Patent Publication Number: US-2015077386-A1

Title: Scanning method with adjustable sampling frequency and touch device using the same

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a scanning method of a capacitive touch device, and more particularly to a scanning method with adjustable sampling frequency and a touch device using the scanning method. 
     2. Description of the Related Art 
     Capacitive touch device detects a location of an object, such as a finger or a stylus, touching on the touch device by a capacitive variation of corresponding sensor traces. To ensure a correct sensing of capacitive variation caused by the object touching the sensor traces, capacitive touch device usually acquires a reference value through an analog-to-digital conversion (ADC) calibration procedure when being turned on or awakened from a hibernation state. The reference value is taken to determine where the object is actually on the capacitive touch device upon subsequent scanning. 
     With reference to  FIG. 10 , a conventional mutual capacitance touch device has a sensing unit  50  and a scanning circuit  60 . The sensing unit  50  has multiple first traces and second traces respectively aligned in a first-axis direction and a second-axis direction. The first traces are connected to a driving unit  61  of the scanning circuit  60  and the second traces are connected to a sensing unit  62  of the scanning circuit  60 . When performing the ADC calibration procedure upon a mutual-capacitance scan mode, the sensing unit  50  sequentially sends a driving signal to each first trace and receives a sensing value of each second trace with an identical sampling frequency. Suppose that the receiving unit  62  is adjacent to a top one of the first traces Y1 and a top end of each second trace X1˜Xm. With reference to  FIGS. 11A and 11B , after the driving signal is outputted to the top first trace Y1, capacitors intersected by the top first trace Y1 and the second traces X1˜Xm are charged by the driving signal to a saturation state or discharged down to zero voltage in a period of time t1 as indicated by two curves L1 and L2. However, after the same driving signal is outputted to a last one of the first traces Yn, capacitors intersected by each second trace X1˜Xm and the last first trace Yn is charged to the saturation state or discharged to the zero voltage in a period of time t2, which is longer than t1. As the receiving unit  62  reads sensed capacitance values of the second traces with a fixed sampling frequency, the fixed sampling frequency should correspond to t2 instead of t1 for receiving correct sensed capacitance. 
     Different charging and discharging times reside in that a resistor-capacitor (RC) load arising from the driving signal outputted to the last first trace is greater than the RC load arising from the driving signal outputted to each of the rest of the first traces. Hence, the times for charging all intersections on the last first trace to the saturation state or discharging all intersections on the last first trace to the zero voltage are relatively longer. To ensure to receive the correct sensed capacitance values of the second traces each time after the driving signal is outputted, the sampling frequency must be lowered. For example, in the case of a double-layered capacitive touch panel with a total resistance under 20K, the sampling frequency is usually configured from 800K to 500K, and in the case of a single-layered capacitive touch panel with a total resistance from 60K to 80K, the sampling frequency is configured from 300K to 150K. The lowered sampling frequency leads to a lower report rate, which causes unsmooth operation. However, if the sampling frequency is not lowered, there is a likelihood that incorrect capacitance values are received when the capacitors intersected by the first traces and the second traces are not yet fully charged to the saturation state or discharged to zero voltage. 
     As far as the conventional mutual capacitance touch device is concerned, to tackle the foregoing problems, one solution is proposed to manually measure the RC loads of all the first traces prior to shipment of the conventional mutual capacitance touch device. The conventional mutual capacitance touch device is then scanned with different sampling frequencies and a preferred sampling frequency for scanning the second traces is determined in the end. However, the solution has the shortcoming of being time-consuming in operation. 
     SUMMARY OF THE INVENTION 
     An objective of the present invention is to provide a scanning method with adjustable frequency and a touch device using the scanning method tackling the issues of spending lots of time and cost in manually measuring and testing for the determination of sampling frequency. 
     To achieve the foregoing object, the scanning method with adjustable sampling frequency for a sensing unit having multiple sensor traces aligned in a first-axis direction and in a second-axis direction, the scanning method comprising steps of: 
     pre-scanning the sensing unit to acquire a capacitance offset corresponding to each sensor trace in at least one of the first-axis direction and the second-axis direction; 
     determining a sampling frequency according to the capacitance offset of the sensor trace; and 
     sampling the sensor unit with the determined sampling frequencies. 
     To achieve the foregoing objective, alternatively, the scanning method with adjustable sampling frequency for a sensing unit having multiple sensor traces aligned in a first-axis direction and in a second-axis direction, has steps of: 
     scanning the sensing unit to acquire capacitance offsets of the sensor traces in at least one of the first-axis direction and the second-axis direction; 
     driving each sensor trace in the first-axis direction; 
     determining sampling frequencies according to the capacitance offsets of the respective driven sensor traces in the first-axis direction; and 
     reading sensed capacitance values at locations intersected by the sensor traces in the second-axis direction and the driven sensor traces in the first-axis direction with the corresponding sampling frequencies. 
