This application is based upon and claims priority under 35 U.S.C. 119 from Taiwan Patent Application No. 103134173 filed on Oct. 1, 2014, which is hereby specifically incorporated herein by this reference thereto.
1. Field of the Invention
The present invention is related to a capacitive touch device, and more particularly to an excitation signal generating circuit and method of the capacitive touch device.
2. Description of the Prior Arts
A conventional capacitive touch device has a touch panel and a scanning circuit. The touch panel has multiple sensing traces, and the scanning circuit provides an excitation signal to parts or all of the sensing traces in sequence. A charging current is generated on the sensing trace to which the excitation signal is supplied based on the influence of capacitance effect. After a period of charging time, the charging current on the sensing trace is stable. At the time, a discharging current on the same sensing trace can be read, or a induced current on another sensing trace crossing to the sensing trace to which the excitation signal is supplied can be read. Then the read discharging current or the read induced current is further converted to a valid capacitance sensing value. A position of a touch on the touch panel can be identified by determining a variation of the capacitance sensing value.
A distance between one end of each sensing trace and the excitation circuit is different since the sensing traces are formed on different positions on the touch panel. Therefore, the charging times of the sensing traces are not the same. For example, with reference to
With reference to
Based on foregoing description, the excitation signal generator can output since wave signal as the excitation signal, but the excitation signal generator requires the amplifiers to amplify the analog sine wave signal. Therefore, the excitation signal generator has higher power consumption and also increases manufacturing cost for a larger size touch panel with more sensing traces. In addition, the analog signal generating unit has an analog circuit, which requires a larger layout area of chip if the analog generating unit is in the form of the integrated circuit.
To overcome the shortcomings, the present invention provides an n excitation signal generating circuit to mitigate or obviate the aforementioned problems.
The objective of the present invention provides an excitation signal generating circuit with a simple circuit structure, a method of generating the excitation signal and a capacitive touch device using the excitation signal generating circuit to solve the signal distortion, high power consumption, and high manufacturing cost etc. drawbacks.
To achieve the objective, the excitation signal generating circuit is electronically connected to multiple sensing traces of the capacitive touch device and has a storage unit and a pulse density modulation (hereinafter PDM) signal generating circuit.
The storage unit stores at least one set of digital data and each set of digital data corresponds to a frequency.
The PDM signal generating circuit is connected to the storage unit and the sensing traces to read the at least one set of digital data. The PDM signal generating circuit converts the set of digital data to a PDM signal according to the frequency of the read set of digital data, and then outputs the PDM signal to the sensing traces.
The excitation signal generating circuit of the present invention directly generates the PDM signal as an excitation signal output to the sensing traces. The PDM signal is recovered to an analog excitation signal since the PDM signal passes through a current transmission path, which is equivalent to a low pass filter. Therefore, a receiving circuit receives an available analog sensing signal with low signal distortion to increase accuracy of the current sensing signal. Further, the excitation signal generating circuit does not require amplifiers to amplify the PDM signal, so a circuit structure of the excitation signal generating circuit is simplified, a manufacturing cost can be reduced, and the power consumption is low.
To achieve the objective, the capacitive touch device has a touch panel, an excitation signal generating circuit having a storage unit, a PDM signal generating circuit and a receiving circuit.
The touch panel has multiple sensing traces.
The storage unit stores at least one set of digital data and each set of digital data corresponds to a frequency. The PDM signal generating circuit is connected to the storage unit and the sensing traces to read the at least one set of digital data. The PDM signal generating circuit converts the set of digital data to a PDM signal according to the frequency of the read set of digital data, and then outputs the PDM signal to the sensing traces.
The receiving circuit is connected to the sensing traces of the touch panel to receive an analog signal of each sensing trace corresponding to the sensing trace to which the PDM signal is supplied.
The capacitive touch device of the present invention directly generates the PDM signal as an excitation signal output to the sensing traces. The PDM signal is recovered to an analog excitation signal since the PDM signal passes through a current transmission path, which is equivalent to a low pass filter. Therefore, a receiving circuit receives an available analog sensing signal with low signal distortion to increase accuracy of the current sensing signal. Further, the capacitive touch device does not require amplifiers to amplify the PDM signal, so a circuit structure of the excitation signal generating circuit is simplified, a manufacturing cost can be reduced, and the power consumption is low.
To achieve the objective, the method of generating excitation signal has steps of: storing at least one set of digital data, wherein each set of digital data corresponds to a frequency; converting the set of digital data to a PDM signal according to the frequency of the set of digital data; and using the PDM signal as an excitation signal and outputting the PDM signal to the sensing traces.
The generating method generates the PDM signal having a frequency which is the same as a frequency of as an excitation signal, so that the PDM signal is directly output to the sensing traces. The PDM signal is recovered to an analog excitation signal since the PDM signal passes through a current transmission path, which is equivalent to a low pass filter. Therefore, an available analog sensing signal with low signal distortion is received to increase accuracy of the current sensing signal and to save power.
