Electrostatic capacity type touch sensor

This application claims priority from Japanese Patent Application No. 2010-33725, the content of which is incorporated herein by reference in its entirety.

1. Field of the Invention

This invention relates to an electrostatic capacity type touch sensor.

2. Description of the Related Art

The electrostatic capacity type touch sensor has been known as a data input device to various kinds of electronic devices such as a mobile phone, a portable audio device, a portable game console, a television and a personal computer.

The electrostatic capacity type touch sensor detects a touch by detecting a change in a capacitance of a capacitor associated with a touch pad induced by touching or approaching the touch pad with a finger of an operator or a pen tip (hereafter referred to as the finger of operator or the like).

Since an output value of the electrostatic capacity type touch sensor varies sensitively to its environment (temperature, humidity, electric field from its surrounding environment or the like), an offset adjustment to an input or an output of the sensor, which is referred to as calibration, is required in general.

Further information on the electrostatic capacity type touch sensor is disclosed in Japanese Patent Application Publication No. 2005-190950, for example.

It is preferable that the calibration is performed while the finger of operator or the like does not touch the touch pad. It is also preferable that the calibration is performed whenever the environment has changed even slightly.

However, while the change in the environment, to which the electrostatic capacity type touch sensor is vulnerable, occurs frequently in daily usage, the calibration take considerable amount of time. In addition, if the finger of operator or the like touches during the calibration, the situation is reflected in the calibration to make it wrong calibration.

Thus, this invention is directed to offer an electrostatic capacity type touch sensor that can be calibrated in a short period of time at a moment when the finger of operator or the like does not touch the touch pad.

This invention provides an electrostatic capacity type touch sensor having a sensor circuit detecting a change in a capacitance of a touch pad, a calibration register to adjust an offset in an output value of the sensor circuit, and a control circuit that makes the sensor circuit operate to obtain a first output value of the electrostatic capacity type touch sensor based on calibration data stored in the calibration register, modifies the calibration data depending on the first output value, makes the sensor circuit operate to obtain a second output value of the electrostatic capacity type touch sensor based on the modified calibration data, makes the sensor circuit operate to obtain a third output value of the electrostatic capacity type touch sensor based on the calibration data before the modification, and judges whether an offset in the second output value is larger than an offset in the third output value when a difference between the first output value and the third output value is smaller than a first threshold value.

FIG. 1 shows a structure of an electrostatic capacity type touch sensor according to an embodiment of this invention.

FIG. 2 shows a structure of a signal processing circuit of the electrostatic capacity type touch sensor according to the embodiment of this invention.

FIG. 3 shows a structure of a sensor circuit.

FIG. 4 shows a structure of a variable capacitor for calibration.

FIGS. 5A and 5B show operations of the sensor circuit.

FIG. 6 shows output waveforms of the sensor circuit.

FIG. 7 is an operation timing chart of the electrostatic capacity type touch sensor.

FIG. 8 is a flow chart showing a calibration control flow of the electrostatic capacity type touch sensor according to the embodiment of this invention.

FIGS. 9A , 9 B and 9 C show output waveforms of the electrostatic capacity type touch sensor according to the embodiment of this invention.

FIG. 10 shows a structure of a signal processing circuit of the electrostatic capacity type touch sensor according to the embodiment of this invention.

FIG. 11 shows a structure of a calibration control circuit.

An electrostatic capacity type touch sensor according to an embodiment of this invention is described referring to the drawings. The electrostatic capacity type touch sensor is formed to include a touch panel and a signal processing circuit. An example of an overall structure of the electrostatic capacity type touch sensor will be described first, and then proceedings of calibration control, which make features of this invention, will be described.

An electrostatic capacity type touch sensor 100 is formed to include a touch panel 1 , signal processing circuits 2 X and 2 Y and a microcomputer 3 , as shown in FIG. 1 .

The touch panel 1 has X sense lines XL 1 -XL 4 (an example of “touch pads” of this invention) and an X drive line DRXL extending in an X direction on a glass substrate 200 . The X drive line DRXL is disposed on both sides of each of the X sense lines XL 1 -XL 4 . The touch panel 1 also has Y sense lines YL 1 -YL 4 (an example of “touch pads” of this invention) and a Y drive line DRYL extending in a Y direction on the glass substrate 200 and intersecting the X sense lines XL 1 -XL 4 . The Y drive line DRYL is disposed on both sides of each of the Y sense lines YL 1 -YL 4 . The X sense lines XL 1 -XL 4 , the X drive line DRXL, the Y sense lines YL 1 -YL 4 and the Y drive line DRYL are electrically insulated from each other with a dielectric layer or the like.

