the crystal are amplified by the driver as the driver approaches full swing. Full swing refers to the difference between maximum and minimum voltages in the oscillating signal at the output of the crystal driver circuit once steady state operation is achieved. Initially the amplitude of the oscillating signal is small and grows larger during startup until full swing is achieved.
The center frequency of the crystal oscillator is described by
The quality factor Q is the peak energy stored in L or C per cycle over the energy dissipated in R per cycle (see L, C, and R in
The bandwidth of the crystal is
As can be seen, as the quality factor Q goes up, the bandwidth narrows. A startup circuit providing an in-band periodic signal reduces the amount of time the signal provided by inverter 201 takes to reach full swing.
Referring to the embodiment of
Once the periodic signal generator begins to supply the in-band signal, that signal is supplied to counter 311. Counter 311 tracks the length of time that the in-band signal is injected into the crystal driver loop at the input of the crystal driver. The counter 311 is coupled to receive the periodic in-band signal 315 supplied by the periodic signal generator 309 and uses the in-band signal to increment (or decrement) its count. The counter may be clocked directly by the periodic in-band signal or be clocked by a signal derived from the periodic in-band signal, e.g., to reduce the clock rate.
The periodic in-band signal 315 is coupled to the input of the crystal driver through a transmission gate or switch 317. The control logic turns on the N-channel and P-channel transistors of the transmission gate or switch to couple the periodic signal to the input of the inverter. The switch turns in response to power-up and the switch turns off when the counter has reached a predetermined value, e.g., by counting up or counting down a predetermined amount. In an embodiment, the counter counts a value that corresponds to the crystal driver achieving full swing. In an embodiment, that length of time is approximately 2 microseconds but in other embodiments the time will vary according to the particular embodiment, e.g., on the gain of the gain stage. Note that once the counter has determined that the crystal driver is operating at full swing (the output of inverter 201) based on the count expiring, the crystal driver is isolated from the circuits that inject the periodic in-band signal by grounding the transmission gate through transistor 321 and turning off the N-channel and P-channel transistors constituting the transmission gate.
Another aspect of the gain element 201 is that the transistors forming the gain element may be designed for increased gain and reduced power consumption during start-up. For an inverter, such as shown in
While circuits and physical structures have been generally presumed in describing embodiments of the invention, it is well recognized that in modern semiconductor design and fabrication, physical structures and circuits may be embodied in a computer readable medium as data structures for use in subsequent design, simulation, test, or fabrication stages. For example, such data structures may encode a functional description of circuits or systems of circuits. The functionally descriptive data structures may be, e.g., encoded in a register transfer language (RTL), a hardware description language (HDL), in Verilog, or some other language used for design, simulation, and/or test. Data structures corresponding to embodiments described herein may also be encoded in, e.g., Graphic Database System II (GDSII) data, and functionally describe integrated circuit layout and/or information for photomask generation used to manufacture the integrated circuits. Other data structures, containing functionally descriptive aspects of embodiments described herein, may be used for one or more steps of the manufacturing process.
Computer-readable media include tangible computer readable media, e.g., a disk, tape, or other magnetic, optical, or electronic storage medium. In addition to computer-readable medium having encodings thereon of circuits, systems, and methods, the computer readable media may store instructions as well as data that can be used to implement embodiments described herein or portions thereof. The data structures may be utilized by software executing on one or more processors, firmware executing on hardware, or by a combination of software, firmware, and hardware, as part of the design, simulation, test, or fabrication stages.
The description of the invention set forth herein is illustrative, and is not intended to limit the scope of the invention as set forth in the following claims. Other variations and modifications of the embodiments disclosed herein may be made based on the description set forth herein, without departing from the scope of the invention as set forth in the following claims.