Other objectives, advantages and novel features of the invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings.
The present invention provides an improved excitation signal generating circuit of a capacitive touch device to simplify the circuit structure thereof.
With reference to
The touch panel 10 has multiple sensing traces 11 including multiple first axis sensing traces and a second axis sensing traces crossing the first axis sensing traces. In the first preferred embodiment, a mutual-capacitive scanning method is used as an example so the first axis sensing traces are used as driving traces TX1˜TXn and the second axis sensing traces are used as sensing traces RX1˜RXm. If a self-capacitive scanning method is used, each sensing trace is used as the driving and receiving trace.
The excitation signal generating circuit has a storage unit 20 and a pulse density modulation (hereinafter PDM) signal generating circuit 21. The storage 20 stores at least one set of digital data. Each set of digital data corresponds to a frequency. The PDM signal generating circuit 21 is connected to the storage unit 20 and multiple sensing traces 11 to read the at least one set of digital data. The PDM signal generating circuit 21 also converts the read set of digital data to a PDM signal according to the frequency of the read set of digital data. The PDM signal is further supplied to the sensing traces as an excitation signal. When the mutual-capacitive scanning is employed, the PDM signal is supplied to the first axis sensing traces TX1˜TXn used as the driving traces. In the first preferred embodiment, the set of digital data stored in storage unit 20 is a set of PDM digital data and each set of PDM digital data is consisted of digital values +1 and −1, so the storage unit 20 can store one or more sets of PDM digital data in one or more look-up tables 201 in the storage unit 20. In the first preferred embodiment, the PDM signal generating circuit 21 has as controller 211 and a switching circuit 212. The switching circuit 212 has a control terminal COL, two switching terminals and multiple output terminals. The controller 211 is connected to the storage unit 20 and the control terminal COL of the switching circuit 212. The multiple output terminals of the switching circuit 212 are respectively connected to the corresponding first axis sensing traces TX1˜TXn. The two switching terminals of the switching circuit 212 are respectively connected to two voltage terminals V+ and V−. Therefore, the controller 211 outputs a control signal to the control terminal COL of the switching circuit 212 and the switch circuit 212 switches each output terminal to one of the two switching terminals according to the control signal.
When the controller 211 reads one set of the PDM digital data from the storage unit 20. The controller 211 outputs the control signal to the control terminal COL of the switching circuit 212 according to the read set of PDM digital data and the frequency thereof. Each output terminal of the switching circuit 212 outputs a PDM signal according to the read set of the PDM digital data, since each output terminal is switched to the voltage terminal V+ if the present read PDM data is +1 or switched to another voltage terminal V− if the present read PDM data is −1. The PDM single is outputted to the corresponding first axis sensing trace TX1, TX2 . . . or TXn. In a preferred embodiment, the set of PDM digital data is stored in 1-bit look-up tale 201 of the storage unit 20. With further reference to
Based on foregoing description, a switching frequency of the switching circuit 212 has to be higher than that of the set of PDM digital data to successfully output a correct PDM signal. Therefore, a switching frequency of the switching circuit 212 and the frequency of the set of PDM digital data has to be complied with an equation: fsw/fs>n, wherein n is an integer and greater than 4 or equal to 4.
The receiving circuit 30 is connected to the sensing traces 11 of the touch panel 10 to receive an analog sensing signal Sc from each sensing trace crossing to the driven sensing trace to which the PDM signal is supplied, and then converts the analog sensing signal Sc to a digital capacitance sensing value. Since the first embodiment employs the mutual-capacitive scanning, the receiving circuit 30 is connected to the second axis sensing traces RX1˜RXm. In the first preferred embodiment, the receiving circuit 30 has a one to many multiplexer 32, an analog to digital converter (hereinafter ADC) 31 and a low pass filter (LPF hereinafter) 33. The one to many multiplexer 32 has multiple output pins and a common pin. The output pins of the one to many multiplexer 32 are respectively connected to the second axis sensing traces RX1˜RXm. The common pin of the one to many multiplexer 32 is connected to the ADC 31 to respectively receive the analog sensing signal Sc from the second axis sensing traces RX1˜RXm. The ADC 31 converts the analog sensing signal Sc to the digital capacitance sensing value. The LPF 33 is connected to the ADC 31.