The signal processing circuits 2 X and 2 Y are disposed adjacent the touch panel 1 on the glass substrate 200 . The signal processing circuit 2 X has first through fourth input terminals CIN 1 -CIN 4 and a drive terminal CDRV outputting an alternating current drive signal SCDRV (amplitude voltage Vref). The first input terminal CIN 1 is connected to the X sense line XL 1 , the second input terminal CIN 2 is connected to the X sense line XL 3 , the third input terminal CIN 3 is connected to the X sense line XL 2 and the fourth input terminal CIN 4 is connected to the X sense line XL 4 . The drive terminal CDRV is connected to the X drive line DRXL.

Similarly, the signal processing circuit 2 Y has first through fourth input terminals CIN 1 -CIN 4 and a drive terminal CDRV outputting an alternating current drive signal SCDRV (amplitude voltage Vref). The first input terminal CIN 1 is connected to the Y sense line YL 1 , the second input terminal CIN 2 is connected to the Y sense line YL 3 , the third input terminal CIN 3 is connected to the Y sense line YL 2 , and the fourth input terminal CIN 4 is connected to the Y sense line YL 4 . The drive terminal CDRV is connected to the Y drive line DRYL.

In addition, each of the signal processing circuits 2 X and 2 Y has a serial clock terminal SCL and a serial data terminal SDA. Both of the serial clock terminals SCL are connected to a serial clock line 4 , while both of the serial data terminals SDA are connected to a serial data line 5 . In this case, the serial clock line 4 and the serial data line 5 constitute an I 2 C (Inter-Integrated Circuit) bus.

A microcomputer 3 , that is a master device, is provided on a PCB substrate (not shown) outside the glass substrate 200 . The serial clock line 4 and the serial data line 5 are connected to the microcomputer 3 through an FPC (Flexible Printed Circuit) or the like. With the structure described above, it is made possible that data communication is performed between the microcomputer 3 and the signal processing circuits 2 X and 2 Y.

The calibration control, which makes the features of this invention, can be carried out by executing a program stored in the microcomputer 3 .

A detailed structure of each of the signal processing circuits 2 X and 2 Y in the electrostatic capacity type touch sensor is hereafter described referring to FIG. 2 . Since the structure of the signal processing circuit 2 X is identical to the structure of the signal processing circuit 2 Y, it is sufficient to describe only the structure of the signal processing circuit 2 Y.

As shown in FIG. 2 , the signal processing circuit 2 Y is formed to include a selection circuit 10 , a control circuit 11 , a drive circuit 12 that generates the alternating current drive signal SCDRV, an inverter 13 , a third electrostatic capacitor C 3 , a fourth electrostatic capacitor C 4 , a differential amplifier 14 , a first feedback capacitor (Cf) 15 , a second feedback capacitor (Cf) 16 , an AD converter 17 , an I 2 C bus interface circuit 18 , a calibration register 19 and an EEPROM 20 . The control circuit 11 controls overall operations (including the calibration control) of the signal processing circuit 2 Y based on commands from the microcomputer 3 .

The selection circuit 10 has a first phase and a second phase. In the first phase, it selects signals from the first input terminal CIN 1 and the second input terminal CIN 2 . That is, the first input terminal CIN 1 is connected to a non-inverting input terminal (+) of the differential amplifier 14 through a wiring 22 , and the second input terminal CIN 2 is connected to an inverting input terminal (−) of the differential amplifier 14 through a wiring 23 .

The selection circuit 10 selects signals from the third input terminal CIN 3 and the fourth input terminal CIN 4 in the second phase. That is, the third input terminal CIN 3 is connected to the non-inverting input terminal (+) of the differential amplifier 14 through the wiring 22 , and the fourth input terminal CIN 4 is connected to the inverting input terminal (−) of the differential amplifier 14 through the wiring 23 .

One end of the third electrostatic capacitor C 3 is connected to the wiring 22 , while one end of the fourth electrostatic capacitor C 4 is connected to the wiring 23 . Another end of the third electrostatic capacitor C 3 and another end of the fourth electrostatic capacitor C 4 are connected together and applied an inverted alternating current drive signal *SCDRV, that is generated by inverting the alternating current drive signal SCDRV from the drive circuit 12 with the inverter 13 .

As a result, there is formed a sensor circuit 30 of a differential input type, as shown in FIG. 3 . FIG. 3 shows a case of the first phase in which the selection circuit 10 selects the signals from the first input terminal CIN 1 and the second input terminal CIN 2 . In this case, a first electrostatic capacitor C 1 is formed between the Y sense line YL 1 connected to the first input terminal CIN 1 and the Y drive line DRYL, while a second electrostatic capacitor C 2 is formed between the Y sense line YL 3 connected to the second input terminal CIN 2 and the Y drive line DRYL, as shown in FIG. 1 .