     To achieve the foregoing objective, alternatively, the scanning method with adjustable sampling frequency of a sensing unit having multiple sensor traces aligned in a first-axis direction and in a second-axis direction, has steps of: 
     a pre-scanning procedure having a step of acquiring capacitance offsets corresponding to at least the sensor traces in the first direction; and 
     a subsequent scanning procedure having steps of: 
     determining a driving and sampling frequency of each sensor trace in the first-axis direction according to the capacitance offset of the sensor trace in the first-axis direction; and 
     driving the sensor trace in the first-axis direction and reading sensed capacitance value of the sensor trace in the first-axis direction with the driving and sampling frequency. 
     To achieve the foregoing objective, the touch device with adjustable sampling frequency has a sensing unit, a driving unit, a receiving unit, and a control unit. 
     The sensing unit has multiple sensor traces aligned in a first-axis direction and in a second-axis direction. 
     The driving unit is connected to the sensor traces in the first-axis direction of the sensing unit. 
     The receiving unit is connected to the sensor traces in the second-axis direction of the sensing unit. 
     The control unit is connected to the driving unit and the receiving unit, controls the driving unit and the receiving unit to scan the sensing unit so as to acquire capacitance offsets of the sensor traces in at least the first-axis direction, and determines sampling frequencies according to the capacitance offsets of the respective driven sensor traces in the first-axis direction. The receiving unit reads sensed capacitance values at locations intersected by sensor traces in the second-axis direction and the driven sensor traces in the first-axis direction with the corresponding sampling frequencies. 
     To achieve the foregoing objective, the touch device with adjustable sampling frequency has a sensing unit, a first driving and receiving unit, a second driving and receiving unit, and a control unit. 
     The sensing unit has multiple sensor traces in the first-axis direction aligned in a first-axis direction and a second-axis direction. 
     The first driving and receiving unit is connected to the sensor traces in the first-axis direction 
     The second driving and receiving unit is connected to the sensor traces in the second-axis direction. 
     The control unit is connected to the first driving and receiving unit and the second driving and receiving unit, controls the first driving and receiving unit and the second driving and receiving unit to pre-scan the sensing unit and at least acquire a capacitance offset of each sensor trace in the first-axis direction, determines a first driving and sampling frequency of each sensor trace in the first-axis direction according to the capacitance offset of the sensor trace in the first-axis direction when subsequently scanning the sensor traces in the first-axis direction, and drives the sensor trace in the first-axis direction and reading sensed capacitance value of the sensor trace in the first-axis direction with the first driving and sampling frequency. 
     After the touch device performs an analog-to-digital conversion (ADC) calibration procedure, the receiving unit automatically generates a capacitance offset corresponding to each sensor trace connected thereto. Such capacitance offset varies with the RC load value of the sensor traces. Hence, prior to formal scanning, the present invention pre-scans the touch device once first to acquire capacitance offsets corresponding to sensors traces in the first-axis direction or the second-axis direction. Upon formal scanning, the receiving unit configures a sampling frequency of a corresponding driven sensor trace according to the capacitance offset of the driven sensor trace. As to the sensor traces with lower RC load values, higher sampling frequencies are used to receive the sensed capacitance values of the sensor traces. As to the sensor traces with higher RC load values, lower sampling frequencies are used to receive the sensed capacitance values of the sensor traces, thereby fulfilling the goal of automatically adjusting sampling frequency and increasing a report rate of the sensing unit. 