With further reference to
Based on the foregoing description of the first preferred embodiment, the excitation signal generating circuit 20 of the present invention stores the sets of digital data, whose frequencies respectively correspond to frequencies of the excitation signals. The set of digital data is the set of the PDM digital data so that a PDM signal can be generated as the excitation signal and outputted to the sensing traces 11. The PDM signal is recovered to the analog sensing signal Sc, since the PDM signal is transmitted to a current transmission path, which is equivalent to a low pass filter and the current transmission path is from one end of the sensing trance 11, to which the PDM signal is outputted, to one end of the sensing trance 11 connected to the receiving circuit 30. With further reference to
With reference to
In the second preferred embodiment, the receiving circuit 30′ has a many to many multiplexer 32′ and ADCs 31 and y LPFs 33. The many to many multiplexer 32′ has multiple common pins and output pins. The output pins of the many to many multiplexer 32′ are connected to the corresponding ADCs 31 and the output pins of the many to many multiplexer 32′ are connected to the second axis sensing traces RX1˜RXm. With further reference to
With reference to
In the third preferred embodiment, the set of digital data stored in the storage unit 20a is a set of digital wave data. The set of digital wave data corresponds to a frequency. The PDM signal generating circuit 21a has a controller 211, a single signal converting unit 22 and a one to many multiplexer 213. The controller is connected between the storage 20a and the signal converting unit 22 to read the set of digital wave data, and generates an output signal S3 according to the set of digital wave data and the frequency thereof.
An input terminal of the signal converting unit 22 is connected to the controller 211 to receive the output signal S3 and then converts the output signal S3 to a PDM signal. Preferably, the set of digital wave data is a digital sine wave data by sampling an analog sine wave signal. An amplitude and frequency of the sampled analog sine wave signal are the same as the amplitude and frequency of the set of digital sine wave data. An output terminal of signal converting unit 22 is connected to a common pin of the one to many multiplexer 213, so the PDM signal is supplied to the first axis sensing trace TX1˜TXn in sequence through the one to many multiplexer 213.
In addition,
With reference to
With further reference to
to accumulate sets of input values x[n] of the input signal S3. The quantizer 222 is connected to an output terminal of the accumulator 221 to quantize each output value of the accumulator 221. The output feedback circuit 223 delays the quantized output value y[n] and then feeds back to an input terminal of the accumulator 221 to calculate a next input value of the accumulator 221 by subtracting quantized output value y[n] from a next external input value x[n+1]. An output terminal of the quantizer 223 is connected to the common pin(s) of the one to many multiplexer 213 and many to many multiplexer 213a in
In addition, the signal converting unit 22 may be a digital converter having a differential equations (a) and (b):
The e[n] is a present quantization error value, e[n−1] is a previous quantization error value and e[−1]=0, and x[n] is a present input value, x[n−1] is a previous input value, y[n] is a present output value and y[n−1] is previous output value. As shown in the above list, the output values y[n] shown in the third column thereof can be obtained by respectively input the input values x[n] in the equations (a) and (b). Therefore, the digital converter also generates the PDM signal.
Based on the foregoing description, the processing frequency (fm) of the signal converting unit 22 must correspond to the frequency (fs) of the digital wave data or higher than the frequency fs of the corresponding digital wave data to successfully convert the correct PDM signal. Therefore, the processing frequency (fm) of the signal converting unit 22 and the frequency (fs) of the digital wave data meet an equation: fm/fs>n, wherein n is an integer and greater than 4 or equal to 4.
In addition, the RC loading circuits 40 can be respectively connected between the first axis sensing traces and the signals, the output pins of the multiplexer 213, 213a in the third to fifth preferred embodiments. The RC loading circuits 40 with RC loads from small to large are respectively connected to the first axis sensing traces with the current transmission paths from long to short, i.e. the RC loading circuit with smaller RC load connected to the first axis sensing trace with longer current transmission path and the RC loading circuit with larger RC load connected to the first axis sensing trace with shorter current transmission path. Therefore, the higher capacitance value of the shorter current transmission path is solved.
As above descriptions for the first to fifth preferred embodiments, the excitation signal generating method of the capacitive touch device of the present invention mainly generates at least one PDM signal according to the frequency of the at least one excitation signal and supplies the at least one PDM signal used as the least one excitation signal to the sensing traces. The preferred excitation signal is sine wave signal. The PDM signal is used as the excitation signal since the frequency of the PDM signal is the same as that of the excitation signal. The PDM signal is recovered to an analog excitation signal since the PDM signal passes through a current transmission path, which is equivalent to a low pass filter. Therefore, a receiving circuit receives an available analog sensing signal with low signal distortion to increase accuracy of the current sensing signal. Further, the capacitive touch device does not require amplifiers to amplify the PDM signal, so a circuit structure of the excitation signal generating circuit is simplified, a manufacturing cost can be reduced, and the power consumption is low. If the controller and the signal converter is implemented by digital circuit and fabricated to an integrated circuit on chip, the layout area is decreased in comparison with implementing the analog circuit. Therefore, the fabricating cost is decreased to implement a low profile and light electric device.
Even though numerous characteristics and advantages of the present invention have been set forth in the foregoing description, together with details of the structure and features of the invention, the disclosure is illustrative only. Changes may be made in the details, especially in matters of shape, size, and arrangement of parts within the principles of the invention to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed.
Number | Date | Country | Kind |
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103134173 | Oct 2014 | TW | national |