Touch detection circuit with detection of water

A touch detection circuit has a first terminal and a second terminal which are respectively coupled to a first electrode and a second electrode located adjacent to each other. The capacitance sensing circuit (i) senses first electrostatic capacitance formed by the first electrode in a space with a periphery including the second electrode, and (ii) senses, by a self-capacitance method, second electrostatic capacitance formed by the first electrode in a space with a periphery in a state where voltage of the second terminal is made to follow voltage of the first terminal. A signal processor detects water over the first electrode and the second electrode on the basis of a difference between the first electrostatic capacitance and the second electrostatic capacitance.

CROSS REFERENCE TO RELATED APPLICATIONS

The present invention claims priority under 35 U.S.C. § 119 to Japanese Application No. 2019-107232 filed Jun. 7, 2019, the entire content of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an electrostatic capacitance detection circuit.

2. Description of the Related Art

A touch input device is mounted as a user interface on a recent electronic apparatus such as a computer, a smartphone, a tablet terminal, a portable audio device, or the like. A touch pad, a pointing device, and the like are known as such touch input devices, and various kinds of input can be performed by making a finger or a stylus contact or be located adjacent to such a touch input device.

The touch input devices are roughly classified into: a resistive film system; and an electrostatic capacitance system. The electrostatic capacitance system detects presence/absence of user input and coordinates thereof by converting, into an electric signal, a change in electrostatic capacitance (hereinafter also simply referred to as capacitance) in accordance with the user input, in which the electrostatic capacitance is formed by a plurality of sensor electrodes.

An electrostatic capacitance detection method roughly is classified into: a self-capacitance method; and a mutual-capacitance method. The self-capacitance method provides extremely high sensitivity and can detect not only a touch but also adjacency of a finger. However, the self-capacitance method has problems that: adhesion of a water droplet cannot be distinguished from a touch; and a two-point touch cannot be detected. On the other hand, the mutual-capacitance method has advantages of being capable of detecting the two-point touch (or a multi-touch more than the two-point touch) and is hardly affected by a water droplet, whereas the mutual-capacitance method is inferior to the self-capacitance method in terms of detection sensitivity. Therefore, the self-capacitance method and the mutual-capacitance method are selected in accordance with its use, or both methods are used in combination.

SUMMARY OF THE INVENTION

The present invention is made in view of above-described situations, and an example of a general purpose of an embodiment of the present invention is to provide a touch detection circuit capable of detecting water.

One embodiment of the present invention relates to a touch detection circuit. A touch detection circuit includes: a first terminal to which a first electrode is to be coupled; a second terminal to which a second electrode located adjacent to the first electrode is to be coupled; a capacitance sensing circuit structured to (i) sense first electrostatic capacitance formed by the first electrode in a space with a periphery including the second electrode, and (ii) sense, by a self-capacitance method, second electrostatic capacitance formed by the first electrode in a space with a periphery in a state where voltage of the second terminal is made to follow voltage of the first terminal; and a signal processor structured to detect, on the basis of a difference between the first electrostatic capacitance and the second electrostatic capacitance, water over the first electrode and the second electrode.

Note that any combination of the aforementioned constituent elements or any wording of the present invention substituted mutually between a method, a device, and so forth may also be effective as an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

OUTLINE OF EMBODIMENT

One embodiment disclosed herein relates to a touch detection circuit. A touch detection circuit includes: a first terminal to which a first electrode is to be coupled; a second terminal to which a second electrode located adjacent to the first electrode is to be coupled; a capacitance sensing circuit structured to (i) sense first electrostatic capacitance formed by the first electrode in a space with a periphery including the second electrode, and (ii) sense, by a self-capacitance method, second electrostatic capacitance formed by the first electrode in a space with a periphery in a state where voltage of the second terminal is made to follow voltage of the first terminal; and a signal processor structured to detect, on the basis of a difference between the first electrostatic capacitance and the second electrostatic capacitance, water over the first electrode and the second electrode.

In a case where the water is present across the first electrode and the second electrode, electrostatic capacitance thereof is included in the first electrostatic capacitance but not included in the second electrostatic capacitance. Accordingly, presence/absence of the water can be determined by obtaining a difference between the first electrostatic capacitance and the second electrostatic capacitance.

The capacitance sensing circuit may sense the first electrostatic capacitance by the self-capacitance method. The capacitance sensing circuit may also sense the first electrostatic capacitance by a mutual-capacitance method.

The capacitance sensing circuit may further include a cancel circuit that drives the second terminal such that the voltage of the second electrode follows the voltage of the first electrode. The second electrostatic capacitance may be measured by the self-capacitance method in a state where the cancel circuit is enabled.

Depending on a configuration and a detection method of the capacitance sensing circuit, detection sensitivity for the first electrostatic capacitance may largely differ from detection sensitivity for the second electrostatic capacitance. In this case, the signal processor may correct an error between the sensitivity of the capacitance sensing circuit for the first electrostatic capacitance and the sensitivity of the capacitance sensing circuit for the second electrostatic capacitance.

The signal processor may determine, on the basis of the second electrostatic capacitance, presence/absence of a touch on the first electrode.

At the time of performing touch determination for a certain frame, the signal processor may generate a correction value on the basis of the second electrostatic capacitance measured in a past frame, correct at least one of second electrostatic capacitance of a current frame and a threshold value for touch detection by using the correction value, and determine presence/absence of a touch on the first electrode on the basis of a comparison result between the second electrostatic capacitance and the threshold value after the correction. Consequently, it is possible to cancel influence of electrostatic capacitance of the water over each of the electrodes.

A determination condition to determine presence/absence of a touch on the first electrode may be changed in accordance with a detection result of the water.

The determination condition to determine presence/absence of a touch on the first electrode for a certain frame may be changed in accordance with a detection result of the water in a frame prior to the certain frame.

The capacitance sensing circuit senses (iii) third electrostatic capacitance formed by the second electrode in a space with a periphery including the first electrode, and also (iv) senses, by the self-capacitance method, fourth electrostatic capacitance formed by the second electrode in a space with a periphery in a state where the voltage of the first terminal is made to follow the voltage of the second terminal, in addition to the first electrostatic capacitance and the second electrostatic capacitance. Consequently, touch input to the first electrode and touch input to the second electrode can be detected.

