On-chip impedance network with digital coarse and analog fine tuning

1. A self calibrating resistance circuit, comprising:

a connection providing a signal from a reference resistor;

a resistor network coupled to said connection, wherein said resistor network provides a configurable resistance across said connection, wherein said resistor network comprises a digital resistor network and an analog resistor network; and

control circuitry for configuring said configurable resistance based on a reference resistance of said reference resistor by coarsely tuning said resistor network through said digital resistor network and fine tuning said resistor network through said analog resistor network.

2. The circuit of claim 1 , wherein said resistor network and said control circuitry is included within an integrated circuit that is coupled to said connection.

3. The circuit of claim 2 , wherein said configurable resistance comprises an internal termination resistance.

4. The circuit of claim 2 , wherein said reference resistor comprises an external reference resistor.

5. The circuit of claim 1 , wherein said configurable resistance is substantially the same as said reference resistance.

6. The circuit of claim 1 , wherein said configurable resistance comprises:

a digital portion of said configurable resistance configured by said digital resistor network; and

an analog portion of said configurable resistance configured by said analog resistor network, wherein said digital portion and said analog portion are coupled in parallel.

7. The circuit of claim 6 , wherein said analog resistor network is configured to dynamically adjust said configurable resistance by dynamically tuning said analog portion of said configurable resistance.

8. The circuit of claim 6 , wherein said control circuitry comprises:

wherein said digital resistor network comprises a plurality of selectable, parallel resistive branches; and

a fixed voltage divider generating a fixed divider voltage; and

a second voltage divider comprising said digital portion of said configurable resistance, said reference resistance, wherein said second voltage divider generates a second divider voltage;

said comparator for comparing said second divider voltage and said fixed divider voltage to generate an output signal; and

a resistor control block for configuring said digital portion of said configurable resistance based on said output signal.

9. The circuit of claim 6 , wherein said control circuitry comprises:

said analog resistor network comprising an adjustable resistive branch in parallel with said digital portion of said configurable resistance;

an operational amplifier having inputs of a fixed voltage divider generating a fixed divider voltage and a second voltage divider comprising said analog resistor network and said reference resistor, wherein said second voltage divider generates a second divider voltage, wherein said operational amplifier generates a bias voltage at an output; and

a feedback loop for said operational amplifier feeding said bias voltage back to said second voltage divider for adjusting said analog portion of said configurable resistance.

10. The circuit of claim 1 , further comprising:

a power controller for configuring said digital portion and said analog portion of said resistor network as open circuits when in a low power state.

11. The circuit of claim 1 , further comprising a second self calibrating resistance circuit comprising:

a second resistor network providing a second configurable resistance, said second resistance network comprising a second digital resistor network and a second analog resistor network, wherein said second digital resistor network is configured similarly as said digital resistor network; and

a second control circuitry for configuring said second configurable resistance based on a second reference resistance by fine tuning said second resistance network through said second analog resistor network, wherein said second reference resistance is configured similarly as said resistor network.

12. A self calibrating resistance circuit, comprising:

a connection providing a signal from a reference resistor;

a resistor network coupled to said connection, wherein said resistor network provides a configurable resistance across said connection and comprises:

a digital resistor network for configuring a digital portion of said configurable resistance across said connection; and

an analog resistor network for configuring an analog portion of said configurable resistance across said connection, wherein said digital portion and said analog portion are coupled in parallel;

control circuitry for configuring said configurable resistance based on a reference resistance of said reference resistor by coarsely tuning said resistor network through said digital resistor network and for dynamically fine tuning said resistor network through said analog resistor network; and

a range selector for configuring a range for setting an initial resistance of said digital portion of said configurable resistance.

13. The circuit of claim 12 , wherein said control circuitry converges to said digital portion before performing said dynamically fine tuning of said analog resistor network.

14. The circuit of claim 12 , wherein said initial resistance of said digital portion is greater than said reference resistance.

15. The circuit of claim 12 , further comprising:

an integrated circuit comprising said resistor network and said control circuitry, wherein said integrated circuit is coupled to said connection.

