Variable delay circuits and methods

1. A variable delay circuit comprising:

first pull-up and first pull-down current paths; and

second pull-up and second pull-down current paths, wherein the variable delay circuit generates first delays in an output signal relative to an input signal in response to the first pull-up and first pull-down current paths being enabled by a first control signal and in response to the second pull-up and second pull-down current paths being disabled by a second control signal, and

wherein the variable delay circuit generates second delays in the output signal relative to the input signal that are different than the first delays in response to the second pull-up and second pull-down current paths being enabled by the second control signal and in response to the first pull-up and first pull-down current paths being disabled by the first control signal.

2. The variable delay circuit of claim 1 further comprising:

third pull-up and third pull-down current paths, wherein the variable delay circuit generates third delays in the output signal relative to the input signal that are different than the first and second delays in response to the third pull-up and third pull-down current paths being enabled by a third control signal.

3. The variable delay circuit of claim 2 further comprising:

fourth pull-up and fourth pull-down current paths, wherein the variable delay circuit generates fourth delays in the output signal relative to the input signal that are different than the first, second, and third delays in response to the fourth pull-up and fourth pull-down current paths being enabled by a fourth control signal.

4. The variable delay circuit of claim 1 , wherein the first pull-up current path comprises first and second transistors coupled between a supply terminal and an output of the variable delay circuit that generates the output signal, and wherein the first pull-down current path comprises third and fourth transistors coupled between the output and a ground terminal.

5. The variable delay circuit of claim 4 , wherein the second pull-up current path comprises fifth and sixth transistors coupled in series, wherein the fifth transistor is coupled between the first and second transistors, wherein the sixth transistor is coupled to the output,

wherein the second pull-down current path comprises seventh and eighth transistors coupled in series, wherein the seventh transistor is coupled between the third and fourth transistors, wherein the eighth transistor is coupled to the output, wherein the second, third, sixth, and eighth transistors are responsive to the first control signal, and wherein the fifth and seventh transistors are responsive to the second control signal.

6. The variable delay circuit of claim 1 , wherein the variable delay circuit is in a delay stage circuit in a ring oscillator circuit.

7. The variable delay circuit of claim 3 , wherein the first pull-up current path comprises first and second transistors, wherein the first pull-down current path comprises third and fourth transistors, wherein the second pull-up current path comprises fifth and sixth transistors, wherein the fifth transistor is coupled between the first and second transistors, wherein the second pull-down current path comprises seventh and eighth transistors, and wherein the seventh transistor is coupled between the third and fourth transistors.

8. The variable delay circuit of claim 7 , wherein the third pull-up current path comprises ninth and tenth transistors coupled in series, wherein the ninth transistor is coupled between the first and second transistors, wherein the third pull-down current path comprises eleventh and twelfth transistors coupled in series, and wherein the eleventh transistor is coupled between the third and fourth transistors.

9. The variable delay circuit of claim 8 , wherein the fourth pull-up current path comprises thirteenth and fourteenth transistors coupled in series, wherein the thirteenth transistor is coupled between the first and second transistors, wherein the fourteenth transistor is coupled between the ninth and tenth transistors, wherein the fourth pull-down current path comprises fifteenth and sixteenth transistors coupled in series, wherein the fifteenth transistor is coupled between the third and fourth transistors, and wherein the sixteenth transistor is coupled between the eleventh and twelfth transistors.

10. A method for varying a delay of a signal using a variable delay circuit, the method comprising:

generating first delays in an output signal relative to an input signal in response to first pull-up and first pull-down current paths in the variable delay circuit being enabled by a first control signal;

generating second delays in the output signal relative to the input signal that are different than the first delays in response to second pull-up and second pull-down current paths in the variable delay circuit being enabled by a second control signal; and

varying a frequency of a signal generated by a ring oscillator circuit comprising the variable delay circuit by varying a logic state of at least one of the first control signal or the second control signal.

11. The method of claim 10 further comprising:

generating third delays in the output signal relative to the input signal that are different than the first and second delays in response to third pull-up and third pull-down current paths in the variable delay circuit being enabled by a third control signal.

12. The method of claim 11 further comprising:

generating fourth delays in the output signal relative to the input signal that are different than the first, second, and third delays in response to fourth pull-up and fourth pull-down current paths in the variable delay circuit being enabled by a fourth control signal.

13. The method of claim 11 further comprising:

preventing current through the first pull-up and first pull-down current paths in response to the first control signal when the second pull-up and second pull-down current paths are enabled by the second control signal; and

preventing current through the second pull-up and second pull-down current paths in response to the first and second control signals when the first pull-up and first pull-down current paths are enabled by the first control signal.

14. The method of claim 13 further comprising:

preventing current through the first pull-up, first pull-down, second pull-up, and second pull-down current paths in response to the first and second control signals when the third pull-up and third pull-down current paths are enabled by the third control signal.

15. The method of claim 11 further comprising:

preventing current through the first pull-up, first pull-down, third pull-up, and third pull-down current paths in response to the first, second, and third control signals when the second pull-up and second pull-down current paths are enabled by the second control signal.

16. The method of claim 10 further comprising:

generating third delays in the output signal relative to the input signal that are different than the first and second delays by changing capacitances of the first pull-up and first pull-down current paths in response to the second control signal.

17. The method of claim 10 further comprising:

disabling the first pull-up and first pull-down current paths in response to the first control signal when the second pull-up and second pull-down current paths are enabled by the second control signal; and

disabling the second pull-up and second pull-down current paths in response to the second control signal when the first pull-up and first pull-down current paths are enabled by the first control signal.

18. An oscillator circuit system comprising:

delay stage circuits coupled together as a ring oscillator circuit, wherein at least one of the delay stage circuits comprises a variable delay circuit that comprises first, second, third, fourth, fifth, sixth, seventh, and eighth transistors,

wherein the first, second, third, and fourth transistors are coupled in series between a supply terminal and a ground terminal, wherein control inputs of the first and fourth transistors are coupled to an input of the variable delay circuit, wherein the second and third transistors are coupled to an output of the variable delay circuit,

wherein the fifth and sixth transistors are coupled in series between the first and second transistors and the output,

wherein the seventh and eighth transistors are coupled in series between the third and fourth transistors and the output, wherein the second, third, sixth, and eighth transistors are responsive to a first control signal, and wherein the fifth and seventh transistors are responsive to a second control signal.

19. The oscillator circuit system of claim 18 , wherein the variable delay circuit further comprises:

ninth and tenth transistors coupled in series between the first and second transistors and the fifth and sixth transistors, wherein the ninth transistor is responsive to a third control signal, and wherein the tenth transistor is responsive to the second control signal.

20. The oscillator circuit system of claim 19 , wherein the variable delay circuit further comprises:

eleventh and twelfth transistors coupled in series between the third and fourth transistors and the seventh and eighth transistors, wherein the eleventh transistor is responsive to the third control signal, and wherein the twelfth transistor is responsive to the second control signal.