     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. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view of a touch device in accordance with the present invention; 
         FIG. 2A  is an electrical functional block of an embodiment of a receiving unit of a scanning circuit in  FIG. 1 ; 
         FIG. 2B  is an electrical functional block of another embodiment of a receiving unit of a scanning circuit in  FIG. 1 ; 
         FIG. 3A  is a circuit diagram of a receiver of the receiving unit in  FIG. 2A ; 
         FIG. 3B  is a circuit diagram of a receiver of the receiving unit in  FIG. 2B ; 
         FIG. 4  is a curve diagram showing a relationship between capacitance offset and sensed capacitance values of three traces driven by the capacitance offset; 
         FIG. 5  is a flow diagram of a scanning method in accordance with the present invention; 
         FIG. 6  is a flow diagram of a first embodiment of the scanning method in  FIG. 5 ; 
         FIG. 7  is a flow diagram of a second embodiment of the scanning method in  FIG. 5 ; 
         FIG. 8  is an electrical block diagram of a self-capacitance scanning circuit in accordance with the present invention; 
         FIG. 9  is a flow diagram of a third embodiment of the scanning method in  FIG. 5 ; 
         FIG. 10  is a schematic view of a conventional touch device; and 
         FIGS. 11  A and  11 B are waveform diagrams showing when the touch device in  FIG. 10  drives the first traces Y 1  and Y n  during a charging process and a discharging process. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     With reference to  FIG. 1 , a touch device in accordance with the present invention has a sensing unit  10  and a scanning circuit  20 . The scanning circuit  20  has a driving unit  21 , a receiving unit  22  and a control unit  23  electrically connected to the driving unit  21  and the receiving unit  22 . With reference to  FIGS. 2A and 3A , an embodiment of the receiving unit  22  has multiple receivers  221  respectively connected to multiple second traces X 1 ˜X m  of the sensing unit  10  aligned in a second-axis direction. Each receiver  221  has a comparator  222 , an analog-to-digital converter (ADC)  223 , and a variable capacitance compensation circuit  224 . One input terminal of the comparator  222  is connected to one end of one of the second traces X 1 ˜X m  and the variable capacitance compensation circuit  224 . An output terminal of the comparator  222  is connected to the control unit  23  through the ADC  223  to convert a sensed capacitive signal of the second trace X 1 ˜X m  into a digital capacitance value and then to output the digital capacitance value to the control unit  23 . With reference to  FIG. 3B , another embodiment of the receiving unit  22  has a multiplexer  24  and a receiver  221 . The multiplexer  24  has multiple select terminals, a control terminal and a common terminal. The select terminals are respectively connected to the second traces X 1 ˜X m . With reference to  FIG. 3B , the receiver  221  has a comparison circuit  222 , an ADC  223 , and a variable capacitance compensation circuit  224 . One input terminal of the comparator  222  is connected to the common terminal COM of the multiplexer  24 . The control terminal CTL of the multiplexer  24  is connected to the control unit  23 . The control unit  23  controls a control terminal CTL of the multiplexer  24  for the multiplexer  24  to select one of the second traces X 1 ˜X m  and receive a sensed capacitance value of the second trace X 1 ˜X m . The variable capacitance compensation circuit  224  has multiple capacitors C 1 ˜C N  and multiple electronic switches SW 1 ˜SW N . One end of each capacitor C 1 ˜C N  is connected to the input terminal of the comparator  222 . Each electronic switch SW 1 ˜SW N  is connected in series between the other end of a corresponding capacitor C 1 ˜C N  and a ground terminal. The control terminal of each electronic switch SW 1 ˜SW N  is connected to the control unit  23 . 
     With further reference to  FIGS. 3A and 3B , the control unit  23  adjusts a capacitance offset of the variable capacitance compensation circuit  24  according to the digital capacitance value transmitted from the ADC  223 . A proper capacitance offset can be determined by turning on or turning off a part of or all the electronic switches SW 1 ˜SW N . As the capacitance offset varies with the RC loads of sensor traces, the capacitance offset can be estimated by directly sensing the RC load of each sensor trace. With reference to  FIG. 4 , three curves respectively correspond to three different capacitance offsets estimated by sensing the capacitance values of a top second trace X 1  after the driving signal is outputted to a top first trace Y 1 , a middle first trace Y 6  and a bottom first trace Y n  aligned in a first-axis direction in  FIG. 1 . The bottom trace Y n  is farther from the receiving unit  22  than the top first trace Y 1  and the middle first trace Y 6 . The RC load for the driving signal to reach the top second trace X 1  through the bottom first trace Y n  ranks the highest among all the RC loads for the driving signal to reach the top second trace X 1  through all the first traces and the capacitance offset corresponding to the highest RC load is therefore the highest among all the capacitance offsets. 
     After the touch device performs the ADC calibration procedure, the receiving unit  22  automatically generates a capacitance offset on each second trace X 1 ˜X m  connected to the receiving unit  22 . With reference to  FIG. 5 , a scanning method with adjustable sampling frequency in accordance with the present invention has the following steps. 
     Step S 10 : Scan the sensing unit  10  to at least acquire capacitance offsets corresponding to the first traces Y 1 ˜Y n  aligned in the first-axis direction. A fixed sampling frequency or different sampling frequencies can be used to scan the sensing unit  10 . If better frame rate is taken into account, a fixed higher sampling frequency is desired. 
     Step S 11 : Drive each first trace Y 1 ˜Y n . 
     Step S 12 : Determine sampling frequencies according to the capacitance offsets of the respective driven first traces Y 1 ˜Y n . 
     Step S 13 : Read sensed capacitance values at locations intersected by the second traces X 1 ˜X m  aligned in the second-axis direction and the first traces Y 1 ˜Y n  with the corresponding sampling frequencies. 
