Low power baseline tracking for fingerprint sensor

1. A method of automatically tracking a baseline input into a capacitive sensor having a plurality of transmitter electrodes and at least one receiver electrode, the method comprising:

acquiring a partial baseline image of the capacitive sensor when no biometric object is present at the capacitive sensor by applying a subset of a plurality of distinct waveforms in a sequence to some or all of the plurality of transmitter electrodes, wherein the subset comprises less than all of the plurality of distinct waveforms applied when acquiring a full baseline image of the capacitive sensor;

comparing the partial baseline image with a stored baseline image;

acquiring the full baseline image of the capacitive sensor when no biometric object is present at the capacitive sensor in response to a difference between the captured partial baseline image and the stored baseline image exceeding a threshold value; and

updating the stored baseline image.

2. The method of claim 1 , wherein the updating the stored baseline image comprises storing the acquired full baseline image as the stored baseline image.

3. The method of claim 1 , wherein the acquiring the full baseline image includes:

applying the plurality of distinct waveforms in a sequence to the plurality of transmitter electrodes, wherein each of the plurality of distinct waveforms is applied to the plurality of transmitter electrodes simultaneously; and

detecting receiver signals from the at least one receiver electrode for each of the plurality of waveforms applied to the plurality of transmitter electrodes.

4. The method of claim 1 , wherein the acquiring a partial baseline image further comprises:

detecting receiver signals from a subset of the at least one receiver electrode.

5. The method of claim 1 , wherein the acquiring a partial baseline image is performed in response to a timer event.

6. The method of claim 1 , wherein the capacitive sensor comprises a capacitive fingerprint sensor.

7. The method of claim 1 , wherein the stored baseline image is a previously stored full baseline image and the comparing the partial baseline image with a stored baseline image includes comparing the partial baseline image with a portion of the previously stored full baseline image.

8. The method of claim 1 , wherein the stored baseline image is a partial baseline image and the comparing the partial baseline image with a stored baseline image includes comparing the partial baseline image with the stored partial baseline image.

9. The method of claim 1 , further comprising subtracting the stored baseline image from a subsequent full fingerprint image to produce a baseline-subtracted fingerprint image.

10. An electronic system for capacitive sensing, the electronic system comprising:

a capacitive sensor configured to capacitively sense an input object in proximity to a plurality of sensor electrodes comprising a plurality of transmitter electrodes, and at least one receiver electrode;

a processor configured to drive the plurality of transmitter electrodes and to detect receiver signals from the at least one receiver electrode, wherein the processor is configured to control the capacitive sensor to:

acquire a partial baseline image of the capacitive sensor when no biometric object is present at the capacitive sensor by applying a subset of a plurality of distinct waveforms in a sequence to some or all of the plurality of transmitter electrodes, wherein the subset comprises less than all of the plurality of distinct waveforms applied when acquiring a full baseline image of the capacitive sensor; and

wherein the processor is further configured to:

compare the partial baseline image with a stored baseline image;

in response to a difference between the captured partial baseline image and the stored baseline image exceeding a threshold value, control the capacitive sensor to acquire the full baseline image of the capacitive sensor when no biometric object is present at the capacitive sensor; and

update the stored baseline image using the acquired full baseline image.

11. The electronic system of claim 10 , wherein the processor is configured to store the acquired full baseline image as the stored baseline image.

12. The electronic system of claim 10 , wherein the processor is configured to control the capacitive sensor to acquire the full baseline image by:

applying the plurality of distinct waveforms in a sequence to the plurality of transmitter electrodes simultaneously; and

detecting receiver signals from the at least one receiver electrode for each of the plurality of waveforms applied to the plurality of transmitter electrodes.

13. The electronic system of claim 10 , wherein the processor is further configured to control the capacitive sensor to acquire the partial baseline image by detecting receiver signals from a subset of the at least one receiver electrode.

14. The electronic system of claim 10 , wherein electrodes in a subset of the at least one receiver electrode are distributed evenly among the subset of the at least one receiver electrode.

15. The electronic system of claim 10 , wherein the processor is configured to control the capacitive sensor to acquire the partial baseline image in response to a timer event.

16. An electronic device for capacitive fingerprint sensing, the electronic device comprising:

a capacitive fingerprint sensor having a plurality of sensor electrodes comprising a plurality of transmitter electrodes, and at least one receiver electrode, wherein the capacitive fingerprint sensor is configured to capacitively sense an input fingerprint on an input surface in proximity to the plurality of sensor electrodes;

one or more processors coupled to the capacitive fingerprint sensor and configured to drive the plurality of transmitter electrodes and to detect receiver signals from the at least one receiver electrode, wherein the one or more processors are configured to control the capacitive fingerprint sensor to:

acquire a partial baseline image of the capacitive sensor when no biometric object is present at the capacitive sensor by applying a subset of a plurality of distinct waveforms in a sequence to some or all of the plurality of transmitter electrodes, wherein the subset comprises less than all of the plurality of distinct waveforms applied when acquiring a full baseline image of the capacitive sensor; and

wherein the one or more processors are further configured to:

compare the partial baseline image with a stored baseline image;

in response to a difference between the acquired partial baseline image and the stored baseline image exceeding a threshold value, control the capacitive sensor to acquire the full baseline image of the capacitive sensor when no biometric object is present at the capacitive sensor; and

update the stored baseline image using the acquired full baseline image.

17. The electronic device of claim 16 , further including a glass or polymer material overlaying the capacitive fingerprint sensor, wherein a surface of the glass or polymer material comprises the input surface.

18. The electronic device of claim 16 , wherein the one or more processors are configured to control the capacitive fingerprint sensor to acquire the full baseline image by:

applying the plurality of distinct waveforms in a sequence to the plurality of transmitter electrodes simultaneously; and

detecting receiver signals from the at least one receiver electrode for each of the plurality of waveforms applied to the plurality of transmitter electrodes.

19. The electronic device of claim 16 , wherein the one or more processors are further configured to control the capacitive fingerprint sensor to acquire the partial baseline image by detecting receiver signals from a subset of the at least one receiver electrode.

20. The electronic device of claim 16 , wherein the one or more processors are configured to control the capacitive fingerprint sensor to acquire the partial baseline image in response to a timer event.