The touch detection circuit may further include a selector that is provided between the capacitance sensing circuit and a portion including the first terminal and the second terminal, and may switch a coupling relation of the capacitance sensing circuit between the first terminal and the second terminal.

EMBODIMENTS

Hereinafter, the present invention will be described with reference to the drawings on the basis of preferred embodiments. The same or equivalent constituent elements, members, and processing illustrated in the drawings will be denoted by the same reference signs, and repetition of the same description will be omitted as appropriate. Additionally, the embodiments are not intended to limit the invention but provided as the examples, and all of features and combinations thereof described in the embodiments are not necessarily essential to the invention.

In the present specification, “a state where a member A is coupled to a member B” includes not only a case where the member A is physically directly coupled to the member B but also a case where the member A and member B are indirectly coupled via another member that does not substantially affect an electric coupling state therebetween or does not impair functions and effects provided by coupling the members.

Similarly, “a state where a member C is provided between the member A and the member B” includes not only a case where the member A is directly coupled to the member C or the member B is directly coupled to the member C but also a case where the member A is indirectly coupled to the member C or the member B is indirectly coupled to the member C via another member that does not substantially affect an electric coupling state therebetween or does not impair functions and effects provided by coupling the members.

First Embodiment

FIG. 1is a block diagram of a touch input device100including a touch detection circuit200according to a first embodiment. The touch input device100is a user interface that detects touch operation (hereinafter, including adjacency) of a finger2of a user (or a stylus).

The touch input device100includes a panel110, a host processor120, and a touch detection circuit200. The panel110is a touch panel or a switch panel, and includes a first electrode E1(hereinafter referred to as a sense electrode ESNS) and a second electrode E2(hereinafter referred to as an auxiliary electrode EAUX). Preferably, the sense electrode ESNSand the auxiliary electrode EAUXare located adjacent to each other to an extent such that both of the electrodes contact a same water droplet or a same water accumulation when water adheres to a surface of the panel110. In the present embodiment, only presence/absence of a touch on (including adjacency to) the sense electrode ESNSis determined, and a touch on the auxiliary electrode EAUXis not to be detected.

The host processor120is a host controller that integrally controls an apparatus, a device, and a system where the touch input device100is mounted. The touch detection circuit200can transmit, to the host processor120, a state of the panel110, more specifically, presence/absence of input (adjacency) to the panel110and presence/absence of water adhesion.

The touch detection circuit200includes a first terminal P1(hereinafter referred to as a sense terminal PSNS) and a second terminal P2(hereinafter referred to as an auxiliary terminal PAUX). The sense terminal PSNSis coupled to the sense electrode ESNSthat is a touch sensing target, and the auxiliary terminal PAUXis coupled to the auxiliary electrode EAUX.

The touch detection circuit200includes a capacitance sensing circuit210, an A/D converter230, a signal processor250, a controller270, and an interface circuit290.

The capacitance sensing circuit210(i) senses first electrostatic capacitance Ca formed by the sense electrode ESNSin a space with a periphery including the auxiliary electrode EAUXand generates a first detection signal VCa indicating the first electrostatic capacitance Ca. Additionally, in a state where voltage Vy of the auxiliary terminal PAUXis made to follow voltage Vx of the sense terminal PSNS, the capacitance sensing circuit210(ii) senses, by the self-capacitance method, second electrostatic capacitance Cb formed by the sense electrode ESNSin a space with a periphery, and generates a second detection signal VCb indicating the second electrostatic capacitance Cb. The sensing of the first electrostatic capacitance Ca and the sensing of the second electrostatic capacitance Cb are performed in a time-sharing manner.

The A/D converter230converts the first detection signal VCa and the second detection signal VCb into a first digital signal DCa and a second digital signal DCb, respectively. The signal processor250determines presence/absence of a touch on the sense electrode ESNSon the basis of at least one of the first digital signal DCa and the second digital signal DCb. For example, when the second electrostatic capacitance Cb exceeds a threshold value THt for touch detection, the signal processor250may determine that a touch is made.

Additionally, the signal processor250determines presence/absence of water adhesion to the panel110on the basis of a difference between the first digital signal DCa and the second digital signal DCb, that is, a difference between the first electrostatic capacitance Ca and the second electrostatic capacitance Cb. Specifically, when a differential capacitance ΔC between the first electrostatic capacitance Ca and the second electrostatic capacitance Cb exceeds a threshold value THw for water detection, the signal processor250may determine that the water adheres.

Note that, depending on a configuration and a detection method of the capacitance sensing circuit210, detection sensitivity for the first electrostatic capacitance Ca may largely differ from detection sensitivity for the second electrostatic capacitance Cb. In this case, preferably, the signal processor250corrects at least one of the first digital signal DCa and the second digital signal DCb, and performs water detection and touch detection on the basis of the first digital signal DCa and the second digital signal DCb after the correction.

The interface circuit290is coupled to the host processor120. Not limited thereto, the interface circuit290includes, for example, an inter-integrated circuit (I2C) interface, a serial peripheral interface (SPI), or the like, and transmits a detection result of the signal processor250to the host processor120.

FIG. 2is a diagram illustrating an exemplary configuration of the capacitance sensing circuit210. The capacitance sensing circuit210includes a self-capacitance sensor211and a cancel circuit240. The self-capacitance sensor211is coupled to the sense electrode ESNSvia the sense terminal PSNSand detects, by the self-capacitance method, electrostatic capacitance Cs formed by the sense electrode ESNSin a space with a periphery thereof.

A circuit form of the self-capacitance sensor211is not particularly limited, and a known technology may be employed. During the sensing by the self-capacitance sensor211, the sense terminal PSNSand the sense electrode ESNShave substantially the same potential, and the voltage Vx of the sense terminal PSNSfluctuates.

The cancel circuit240has an output coupled to the auxiliary electrode EAUXvia the auxiliary terminal PAUX. The cancel circuit240can be switched between an enable state and a disable state under the control of the controller270. The cancel circuit240is switched to the disable state at the time of measuring the first electrostatic capacitance Ca, and switched to the enable state at the time of measuring the second electrostatic capacitance Cb.

