Fingerprint sensing system with finger detect

1. A method of sensing a fingerprint pattern of a finger using a fingerprint sensing device comprising:

providing an array of sensing elements for sensing said fingerprint pattern, each sensing element comprising a sensing structure for capacitive coupling with the finger;

controlling a group of sensing elements to change a potential of the group of sensing structures comprised in said group of sensing elements;

acquiring, in response to said variation in potential, a response signal from a finger detecting circuitry connected to an electrically conductive finger detecting structure arranged adjacent to said array of sensing elements, the response signal is indicative of the capacitive coupling between the group of sensing structures and the finger detecting structure;

comparing a variation in the response signal with a predefined threshold value; and

providing a signal indicating that a finger is present when the variation in response signal is greater than the threshold value.

2. The method according to claim 1 , further comprising:

activating, when the variation in response signal is greater than the threshold value, at least a subset of said sensing elements to sense at least a portion of said fingerprint pattern.

3. The method according to claim 2 , wherein said step of activating is only carried out when said variation in the response signal is greater than the threshold value.

4. The method according to claim 1 , further comprising:

evaluating said response signal to determine a background capacitive coupling between the group of sensing structures and the finger detecting structure when a finger is not coupled to the finger detecting structure, wherein said predefined threshold value is indicative of the background capacitive coupling.

5. The method according to claim 1 , wherein said fingerprint sensing device comprises at least two electrically conductive finger detecting structures, wherein said response signal is indicative of the capacitive coupling between the group of sensing elements and a first finger detecting structure, and wherein a second group of sensing elements neighboring to a second finger detecting structure are at least partly inactive, wherein the method comprises:

acquiring, a second response signal from the finger detecting circuitry of the second finger detecting structure; and

providing a response signal from which the second response signal has been subtracted.

6. The method according to claim 1 , wherein said fingerprint sensing device comprises at least two electrically conductive finger detecting structures, wherein said response signal is indicative of the capacitive coupling between the group of sensing elements and a first finger detecting structure, wherein a second group of the sensing elements close to a second finger detecting structure are at least partly inactive, wherein the method comprises:

acquiring, a second signal from the finger detecting circuitry of the second finger detecting structure; and

discarding the response signal when the second signal is greater than a background threshold level, whereby a further response signal is acquired.

7. The method according to claim 1 , wherein the group of sensing structures and the electrically conductive finger detecting structure are separated from each other such that the capacitive coupling between the group of sensing structures and the finger detecting structure is relatively low when no finger is present and relatively high when a finger is present, wherein said threshold value is indicative of the relatively low capacitive coupling.

8. A fingerprint sensing device comprising:

an array of sensing elements for sensing a fingerprint pattern, each sensing element comprising a sensing structure for capacitive coupling with a finger;

an electrically conductive finger detecting structure arranged adjacent to said array of sensing elements; and

finger detecting circuitry connected to said finger detecting structure for providing a finger detection signal indicative of a capacitive coupling between a group of sensing structures and the finger detecting structure,

wherein the fingerprint sensing device is configured to:

control a group of sensing elements to change a potential of the group of sensing structures comprised in said group of sensing elements;

acquire, in response to said change in potential, a response signal from the finger detecting circuitry indicative of the capacitive coupling between the group of sensing structures and the finger detecting structure;

compare a variation in the response signal with a predefined threshold value; and

provide a signal indicating that a finger is present when the variation in the response signal is greater than the threshold value.

9. The fingerprint sensing device according to claim 8 , wherein said fingerprint sensing device comprises at least two finger detecting structures, wherein said response signal is indicative of the capacitive coupling between the group of sensing elements and a first of the finger detecting structures, wherein a second group of sensing elements neighboring to a second finger detecting structure are at least partly inactive, wherein the fingerprint sensing device is configured to:

acquire, a second response signal from the finger detecting circuitry of the second finger detecting structure; and

provide a response signal from which the second response signal has been subtracted.

10. The fingerprint sensing device according to claim 8 , wherein said fingerprint sensing device comprises at least two finger detecting structures, wherein said response signal is indicative of the capacitive coupling between the group of sensing elements and a first of the finger detecting structures, wherein a second group of the sensing elements close to a second finger detecting structure are at least partly inactive, wherein the fingerprint sensing device is configured to:

acquire, a background signal from the finger detecting circuitry of the second finger detecting structure; and

when the background signal is greater than a background threshold level, discard the response signal, whereby a further response signal is acquired.

11. The fingerprint sensing device according to claim 8 , wherein the group of sensing structures and the electrically conductive finger detecting structure are separated from each other such that the capacitive coupling between the group of sensing structures and the finger detecting structure is relatively low when no finger is present and relatively high when a finger is present.

12. The fingerprint sensing device according to claim 11 , wherein said group of sensing structures is surrounded in a sensing structure plane by at least partly inactive sensing structures.

13. The fingerprint sensing device according to claim 8 , wherein the electrically conductive finger detecting structure is arranged to extend around the circumference of the array of sensing structures.