Then, the first electrostatic capacitor C 1 is connected in series with the third electrostatic capacitor C 3 , while the second electrostatic capacitor C 2 is connected in series with the fourth electrostatic capacitor C 4 , as shown in FIG. 3 . The alternating current drive signal SCDRV from the drive circuit 12 is applied to a connecting node between the first electrostatic capacitor C 1 and the second electrostatic capacitor C 2 , which is the Y drive line DRYL.

A connecting node N 2 between the first electrostatic capacitor C 1 and the third electrostatic capacitor C 3 is connected to the non-inverting input terminal (+) of the differential amplifier 14 . A connecting node N 1 between the second electrostatic capacitor C 2 and the fourth electrostatic capacitor C 4 is connected to the inverting input terminal (−) of the differential amplifier 14 .

The first feedback capacitor 15 and a switch SW 1 are connected between an inverting output terminal (−) and the non-inverting input terminal (+) of the differential amplifier 14 , while the second feedback capacitor 16 and a switch SW 2 are connected between a non-inverting output terminal (+) and the inverting input terminal (−) of the differential amplifier 14 . It is preferable that the first and second feedback capacitors 15 and 16 have the same capacitance CAf.

The sensor circuit 30 of the differential input type outputs an output voltage Vout that corresponds to a difference between a capacitance CA 1 of the first electrostatic capacitor C 1 and a capacitance CA 2 of the second electrostatic capacitor C 2 . Its detailed operations are to be described.

Since the output voltage Vout of the sensor circuit 30 is an analog signal, it can be not digitally processed as it is. Thus, the output voltage Vout is converted into a digital signal with the AD converter 17 . An output of the AD converter 17 is converted into serial data in a predetermined format by the I 2 C bus interface circuit 18 , and transmitted to the microcomputer 3 through the serial clock terminal SCL and the serial data terminal SDA. The microcomputer 3 processes the serial data it received, and determines a touch position on the touch panel 1 .

The commands from the microcomputer 3 are transmitted to the control circuit 11 through the I 2 C bus interface circuit 18 . The control circuit 11 controls overall operations (including the calibration control) of the signal processing circuit 2 Y based on the commands it received.

The proceedings of the calibration of the sensor circuit 30 described above are explained referring to FIGS. 2 , 3 and 4 . An offset is caused in the output voltage Vout of the sensor circuit 30 when there is an imbalance between the capacitance CA 1 of the first electrostatic capacitor C 1 and the capacitance CA 2 of the second electrostatic capacitor C 2 , that is, when there is a difference between the capacitances, in an initial state (a state in which the finger of operator or the like is too far away from the touch panel 1 to be detected).

In this case, the offset in the output voltage Vout means a deviation voltage from 0 V that is a center value of the output voltage Vout. When the offset is caused in the output voltage Vout, detection accuracy of the touch sensor is degraded.

In addition, since the output voltage Vout varies sensitively to its environment (temperature, humidity, electric field from its surrounding environment or the like) as described above, an offset adjustment corresponding to a change in the environment is required.

Thus, the third and fourth electrostatic capacitors C 3 and C 4 are formed of variable capacitors so that the offset in the output voltage Vout can be adjusted. That is, a capacitance CA 3 of the third electrostatic capacitor C 3 and a capacitance CA 4 of the fourth electrostatic capacitor C 4 are adjusted based on calibration data written into the calibration register 19 , so that the offset in the output voltage Vout of the sensor circuit 30 (preferably the digital value after the AD conversion) becomes a desired value, preferably a minimum value.

As for the calibration of the sensor circuit 30 (Refer to FIG. 3 ), it is preferable that the capacitances CA 1 -CA 4 of the first through fourth electrostatic capacitors C 1 -C 4 are equal to each other (CA 1 =CA 2 =CA 3 =CA 4 =C) in the initial state.

However, when the capacitance CA 1 of the first electrostatic capacitor C 1 becomes larger than the capacitance CA 2 of the second electrostatic capacitor C 2 by ΔC (CA 1 =C+ΔC, CA 2 =C) due to the change in the environment, for example, there is caused the offset in the output voltage Vout. In this case, the offset can be reduced to the minimum value (0 V) by adjusting the capacitances CA 3 and CA 4 of the third and fourth electrostatic capacitors C 3 and C 4 so that the capacitance CA 3 is larger than the capacitance CA 4 by ΔC (CA 3 =C+ΔC, CA 4 =C).

