Fingerprint sensor with IO voltage determination circuit

Abstract

A fingerprint sensor comprising a fingerprint sensing circuit configured to sense a fingerprint of a finger and provide a representation of the fingerprint; a sensor interface coupled to the fingerprint sensing circuit; a fingerprint sensor includes a fingerprint sensor and includes at least a first input pad for inputting at least a first digital signal to the fingerprint sensor and at least one output pad for outputting a digital data signal encoding a representation of a fingerprint from the fingerprint sensor; and an IO voltage determination circuit coupled to the first input pad and configured to sense a voltage level of the first digital signal and determine a nominal voltage level of the digital data signal to be used for encoding the representation of the fingerprint based on the sensed voltage level of the first digital signal.

Description

Technical Field

[0001] The present invention relates to a fingerprint sensor and to an electronic device comprising the fingerprint sensor. Background Art

[0002] Biometric systems are widely used as a means for improving the convenience and security of personal electronic devices, such as mobile phones, etc. In particular, fingerprint sensors are now included in most of all newly released personal communication devices, such as mobile phones.

[0003] For the operation of the fingerprint sensing circuit and other circuits inside the fingerprint sensor, the fingerprint sensor is usually provided with a power supply voltage, which is the difference between the reference potential and the power supply potential. In existing electronic devices with fingerprint sensors, the fingerprint sensor is additionally provided with an IO power supply voltage, which is the difference between the reference potential and the IO power supply potential. The IO power supply voltage (IO power supply potential) determines how the fingerprint sensor interprets the digital signal supplied to the fingerprint sensor, and also defines the nominal voltage level used by the fingerprint sensor for digital data output by the fingerprint sensor (e.g., digital data encoding a representation of a fingerprint acquired by the fingerprint sensor).

[0004] According to another more flexible method for setting the IO supply voltage, the integrated circuit may include a dedicated input for receiving a signal indicating the desired IO supply voltage. Based on this signal, the programmable input / output circuit in the integrated circuit can generate the desired IO supply voltage internally. This method is described in US8179161.

[0005] A consequence of each of the two approaches outlined above is that dedicated bond pads are required, and another consequence is that specific requirements are imposed on the circuitry that communicates with the integrated circuit. Neither of these consequences is desirable, particularly for fingerprint sensors that have very limited available space for bond pads. Summary of the invention

[0006] In view of the above, an object of the present invention is to provide an improved fingerprint sensor, in particular a fingerprint sensor for determining an IO voltage of the improved fingerprint sensor.

[0007] According to the present invention, a fingerprint sensor is provided, which includes: a fingerprint sensing circuit, which is configured to sense a fingerprint of a finger and provide a representation of the fingerprint; a sensor interface, which is coupled to the fingerprint sensing circuit and includes at least a first input pad and at least one output pad, the first input pad being used to input at least a first digital signal for controlling the operation of the fingerprint sensor to the fingerprint sensor, and the at least one output pad being used to output a digital data signal encoding the representation of the fingerprint from the fingerprint sensor; and an IO voltage determination circuit, which is coupled to the first input pad and configured to sense a voltage level of the first digital signal, and determine a nominal voltage level of the digital data signal to be used to encode the representation of the fingerprint based on the voltage level of the sensed first digital signal.

[0008] The nominal voltage level of the digital data signal to be used to encode the representation of the fingerprint is the voltage level relative to the reference potential, which is taken to represent the logical high level of the digital data signal. Common nominal voltage levels currently used include, for example, 1.2V, 1.8V, 3.3V, etc.

[0009] The present invention is based on the recognition that the digital signal provided to the fingerprint sensor contains information about the nominal voltage level used by the circuit communicating with the fingerprint sensor. A further realization is that in substantially every meaningful use case, any output of data from the fingerprint sensor will be before the fingerprint sensor receives at least one digital signal for controlling the operation of the fingerprint sensor. Therefore, the inventors have recognized that the voltage level of an appropriate digital signal for controlling the operation of the fingerprint sensor can be sensed, and the nominal voltage level to be used for outputting the data can be determined based on the sensed voltage level. The digital signal for controlling the operation of the fingerprint sensor can be, for example, a data signal or a control signal containing instructions for the fingerprint sensor.

[0010] Thus, no pads dedicated to supplying or determining the IO voltage are required, and no requirement for explicit communication about the desired IO voltage / nominal voltage level is imposed. This simplifies the design and integration of the fingerprint sensor, and also provides for a more compact fingerprint sensor and / or an elongated fingerprint sensor with a large aspect ratio.

