Termination impedance apparatus with calibration circuit and method therefor

1. A termination impedance apparatus comprising:

a variable pull-up resistor coupled between a first power supply voltage terminal and an output terminal;

a variable pull-down resistor coupled between said output terminal and a second power supply voltage terminal; and

a small-signal calibration circuit for alternately calibrating said variable pull-up resistor and said variable pull-down resistor to achieve a desired small-signal voltage swing.

2. The termination impedance apparatus of claim 1 , wherein said small-signal calibration circuit comprises:

a first reference resistor;

a second reference resistor; and

a control circuit for coupling said first reference resistor between said first power supply voltage terminal and said output terminal and changing a resistance of said variable pull-down resistor until a voltage on said output terminal is approximately equal to an output high voltage in a pull-down calibration mode, and for coupling said second reference resistor between said output terminal and said second power supply voltage terminal and changing a resistance of said variable pull-up resistor until a voltage on said output terminal is approximately equal to an output low voltage in a pull-up calibration mode.

3. The termination impedance apparatus of claim 2 wherein said variable pull-up resistor and said variable pull-down resistor are characterized as being non-linear resistors.

4. The termination impedance apparatus of claim 3 wherein said first reference resistor and said second reference resistor are characterized as being non-linear resistors.

5. The termination impedance apparatus of claim 2 , wherein said control circuit comprises:

a comparator having a first input coupled to said output terminal, a second input, and an output; and

a first switch having a first terminal coupled to said second input of said comparator, a second terminal for receiving said output high voltage, a third terminal for receiving said output low voltage, and a control terminal; and

a controller having an input coupled to said output of said comparator, and an output coupled to said control terminal of said first switch.

6. The termination impedance apparatus of claim 5 , wherein said control circuit further comprises:

a second switch having a first terminal coupled to a terminal of said first reference resistor, a second terminal coupled to said output terminal, and a control terminal; and

a third switch having a first terminal coupled to a terminal of said reference resistor, a second terminal coupled to said output terminal, and a control terminal,

wherein said controller further has output terminals coupled to said control terminals of said second and third switches.

7. The termination impedance apparatus of claim 2 , wherein said control circuit repetitively operates in said pull-down calibration mode and said pull-up calibration mode until said voltage on said output terminal is approximately equal to said output high voltage in said pull-down calibration mode and said voltage on said output terminal is approximately equal to said output low voltage in said pull-up calibration mode without changing said resistances of said variable pull-up resistor and said variable pull-down resistor.

8. The termination impedance apparatus of claim 1 wherein said variable pull-up resistor and said variable pull-down resistor are active in a normal operation mode in response to an on-die termination control signal.

9. A system comprising:

a bus circuit capable of providing data to or receiving data from an external bus;

a termination impedance apparatus coupled to said bus circuit and comprising:

a variable pull-up resistor coupled between a first power supply voltage terminal and an output terminal;

a variable pull-down resistor coupled between said output terminal and a second power supply voltage terminal; and

a small-signal calibration circuit for alternately calibrating said variable pull-up resistor and said variable pull-down resistor to achieve a desired small-signal voltage swing.

10. The system of claim 9 , further comprising said external bus.

11. The system of claim 10 , wherein said bus circuit is a physical interface for a dynamic random access memory (DRAM) controller, and the system further comprises a DRAM coupled to said DRAM controller over said external bus.

12. The system of claim 10 wherein said bus circuit is a data input/output circuit for a dynamic random access memory (DRAM), and the system further comprises a DRAM controller having a physical interfacde coupled to said DRAM over said external bus.

13. The system of claim 9 , wherein said small-signal calibration circuit comprises:

a first reference resistor;

a second reference resistor; and

a control circuit for coupling said first reference resistor between said first power supply voltage terminal and said output terminal and changing a resistance of said variable pull-down resistor until a voltage on said output terminal is approximately equal to an output high voltage in a pull-down calibration mode, and for coupling said second reference resistor between said output terminal and said second power supply voltage terminal and changing a resistance of said variable pull-up resistor until a voltage on said output terminal is approximately equal to an output low voltage in a pull-up calibration mode.

14. The system of claim 13 , wherein said variable pull-up resistor and said variable pull-down resistor are characterized as being non-linear resistors.

15. The system of claim 14 , wherein said first reference resistor and said second reference resistor are characterized as being non-linear resistors.

