Apparatus and method of tuning a digitally controlled input/output driver

1. Apparatus for tuning a digitally controlled input/output (I/O) driver of an integrated circuit for parameter variation compensation, said I/O driver including a first switching network coupled between a bus and a fixed voltage potential, the impedance of said first switching network being controlled by a first digital code, said apparatus comprising:

a second switching network coupled between a desired impedance and said fixed voltage potential, the impedance of said second switching network being controlled by a second digital code;

a first control circuit coupled to said second switching circuit and said desired impedance for monitoring a voltage potential of said desired impedance and for generating said second digital code based on the monitored voltage potential; and

a second control circuit coupled to said first control circuit for tuning said second digital code to generate said first digital code, wherein the second control circuit comprises:

an adder circuit for adding a digital code to said second digital code to generate the first digital code;

a subtractor circuit for subtracting a digital code from said second digital code to generate the first digital code; and

a selector circuit for selecting between said adder circuit and said subtractor circuit for tuning the second digital code to generate the first digital code.

2. The apparatus of claim 1 wherein the desired impedance includes a fixed resistance coupled between a pin of the integrated circuit and a second voltage potential.

3. The apparatus of claim 1 wherein the second control circuit includes a circuit for circumventing the tuning of the second digital code and setting the first digital code to the second digital code.

4. The apparatus of claim 1 wherein the second control circuit includes a circuit for circumventing the tuning of the second digital code and setting the first digital code to a digital code generated independent of the second digital code.

5. The apparatus of claim 1 wherein the second control circuit comprises a programmed controller operative to determine operating bus conditions and to control the tuning of the second digital code to generate the first digital code based on the determined bus operating conditions.

6. The apparatus of claim 1 wherein the second controller is operative to select among: (1) tuning of the second digital code to generate the first digital code; (2) circumventing the tuning of the second digital code and setting the first digital code to the second digital code; and (3) circumventing the tuning of the second digital code and setting the first digital code to a digital code generated independent of the second digital code.

7. The apparatus of claim 1 wherein the first switching network is operative to provide drive impedance to the bus controlled by the first digital code.

8. The apparatus of claim 1 wherein the first switching network is operative to provide termination impedance to the bus controlled by the first digital code.

9. The apparatus of claim 1 wherein the second switching network comprises a parallel combination of field effect transistors.

10. The apparatus of claim 1 wherein the parameter variation comprises integrated circuit processing, voltage and temperature variation.

11. Method of tuning a digitally controlled input/output (I/O) driver of an integrated circuit for parameter variation compensation, a bus impedance of said I/O driver being controlled by a first digital code, said method comprising the steps of:

controlling a bus impedance of a reference I/O driver network by a second digital code;

monitoring a voltage potential of a desired impedance and generating said second digital code based on the monitored voltage potential; and

tuning said second digital code to generate said first digital code, wherein the step of tuning includes the steps of:

adding a digital code to the second digital code to generate the first digital code;

subtracting a digital code from the second digital code to generate the first digital code; and

selecting between the steps of adding and subtracting for tuning the second digital code to generate the first digital code.

12. The method of claim 11 including coupling a fixed termination resistance to a pin of the integrated circuit to form the desired impedance.

13. The method of claim 11 including the steps of circumventing the tuning of the second digital code and setting the first digital code to the second digital code.

14. The method of claim 11 including the step of circumventing the tuning of the second digital code, and setting the first digital code to a digital code generated independent of the second digital code.

15. The method of claim 11 including the steps of determining operating bus conditions, and controlling the tuning of the second digital code to generate the first digital code based on the determined bus operating conditions.

16. The method of claim 11 including the step of selecting among the steps of: (1) tuning of the second digital code to generate the first digital code; (2) circumventing the tuning of the second digital code and setting the first digital code to the second digital code; and (3) circumventing the tuning of the second digital code and setting the first digital code to a digital code generated independent of the second digital code.

17. The method of claim 11 wherein the step of controlling includes controlling a bus drive impedance of the reference I/O driver network by the second digital code.

18. The method of claim 11 wherein the step of controlling includes controlling a bus termination impedance of the reference I/O driver network by the second digital code.

19. The method of claim 11 wherein the parameter variation being compensated comprises integrated circuit processing, voltage and temperature variation.

20. Apparatus for tuning a digitally controlled input/output (I/O) driver of an integrated circuit for parameter variation compensation, said I/O driver including at least one first switching network coupled between a bus and a corresponding voltage potential, the impedance of each first switching network being controlled by a corresponding first digital code, said apparatus comprising:

a second switching network corresponding to each first switching network, each second switching network coupled between a desired impedance and said corresponding voltage potential, the impedance of each second switching network being controlled by a corresponding second digital code;

a first control circuit coupled to each second switching circuit and corresponding desired impedance for monitoring a voltage potential of said corresponding desired impedance and for generating said corresponding second digital code based on the monitored voltage potential; and

a second control circuit coupled to each first control circuit for tuning said second digital code generated by each first control circuit to generate said corresponding first digital code, wherein each second control circuit comprises:

an adder circuit for adding a digital code to said second digital code to generate the first digital code;

a subtractor circuit for subtracting a digital code from said second digital code to generate the first digital code; and

a selector circuit for selecting between said adder circuit and said subtractor circuit for tuning the second digital code to generate the first digital code.

21. Apparatus for tuning a digitally controlled input/output (I/O) driver of an integrated circuit for parameter variation compensation, a bus impedance of said I/O driver being controlled by a first digital code, said apparatus comprising:

means for controlling a bus impedance of a reference I/O driver network by a second digital code;

means for monitoring a voltage potential of a desired impedance and generating said second digital code based on the monitored voltage potential; and

means for tuning said second digital code to generate said first digital code, wherein the tuning means includes:

means for adding a digital code to the second digital code to generate the first digital codes;

means for subtracting a digital code from the second digital code to generate the first digital code; and

means for selecting between the steps of adding and subtracting for tuning the second digital code to generate the first digital code.

22. Apparatus for tuning a digitally controlled input/output (I/O) driver of an integrated circuit for parameter variation compensation, said I/O driver including a first switching network coupled between a bus and a fixed voltage potential, the impedance of said first switching network being controlled by a first digital code, said apparatus comprising:

a second switching network coupled between a desired impedance and said fixed voltage potential, the impedance of said second switching network being controlled by a second digital code;

a first control circuit coupled to said second switching circuit and said desired impedance for monitoring a voltage potential of said desired impedance and for generating said second digital code based on the monitored voltage potential; and

a second control circuit coupled to said first control circuit for tuning said second digital code to generate said first digital code, wherein the second control circuit includes a circuit for circumventing the tuning of the second digital code and setting the first digital code to a digital code generated independent of the second digital code.

23. Method of tuning a digitally controlled input/output (I/O) driver of an integrated circuit for parameter variation compensation, a bus impedance of said I/O driver being controlled by a first digital code, said method comprising the steps of:

controlling a bus impedance of a reference I/O driver network by a second digital code;

monitoring a voltage potential of a desired impedance and generating said second digital code based on the monitored voltage potential;

tuning said second digital code to generate said first digital code; and

selectively circumventing the tuning of the second digital code, and setting the first digital code to a digital code generated independent of the second digital code.