The output of the cancel circuit240has high impedance in the disabled state. In this state, electrostatic capacitance Cs1detected by the self-capacitance sensor211includes: electrostatic capacitance Cf between the sense electrode ESNSand the finger2; and parasitic capacitance Cp between the sense electrode ESNSand the auxiliary electrode EAUX. Additionally, as described later, in a case where water adheres across the sense electrode ESNSand the auxiliary electrode EAUX, electrostatic capacitance Cw caused by the water is also included.
Cs1=Cf+Cp+Cw

This electrostatic capacitance Cs1corresponds to the above-described first electrostatic capacitance Ca.

In the enable state, the cancel circuit240changes the voltage Vy of the auxiliary terminal PAUXwhile following the voltage Vx of the sense terminal PSNS. Consequently, the voltage of the sense electrode ESNSand the voltage of the auxiliary electrode EAUXbecome equal while the sensing is performed by the self-capacitance sensor211. Therefore, a parasitic capacitance Cp component and an electrostatic capacitance Cw component formed by the sense electrode ESNSin a space with the auxiliary electrode EAUXbecome invisible to the self-capacitance sensor211, and electrostatic capacitance Cs2detected by the self-capacitance sensor211includes only a component (for example, an electrostatic capacitance Cf component in the space with the finger) other than these components.
Cs2=Cf

This electrostatic capacitance Cs2corresponds to the above-described second electrostatic capacitance Cb.

Thus, according to the capacitance sensing circuit210ofFIG. 2, the first electrostatic capacitance Ca and the second electrostatic capacitance Cb can be measured.

Subsequently, operation of the touch input device100will be described. First, detection in an ideal state will be described.FIGS. 3A to 3Dare diagrams each illustrating a relation between a panel state, the first electrostatic capacitance Ca, the second electrostatic capacitance Cb, and a differential capacitance ΔC therebetween (=Ca−Cb). A length of each bar represents a capacitance value, but a relative magnitude relation therebetween may be illustrated differently from actual one for easy understanding.

FIG. 3Aillustrates a state of having neither water nor adjacency of the finger2. The first electrostatic capacitance Ca includes the parasitic capacitance Cp between the electrodes ESNSand EAUX, and the second electrostatic capacitance Cb is zero. The differential capacitance ΔC between the first electrostatic capacitance Ca and the second electrostatic capacitance Cb is the parasitic capacitance Cp. In this example, the differential capacitance ΔC is smaller than the threshold value THw for water detection, and therefore, it is determined that there is no water.

FIG. 3Billustrates a state where the finger2is located in an adjacent manner in the state ofFIG. 3A. The first electrostatic capacitance Ca includes the parasitic capacitance Cp and the electrostatic capacitance Cf in a space with the finger, and the second electrostatic capacitance Cb includes the electrostatic capacitance in the space with the finger2. The differential capacitance ΔC between the first electrostatic capacitance Ca and the second electrostatic capacitance Cb is the parasitic capacitance Cp. In this example also, since the differential capacitance ΔC is smaller than the threshold value THw for water detection, it can be determined that there is no water. Additionally, since the second electrostatic capacitance Cb exceeds the threshold value THt for touch detection, it can be determined that a touch is made.

FIG. 3Cillustrates a case where water4adheres across the sense electrode ESNSand the auxiliary electrode EAUX. The first electrostatic capacitance Ca includes: the parasitic capacitance Cp between the sense electrode ESNSand the auxiliary electrode EAUX; and the electrostatic capacitance Cw of the water. The second electrostatic capacitance Cb is zero. The differential capacitance ΔC between the first electrostatic capacitance Ca and the second electrostatic capacitance Cb is the parasitic capacitance Cp+the electrostatic capacitance Cw. In this example, the differential capacitance ΔC exceeds the threshold value THw for water detection, and therefore, it can be determined that the water4is in an adhering state. Additionally, since the second electrostatic capacitance Cb is smaller than the threshold value THt for touch detection, it can be determined that no touch is made.

FIG. 3Dillustrates a state where the finger2is located in an adjacent manner in the state ofFIG. 3C. The first electrostatic capacitance Ca includes the parasitic capacitance Cp, the electrostatic capacitance Cw, and the electrostatic capacitance Cf. The second electrostatic capacitance Cb includes the electrostatic capacitance Cf. The differential capacitance ΔC includes the parasitic capacitance Cp and the electrostatic capacitance Cw of the water. In this example, the differential capacitance ΔC exceeds the threshold value THw for water detection, and therefore, it can be determined that the water4is in the adhering state. Additionally, since the second electrostatic capacitance Cb exceeds the threshold value THt for touch detection, it can be determined that a touch is made.

The above is basic operation of the touch input device100. According to the touch input device100, the presence/absence of adhesion of the water4can be determined, and also the presence/absence of a touch on the sense electrode ESNScan be determined.

Next, a modified example related to the touch determination will be described.

First Modified Example

There is a case where sensitivity of finger detection may be changed depending on presence/absence of the water.FIGS. 4A and 4Bare diagrams schematically illustrating touch detection in the absence of water, andFIGS. 4C and 4Dare diagrams schematically illustrating touch detection in the presence of the water.

As illustrated inFIGS. 4A and 4B, in a case where the finger approaches to the dry sense electrode ESNS, a space between the finger and the sense electrode ESNSis air. Therefore, electrostatic capacitance CAIRof the space therebetween is extremely small. Therefore, the second electrostatic capacitance Cb does not exceed the threshold value THt for touch detection unless a distance between the finger2and the sense electrode ESNSbecomes considerably close.

Now, refer toFIGS. 4C and 4D. The water4has relative permittivity larger than that of the air. Therefore, as illustrated inFIG. 4C, in a case where the water4adheres to the sense electrode ESNS, when the finger2contacts the water4, the second electrostatic capacitance Cb exceeds the threshold value THt for touch detection even though the finger2does not contact the sense electrode ESNS, and it is determined that a touch is made. As illustrated inFIG. 4D, when the finger2contacts the sense electrode ESNS, the second electrostatic capacitance Cb is further increased. Thus, in the case where the water adheres, the detection sensitivity for the finger may be excessively high, compared to the case where the water does not adhere.

