Low-power crystal circuit
This is a continuation of application Ser. No. 704,745 filed May 28, 1991, now U.S. Pat. No. 5,155,453. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates generally to crystal oscillator circuits and, more particularly, relates to a crystal oscillator circuit employing a power saving sleep mode. 
     Crystal oscillator circuits have many applications, one of the most common being to provide a clock signal required by a digital system. Most systems have required electrical characteristics for the clock signal such as the voltage swing, typically from 0 to 5 volts, and the output current drive. 
     The crystal oscillator circuit consumes a substantial amount of current, typically on the order of milliamps, to achieve the required characteristics. For systems utilizing a battery as a power supply the current consumed by the crystal oscillator circuit is a substantial load on the battery and reduces battery life. 
     Accordingly, many battery-powered portable systems, such as laptop computers, turn off the crystal oscillator circuit when the clock signal is not required. However, when the crystal oscillator is turned back there is a delay, typically on the order of milliseconds, before the output signal is stable and exhibits the required electrical characteristics. In certain applications, this delay is undesirable and adversely affects the operation of the system. 
     SUMMARY OF THE INVENTION 
     The present invention is a crystal oscillator having a sleep mode for reducing power consumption and that almost instantaneously generates an oscillating output signal having required electrical characteristics when switched back to a normal mode of operation. 
     In the sleep mode the amount of current supplied to the amplifier is sufficient to maintain oscillation of the crystal, but the current level is not sufficient to generate an oscillator output signal having required electrical characteristics. However, this maintained oscillation facilitates a very fast transition to a normal mode when the amount of current supplied to the amplifier is increased to a level sufficient to generate an oscillator output signal having required electrical characteristics. 
     Other advantages and features of the invention will become apparent in view of the appended drawings and following detailed description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram of a preferred embodiment of the invention; and 
     FIG. 2 is a circuit diagram of an implementation of the embodiment depicted in FIG. 1. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 is a functional schematic diagram of a preferred embodiment of the invention. A crystal oscillator circuit 10 includes a crystal 11, disposed between capacitive crystal input plates 12 and 14, a first amplifier 16 having its input coupled to the first crystal input 12 and its output coupled to the second crystal input 14. An RC feedback network comprising a feedback resistor 18 and a capacitor 20 couples the output of the first amplifier 16 to its input. A second amplifier 22 has its input coupled to the output of the first amplifier 16 and it output coupled to an oscillator output terminal 24 by a driver circuit 26 and to ground via a first switching transistor 28. 
     First, second, and third current sources 30, 32, and 34 are coupled in a current mirror configuration. The first and second current sources 30 and 32 provide current to drive the first and second amplifiers 16 and 22. The third current source is coupled to a normal mode biasing current source 36 via a second switching transistor 38 and to a sleep mode bleeder resistor 40 via a third switching transistor 42. The control inputs of the first and third switching transistors 28 and 42 are connected to receive a sleep mode signal (SL) and the control input of the second switching transistor is connected to receive an inverted sleep mode signal (SL*). 
     The operation of the system depicted in FIG. 1 will now be described. In normal operation SL is low and SL* is high so that the output of the second amplifier is isolated from ground and the third current source 34 is coupled to the normal mode biasing source 36 to bias the third current source 34 to conduct a large, normal mode current. The third current source then biases the first and second current sources provide a large amount of current to the first and second amplifiers 16 and 22. These large currents cause the output voltages generated by the first and second amplifiers to be at sufficiently high levels to cause voltage swing of the oscillator output signal at the output terminal 24 to be at the operational level, typically between 0 and 5 volts. 
     In the sleep mode SL is high and SL* is low so that the output of the second amplifier 22 is tied to ground and the third current source 34 is coupled to the bleeder resistor 40. A very small amount of current flows through the resistor 40 so that the current flow through the current mirror coupled current sources 30, 32, and 34 is substantially reduced below the normal mode levels. This current reduction lowers the output voltage level of the first amplifier 16 to a level sufficient to maintain the oscillation the crystal oscillator but not at the level required to maintain the operational voltage swing of the output signal. Further, the second amplifier is shut down because its output is coupled to ground. 
     The low level of current supplied by the current sources 30, 32, and 34 during the sleep mode substantially reduces the power consumed by the crystal oscillator 10. However, because oscillation of the crystal is maintained, the output voltage can be returned to the full operational level in a matter of microseconds. Thus, the system reduces the delay caused by switching from the sleep mode to the normal mode by an order of magnitude. 
     Typically, digital circuitry is coupled to the oscillator output and is toggled by the output signal. Each time the coupled digital circuitry is toggled power is consumed. It is possible that the low voltage output signal provided during the sleep mode could still cause this coupled circuitry to toggle and consume power. The coupling of the output of the second amplifier 22 to ground during the sleep mode assures that no oscillating output signal is present at the output terminal 24 to prevent toggling of the coupled digital circuitry. 
     FIG. 2 is a circuit diagram of a CMOS circuit implementing the system depicted in FIG. 1. The same reference numerals are utilized to depict identical or corresponding parts. In FIG. 1, the feedback transistor 18, capacitor 20, and switching transistor 28 are implemented as NMOS transistors I19, I16, I31 respectively. A bias current is provided at the pb input of the circuit. The remaining circuit elements are implemented in a standard manner. 
     In the circuit depicted in FIG. 2, it would take several milliseconds to generate a stable output signal after completely turning off the oscillator, however, when utilizing the sleep mode disclosed herein the recovery time is a few microseconds. Further, during normal operation the power consumption is about 3 milliamps and is sleep mode it is about 50 microamps. 
     The invention has now been described with reference to the preferred embodiments. Alternatives and substitutions will now be apparent to persons of ordinary skill in the art. For example, the various functional elements depicted in FIG. 1 may be realized in a variety of standard configurations. Accordingly, it is not intended to limit the invention except as provided by the appended claims.