Low voltage crystal oscillator (XTAL) driver with feedback controlled duty cycling for ultra low power

Patent metadata

Field	Value
Publication number	US-9350294-B2
Application number	US-201514594814-A
Country	US
Kind code	B2
Filing date	Jan 12, 2015
Priority date	Jan 10, 2014
Publication date	May 24, 2016
Grant date	May 24, 2016

Abstract

A low voltage crystal oscillator (XTAL) driver with feedback controlled duty cycling for ultra low power biases an amplifier for an XTAL in the sub-threshold operating regime. A feedback control scheme can be used to bias the amplifier for an XTAL biased in the sub-threshold operating regime. The amplifier of a XTAL oscillator can be duty cycled to save power, e.g., the XTAL driver can be turned off to save power when the amplitude of the XTAL oscillation reaches a maximum value in range; but be turned back on when the amplitude of the XTAL oscillation starts to decay, to maintain the oscillation before it stops. In addition or alternatively, a feedback control scheme to duty cycle the amplifier of a XTAL oscillator can be used to monitor the amplitude of the oscillation.

First claim

What is claimed is:

1. An apparatus, comprising:
a crystal oscillator (XTAL) driver having an amplifier with metal-oxide-semiconductor (MOS) transistors, the XTAL driver configured to produce an operating signal to operate a XTAL while the MOS transistors are operated at a sub-threshold operating regime; and
a feedback control unit operatively coupled to the XTAL driver, the feedback control unit configured to receive an output from the XTAL driver, the feedback control unit configured to generate an adjustment signal based on a comparison of the output and a voltage reference to adjust effective lengths of the MOS transistors, the feedback control unit configured to send the adjustment signal to the XTAL driver,
the XTAL driver configured to adjust circuit properties of the XTAL driver in response to the adjustment signal such that a negative resistance of the amplifier of the XTAL driver is calibrated to a value for operation of the MOS transistors at the sub-threshold operating regime.

2. The apparatus of claim 1 , wherein the feedback control unit is configured to bias the amplifier to be operated at the sub-threshold operating regime.

3. The apparatus of claim 1 , wherein the amplifier includes an inverter having the MOS transistors.

4. The apparatus of claim 1 , wherein:
the amplifier includes an inverter,
the negative resistance of the amplifier is calibrated by tuning a transistor strength of the inverter based on the output as feedback control.

5. The apparatus of claim 1 , further comprising:
an off-chip resistor with a reference value of resistance and operatively coupled to the XTAL driver,
the XTAL driver configured to maintain a voltage reference based on the reference value of the off-chip resistor.

6. The apparatus of claim 1 , wherein the value is sufficient to satisfy a condition for XTAL oscillation.

7. An apparatus, comprising:
a crystal oscillator (XTAL) configured to generate an oscillation;
a XTAL driver communicatively coupled to the XTAL, the XTAL driver configured to generate a duty cycle signal to modulate an envelope of the oscillation; and
a feedback control unit communicatively coupled to the XTAL driver and the XTAL, the feedback control unit configured to generate an adjustment signal to adjust effective lengths of metal-oxide-semiconductor (MOS) transistors of the XTAL driver, the feedback control unit configured to use feedback from the oscillation to repeatedly turn on the XTAL driver when an amplitude of the envelope of the oscillation decays to a minimum value and turn off the XTAL driver when the amplitude of the envelope of the oscillation reaches a maximum value.

8. The apparatus of claim 7 , wherein the XTAL is configured to operate at a sub-threshold operating regime of metal-oxide-semiconductor (MOS) transistors.

9. The apparatus of claim 7 , wherein:
the XTAL driver is configured to modulate the envelope of the oscillation without stopping the oscillation during a duty cycle, and
the feedback control unit turns the XTAL driver on in time to sustain oscillation when the amplitude of the envelope of the oscillation decreases.

10. The apparatus of claim 7 , wherein the XTAL driver comprises an amplifier, the amplifier configured to be switched repeatedly while maintaining the amplitude of the envelope of the oscillation above a minimum value.

11. The apparatus of claim 7 , wherein the XTAL driver is configured to be turned off for a time proportional to a fall time of the XTAL oscillation.

12. The apparatus of claim 7 , wherein the XTAL driver is configured to be turned on for a time proportional to a rise time of the XTAL oscillation.

13. A method for duty cycling a XTAL, comprising:
obtaining measurements of a rise time and a fall time of a XTAL oscillation envelope;
sending an initiation signal to the XTAL to initiate an XTAL oscillation that is configured to reach a maximum amplitude; and
repeatedly performing the following:
sending a first signal to turn off the XTAL driver for a first time proportional to the fall time of the XTAL oscillation envelope, and
sending a second signal to turn on the XTAL driver for a second time proportional to the rise time of the XTAL oscillation envelope;

the first time and the second time being relative to growing and decaying delays of the XTAL oscillation envelope such that the XTAL oscillation is preserved.

14. The method of claim 13 , wherein the measurements of the rise time and the fall time of the XTAL oscillation envelope are obtained via a measurement circuit, the measurement circuit having a first comparator configured to set a high value for a counter to start counting when the amplitude of the XTAL oscillation increases, a second comparator configured to set a zero value for the counter to stop counting when the amplitude of the XTAL oscillation decreases.

15. The method of claim 13 , further comprising:
monitoring and controlling the first time and the second time relative to the growing and the decaying delays of the XTAL oscillation envelope.

16. The method of claim 13 , wherein the XTAL driver operates the XTAL at a sub-threshold operating regime of MOS transistors of the XTAL driver.

17. The method of claim 13 , wherein the XTAL driver modulates the XTAL oscillation envelope without stopping the XTAL oscillation during a duty cycle.

18. The method of claim 13 , further comprising using feedback from the XTAL oscillation to periodically turn on and off the XTAL driver.

19. The method of claim 13 , further comprising:
turning off the XTAL driver until the XTAL oscillation envelope reaches a minimum value;
turning on the XTAL driver until the XTAL oscillation envelope reaches a maximum value; and
repeatedly turning on and off the XTAL driver such that the XTAL oscillation is preserved.

Assignees

• Calhoun Benton
• Shrivastava Aatmesh
• Univ Virginia Patent Found

Inventors

• Calhoun Benton
• Shrivastava Aatmesh

Classifications

• H03B5/364 Primary

  the amplifier comprising field effect transistors (H03B5/366 takes precedence) · CPC title

• H03B2200/0082

  Lowering the supply voltage and saving power · CPC title

• H03B2200/0046

  including measures to switch the gain of an amplifier · CPC title

• H03B2200/0064

  Pulse width, duty cycle or on/off ratio · CPC title

• H03B2200/0012

  Pierce oscillator · CPC title