When the capacitance CA 1 of the first electrostatic capacitor C 1 is smaller than the capacitance CA 2 of the second electrostatic capacitor C 2 by ΔC (CA 1 =C−ΔC, CA 2 =C), on the other hand, the capacitances CA 3 and CA 4 of the third and fourth electrostatic capacitors C 3 and C 4 are adjusted so that the capacitance CA 3 is smaller than the capacitance CA 4 by ΔC (CA 3 =C−ΔC, CA 4 =C).

As an example structure of the third electrostatic capacitor C 3 in this case, the third electrostatic capacitor C 3 is formed to include m electrostatic capacitors C 31 -C 3 m and m switches S 31 -S 3 m , as shown in FIG. 4 . It is preferable that capacitances CA 31 -CA 3 m of the electrostatic capacitors C 31 -C 3 m are weighted so that the capacitance CA 3 of the third electrostatic capacitor C 3 can be fine-adjusted. For example, when the capacitance CA 31 of the capacitor C 31 is denoted as C 0 , CA 32 =½ C 0 , CA 33 =¼ C 0 , CA 34 =⅛ C 0 , CA 3 m= ½ m-1 C 0 . Each of the switches S 31 -S 3 m is turned on and off by corresponding each of m-bits of the calibration data from the calibration register 19 . The same applies to the fourth electrostatic capacitor C 4 .

With the structure described above, the capacitances CA 3 and CA 4 of the third and fourth electrostatic capacitors C 3 and C 4 can be adjusted by the corresponding 2m-bits of the calibration data from the calibration register 19 . The 2m-bits of the calibration data can be determined based on the output voltage Vout of the sensor circuit 30 so that the offset in the output voltage Vout becomes the desired value, that is preferably the minimum value. The determined calibration data is written into an electrically writable and erasable non-volatile memory such as the EEPROM 20 , and stored in it.

Next, the operations of the sensor circuit 30 of the differential input type described above (Refer to FIG. 3 ) will be explained referring to FIG. 5A through FIG. 7 . In this case, the alternating current drive signal SCDRV is a clock signal alternating between a high level (Vref) and a low level (ground voltage=0 V). A voltage difference between an output voltage Vom from the inverting output terminal (−) of the differential amplifier 14 and an output voltage Vop from the non-inverting output terminal (+) of the differential amplifier 14 is the output voltage Vout (=Vop−Vom).

The sensor circuit 30 has a charge accumulation mode and a charge transfer mode that alternate between each other.

First, when the sensor circuit 30 is in the charge accumulation mode, which is shown in FIG. 5A , Vref is applied to the first and second electrostatic capacitors C 1 and C 2 . Also, the ground voltage (0 V) is applied to the third and fourth electrostatic capacitors C 3 and C 4 .

The switches SW 1 and SW 2 are turned on. With this, the inverting output terminal (−) and the non-inverting input terminal (+) of the differential amplifier 14 are short-circuited, while the non-inverting output terminal (+) and the inverting input terminal (−) are short-circuited. As a result, a voltage at the node N 1 (node of the wiring connected to the inverting input terminal (−)), a voltage at the node N 2 (node of the wiring connected to the non-inverting input terminal (+)), a voltage at the inverting output terminal (−) and a voltage at the non-inverting output terminal (+) are all set to ½ Vref. A common mode voltage of the differential amplifier 14 in this case is ½ Vref.

Next, when the sensor circuit 30 is in the charge transfer mode, which is shown in FIG. 5B , the ground voltage (0 V) is applied to the first and second electrostatic capacitors C 1 and C 2 , to the contrary of the case in the charge accumulation mode. Also, Vref is applied to the third and fourth electrostatic capacitors C 3 and C 4 . The switches SW 1 and SW 2 are turned off.

The capacitances CA 1 , CA 2 , CA 3 and CA 4 of the electrostatic capacitors C 1 , C 2 , C 3 and C 4 are equal to each other in the initial state (CA 1 =CA 2 =CA 3 =CA 4 =C). A difference between the capacitances CA 1 and CA 2 when the finger of operator approaches the touch pad is represented by ΔC (CA 1 −CA 2 =ΔC). In this case, CA 1 =C+½ ΔC, and CA 2 =C−½ ΔC.

In the charge accumulation mode shown in FIG. 5A , an amount of electric charges at the node N 1 is given by the following equation:

where (C−½ΔC)·(−½ Vref) represents an amount of electric charges stored in C 2 , C·(½Vref) represents an amount of electric charges stored in C 4 and CAf·0 (=0) represents an amount of electric charges stored in Cf.