[0011] According to an embodiment, the determination of the nominal voltage level may be performed after activation / activation of the fingerprint sensor, and the determined nominal voltage level may be considered valid until the fingerprint sensor is powered off / deactivated. According to other embodiments, the voltage level of the first digital signal may be repeatedly sensed between activation / activation and power off / deactivation of the fingerprint sensor, and a new determination of the nominal voltage level may be performed each time the voltage level has been sensed. For example, a new determination of the nominal voltage level may be performed when preparing each predetermined or requested output of digital data from the fingerprint sensor.

[0012] In either case, the first input pad for inputting the first digital signal may be selected so that the first digital signal is at a logic high level prior to or at the beginning of a predetermined or requested period for outputting a digital data signal encoding a representation of the fingerprint from the fingerprint sensor.

[0013] This enables the correct nominal voltage to be determined before digital data is output from the fingerprint sensor, thereby reducing the risk of misinterpretation of the digital data, or potentially damage to other external circuitry connected to the sensor interface used to receive digital data from the fingerprint sensor.

[0014] Advantageously, the first digital signal may be one of a reset signal and an enable signal for enabling or selecting the fingerprint sensor for communication.

[0015] According to an embodiment, the IO voltage determination circuit may be configured to determine the nominal voltage level by selecting between a predetermined first nominal voltage level and a predetermined second nominal voltage level higher than the first nominal voltage level.

[0016] Furthermore, the sensor interface may advantageously be configured to control at least one output pad in the sensor interface to a logic low level, a tri-state level, or a predetermined first nominal voltage level before the IO voltage determination circuit has determined the nominal voltage level of the digital data signal to be used to encode the representation of the fingerprint.

[0017] According to various embodiments, the first digital signal can advantageously be selected as a low-active reset signal. "Low-active" is an established term that means that control of the first digital signal is achieved by a transition from a logic high level to a logic low level. In the context of embodiments of the present invention, this means that the expected information about the nominal voltage level to be used can be obtained and used before the circuit communicating with the fingerprint sensor requires any action of the fingerprint sensor. This reduces the risk of using an incorrect IO voltage (nominal voltage level) for the fingerprint sensor, which would otherwise result in incorrect interpretation of the output data and / or damage to the connected circuitry.

[0018] A fingerprint sensor according to an embodiment of the present invention may advantageously be included in an electronic device which also includes a device controller coupled to a sensor interface of the fingerprint sensor, the device controller being used to control the operation of the fingerprint sensor and to obtain digital data encoding a representation of the fingerprint from the fingerprint sensor. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] These and other aspects of the present invention will now be described in more detail with reference to the accompanying drawings showing example embodiments of the invention, in which:

[0020] Figure 1 is an illustration of an exemplary electronic device in the form of a mobile phone including a fingerprint sensor in accordance with an embodiment of the present invention;

[0021] Figure 2 is included in Figure 1 A partially opened perspective diagram of a fingerprint sensor in an electronic device; and

[0022] Figure 3 is a simplified schematic block diagram of a fingerprint sensor according to an example embodiment of the present invention. DETAILED DESCRIPTION

[0023] In this detailed description, various embodiments of fingerprint sensors according to the present invention are described primarily with reference to fingerprint sensor components including semiconductor-based capacitive fingerprint sensor integrated circuits (ICs). The finger receiving surface of the fingerprint sensor is primarily illustrated as a convex surface. It should be noted that the present invention as defined in the claims is not limited to any particular shape or configuration of the finger receiving surface. Furthermore, the fingerprint sensor need not be elongated, but may be any other shape, such as square or circular, etc. Furthermore, it should be understood that the mobile phone in the figure is only an example of an electronic device that may include a fingerprint sensor according to an embodiment of the present invention. Fingerprint sensors according to embodiments of the present invention may be advantageously included in many other electronic devices, including, for example, computers, electronic watches and other gadgets, as well as smart cards, etc.

[0024] Figure 1 Schematically shown is an electronic device, here in the form of a mobile phone 1, comprising a device housing 3 and an exemplary fingerprint sensor component 5. Figure 1 As can be seen, the device housing 3 has a raised portion 7 with an opening 9. The fingerprint sensor component 5 is arranged in the opening 9 and also presents a convex shape in this particular example configuration. The convex shape of the fingerprint sensor component 5 can substantially follow the convex shape of the raised portion 7 of the device housing 3, at least at the opening 9.

