Circuits and methods for sensing resistance

1. A method comprising:

generating a current into a node, wherein the node is coupled to a first terminal of a resistor, and wherein a second terminal of the resistor is coupled in series with a capacitance and a reference voltage;

turning off the current when a voltage on the node meets a threshold, the threshold corresponding to the voltage on the node having a first voltage;

detecting a second voltage on the node after the current is turned off; and

determining a resistance value of said resistor based on said current, the first voltage on the node and the second voltage on the node.

2. The method of claim 1 wherein the current is generated by an integrated circuit, and wherein the resistor and the capacitance are external to the integrated circuit.

3. The method of claim 2 wherein the integrated circuit is coupled to the resistor through a single pin of the integrated circuit.

4. The method of claim 1 further comprising:

configuring a digital-to-analog converter to generate a third voltage, wherein the third voltage sets said threshold; and

comparing the third voltage to the voltage on the node when the current is being generated into said node, and in accordance therewith, turning off the current when the voltage on the node has said first voltage equal to said third voltage.

5. The method of claim 1 , said detecting the second voltage on the node comprising:

configuring a digital-to-analog converter to generate a plurality of voltages; and

comparing the plurality of voltages to the voltage on the node.

6. The method of claim 5 wherein the plurality of voltages are generated on a single output of the digital-to-analog converter, the plurality of voltages changing from an initial voltage to a final voltage, wherein the final voltage occurs after the plurality of voltages crosses the voltage on the node.

7. The method of claim 1 further comprising:

generating a third voltage based on first digital bits, wherein the third voltage sets said threshold;

comparing the third voltage to the voltage on the node when the current is being generated into said node, and in accordance therewith, turning off the current when the voltage on the node has said first voltage equal to said third voltage;

generating a plurality of voltages based on a plurality of digital bits;

comparing the plurality of voltages to the voltage on the node, wherein the second voltage is detected based on a crossover voltage of the plurality of voltages and the voltage on the node, the detected second voltage corresponding to second digital bits,

wherein determining the resistance is based on a difference between the first digital bits and the second digital bits.

8. The method of claim 1 further comprising:

detecting a time period between starting the current into the node and turning off the current when the voltage on the node meets the threshold; and

turning off the current, increasing the threshold, and restarting the current if the time period is less than a first predefined time period.

9. The method of claim 1 further comprising:

detecting a time period between starting the current into the node and turning off the current when the voltage on the node meets the threshold; and

changing the threshold if the time period is outside a predefined time window.

10. The method of claim 1 further comprising waiting a period of time between turning off the current and detecting the second value of the voltage on the node.

11. A resistance sensing circuit comprising:

a current source to generate a current into a node, wherein the node is coupled to a first terminal of a resistor, and wherein a second terminal of the resistor is coupled in series with a capacitance and a reference voltage;

a threshold detection circuit coupled to the node and to the current source, wherein the threshold detection circuit turns off the current when a voltage on the node meets a threshold, the threshold corresponding to a first voltage on the node; and

a voltage measurement circuit coupled to the node to detect a second voltage on the node after the current is turned off,

wherein said current and a difference between the first voltage on the node and the second voltage on the node corresponds to a resistance value of said resistor.

12. The circuit of claim 11 wherein said resistance sensing circuit is an integrated circuit, and wherein the resistor and the capacitance are external to the integrated circuit.

13. The circuit of claim 12 wherein the integrated circuit is coupled to the resistor through a single pin of the integrated circuit.

14. The circuit of claim 11 wherein the threshold detection circuit comprises:

a digital-to-analog converter configured to generate a third voltage, wherein the third voltage sets said threshold; and

a comparator to compare the third voltage to the voltage on the node when the current is being generated into said node, and in accordance therewith, turning off the current when the voltage on the node has said first voltage equal to said third voltage.

15. The circuit of claim 11 wherein the voltage measurement circuit comprises:

a digital-to-analog converter configured to generate a plurality of voltages; and

at least one comparator to compare the plurality of voltages to the voltage on the node after the current is turned off.

16. The circuit of claim 15 wherein the plurality of voltages are generated on a single terminal of the digital-to-analog converter that changes from an initial voltage and a final voltage, wherein the final voltage occurs after one of the plurality of voltages on the single terminal crosses the voltage on the node.

17. The circuit of claim 11 wherein the threshold detection circuit and the voltage measurement circuit comprise:

a digital-to-analog converter; and

a comparator;

wherein, when the current is being generated into said node,

the digital-to-analog converter generates a third voltage based on first digital bits, wherein the third voltage sets said threshold; and

the comparator compares the third voltage to the voltage on the node, and in accordance therewith, turns off the current when the voltage on the node has said first voltage equal to said third voltage;

and wherein, after the current is turned off,

the digital-to-analog converter generates a plurality of voltages based on a plurality of digital bits; and

the comparator compares the plurality of voltages to the voltage on the node, wherein the second voltage is detected based on a crossover voltage of the plurality of voltages and the voltage on the node, the detected second voltage corresponding to second digital bits.

18. The circuit of claim 11 further comprising:

a timer to detect a time period between starting the current into the node and turning off the current when the voltage on the node meets the threshold; and

wherein if the time period is less than a first predefined time period, the current is turned off, the threshold is increased, and the current is restarted.

19. The circuit of claim 11 further comprising:

a timer to detect a time period between starting the current into the node and turning off the current when the voltage on the node meets the threshold; and

wherein if the time period is outside a predefined time window, the threshold is changed.

20. The circuit of claim 11 further comprising a timer to wait a period of time between turning off the current and detecting the second value of the voltage on the node.