In the charge transfer mode shown in FIG. 5B , an amount of electric charges at the node N 1 is given by the following equation:

where (C−½ ΔC)·(½Vref) represents an amount of electric charges stored in C 2 , C·(−½ Vref) represents an amount of electric charges stored in C 4 and CAf·(Vop−½Vref) represents an amount of electric charges stored in Cf.

[Equation 1]=[Equation 2] holds, since the amount of electric charges at N 1 in the charge accumulation mode is equal to the amount of electric charges at N 1 in the charge transfer mode according to the law of conservation of electric charge.

Following equation is derived by solving [Equation 1]=[Equation 2] for Vop:

Similarly, following equation is derived by solving for Vom an equation obtained by applying the law of conservation of electric charge to an amount of electric charges at the node N 2 in the charge accumulation mode and an amount of electric charges at the node N 2 in the charge transfer mode:

Vout is obtained from [Equation 3] and [Equation 4]:

It is understood that the output voltage Vout of the sensor circuit 30 varies in proportion to the difference ΔC between the capacitance CA 1 of the first electrostatic capacitor C 1 and the capacitance CA 2 of the second electrostatic capacitor C 2 .

It is assumed in the above calculations that CA 1 =CA 2 =CA 3 =CA 4 =C. When there is caused a difference between CA 1 and CA 2 due to the change in the environment or the like, the offset in the output voltage Vout can be made to a predetermined value or to the minimum value by adjusting CA 3 and CA 4 using the calibration register 19 as described above so that there is the same amount of difference between CA 3 and CA 4 .

Next, characteristics of the output voltage Vout of the sensor circuit 30 used in the touch sensor are explained referring to Table 1 and FIG. 6 . As described above, the selection circuit 10 has the first phase in which it selects the signals from the first input terminal CIN 1 and the second input terminal CIN 2 and the second phase in which it selects the signals from the third input terminal CIN 3 and the fourth input terminal CIN 4 .

The output voltage Vout of the sensor circuit 30 in the first phase is denoted as V 1 , while the output voltage Vout of the sensor circuit 30 in the second phase is denoted as V 2 . In this case, the output voltage V 1 is proportional to the difference between the capacitance of the capacitor formed between the Y sense line YL 1 and the Y drive line DRYL and the capacitance of the capacitor formed between the Y sense line YL 3 and the Y drive line DRYL.

Also, the output voltage V 2 is proportional to the difference between a capacitance of a capacitor formed between the Y sense line YL 2 and the Y drive line DRYL and a capacitance of a capacitor formed between the Y sense line YL 4 and the Y drive line DRYL. Then, the finger of operator or the like makes a single-touch on the touch panel 1 in a range between the Y sense line YL 1 and the Y sense line YL 4 .

First, when the finger of operator or the like touches the Y sense line YL 1 , the first output voltage V 1 in the first phase becomes a positive (+) value. This is because the capacitance of the capacitor formed between the Y sense line YL 1 and the Y drive line DRYL becomes larger than the capacitance of the capacitor formed between the Y sense line YL 3 and the Y drive line DRYL. And the second output voltage V 2 in the second phase becomes 0 V. This is because no change is caused in the capacitance related to the Y sense line YL 2 or YL 4 , since the finger of operator or the like touches the Y sense line YL 1 only.

Next, when the finger of operator or the like touches the Y sense line YL 2 , the first output voltage V 1 in the first phase becomes 0 V. This is because no change is caused in the capacitance related to the Y sense line YL 1 or YL 3 . On the other hand, the second output voltage V 2 in the second phase becomes a positive (+) value. This is because the capacitance of the capacitor formed between the Y sense line YL 2 and the Y drive line DRYL becomes larger than the capacitance of the capacitor formed between the Y sense line YL 4 and the Y drive line DRYL.

Next, when the finger of operator or the like touches the Y sense line YL 3 , the first output voltage V 1 in the first phase becomes a negative (−) value. This is because the capacitance of the capacitor formed between the Y sense line YL 3 and the Y drive line DRYL becomes larger than the capacitance of the capacitor formed between the Y sense line YL 1 and the Y drive line DRYL. On the other hand, the second output voltage V 2 in the second phase becomes 0 V. This is because no change is caused in the capacitance related to the Y sense line YL 2 or YL 4 , since the finger of operator or the like touches the Y sense line YL 3 only.