[0025] Figure 2 is included in Figure 1A partially opened perspective diagram of an exemplary fingerprint sensor component 5 in a mobile phone 1. Figure 2 The fingerprint sensor component 5 includes a substrate 11, a fingerprint sensor 13 and a dielectric material 15. The substrate 11 has a substrate top surface 17 and a substrate bottom surface 19. The substrate top surface 17 has a top surface conductor pattern, including Figure 2 The bonding pad 21 is visible in FIG. 1 , and (although in Figure 2 The substrate bottom surface 19 (not visible in the figure) may have a bottom conductor pattern, which may form a component conductor pattern for electrically connecting the fingerprint sensor component 5 to the circuit of the electronic device 1. The fingerprint sensor 13 has a top surface 23, a bottom surface 25, and a side surface 27 connecting the top surface 23 and the bottom surface 25. The top surface 23 includes a planar sensing surface 29. In this example configuration, the bottom surface 25 of the fingerprint sensor 13 is bonded to the substrate top surface 17 of the substrate 11. Figure 2 As schematically shown in FIG. 1 , the fingerprint sensor 13 further includes connection pads collectively indicated by reference numeral 31 , which embody a sensor interface of the fingerprint sensor 13 .

[0026] exist Figure 2 In the example configuration of the fingerprint sensor component 5 in FIG. 1 , the dielectric material 15 covers the sensing surface 29 and the side surface 27 of the fingerprint sensor 13, and a portion of the substrate top surface 17 of the substrate 11 that is not covered by the fingerprint sensor 13. Figure 2 As schematically shown in FIG. 1 , the dielectric material 15 presents a convex shape on the sensing surface 29 of the fingerprint sensor 13. The fingerprint sensor component 5 may optionally additionally include a colored coating 33 on top of the dielectric material 15.

[0027] Figure 3 is a simplified schematic block diagram of a fingerprint sensor 13 according to an example embodiment of the present invention. Figure 3 The fingerprint sensor 13 includes a fingerprint sensing circuit 35, a sensor interface 37 and an IO voltage determination circuit 39. Figure 3 As schematically shown in FIG. 1 , the fingerprint sensor 13 may optionally further include an IO voltage providing circuit 41 .

[0028] The fingerprint sensing circuit 35 is configured to sense the fingerprint of a finger. Various types of suitable fingerprint sensing circuits 35 are known to those skilled in the art of fingerprint sensing. Therefore, the fingerprint sensing circuit 35 will not be described in detail here.

[0029] The sensor interface 37 is coupled to the fingerprint sensing circuit 35 and includes at least a first input pad 43a and at least one output pad 45. The first input pad 43a is adapted to receive a first digital signal for controlling the operation of the fingerprint sensor 13, and the output pad 45 is adapted to output from the fingerprint sensor 13 digital data encoding a representation of the fingerprint provided by the fingerprint sensing circuit 35.

[0030] The IO voltage determination circuit 39 is coupled to the first input pad 43a and is configured to sense the voltage level of the first digital signal and determine a nominal voltage level of a digital data signal 49 to be used to encode a representation of a fingerprint based on the sensed voltage level of the first digital signal, which is output from the fingerprint sensor 13 via the output pad 45.

[0031] The IO voltage determination circuit may determine in various ways the nominal voltage level to be used for the digital output from the fingerprint sensor 13. Some examples are provided below, and based on these examples and the skilled person's knowledge in the art, the skilled person will be able to come up with alternative solutions as required.

[0032] According to one example configuration, the IO voltage determination circuit 39 may be configured to sense the voltage level of the first digital signal 47a during at least a predetermined time period before determining the nominal voltage level of the digital data signal 49 to be used to encode the representation of the fingerprint. For example, the voltage level of the first digital signal 47a may be repeatedly sampled, thereby generating a time series of voltage level samples, and the voltage samples acquired during at least the predetermined time period may be filtered to obtain a stable and reliable sensed voltage level. Various filtering techniques are known per se to those skilled in the art, and may include averaging, for example. The filtered sensed voltage level may then be compared to (one or more) predetermined thresholds to determine the nominal voltage level of the digital data signal 49 to be used to encode the representation of the fingerprint.

[0033] In an embodiment, the IO voltage determination circuit 39 may be configured to determine the nominal voltage level by selecting between a predetermined first nominal voltage level and a predetermined second nominal voltage level higher than the first nominal voltage level. A non-limiting example of such a predetermined first nominal voltage level may be 1.2V, and a non-limiting example of such a predetermined second nominal voltage level may be 1.8V.