The detection sensitivity at the time of the water adhering has a correlation with an amount of the water over the sense electrode ESNS. Accordingly, in the first modified example, correction processing is performed in accordance with presence/absence of water adhesion, and a touch determination condition is changed. More preferably, in a case where it is determined that the water adheres (that is, in a case of ΔC>THw), the electrostatic capacitance of the water which currently adheres is estimated, and the correction processing based on the estimated value Cw{circumflex over ( )} is performed. As already described above, the differential capacitance ΔC between the first electrostatic capacitance Ca and the second electrostatic capacitance Cb has the correlation with the amount of the water, in other words, the electrostatic capacitance Cw of the water. Accordingly, the electrostatic capacitance of the water may be estimated by multiplying the differential capacitance ΔC by a predetermined coefficient k.
Cw{circumflex over ( )}=k×(Ca−Cb)

Alternatively, the estimated value Cw{circumflex over ( )} of the electrostatic capacitance of the water may be calculated by using a more complicated arbitrary function f (Ca−Cb).
Cw{circumflex over ( )}=f(Ca−Cb)

Alternatively, a relation between the differential capacitance ΔC and the estimated value Cw{circumflex over ( )} may be held in a table.

Some of the correction processing based on the estimated value of the electrostatic capacitance of the water will be described.

In first correction processing, the threshold value THt for touch detection is dynamically shifted on the basis of the estimated value Cw{circumflex over ( )}. The shifted threshold value THt′ is represented as follows.
THt′=THt+Cw{circumflex over ( )}

The first correction processing is illustrated inFIGS. 4C and 4D. The threshold value at the time of the water adhesion is represented as THt′. In the case where the water adheres, the threshold value THt′ is increased. Consequently, the case ofFIG. 4Ccan be determined as a non-touched state and the case ofFIG. 4Dcan be determined as a touched state. That is, the detection sensitivity can be made uniform between in the presence of the water and in the absence of the water.

In second correction processing, influence of the water4is reduced by subtracting the estimated value Cw{circumflex over ( )} from the second electrostatic capacitance Cb obtained by measurement, and corrected capacitance Cb′ is compared with the fixed threshold value THt for touch detection.
Cb′=Cb−Cw{circumflex over ( )}

Those skilled in the art understand that the first correction processing and the second correction processing are equivalent. That is, at least one of the electrostatic capacitance Cb and the threshold value THt is corrected on the basis of the estimated value Cw{circumflex over ( )} of the electrostatic capacitance of the water, and the touch determination is performed on the basis of a comparison result between the electrostatic capacitance Cb and the threshold value THt after the correction.

Second Modified Example

As described above, the touch input device100according to the present embodiment can determine presence/absence of the water. However, such water detection is accurate when the finger2is in the non-touched state, but when the finger2is moved adjacent to the panel110(the sense electrode ESNS) to a certain extent, the water can be hardly detected. Transition when the finger is moved adjacent from a distance will be described with reference toFIGS. 5A to 5C.FIG. 5Ais a diagram illustrating a state where the finger2is located extremely distant,FIG. 5Bis a diagram illustrating a state where the finger approaches, andFIG. 5Cis a diagram illustrating a touched state of the finger.

FIG. 5Aillustrates the state where the water4adheres to the panel, and corresponds toFIG. 3C. At this time, the water can be detected on the basis of the differential capacitance ΔC.

FIG. 5Billustrates a process in which the finger2approaches to the panel to which the water4adheres. In this state, detection sensitivity may be different between at the time of measuring the first electrostatic capacitance Ca and at the time of measuring the second electrostatic capacitance Cb. In this example, a measurement value of the electrostatic capacitance Cf component of the finger2is relatively large at the time of measuring the second electrostatic capacitance Cb, but the measurement value is relatively small at the time of measuring the first electrostatic capacitance Ca. As a result, the differential capacitance Ca−Cb becomes lower than the threshold value THw for water detection, and the water4cannot be detected. Since the second electrostatic capacitance Cb is lower than threshold value THt for water detection, no touch is detected.

FIG. 5Cillustrates the state where the finger2is in contact with the sense electrode ESNS. In this state also, a measurement value of the electrostatic capacitance Cf component of the finger2is small at the time of measuring the first electrostatic capacitance Ca, and the measurement value of the electrostatic capacitance Cf component of the finger2is large at the time of measuring the second electrostatic capacitance Cb. Therefore, the differential capacitance Ca−Cb becomes lower than the threshold value THw for water detection, and the water4cannot be detected. Since the second electrostatic capacitance Cb is larger than threshold value THt for touch detection, a touch can be detected.

Thus, the presence of the water4can be detected in the state ofFIG. 5A, but when the finger2approaches to the panel, the touch input device100misses this presence of the water. As described in the first and second modified examples, in a case of reflecting presence/absence of the water to a determination criterion for touch detection (the threshold value THt′ for touch detection), touch detection accuracy is degraded when the presence/absence of the water is erroneously determined.

In the second modified example, the above-described problem is prevented as follows. Water adhesion and finger approach seldom occur at the same time, and in many cases, the water first adheres to the panel, and then the finger approaches to the panel and finally contacts the same. That is, direct transition from the state ofFIG. 3Cto the state ofFIG. 5Bis practically impossible, and normally, the state ofFIG. 3Ctransitions to the state ofFIG. 5Bvia the state ofFIG. 5A. This principle is preferably incorporated in algorithms of the water detection and the touch detection in the signal processor250.

Assume that the touch input device100detects the first electrostatic capacitance Ca and the second electrostatic capacitance Cb at a predetermined frame rate. At this time, it is preferable to determine, for a certain frame, the determination condition to determine presence/absence of a touch (for example, the threshold value THt for touch detection) is determined in accordance with a determination result on presence/absence of the water and an amount of the water obtained in a frame prior to the certain frame (for example, several frames before).

The water is accurately detected in the certain frame in the state ofFIG. 5A, and the states ofFIGS. 5B and 5Care determined in subsequent frames thereto. In the frames where the states ofFIGS. 5B and 5Care sensed, the detected state in the past frame at the time of sensing the state ofFIG. 5Ais referenced. Therefore, the processing is executed while assuming that the water4is present.

