Circuit arrangement and method for operating a fingerprint sensor
View Patent ↗A circuit and method are provided for ensuring that a fingerprint sensor is started automatically when a finger is rested thereon such that a sufficiently contrasting image can be produced. A difference between a maximum and a minimum value of the sensor signals is generated wherein, if the difference is sufficiently large, indicating a sufficient contrast of an image to be produced, a normal scanning operation of the fingerprint sensor is initiated, thus ensuring that the complete fingerprint image is produced, a normal scanning operation of the fingerprint sensor is initiated, thus ensuring that the complete fingerprint images produced are of satisfactory quality.
1. A circuit for operating a fingerprint sensor, comprising:
a circuit portion for detecting sensor signals of individual pixels and for controlling a sequence in which the sensor signals are detected;
a first comparing element for determining whether a supplied sensor signal is greater than a stored first value;
a second comparing element for determining whether the supplied sensor signal is smaller than a stored second value;
a first memory register for storing the first value;
a second memory register for storing the second value;
a first gate circuit for causing, if the supplied sensor signal has a larger value than a value stored in the first memory register, the larger value to be stored in the first memory register;
a second gate circuit for causing, if the supplied sensor signal has a smaller value than a value stored in the second memory register, the smaller value to be stored in the second memory register;
a subtracting element in which a difference between the values stored in the first and second memory registers is formed; and
a third comparing element for comparing the difference with a prescribed value and for initiating production of a fingerprint image when the difference is greater than the prescribed value.
2. A method for operating a fingerprint sensor, the method comprising the steps of:
generating a sensor signal for each pixel of a previously selected group of pixels;
determining if each of the sensor signals are greater than a maximum value or smaller than a minimum value;
repeating the steps of generating and determining in repeated runs, with account being taken in the step of determining only of values of a run just performed; and
initiating production of a fingerprint image when a difference between the maximum and minimum values overshoots a prescribed value.
3. A method for operating a fingerprint sensor as claimed in claim 2 , wherein the step of generating includes determining the sensor signals relating to the pixels in each case in a permanently prescribed sequence, wherein the step of determining includes storing a respective current maximum value and a respective current minimum value, and wherein each newly determined sensor signal is compared with the stored values and, upon undershooting or overshooting one of the stored values, the current sensor signal is stored so as to replace a respectively associated stored value.
4. A method for operating a fingerprint sensor as claimed in claim 3 , wherein there are determined for a matrix-like arrangement of pixels the sensor signals relating to pixels of one of a row, a column and a diagonal in a sequence corresponding to the arrangement of the pixels in the respective row, column and diagonal.