Method and apparatus to improve noise immunity of a touch sense array

1. A method, performed at a capacitive sensing circuit for a touch sense array, comprising:

scanning the touch sense array using a first sensing frequency passband;

during a noise detection state in which transmit TX signals are absent on TX electrodes:

receiving a response signal on an RX channel;

measuring a noise component of the response signal; and

when a level of noise of the noise component within the first sensing frequency passband is greater than a threshold, selecting a second sensing frequency passband having a lower level of noise from among a plurality of alternative sensing frequency passbands; and

scanning the touch sense array using the second sensing frequency passband.

2. The method of claim 1 , further comprising while selecting the second sensing frequency passband, changing at least one parameter of the capacitance sensing circuit.

3. The method of claim 1 , wherein selecting the second sensing frequency passband comprises forcing synchronization of the second sensing frequency passband to cause the frequency spectrum of the noise component to substantially match a passband of the capacitive sensing circuit.

4. The method of claim 2 , wherein changing at least one parameter includes at least one of changing an integration time, changing a driving signal, changing a number of subconversions per conversion cycle, and changing a number of TX cycles applied in an individual subconversion.

5. The method of claim 1 , wherein determining whether the level of noise is greater than the threshold comprises analyzing raw counts levels during several conversions around a touch area of a conductive object proximate to the touch sense array.

6. The method of claim 1 , wherein determining whether the level of noise is greater than the threshold comprises analyzing touch position jitter history of a touch position of a conductive object proximate to the touch sense array.

7. The method of claim 1 , wherein determining whether the level of noise is greater than the threshold comprises measuring a level of noise when all receive (RX) electrodes are tied together in a single channel.

8. The method of claim 7 , further comprising alternating between listening to the level of noise and performing a full panel scan of the touch sense array.

9. The method of claim 7 , further comprising interleaving between listening to the level of noise and performing a partial scan of the touch sense array.

10. A circuit comprising:

a capacitive sensing circuit for a touch sense array, wherein the capacitive sensing circuit is configured to:

scan the touch sense array using a first sensing frequency passband;

during a noise detection state in which transmit TX signals are absent on TX electrodes:

receive a response signal on an RX channel;

measure a noise component of the response signal; and

when a level of noise of the noise component within the first sensing frequency passband is greater than a threshold, select a second sensing frequency passband having a lower level of noise from among a plurality of alternative sensing frequency passbands; and

scan the touch sense array using the second sensing frequency passband.

11. The circuit of claim 10 , wherein the capacitive sensing circuit comprises a circuit for forcing synchronization of a sensing frequency passband to cause the frequency spectrum of the noise component to substantially match the passband of the capacitive sensing circuit.

12. The circuit of claim 11 , wherein the circuit for forcing synchronization of the sensing frequency passband comprises a clocking circuit.

13. The circuit of claim 12 , wherein the capacitive sensing circuit further comprises a pair of current sources each coupled to the clocking circuit and a one shot coupled between the clocking circuit and one of the current sources to determine whether the level of noise is above the threshold.

14. The circuit of claim 12 , wherein the capacitive sensing circuit further comprises a current source coupled to the clocking circuit and a delay circuit coupled between the clocking circuit and the current source to determine whether the level of noise is above the threshold.

15. A processing device, comprising:

a capacitive sensing circuit for a touch sense array, wherein the capacitive sensing circuit is configured to:

scan the touch sense array using a first sensing frequency passband;

during a noise detection state in which transmit TX signals are absent on TX electrodes:

receive a response signal on an RX channel;

measure a noise component of the response signal; and

when a level of noise of the noise component within the first sensing frequency passband is greater than a threshold, select a second sensing frequency passband having a lower level of noise from among a plurality of alternative sensing frequency passbands; and

scan the touch sense array using the second sensing frequency passband; and

processing logic coupled to the capacitive sensing circuit and configured to detect a conductive object proximate to the touch sense array using the response signal.

16. The processing device of claim 15 , wherein the processing device further comprises a circuit for forcing synchronization of a sensing frequency passband to cause a frequency spectrum of the noise component to substantially match the passband of the capacitive sensing circuit.

17. The processing device of claim 15 , wherein the processing logic is configured to interleave between listening to the level of noise and performing a partial panel scan.