FIGS. 6A and 6Bare diagrams to describe correction processing utilizing a past frame. The first electrostatic capacitance Ca and the second electrostatic capacitance Cb obtained in an ith(i=1, 2 . . . ) frame are represented as Caiand Cbi.

FIG. 6Aillustrates the ithframe in the past, and since Cai−Cbi>THw is satisfied, the water4is detected. A value of a differential capacitance ΔCi=Cai−Cbiat this time or an estimated value Cw{circumflex over ( )}iof the electrostatic capacitance of the water obtained from the value are retained for several frames without being discarded.

FIG. 6Billustrates a jthframe located a few frames afterFIG. 6A. A difference in the differential capacitance ΔCjis smaller than the threshold value THw for water detection due to the approach of the finger2, but the detection result of the ithframe in the past is referenced and the processing is performed assuming that the water4is present. Therefore, there is no differential capacitance ΔCj(or no estimated value Cw{circumflex over ( )}j) of the current frame, and calculation is performed on the basis of the retained differential capacitance ΔCi(or the estimated value Cw{circumflex over ( )}i) of the past frame.

Thus, by referring to the detection result of the past frame, the presence/absence of the touch of the finger2can be correctly determined in consideration of the amount of the water4.

Subsequently, implementation examples of the capacitance sensing circuit210will be described.

FIG. 7is a circuit diagram illustrating a first implementation example (210A) of the capacitance sensing circuit210. The capacitance sensing circuit210A includes a capacitance sensor211A and a cancel circuit240A. The capacitance sensor211A includes a plurality of switches SW21to SW26, an operational amplifier212, a reference capacitor Cref, and a feedback capacitor Cfb. The reference capacitor Cref has one end grounded. The reference capacitor Cref has the other end coupled to the sense terminal PSNSvia the charge transfer switch SW25, and is coupled to an inverting input terminal (−) of the operational amplifier212via the amplification switch SW26.

The charge transfer switch SW25, the amplification switch SW26, the reference capacitor Cref, the feedback capacitor Cfb, and the operational amplifier212form an integrator218using a switched capacitor. Reference voltage Vref is applied to a non-inverting input terminal (+) of the operational amplifier212, and the feedback capacitor Cfb is provided between an output of the operational amplifier212and the inverting input terminal (−).

A pair of the upper switch SW21and the lower switch SW22forms a first driver214, and changes the voltage Vx of the sense terminal PSNSin a range between two values of power supply voltage Vdd and ground voltage 0 V.

A pair of the upper switch SW23and the lower switch SW24forms a second driver216, and changes voltage Vi of the reference capacitor Cref in a range between the two values of the power supply voltage Vdd and the ground voltage 0 V.

The switches SW21to SW26are controlled by the controller270. The controller270may be a part of the signal processor250. It is preferable to satisfy Vref=Vdd/2. An initialization switch (not illustrated) may also be provided in parallel to the feedback capacitor Cfb.

During a drive period (i), the capacitance sensor211A applies one of the power supply voltage Vdd and the ground voltage 0 V to the sense terminal PSNSand applies the other one of the power supply voltage Vdd and the ground voltage 0 V to the reference capacitor Cref in a state where the charge transfer switch SW25is turned off and the sense terminal PSNSis disconnected from the reference capacitor Cref.

During a subsequent sensing period, the capacitance sensor211A has only the charge transfer switch SW25turned on, and the sense terminal PSNSis coupled to the reference capacitor Cref. As a result, electric charge is transferred between the electrostatic capacitance Cs and the reference capacitor Cref. Assuming that the power supply voltage Vdd is applied to the sense terminal PSNSand the ground voltage 0 V is applied to the reference capacitor Cref during the drive period immediately before, following Expressions are established from the principle of electric charge conservation.
Cs×Vdd=Vi×(Cs+Cref)  (1)
Vi=Vdd×Cs/(Cs+Cref)  (2)

Vi represents voltage of the reference capacitor Cref after completion of the electric charge transfer. Provided that Cs=Cref, Vi=Vdd/2 is satisfied.

During a subsequent amplification period, the amplification switch SW26is turned on. As a result, the feedback capacitor Cfb is charged such that the voltage of the inverting input terminal (−) of the operational amplifier212becomes the reference voltage Vref, and it is possible to obtain detection voltage Vs as follows.
Vs=Vref−Cref/Cfb×(Vi−Vref)  (3)

It is possible to find, from the Expressions (2) and (3), that the detection voltage Vs depends on the electrostatic capacitance Cs.

The drive auxiliary circuit244of the cancel circuit240A includes a first switch SW11and a second switch SW12. The first switch SW11is provided between the auxiliary terminal PAUXand a power supply line, and the second switch SW12is provided between the auxiliary terminal PAUXand a ground line. The first switch SW11is turned on in conjunction with the upper switch SW21of the first driver214, and pulls up the voltage Vy of the auxiliary terminal PAUXto the power supply voltage Vdd. Furthermore, the second switch SW12is turned on in conjunction with the lower switch SW22of the first driver214, and pulls down the voltage Vy of the auxiliary terminal PAUXto the ground voltage 0 V.

In the cancel circuit240A, the switches SW11to SW13are each fixed to an off state in the disabled state.

FIG. 8is an operating waveform diagram of the capacitance sensor211A inFIG. 7. During a drive period T1, the upper switch SW21and the lower switch SW24are turned on, the power supply voltage Vdd is applied to the sense terminal PSNS, and the ground voltage 0 V is applied to the reference capacitor Cref. During a subsequent transfer period T2, the charge transfer switch SW25is turned on, and electric charge of the electrostatic capacitance Cs and electric charge of the reference capacitor Cref are averaged. The voltage Vi of the reference capacitor Cref is represented by a following Expression.
Vi=Vdd×Cs/(Cs+Cref)

During a subsequent amplification period T3, the charge transfer switch SW25is turned off, and the voltage Vi is held. When the amplification switch SW26is turned on, the detection voltage Vs is generated.