Finally, when the finger of operator or the like touches the Y sense line YL 4 , the first output voltage V 1 in the first phase becomes 0 V. This is because no change is caused in the capacitance related to the Y sense line YL 1 or YL 3 . On the other hand, the second output voltage V 2 in the second phase becomes a negative (−) value. This is because the capacitance of the capacitor formed between the Y sense line YL 4 and the Y drive line DRYL becomes larger than the capacitance of the capacitor formed between the Y sense line YL 2 and the Y drive line DRYL. Note that maximum absolute values of the first and second output voltages V 1 and V 2 are normalized to “1” in Table 1 and in FIG. 6 .

Also, FIG. 6 shows that the first and second output voltages V 1 and V 2 vary continuously in accordance with the change in the touch position. That is, making a point on the Y line YL 1 in FIG. 1 as an origin in FIG. 6 and making the X coordinate axis in FIG. 1 as a horizontal axis in FIG. 6 , the first output voltage V 1 is approximated by V 1 =cos X, and the second output voltage V 2 is approximated by V 2 =sin X. Therefore, it is possible to detect the touch position (X coordinate) based on the first and second output voltages V 1 and V 2 .

To show an example, since an equation V 2 /V 1 =tan X holds, the X coordinate of the touch position can be obtained using an equation X=arctan (V 2 /V 1 ) and polarities (+, −) of the first and second output voltages V 1 and V 2 . Here, arctan is an inverse function of tan. In this case, the first and second output voltages V 1 and V 2 are converted into digital values with the A/D converter 17 and transmitted to the microcomputer 3 through the I 2 C bus interface circuit 18 as described above. The X coordinate of the touch position can be obtained by performing the calculation described above with the microcomputer 3 .

Similarly, the Y coordinate of the touch position on the X sense lines XL 1 -XL 4 can be detected based on the first and second output voltages V 1 and V 2 through the operations of the signal processing circuit 2 X. The X and Y coordinates of the touch position can be obtained through time-series operations of the signal processing circuits 2 X and 2 Y as shown in FIG. 7 , for example.

Next, the calibration control of the electrostatic capacity type touch sensor 100 is explained referring to FIG. 8 . The calibration control is directed to reducing the time for calibration as well as performing the calibration corresponding to the change in the environment at a moment when the finger of operator or the like does not touch the sense line (Y sense line YL 1 , for example), and is carried out according to the program stored in the microcomputer 3 as described above.

First, in step S 1 , the electrostatic capacity type touch sensor 100 goes into a calibration mode based on a start command from the microcomputer 3 .

Next, in step S 2 , the sensor circuit 30 is put into operation based on 2m-bits of first calibration data X 0 (initial setting, or current setting) stored in the calibration register 19 , and a first output voltage AD 0 (an example of “first output value of the electrostatic capacity type touch sensor” of this invention, and a digital value of the output voltage Vout of the sensor circuit 30 after the AD conversion) is obtained.

Next, in step S 3 , the microcomputer 3 judges whether the first output voltage AD 0 is larger than a predetermined touchdown threshold value. When the first output voltage AD 0 is larger than the touchdown threshold value, it is regarded as the sense line (X sense line XL 1 , for example) has been touched, and touchdown processing is carried out (step S 4 ).

The touchdown processing means manipulating electronic equipment in which the electrostatic capacity type touch sensor 100 is used as an input device (turning on/off or changing the channel of TV, for example). When the first output voltage AD 0 is smaller than the touchdown threshold value, on the other hand, it is regarded as there has been no touchdown, and followed by the calibration as described below.

Next, in step S 5 , the first calibration data X 0 in the calibration register 19 is modified into second calibration data X 1 based on the first output voltage AD 0 . In this case, the second calibration data X 1 is determined through calculation by the microcomputer 3 so that the first output voltage AD 0 of the sensor circuit 30 approaches the center value (0 V in this embodiment, since the sensor circuit 30 is capable of outputting positive and negative voltages around its center value of 0 V) of the output voltage Vout of the sensor circuit 30 .

In this case, assuming that the change in the output voltage Vout of the sensor circuit 30 due to the change in the environment is not steep, it is preferable for reducing the calibration time that the modification on the first calibration data X 0 is a minimum modification, which is increasing by 1 or reducing by 1.

Next, in step S 6 , the sensor circuit 30 is put into operation based on the modified second calibration data X 1 and under modified capacitance balance, and a second output voltage AD 1 (an example of “second output value of the electrostatic capacity type touch sensor” of this invention, and a digital value of the output voltage Vout of the sensor circuit 30 after the AD conversion) is obtained.

Next, in step S 7 , the data in the calibration register 19 is set back to the first calibration data X 0 . This is made because validity of the second calibration data X 1 has not been confirmed yet.