[0034] In such an embodiment, the IO voltage determination circuit 39 may include a comparator ( Figure 3 (not explicitly shown in the figure), the comparator is configured to compare the sensed voltage level of the first digital control signal 43a with a predetermined threshold voltage that is higher than the first nominal voltage level and lower than the second nominal voltage level.

[0035] The sensed voltage level for comparison may be the sensed voltage level for filtering described above. Alternatively or in combination, the IO voltage determination circuit 39 may be configured to select one of the first nominal voltage level and the second nominal voltage level only after at least two sequential comparisons by the comparator (or two sequential samplings of the analog comparator output signal) provide the same comparison result. In practice, this is also a filtering process, but integrated in the comparison step.

[0036] To reduce the risk of the fingerprint sensor 13 using an incorrect nominal voltage level which may have undesirable results, the first digital signal 47a may be advantageously selected to be a signal that can be expected to be at a logic high level when the fingerprint sensor is operating. An advantageous example of such a signal is a low active reset signal, which may be denoted as RST_N. Because the RST_N signal changes from a logic low level to a logic high level to release the reset of the fingerprint sensor 13, and only changes to a logic low level to reset the fingerprint sensor 13, the logic high level used by the RST_N signal may be sampled and / or compared at any time after the fingerprint sensor 13 is activated (released from reset).

[0037] In an example embodiment, for example Figure 3 In the example embodiment of the sensor interface, the sensor interface may include the above-mentioned reset input 47a and an SPI (serial peripheral interface) interface, which is well known to those skilled in the art. Therefore, the first digital control signal 47a may advantageously be a RST_N signal or a CS_N signal (in typical terminology of an SPI interface).

[0038] In order to further improve the accuracy of determining the nominal voltage level to be used, the sensor interface 37 may also optionally include a second control input pad 43b for inputting a second digital control signal 47b to the fingerprint sensor 13, and the IO voltage determination circuit 39 may also optionally be configured to sense the voltage level of the second digital control signal 47b, and further determine the nominal voltage level to be used for the digital data signal 49 based on the sensed voltage level of the second digital control signal 47b. The further description above regarding the sensing of the voltage level of the first digital control signal 47a naturally also applies to the sensing of the voltage level of the second digital control signal 47b. For example, the first digital control signal 47a may be the above-mentioned RST-N signal, and the second digital control signal 47b may be the above-mentioned CS_N signal, or the first digital control signal 47a may be the above-mentioned CS_N signal, and the second digital control signal 47b may be the above-mentioned RST-N signal.

[0039] In order to reduce the risk of outputting incorrect data and / or damaging connected circuits, etc., the sensor interface 37 can be configured to control at least one output pad 45 in the sensor interface 17 to a logic low level or a tri-state level or the above-mentioned predetermined first nominal voltage level before the IO voltage determination circuit has determined the nominal voltage level of the digital data signal to be used to encode the representation of the fingerprint.

[0040] like Figure 3 As schematically shown by the dashed arrow 51 from the IO voltage determination circuit 39 to the sensor interface 37, the IO voltage determination circuit 39 may control the sensor interface 37 according to the determined nominal voltage level. Specifically, the sensor interface 37 may be configured to convert the first digital signal 47a (and optionally any other digital signal) into a logic value for controlling the fingerprint sensor 13 based on the nominal voltage level determined by the IO voltage determination circuit 39.

[0041] Alternatively or in combination, the fingerprint sensing circuit 35 may be controllable to convert the received digital signal into a logic value and / or to use a nominal voltage determined by the IO voltage determination circuit 39 for the digital data signal 49 encoding a representation of the fingerprint. Figure 3 As schematically shown by the dashed arrow 53 from the IO voltage determination circuit 39 to the fingerprint sensing circuit 35, the IO voltage determination circuit 39 may provide the determined nominal voltage level to the fingerprint sensing circuit 35. Alternatively, the nominal voltage level may be provided to the fingerprint sensing circuit 35 via the sensor interface 37, as shown in FIG. Figure 3 This is schematically shown by a dashed arrow 55 from the sensor interface 37 to the fingerprint sensing circuit 35 .

[0042] Furthermore, according to an embodiment, the sensor interface 37 may include a first power supply pad 57a for receiving a reference potential and a second power supply pad 57b for receiving a power supply potential, the difference between the power supply potential and the reference potential being the power supply voltage of the fingerprint sensor 13, and the fingerprint sensor 13 may also include the above-mentioned optional IO voltage providing circuit 41 coupled to the IO voltage determination circuit 39 and to the first power supply pad 57a and the second power supply pad 57b. The IO voltage providing circuit 41 may be configured to provide a nominal voltage level of the digital data signal 49 to be used for encoding a representation of the fingerprint.