During a subsequent drive period T4, the lower switch SW22and the upper switch SW23are turned on, the ground voltage 0 V is applied to the sense terminal PSNS, and the power supply voltage Vdd is applied to the reference capacitor Cref. During a subsequent transfer period T5, the charge transfer switch SW25is turned on, and the electric charge of the electrostatic capacitance Cs and the electric charge of the reference capacitor Cref are averaged.
Vi=Vdd×Cref/(Cs+Cref)

During a subsequent amplification period T6, the charge transfer switch SW25is turned off and the voltage Vi is held. When the amplification switch SW26is turned on, the detection voltage Vs is generated.

FIG. 9is a diagram to describe sensing of the second electrostatic capacitance Cb by the capacitance sensing circuit210A. During the drive period T1, the voltage Vx of the sense terminal PSNSrises to the power supply voltage Vdd. Simultaneously, the voltage Vy of the auxiliary terminal PAUXrises to the power supply voltage Vdd while following the voltage Vx by turning on the first switch SW11.

During the transfer period T2and the amplification period T3, the third switch SW13is turned on, and the auxiliary terminal PAUXis coupled to an output of a buffer242. As a result, the voltage Vy of the auxiliary terminal PAUXis made equal to the voltage Vx of the sense terminal PSNSby the buffer242.

During the drive period T4, the voltage Vx of the sense terminal PSNSdrops to the ground voltage 0 V. Simultaneously, the voltage Vy of the auxiliary terminal PAUXdrops to the ground voltage 0 V while following the voltage Vx by turning on the second switch SW12.

During the transfer period T5and the amplification period T6, the third switch SW13is turned on, and the auxiliary terminal PAUXis coupled to the output of the buffer242. As a result, the voltage Vy of the auxiliary terminal PAUXis made equal to the voltage Vx of the sense terminal PSNSby the buffer242.

The above is the operation of the capacitance sensing circuit210A. According to this capacitance sensing circuit210A, the voltage Vy of the auxiliary terminal PAUXcan be made to follow the voltage Vx of the sense terminal PSNSwith a high speed, influence of the parasitic capacitance Cp and the electrostatic capacitance Cw between the sense electrode ESNSand the auxiliary electrode EAUXcan be canceled, and the second electrostatic capacitance Cb can be measured.

In start timing of the drive period T1, the voltage Vy can be rapidly raised by the drive auxiliary circuit244instead of by the buffer242. Additionally, in start timing of the drive period T4, the voltage Vy can be rapidly made to drop by the drive auxiliary circuit244instead of by the buffer242.

FIG. 10is a diagram illustrating a modified example of an operation sequence of the capacitance sensing circuit210A. In this modified example, a simultaneous ON period of the first switch SW11and the second switch SW12is provided immediately after transition from the drive period T1to the transfer period T2. When ON resistance is equal between the first switch SW11and the second switch SW12, the voltage Vy of the auxiliary terminal PAUXinstantly drops to middle-point voltage between the power supply voltage Vdd and 0 V (that is, the reference voltage Vref) by the drive auxiliary circuit244. Then, when both the first switch SW11and the second switch SW12are turned off, the voltage Vy of the auxiliary terminal PAUXis made equal to the voltage Vx of the sense terminal PSNSby the buffer242.

Similarly, a simultaneous ON period for the first switch SW11and the second switch SW12is also provided immediately after transition from the drive period T4to the transfer period T5. Consequently, the voltage Vy of the auxiliary terminal PAUXinstantly rises to the middle-point voltage between the power supply voltage Vdd and 0 V (that is, the reference voltage Vref) by the drive auxiliary circuit244. Then, when both the first switch SW11and the second switch SW12are turned off, the voltage Vy of the auxiliary terminal PAUXis made equal to the voltage Vx of the sense terminal PSNSby the buffer242.

According to this modified example, the voltage Vy can be rapidly made to drop at end timing of the drive period T1by the drive auxiliary circuit244instead of by the buffer242. Additionally, the voltage Vy can be rapidly made to rise also at end timing of the drive period T4by the drive auxiliary circuit244instead of by the buffer242. Consequently, drive capacity required in the buffer242can be further reduced, and the circuit area and power consumption can be reduced.

FIG. 11is a circuit diagram illustrating a second implementation example (210B) of the capacitance sensing circuit210. A cancel circuit240B includes a bias circuit246in addition to the cancel circuit240A ofFIG. 7. The bias circuit246supplies bias voltage Vbias to an input of the buffer242when the drive auxiliary circuit244is in the off state (an inactive state, that is, both SW11and SW12are turned off). The bias voltage Vbias is preferably made equal to or close to the reference voltage Vref.

The bias circuit246includes a fourth switch SW14, a fifth switch SW15, and a voltage source248. For example, when Vbias=Vdd/2, the voltage source248may include a resistance voltage dividing circuit that divides the power supply voltage Vdd at a voltage division ratio of ½. The fourth switch SW14is provided between the input of the buffer242and the sense terminal PSNS. The fifth switch SW15is provided between the input of the buffer242and the voltage source248.

FIG. 12is an operating waveform diagram of the capacitance sensing circuit210B ofFIG. 11.FIG. 12illustrates output voltage Vz of the buffer242. During the drive periods T1and T4, the fourth switch SW14is turned off, and the fifth switch SW15is turned on. As a result, the output voltage Vz of the buffer242is kept at the bias voltage Vbias. During the transfer periods T2and T5and during the amplification periods T3and T6, the fourth switch SW14is turned on and the fifth switch SW15is turned off, and the output voltage Vz of the buffer242becomes equal to the voltage Vx.

Thus, according to the second implementation example, a fluctuation range of the output voltage Vz of the buffer242can be narrowed. Consequently, the drive capacity of the buffer242can be reduced, and the circuit area and power consumption can be further reduced.

FIG. 13is a circuit diagram of a third implementation example (210C) of the capacitance sensing circuit210. A bias circuit246C of a cancel circuit240C includes a sample hold circuit247. The sample hold circuit247samples and holds the voltage Vx of the sense terminal PSNSduring the transfer period T2(T5) and the amplification period T3(T6). The bias circuit246C outputs, as the bias voltage Vbias, the held voltage during the drive periods T1and T4, and outputs the voltage Vx of the sense terminal PSNSduring the transfer period T2(T5) and the amplification period T3(T6).

