Noise measurement in capacitive touch sensors

1. An apparatus comprising:

a drive electrode;

a sense electrode; and

circuitry configured to perform operations comprising:

making, in a noise-measurement phase, a first measurement by performing operations comprising:

providing a pre-determined amount of charge to a charge-measurement capacitor;

transferring a first amount of charge on the sense electrode to the charge-measurement capacitor, thereby modulating the first amount of charge by the pre-determined amount of charge provided to the charge-measurement capacitor; and

measuring a first voltage of the charge-measurement capacitor to determine the first amount of charge; and

making, in a proximity-detection phase, a second measurement by performing operations comprising:

providing a drive signal to the drive electrode;

transferring a second amount of charge on the sense electrode to the charge-measurement capacitor; and

measuring a second voltage of the charge-measurement capacitor to determine the second amount of charge.

2. The apparatus of claim 1 , wherein the circuitry is configured to perform operations comprising calculating a noise factor to configure a filter based on the first amount of charge.

3. The apparatus of claim 2 , wherein the circuitry is configured to:

determine whether the noise factor has a predetermined relationship with a threshold; and

apply, based on whether the noise factor has the predetermined relationship with the threshold, the filter.

4. The apparatus of claim 2 , wherein:

the circuitry is part of a controller of a touch sensor; and

the filter is provided by a processor of a device that comprises the touch sensor.

5. The apparatus of claim 1 , wherein the circuitry is configured to perform, before measuring the second voltage of the charge-measurement capacitor to determine the second accumulated amount of charge, multiple cycles of operations comprising:

providing the drive signal to the drive electrode;

transferring the second amount of charge on the sense electrode to the charge-measurement capacitor.

6. The apparatus of claim 1 , wherein the circuitry is configured to perform one or more of the following:

measure the first voltage of the charge-measurement capacitor to determine the first amount of charge by performing operations comprising measuring an amount of time for the first amount of charge on the charge-measurement capacitor to reduce to a predetermined value; and

measure the second voltage of the charge-measurement capacitor to determine the second amount of charge by performing operations comprising measuring an amount of time for the second amount of charge on the charge-measurement capacitor to reduce to a predetermined value.

7. The apparatus of claim 1 , wherein:

the first amount of charge on the sense electrode is due at least in part to noise; and

the second amount of charge on the sense electrode is due at least in part to the applied drive signal.

8. The apparatus of claim 1 , wherein the noise-measurement phase occurs before the proximity-detection phase.

9. An apparatus comprising:

a charge-measurement capacitor; and

circuitry configured to perform operations comprising:

providing a pre-determined amount of charge to the charge-measurement capacitor;

transferring an accumulated amount of charge on a sense electrode to the charge-measurement capacitor, the accumulated amount of charge having accumulated on the sense electrode due at least in part to noise;

measuring a voltage of the charge-measurement capacitor to determine the accumulated amount of charge; and

calculating a noise factor to configure a filter based on the accumulated amount of charge.

10. The apparatus of claim 9 , wherein the circuitry is configured to provide the predetermined amount of charge through a resistor coupled to a voltage source by coupling the resistor to the voltage source for a predetermined amount of time.

11. The apparatus of claim 9 , wherein:

the circuitry is part of a controller of a touch sensor; and

the filter is provided by a processor of a device that comprises the touch sensor.

12. The apparatus of claim 9 , wherein the circuitry is configured to perform, before measuring a voltage of the charge-measurement capacitor to determine the accumulated amount of charge, multiple cycles of operations comprising:

providing the pre-determined amount of charge to the charge-measurement capacitor; and

transferring the accumulated amount of charge on the sense electrode to the charge-measurement capacitor.

13. Circuitry comprising logic embodied in a non-transitory computer-readable medium, the logic when executed configured to perform operations comprising:

providing a first control signal to cause a voltage to be applied, providing a pre-determined amount of charge to a charge-measurement capacitor;

providing a second control signal to cause an accumulated amount of charge on a sense electrode to be transferred to the charge-measurement capacitor, thereby modulating the accumulated amount of charge by the pre-determined amount of charge provided to the charge-measurement capacitor, the accumulated amount of charge having accumulated on the sense electrode due at least in part to noise; and

providing a third control signal to cause a voltage of the charge-measurement capacitor to be measured to determine the accumulated amount of charge.

14. The circuitry of claim 13 , wherein the logic when executed is configured to perform operations comprising calculating a noise factor to configure a filter based on the accumulated amount of charge.

15. The circuitry of claim 14 , wherein the logic when executed is configured to:

determine whether the noise factor has a predetermined relationship with a threshold; and

apply, based on whether the noise factor has the predetermined relationship with the threshold, the filter.

16. The circuitry of claim 14 , wherein:

the circuitry is part of a controller of a touch sensor; and

the filter is provided by a processor of a device that comprises the touch sensor.

17. The circuitry of claim 13 , wherein the logic when executed is configured to measure the voltage of the charge-measurement capacitor to determine the accumulated amount of charge by performing operations comprising measuring an amount of time for the accumulated amount of charge on the charge-measurement capacitor to reduce to a predetermined value.