[0043] exist Figure 3 , the provision of the nominal voltage level from the IO providing circuit 41 to the sensor interface 37 is illustrated by dashed arrow 59. Alternatively or in combination, the nominal voltage level may be provided to the IO voltage determination circuit 39 and / or the fingerprint sensing circuit 35.

[0044] In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

Claims

1. A fingerprint sensor, comprising: fingerprint sensing circuitry configured to sense a fingerprint of a finger and provide a representation of the fingerprint; a sensor interface coupled to the fingerprint sensing circuit and comprising at least a first input pad for inputting at least a first digital signal for controlling the operation of the fingerprint sensor to the fingerprint sensor and at least one output pad for outputting a digital data signal encoding a representation of the fingerprint from the fingerprint sensor; as well as an IO voltage determination circuit coupled to the first input pad and configured to sense a voltage level of the first digital signal and determine a nominal voltage level of the digital data signal to be used to encode a representation of the fingerprint based on the sensed voltage level of the first digital signal, Wherein, the first input pad is selected so that the first digital signal is at a logic high level before or at the beginning of a period in which the digital data signal encoding a representation of the fingerprint is output from the fingerprint sensor.

2. The fingerprint sensor according to claim 1, wherein: The first digital signal is one of a reset signal and an enable signal.

3. The fingerprint sensor according to claim 1 or 2, wherein: The IO voltage determination circuit is configured to determine the nominal voltage level by selecting between a predetermined first nominal voltage level and a predetermined second nominal voltage level higher than the first nominal voltage level.

4. The fingerprint sensor according to claim 3, wherein: The IO voltage determination circuit includes a comparator configured to compare a voltage level of the sensed first digital signal with a predetermined threshold voltage that is higher than the first nominal voltage level and lower than the second nominal voltage level.

5. The fingerprint sensor according to claim 4, wherein: The IO voltage determination circuit is configured to select one of the first nominal voltage level and the second nominal voltage level only after at least two sequential comparisons performed by the comparator provide the same comparison result.

6. The fingerprint sensor according to any one of claims 3 to 5, wherein: The sensor interface is configured to control at least one output pad in the sensor interface to a logic low level, a tri-state level, or the predetermined first nominal voltage level before the IO voltage determination circuit has determined the nominal voltage level of the digital data signal to be used to encode the representation of the fingerprint.

7. A fingerprint sensor according to any one of the preceding claims, wherein: The first digital signal is an active low reset signal.

8. A fingerprint sensor according to any one of the preceding claims, wherein: The sensor interface is configured to convert the first digital signal into a logic value for control of the fingerprint sensor based on the nominal voltage level determined by the IO voltage determination circuit.

9. A fingerprint sensor according to any one of the preceding claims, wherein: The sensor interface further comprises a second input pad, wherein the second input pad is used to input a second digital signal to the fingerprint sensor; and The IO voltage determination circuit is configured to sense a voltage level of the second digital signal and determine the nominal voltage level to be used for the digital data signal based additionally on the sensed voltage level of the second digital signal.

10. The fingerprint sensor according to claim 9, wherein: The second digital signal is an enable signal.

11. A fingerprint sensor according to any one of the preceding claims, wherein: The sensor interface further comprises a first power pad for receiving a reference potential and a second power pad for receiving a power potential, wherein the difference between the power potential and the reference potential is a power voltage of the fingerprint sensor; And the fingerprint sensor also includes: An IO voltage providing circuit is coupled to the IO voltage determination circuit, the first power supply pad, and the second power supply pad and is configured to provide the nominal voltage level of the digital data signal to be used to encode the representation of the fingerprint.

12. A fingerprint sensor according to any one of the preceding claims, wherein: The IO voltage determination circuit is configured to sense a voltage level of the first digital signal during at least a predetermined period of time before determining the nominal voltage level of the digital data signal to be used to encode a representation of the fingerprint.

13. A fingerprint sensor according to any one of the preceding claims, wherein: The sensor interface comprises an SPI interface and a reset signal input configured to receive an active low reset signal; and The first digital signal is the active low reset signal or the CS_N signal included in the SPI interface.

14. An electronic device, comprising: A fingerprint sensor according to any one of the preceding claims; as well as A device controller is coupled to the sensor interface of the fingerprint sensor for controlling the operation of the fingerprint sensor and for acquiring digital data encoding a representation of the fingerprint from the fingerprint sensor.

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