FIG. 14is a circuit diagram of a fourth implementation example (210E) of the capacitance sensing circuit210. A circuit form of the capacitance sensor211E differs from that of the capacitance sensor211A inFIG. 7. The capacitance sensor211E includes a reset switch SW41, a current mirror circuit274, and an integrator276.

The reset switch SW41is provided between the sense terminal PSNSand a ground line. The current mirror circuit274has an input-side transistor M41coupled to the sense terminal PSNS. The current mirror circuit274may include a sense switch SW42. The integrator276outputs detection voltage Vs obtained by integrating electric current Is flowing in an output-side transistor M42of the current mirror circuit274.

FIG. 15is an operating waveform diagram of the capacitance sensor211E ofFIG. 14. During a reset period T11, the reset switch SW41is turned on, 0 V is applied to the sense terminal PSNS, and the electrostatic capacitance Cs is discharged. Subsequently, when the sense switch SW42is turned on during a sense period T12, charging current ICHGstarts flowing in the input-side transistor of the current mirror circuit274, and the electrostatic capacitance Cs is charged by the charging current ICHG. Then, when the voltage Vx rises close to the power supply voltage Vdd, the input-side transistor M41of the current mirror circuit274cuts off the charging current, and the charging is stopped. A change width ΔV of the voltage Vx is substantially equal to that of the power supply voltage Vdd, and total electric charge Q flowing into the electrostatic capacitance Cs at this time is: Q=Cs×ΔV=Cs×Vdd.

The charging current ICHGis copied by the current mirror circuit274, and the copied current Is is integrated by the integrator276. A voltage change proportional to an amount of the total electric charge Q, in other words, proportional to the electrostatic capacitance Cs is generated in the detection voltage Vs.

Refer toFIG. 14again. The cancel circuit240E changes the voltage Vy of the auxiliary terminal PAUXwhile following the voltage Vx illustrated inFIG. 15. When the reset switch SW41is turned on, the voltage Vx is rapidly changed. Preferably, such a rapid change is caused by the drive auxiliary circuit244E, and a gentle change of the voltage Vx after turning on the sense switch SW42is caused by the buffer242. In this case, the drive auxiliary circuit244E can include the second switch SW12provided between the auxiliary terminal PAUXand the ground.

FIG. 16is an operating waveform diagram of the capacitance sensing circuit210E ofFIG. 14. During the reset period T11, the reset switch SW41is turned on, and the voltage Vy of the auxiliary terminal PAUXis pulled down to 0 V. Shifting to the sense period T12, the third switch SW13is turned on, and voltage Vy of the auxiliary terminal PAUXis driven by the buffer242so as to be equal to the voltage Vx.

In the capacitance sensing circuit210E ofFIG. 14, the bias circuit246can be added to the input side of the buffer242.

Second Embodiment

FIG. 17is a block diagram of a touch input device100including a touch detection circuit200F according to a second embodiment. A panel110includes a first electrode E1and a second electrode E2. The touch detection circuit200F includes: a first terminal P1coupled to the first electrode E1; and a second terminal P2coupled to the second electrode E2.

The touch detection circuit200F further includes a selector280in addition to a touch detection circuit200ofFIG. 1. The selector280is provided between a capacitance sensing circuit210and a portion including the first terminal P1and the second terminal P2. The selector280can switch a coupling relation of the capacitance sensing circuit210between the first terminal P1and the second terminal P2. That is, the capacitance sensing circuit210can perform switching between: a measurement mode in which touch input to the first electrode E1is to be detected; and a measurement mode in which touch input to the second electrode E2is to be detected.

The above is the configuration of the touch detection circuit200F. Subsequently, operation thereof will be described. First, the capacitance sensing circuit210measures first electrostatic capacitance Ca1and second electrostatic capacitance Cb1while using the first electrode E1as a sense electrode ESNSand the second electrode E2as an auxiliary electrode EAUX. Then, presence/absence of water and presence/absence of a touch are determined on the basis of the first electrostatic capacitance Ca1and the second electrostatic capacitance Cb1. A method relating to touch determination and correction may be similar to those of a first embodiment.

Similarly, the capacitance sensing circuit210measures third electrostatic capacitance Ca2and fourth electrostatic capacitance Cb2while using the second electrode E2as the sense electrode ESNSand the first electrode E1as the auxiliary electrode EAUX. The third electrostatic capacitance Ca2is electrostatic capacitance formed by the second electrode E2in a space with a periphery including the first electrode E1. The fourth electrostatic capacitance Cb2is electrostatic capacitance formed by the second electrode E2using a self-capacitance method in a space with a periphery thereof in a state where voltage of the first terminal P1is made to follow voltage of the second terminal P2, and also the fourth electrostatic capacitance Cb2is electrostatic capacitance excluding electrostatic capacitance in a space with the first electrode E1.

FIGS. 18A to 18Care diagrams to describe exemplary sensing by the touch detection circuit200F ofFIG. 17. Here, a first correction method is adopted as correction considering electrostatic capacitance of water.

FIG. 18Aillustrates a state where the first electrode E1is touched. First, the first electrostatic capacitance Ca1and the second electrostatic capacitance Cb1are measured while using the first electrode E1as the sense electrode ESNS. Since the second electrostatic capacitance Cb1exceeds a threshold value THt1′ for touch detection, the touch on the first electrode E1can be detected.

Subsequently, the third electrostatic capacitance Ca2and the fourth electrostatic capacitance Cb2are measured while using the second electrode E2as the sense electrode ESNS. Since the second electrostatic capacitance Cb2is substantially zero and smaller than a threshold value THt2′ for touch detection, a touch on the second electrode E2is not detected.

FIG. 18Billustrates a state where the second electrode E2is touched. First, the first electrostatic capacitance Ca1and the second electrostatic capacitance Cb1are measured while using the first electrode E1as the sense electrode ESNS. Since the second electrostatic capacitance Cb1is substantially zero and smaller than the threshold value THt1′ for touch detection, a touch on the first electrode E1is not detected.

Subsequently, the third electrostatic capacitance Ca2and the fourth electrostatic capacitance Cb2are measured while using the second electrode E2as the sense electrode ESNS. Since the fourth electrostatic capacitance Cb2exceeds the threshold value THt2′ for touch detection, a touch on the second electrode E2is detected.