16. The system of claim 13 , wherein said control circuit comprises:

a comparator having a first input coupled to said output terminal, a second input, and an output; and

a first switch having a first terminal coupled to said second input of said comparator, a second terminal for receiving said output high voltage, a third terminal for receiving said output low voltage, and a control terminal; and

a controller having an input coupled to said output of said comparator, and an output coupled to said control terminal of said first switch.

17. The system of claim 16 , wherein said control circuit further comprises:

a second switch having a first terminal coupled to a terminal of said first reference resistor, a second terminal coupled to said output terminal, and a control terminal; and

a third switch having a first terminal coupled to a terminal of said reference resistor, a second terminal coupled to said output terminal, and a control terminal,

wherein said controller further has output terminals coupled to said control terminals of said second and third switches.

18. The system of claim 13 , wherein said control circuit repetitively operates in said pull-down calibration mode and said pull-up calibration mode until said voltage on said output terminal is approximately equal to said output high voltage in said pull-down calibration mode and said voltage on said output terminal is approximately equal to said output low voltage in said pull-up calibration mode without changing said resistances of said variable pull-up resistor and said variable pull-down resistor.

19. The system of claim 13 , wherein said bus circuit, said variable pull-up resistor, said variable pull-down resistor, and said small-signal calibration circuit are combined on a single integrated circuit.

20. The system of claim 9 , wherein said variable pull-up resistor and said variable pull-down resistor are active in a normal operation mode in response to an on-die termination control signal.

21. A method comprising:

coupling a variable pull-up resistor between a first power supply voltage terminal and an output terminal;

coupling a variable pull-down resistor between said output terminal and a second power supply voltage terminal;

coupling a first reference resistor between said first power supply voltage terminal and said output terminal; and

changing a value of said variable pull-down resistor until a voltage on said output terminal falls below an output high voltage of a small-signal voltage swing in a pull-down configuration.

22. The method of claim 21 further comprising:

decoupling said first reference resistor from said output terminal;

coupling a second reference resistor between said output terminal and said second power supply voltage terminal; and

changing a value of said variable pull-up resistor until a voltage of an output terminal rises above an output low voltage of said small-signal voltage swing in a pull-up configuration.

23. The method of claim 22 further comprising:

repeating said coupling said first reference resistor between said first power supply voltage terminal and said output terminal, said changing said value of said variable pull-down resistor, said decoupling, said coupling said second reference resistor between said output terminal and said first power supply voltage terminal, and said changing a value of said variable pull-up resistor until said voltage on said output terminal is both approximately equal to said output low voltage in said pull-down configuration and approximately equal to said output high voltage in said pull-up configuration.

24. The method of claim 21 further comprising:

large-signal calibrating a resistance of said variable pull-up resistor and a resistance of said variable pull-down resistor to be substantially equal to a reference value plus a predetermined amount.

25. The method of claim 21 further comprising:

calibrating a resistance of said first reference resistor to conduct a predetermined current at said output high voltage.

26. A method comprising:

coupling a variable pull-up resistor between a first power supply voltage terminal and an output terminal;

coupling a variable pull-down resistor between said output terminal and a second power supply voltage terminal;

coupling a first reference resistor between said output terminal and said second power supply voltage terminal; and

changing a value of said variable pull-up resistor until a voltage on said output terminal rises above an output low voltage of a small-signal voltage swing in a pull-up configuration.

27. The method of claim 26 further comprising:

decoupling said first reference resistor from said output terminal;

coupling a second reference resistor between said first power supply voltage terminal and said output terminal; and

changing a value of said variable pull-down resistor until a voltage of an output terminal falls below an output high voltage of said small-signal voltage swing in a pull-down configuration.

28. The method of claim 27 further comprising:

repeating said coupling said first reference resistor, said changing said value of said variable pull-up resistor, said decoupling, said coupling said second reference resistor, and said changing said value of said variable pull-down resistor until said voltage on said output terminal is both approximately equal to said output low voltage in said pull-down configuration and approximately equal to said output high voltage in said pull-up configuration.

29. The method of claim 26 further comprising:

large-signal calibrating a resistance of said variable pull-up resistor and a resistance of said variable pull-down resistor to be substantially equal to a reference value plus a predetermined amount.

30. The method of claim 26 further comprising:

calibrating a resistance of said first reference resistor to conduct a predetermined current at said output low voltage.