Next, in step S 8 , there is waiting for a predetermined period of time T 1 . This is effective to find out whether the change in the output voltage Vout of the sensor circuit 30 is due to the change in the environment or not, and is based on experimental finding that the change in the output voltage Vout of the sensor circuit 30 during the predetermined period of time T 1 is not steep when the change in the output voltage Vout of the sensor circuit 30 is due to the change in the environment. The predetermined period of time T 1 can be set by a counter incorporated in the microcomputer 3 .

Next, in step S 9 , the sensor circuit 30 is put into operation again after the predetermined period of time T 1 under capacitance balance based on the original first calibration data X 0 , and a third output voltage AD 2 (an example of “third output value of the electrostatic capacity type touch sensor” of this invention, and a digital value of the output voltage Vout of the sensor circuit 30 after the AD conversion) is obtained.

Next, in step S 10 , whether the first output voltage AD 0 and the third output voltage AD 2 are very close to each other is judged. For that purpose, an absolute value of a difference (AD 0 −AD 2 ) between the first output voltage AD 0 and the third output voltage AD 2 is compared with a first threshold value Vtr 1 . When the absolute value of the difference (AD 0 −AD 2 ) between the output voltages is smaller than the first threshold value Vtr 1 , it is regarded as there has been no steep change in the output voltage Vout of the sensor circuit 30 during the waiting time, that is, the finger of operator or the like has not touched the sense line.

When the absolute value of the difference (AD 0 −AD 2 ) between the output voltages is larger than the first threshold value Vtr 1 , on the other hand, it is regarded as there has been a steep change in the output voltage Vout of the sensor circuit 30 during the waiting time, that is, the finger of operator or the like has touched the sense line, and modifying the data in the calibration register 19 is not permitted. That is, the data in the calibration register 19 is retained to be the first calibration data X 0 .

In this case, it is preferable that the first threshold value Vtr 1 is 1/10 of the maximum value of the output voltage Vout of the sensor circuit 30 , that is, the output voltage Vout at the time when the finger of operator or the like touches the sense line. The maximum value of the output voltage Vout of the sensor circuit 30 in this case is the maximum value of each of the first and second output voltages V 1 and V 2 shown in FIG. 6 , for example. When the absolute value of the difference (AD 0 −AD 2 ) between the output voltages is equal to or smaller than the first threshold value Vtr 1 described above, the difference (AD 0 −AD 2 ) between the output voltages can be empirically judged to be due to the change in the environment.

When it is judged in the step S 10 that the finger of operator or the like has not touched, the proceedings advance to next step S 11 in which the second output voltage AD 1 and the third output voltage AD 2 are compared with each other to judge which is more suitable. To describe more specifically, which of the second output voltage AD 1 and the third output voltage AD 2 has smaller offset (difference between the sensor output value and the center value (0 V) of the output voltage Vout of the sensor circuit 30 ) is judged.

When the offset in the second output voltage AD 1 is smaller than the offset in the third output voltage AD 2 , the modification to the second calibration data X 1 is permitted and the data in the calibration register 19 is modified to the second calibration data X 1 in step S 12 .

When the offset in the third output voltage AD 2 is smaller than the offset in the second output voltage AD 1 , the modification to the second calibration data X 1 is not permitted. That is, the data in the calibration register 19 is retained to be the first calibration data X 0 .

Although the second output voltage AD 1 is compared with the third output voltage AD 2 in the step S 11 as described above, it is also possible that the first output voltage AD 0 is compared with the second output voltage AD 1 . However, comparing the second output voltage AD 1 with the third output voltage AD 2 is preferable. Because while the first output voltage AD 0 and the third output voltage AD 2 are based on the same calibration data X 0 , the third output voltage AD 2 reflects the environment in which the electrostatic capacity type touch sensor 100 is currently placed more appropriately than the first output voltage AD 0 .

Again, there is waiting for a predetermined period of time T 2 in subsequent step S 13 . After that, the proceedings return to the step S 1 , and the calibration control flow described above is carried out again when the start command is outputted from the microcomputer 3 .

With the electrostatic capacity type touch sensor 100 according to the embodiment of this invention, the calibration time can be substantially reduced compared with the case in which the offset is adjusted by increasing or decreasing the calibration data step by step from an initial data (all bits are zero, for example), since the second calibration data X 1 is obtained by fine-adjusting the first calibration data X 0 by about ±1 based on the first output voltage AD 0 of the sensor circuit 30 obtained in most recent measurement, before making measurement again.

Also, whether the finger of operator or the like touches the sense line (touch pad) is accurately judged based on the comparison between the first output voltage AD 0 and the third output voltage AD 2 , and the calibration corresponding to the change in the environment can be carried out.