FIG. 18Cillustrates a state where the first electrode E1and the second electrode E2are simultaneously touched (also referred to as multi-touch) with the two fingers2_1and2_2. In this case, both the second electrostatic capacitance Cb1and the fourth electrostatic capacitance Cb2exceed the threshold value THt′ for touch detection. Therefore, the multi-touch can be detected.

Thus, according to the touch input device100ofFIG. 17, touch input to the first electrode E1and touch input to the second electrode E2can be individually detected. Note that threshold values THt1′ and THt2′ are illustrated to be equal in this example, but in a case where an amount of adhering water is different, the first electrode E1and the second electrode E2may have different threshold values THt1′ and THt2′ for touch detection.

Note that, in the touch input device100ofFIG. 17, a single touch may be erroneously determined as a multi-touch, depending on the amount of the water over each of the electrodes.FIGS. 19A and 19Bare diagrams to describe such erroneous detection of the multi-touch.FIG. 19Aillustrates an example in which the water adheres to the electrodes E1and E2, and a large amount of water4adheres to the first electrode E1side. Additionally, a finger2touches the second electrode E2side.

FIG. 19Billustrates a combination of the electrostatic capacitance Cb1and the electrostatic capacitance Cb2which may be measured in the situation likeFIG. 19A. Here, second correction processing is adopted as correction based on an estimated value Cw{circumflex over ( )} of electrostatic capacitance of the water. That is, as for the first electrode E1side, an estimated value Cw{circumflex over ( )}1of the water is subtracted from the second electrostatic capacitance Cb1, and the corrected second electrostatic capacitance Cb1′ is compared with a fixed threshold value THt for touch detection. Similarly, as for the second electrode E2side, an estimated value Cw{circumflex over ( )}2of the water is subtracted from the fourth electrostatic capacitance Cb2, and corrected fourth electrostatic capacitance Cb2′ is compared with the fixed threshold value THt for touch detection.
Cb1′=Cb1−Cw{circumflex over ( )}1
Cb2′=Cb2−Cw{circumflex over ( )}2

In the case ofFIG. 19B, both the first electrostatic capacitance Cb1′ and the second electrostatic capacitance Cb2′ exceed the threshold value THt for touch detection. That is, a single touch is erroneously determined as a multi-touch.FIG. 20is a flowchart of touch determination to prevent such erroneous determination of the multi-touch.

First, determination is made on whether or not the amount of the water adhering to each of the electrodes E1and E2exceeds a predetermined amount (S100). In processing of S100, it may be possible to determine whether or not one of the estimated amounts Cw{circumflex over ( )}1and Cw{circumflex over ( )}2of the water at the electrodes E1and E2are larger than a predetermined threshold value THw2(larger than a threshold value THw for water detection).

Then, in a case where the amount of the water is less than the predetermined amount (N in S100), the processing shifts to touch determination in which a multi-touch is allowable (S102). Specifically, it is determined that:

(i) no touch is made in a case of Cb1′<THt and Cb2′<THt;

(ii) a single touch is made only on E1in a case of Cb1′>THt and Cb2′<THt;

(iii) a single touch is made only on E2in a case of Cb1′<THt and Cb2′>THt; and

(iv) a multi-touch is made on E1and E2in a case of Cb1′>THt and Cb2′>THt.

In the processing of S100, in a case where the amount of the water is larger than the predetermined amount (Y in S100), the processing shifts to touch determination in which the multi-touch is not allowable (S104).

Each of the first electrostatic capacitance Cb1′ and the second electrostatic capacitance Cb2′ is compared with the threshold value THt for touch detection (S106). In the cases of (i) to (iii), it is determined that a single touch is made under the same condition as that in the case where the multi-touch is allowable.

In a case of (iv) Cb1′>THt and Cb2′>THt, following determination is performed.

The determination is made on whether or not a difference |Cb1′−Cb2′| between the first electrostatic capacitance Cb1′ and the second electrostatic capacitance Cb2′ is larger than a predetermined threshold value CTH(S108). Then, in a case where the difference |Cb1′−Cb2′| is larger than the threshold value Cm (Y in S108), it is determined that a single touch is made on an electrode having a larger value among the first electrostatic capacitance Cb1′ and the second electrostatic capacitance Cb2(S110).

On the contrary, in a case where the difference |Cb1′−Cb2′| is smaller than the predetermined threshold value CTH(N in S106), it is determined that no touch is made on either of the electrodes E1and E2among the first electrostatic capacitance Cb1′ and the second electrostatic capacitance Cb2′ (S112).

FIGS. 21A to 21Care diagrams to describe determination based on the flowchart ofFIG. 20. An example ofFIG. 21Ais a case of Cb1′<Cth and Cb1′<Cth, in which the adhering amount of the water is determined to be less than the predetermined amount, and it is determined that a multi-touch is made.

Each of the examples ofFIGS. 21B and 21Cis a case of Cb1′>Cth, in which the adhering amount of the water is larger than the predetermined amount. InFIG. 21B, since the difference between the first electrostatic capacitance Cb1′ and the second electrostatic capacitance Cb2′ is large and Cb2′>Cb1′ is satisfied, it is determined that a single touch is made on the second electrode E2. InFIG. 21C, since the difference between the first electrostatic capacitance Cb1′ and the second electrostatic capacitance Cb2′ is small, it is determined that no touch is made.

The present invention has been described on the basis of the embodiments. Note that the embodiments are examples and it is understood by those skilled in the art that: various modified examples can be achieved by combining the respective constituent elements and the processes of the processing thereof; and such modified examples are also included in the scope of the present invention. Hereinafter, such modified examples will be described.

FIG. 22is a circuit diagram illustrating a modified example (210G) of the capacitance sensing circuit210. The capacitance sensing circuit210G further includes a mutual-capacitance sensor213. In this modified example, the first electrostatic capacitance Ca (or the third electrostatic capacitance Ca2) is measured by the mutual-capacitance sensor213. Also, the second electrostatic capacitance Cb (or the fourth electrostatic capacitance Cb2) is measured by the combination of the self-capacitance sensor211and the cancel circuit240.