Method and device for detecting if a finger is present and sensing a fingerprint pattern

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

an array of sensing elements for sensing said fingerprint pattern;

an electrically conductive finger detecting structure;

finger detecting circuitry connected to said finger detecting structure for providing a finger detection signal indicative of a capacitive coupling between said finger detecting structure and said finger; and

operation control circuitry connected to said sensing elements for activating said sensing elements to sense said fingerprint pattern, and connected to said finger detecting circuitry for evaluating said finger detection signal,

wherein said method comprises the steps of:

comparing said finger detection signal with a first threshold value indicating a first capacitive coupling between said finger detecting structure and said finger, and a second threshold value indicating a second capacitive coupling between said finger detecting structure and said finger, stronger than said first capacitive coupling; and

activating, when said finger detection signal changes from a first value indicating a capacitive coupling weaker than said first capacitive coupling to a second value indicating a capacitive coupling stronger than said second capacitive coupling, at least a subset of said sensing elements to sense at least a portion of said fingerprint pattern.

2. The method according to claim 1 , wherein said step of activating is only carried out when said finger detection signal changes from said first value to said second value within a time duration shorter than a first predefined time duration.

3. The method according to claim 2 , wherein said first predefined time duration is shorter than 0.1 second.

4. The method according to claim 1 , wherein said step of activating is only carried out when said finger detection signal indicates a capacitive coupling weaker than said first capacitive coupling for a time duration longer than a second predefined time duration before changing from said first value to said second value.

5. The method according to claim 4 , wherein said second predefined time duration is longer than one second.

6. The method according to claim 1 , wherein said step of activating is only carried out when said finger detection signal indicates a capacitive coupling stronger than said second capacitive coupling for a time duration longer than a third predefined time duration.

7. The method according to claim 6 , wherein said third predefined time duration is longer than 10 milliseconds.

8. The method according to claim 1 , wherein said fingerprint sensing device comprises at least a first and a second electrically conductive finger detecting structure, each being connected to said finger detecting circuitry, said method further comprising the steps of:

evaluating a first finger detection signal indicative of a capacitive coupling between said first finger detecting structure and said finger; and

evaluating a second finger detection signal indicative of a capacitive coupling between said second finger detecting structure and said finger,

wherein said step of activating is only carried out when each of said first finger detection signal and said second finger detection signal changes from said first value to said second value.

9. The method according to claim 1 , further comprising the steps of:

evaluating said finger detection signal to determine a noise level of said finger detection signal; and

determining said first threshold level based on said noise level.

10. The method according to claim 1 , wherein said step of activating comprises the steps of:

determining at least one sensing setting for at least one of said sensing elements based on a value of said finger detection signal indicating a capacitive coupling stronger than said second capacitive coupling; and

sensing said portion of the fingerprint pattern using said at least one sensing setting.

11. The method according to claim 1 , wherein said step of activating comprises the steps of:

activating, when said finger detection signal changes from said first value to said second value, a subset of said sensing elements;

evaluating a partial image resulting from said subset; and

activating, when the evaluation of said partial image indicates that a finger can be sensed by said fingerprint sensing device, all sensing elements in said array of sensing elements to sense said fingerprint pattern.

12. A fingerprint sensing device for sensing a fingerprint pattern of a finger, said fingerprint sensing device comprising:

an array of sensing elements for sensing said fingerprint pattern;

an electrically conductive finger detecting structure;

finger detecting circuitry connected to said finger detecting structure for providing a finger detection signal indicative of a capacitive coupling between said finger detecting structure and said finger; and

operation control circuitry connected to said sensing elements and to said finger detecting circuitry for:

comparing said finger detection signal with a first threshold value indicating a first capacitive coupling between said finger detecting structure and said finger, and a second threshold value indicating a second capacitive coupling between said finger detecting structure and said finger, stronger than said first capacitive coupling; and

activating, when said finger detection signal changes from a first value indicating a capacitive coupling weaker than said first capacitive coupling to a second value indicating a capacitive coupling stronger than said second capacitive coupling, at least a subset of said sensing elements to sense said fingerprint pattern.

13. The fingerprint sensing device according to claim 12 , comprising at least a first and a second electrically conductive finger detecting structure, each being connected to said finger detecting circuitry.

14. The fingerprint sensing device according to claim 13 , wherein said array of sensing elements is arranged between said first and said second conductive finger detecting structure.

15. The fingerprint sensing device according to claim 12 , wherein said operation control circuitry comprises:

finger detection evaluation circuitry; and

image acquisition control circuitry,

wherein said finger detection evaluation circuitry is connected to said finger detecting circuitry and to said image acquisition control circuitry for:

receiving said finger detection signal;

comparing said finger detection signal with the first threshold value indicating said first capacitive coupling and the second threshold value indicating said second capacitive coupling;

providing, when said finger detection signal changes from a first value indicating a capacitive coupling weaker than said first capacitive coupling to a second value indicating a capacitive coupling stronger than said second capacitive coupling, a signal to said image acquisition control circuitry indicating that said finger has been detected; and

wherein said image acquisition control circuitry is connected to said finger detection evaluation circuitry and to said sensing elements for:

receiving, from the finger detection evaluation circuitry, said signal indicating that said finger has been detected; and

controlling at least a subset of said sensing elements to acquire a representation of at least a part of said fingerprint pattern.