In order to more accurately judge whether the output voltage difference (AD 0 −AD 2 ) between the first output voltage AD 0 and the third output voltage AD 2 is due to the change in the environment, it is preferable that another judgment criteria of a gradient of the output value of the sensor circuit 30 , that is a rate of change, is added to the judgment criteria in the step 10 .

That is, when the absolute value of the output voltage difference (AD 0 −AD 2 ) is equal to or smaller than the first threshold value Vtr 1 , and the gradient of the absolute value of the output voltage difference (AD 0 −AD 2 ), that is, a quotient obtained by dividing the absolute value of the output voltage difference (AD 0 −AD 2 ) by the waiting time T 1 is smaller than a second threshold value, it is judged that the output voltage difference (AD 0 −AD 2 ) is due to the change in the environment.

As shown in FIGS. 9A , 9 B and 9 C, the judgment is based on experimental data taken by the inventors (Refer to FIG. 9C ) which shows that a gradient b (gradient b in FIG. 9B ) of the output voltage Vout of the sensor circuit 30 due to the change in the environment is much smaller than a gradient a (gradient a in FIG. 9A ) of the output voltage Vout of the sensor circuit 30 when the finger of operator or the like touches the sense line.

Although the calibration control described above is executed by the microcomputer 3 , the software functions of the microcomputer 3 can be also realized with hardware that is a calibration control circuit 50 , as shown in FIG. 10 . The calibration control circuit 50 is formed to include a switch SW, an AD 0 latch circuit 51 , an AD 1 latch circuit 52 , a counter 53 , an arithmetic circuit 54 , an AD 2 latch circuit 55 , a comparator 56 , and register write circuits 57 , 58 and 59 , as shown in FIG. 11 . The calibration control circuit 50 may be incorporated in the signal processing circuits 2 X and 2 Y.

The calibration control circuit 50 is structured corresponding to the calibration control flow shown in FIG. 8 . The switch SW outputs the output of the AD converter 17 to either a terminal P 1 or a terminal P 2 . When the calibration control is to be carried out, the switch SW applies the output of the AD converter 17 to the terminal P 1 . The calibration control circuit 50 is configured to be put into operation based on the output of the AD converter 17 .

When the calibration control is not to be carried out, on the other hand, the switch SW applies the output of the AD converter 17 to the terminal P 2 . In this case, the output of the AD converter 17 is outputted without going through the calibration control circuit 50 .

Each of the AD 0 latch circuit 51 , the AD 1 latch circuit 52 and the AD 2 latch circuit 55 latches each of the first through third output voltages AD 0 -AD 2 , respectively. The arithmetic circuit 54 calculates the second calibration data X 1 based on the first output voltage AD 0 stored in the AD 0 latch circuit 51 .

The register write circuit 57 corresponds to the step S 5 , and writes the second calibration data X 1 into the calibration register 19 . As a result, the data in the calibration register 19 is modified to the second calibration data X 1 .

The register write circuit 58 corresponds to the step S 7 , and writes the first calibration data X 0 into the calibration register 19 when the second output voltage AD 1 is latched in the AD 1 latch circuit 52 . As a result, the data in the calibration register 19 is set back to the first calibration data X 0 . The counter 53 corresponds to the step S 8 , and counts the predetermined period of time T 1 .

The comparator 56 corresponds to the step S 10 , and compares the absolute value of the difference (AD 0 −AD 2 ) between the first output voltage AD 0 and the third output voltage AD 2 with the first threshold value Vtr 1 . The comparator 56 also corresponds to the step S 11 , and compares the offset in the second output voltage AD 1 with the offset in the third output voltage AD 2 .

The register write circuit 59 corresponds to the step S 12 , and writes the second calibration data X 1 into the calibration register 19 . As a result, the data in the calibration register 19 is modified to the second calibration data X 1 .

It is apparent that this invention is not limited to the embodiments described above, and may be modified within the scope of the invention. For example, the touch panel 1 is formed using the sense lines. Not limited to the above, the touch panel 1 may be formed of touch pads, each of which makes a capacitor that varies its capacitance when touched by the finger of operator or the like. Also, the sensor circuit 30 is formed of the differential type circuit that detects the difference in the capacitances of a pair of sense lines. Not limited to the above, the sensor circuit 30 may be formed of a single type circuit that directly detects a change in a capacitance of a single sense line (or a touch pad).

With the electrostatic capacity type touch sensor according to the embodiment of this invention, the calibration time can be reduced, while it is made possible that the calibration corresponding to the change in the environment is carried out at the moment when the finger of operator or the like does not touch the touch pad.