16. The circuit of claim 12 , wherein said configurable resistance is substantially the same as said reference resistance.

17. The circuit of claim 12 , wherein said analog resistor network is configured to dynamically adjust said configurable resistance by dynamically tuning said analog portion of said configurable resistance.

18. The circuit of claim 12 , wherein said control circuitry for configuring said digital resistor network comprises:

wherein said digital resistor network comprises a plurality of selectable, parallel resistive branches; and

a fixed voltage divider generating a fixed divider voltage; and

a second voltage divider comprising said digital portion of said configurable resistor, said reference resistor, wherein said second voltage divider generates a second divider voltage;

said comparator for comparing said second divider voltage and said fixed divider voltage to generate an output signal; and

a resistor control block for configuring said digital portion of said configurable resistance based on said output signal.

19. The circuit of claim 18 , wherein said resistor control block comprises:

a register for adding or removing one of said plurality of parallel resistive branches as determined by said output signal, wherein shift register adds a resistive branch when said output signal indicates said second divider voltage is lower than said fixed divider voltage and removes a resistive branch when said output signal indicates said second divider voltage is higher than said reference divider voltage.

20. The circuit of claim 12 , wherein said control circuitry for configuring said analog resistor network comprises:

said analog resistor network comprising an adjustable resistive branch in parallel with said digital portion of said configurable resistance;

an operational amplifier having inputs of a fixed voltage divider generating a fixed divider voltage and a second voltage divider comprising said analog resistor network and said reference resistor, wherein said operational amplifier generates a bias voltage at an output; and

a feedback loop for said operational amplifier feeding said bias voltage back to said second voltage divider for adjusting said analog portion of said configurable resistance.

21. A method for self calibrating a resistance, comprising:

coarsely tuning a digital resistor network to configure a digital portion of a configurable resistance; and

fine tuning an analog resistor network to configure an analog portion of said configurable resistance, wherein a resistor network comprises said digital resistor network and said analog resistor network coupled in parallel to configure said configurable resistance, wherein said configurable resistance is related to a reference resistance provided by a connection.

22. The method of claim 21 , wherein said coarsely tuning comprises:

comparing a fixed divider voltage configured by a fixed voltage divider and a second divider voltage configured by a second voltage divider comprising said digital portion of said configurable resistance and said reference resistance;

adding one of a plurality of parallel resistive branches when said second divider voltage is lower than said fixed divider voltage, wherein said digital resistor network comprises said plurality of parallel resistive branches; and

removing one of said plurality of parallel resistive branches when said second divider voltage is higher than said fixed divider voltage.

23. The method of claim 22 , wherein said coarsely tuning further comprises:

determining when said second divider voltage is higher than said fixed divider voltage after a last parallel resistive branch is added;

digitally converging said digital portion by removing said last parallel resistive branch; and

storing a configuration of said digital portion.

24. The method of claim 21 , further comprising:

configuring an initial value of said digital portion of said configurable resistance to be greater than a reference resistance of said reference resistor.

25. The method of claim 21 , wherein said analog fine tuning comprises:

dynamically adjusting said analog portion of said configurable resistance in order to dynamically adjust said configurable resistance.

26. The method of claim 25 , further comprising:

generating a bias voltage through an operational amplifier configured with inputs coupled to said fixed voltage divider and said second voltage divider; and

feeding said bias voltage back to said second voltage divider for adjusting said analog portion of said configurable resistance, wherein said analog portion comprises an adjustable resistive branch coupled in parallel with said digital portion of said configurable resistance.

27. The method of claim 21 , further comprising:

configuring said configurable resistance as being substantially the same as said reference resistance.

28. The method of claim 21 , further comprising:

configuring said digital portion and said analog portion of said configurable resistance as an open circuit when in a low power state.

29. The method of claim 28 , further comprising:

retrieving a stored configuration of said digital portion to re-enable said digital portion of said configurable resistance; and

performing said fine tuning of said resistor network to configure said configurable resistance when exiting said low power state.

30. The method of claim 21 , further comprising:

providing a secondary resistance that is a ratio of said configurable resistance.