     With reference to  FIG. 6 , a first embodiment of the scanning method in accordance with the present invention is applied to a mutual-capacitance scan circuit, is performed by the touch device under a mutual-capacitance scan mode and has the following steps. 
     Step S 20 : The control unit  23  performs a pre-scanning procedure, which first sequentially drives the first traces Y 1 ˜Y n , each time after controlling the driving unit  21  to output a driving signal to one of the first traces Y 1 ˜Y n , reads the sensed capacitance values at sensed points intersected by the driven first traces Y 1 ˜Y n  and all the second traces X 1 ˜X m  with a fixed frequency or preset different frequencies, and acquires capacitance offsets corresponding to the sensed points. After all the first traces Y 1 ˜Y n  are driven, the capacitance offsets of all the sensed points are acquired. 
     Step S 21 : The control unit  23  calculates the capacitance offset of each first trace Y 1 ˜Y n  with the capacitance offsets of the sensed points on the first trace Y 1 ˜Y n . For example, the capacitance offset of any sensed point on each first trace Y 1 ˜Y n  or an average value of the capacitance offsets of all the sensed points on the first trace Y 1 ˜Y n  is taken as the capacitance offset of the first trace Y 1 ˜Y n . 
     Step S 22 : The mutual-capacitance scan circuit performs an analog-to-digital conversion (ADC) calibration procedure with respect to the sensing unit  10  under the mutual-capacitance scan mode, and sequentially outputs the driving signal to the first traces Y 1 ˜Y n . 
     Step S 23 : The receiving unit  22  determines a sampling frequency of each first trace according to the capacitance offset of the first traces when the driving unit  21  is controlled to output the driving signal, and stores the sampling frequency in the receiving unit  22 . 
     Step S 24 : The receiving unit  22  senses capacitive signals at the sensed points intersected by the second traces X 1 ˜X m  and the driven first traces Y 1 ˜Y n  with the corresponding sampling frequencies, respectively converts the sensed capacitive signals into sensed capacitance values, and outputs the sensed capacitance values to the control unit  23 . 
     With reference to  FIGS. 1 and 7 , a second embodiment of the scanning method in accordance with the present invention is applied to a self-capacitance and mutual-capacitance scan circuit. Although a driving signal is outputted to a sensor trace and a sensed capacitance signal is received from the same sensor trace under a self-capacitance scan mode, the issue of different RC loads does not seemingly exist. However, each connection wire L between the driving unit  21  and a corresponding sensor trace varies with a mounting location of the driving unit  21 . Hence, different RC loads still arise from the connection wires with different lengths, and the capacitance offset of each sensor trace can still be acquired by scanning the sensor trace under the self-capacitance scan mode. The second embodiment of the scanning method is performed by the touch device and has the following steps. 
     Step S 30 : The control unit  23  performs a pre-scanning procedure with respect to the sensing unit  10  under the self-capacitance scan mode. In the pre-scanning procedure, the control unit  23  sequentially outputs the driving signal to the first traces Y 1 ˜Y n  with a fixed frequency or preset different frequencies and receives the sensed capacitance values from the driven first traces Y 1 ˜Y n , and then acquires a capacitance offset of each first trace Y 1 ˜Y n . 
     Step S 31 : The control unit  23  further performs the ADC calibration procedure and a subsequent mutual-capacitance scanning procedure with respect to the sensing unit  10  under the mutual-capacitance scan mode, and sequentially outputs the driving signal to the first traces Y 1 ˜Y n . 
     Step S 32 : The receiving unit  22  determines a sampling frequency according to the capacitance offset of each driven first trace when the driving unit  21  is controlled to output the driving signal, and stores the sampling frequencies in the receiving unit  22 . 
     Step S 33 : The receiving unit  22  senses capacitive signals at the sensed points intersected by the second traces X 1 ˜X m  and the driven first traces Y 1 ˜Y n  with the corresponding sampling frequencies, converts the sensed capacitive signals into sensed capacitance values, and outputs the sensed capacitance values to the control unit  23 . 
     With reference to  FIGS. 1 ,  8  and  9 , a third embodiment of the scanning method in accordance with the present invention is applied to a self-capacitance scan circuit. The self-capacitance scan circuit has a first driving and receiving unit  21   a , a second driving and receiving unit  22   a , and a control unit  23 . The third embodiment of the scanning method has the following steps. 
     Step S 40 : The control unit  23  performs a pre-scanning procedure with respect to the sensing unit  10  under the self-capacitance scan mode. In the pre-scanning procedure, the control unit  23  sequentially outputs the driving signal to the first traces Y 1 ˜Y n  and the second traces X 1 ˜X m  with a fixed frequency or preset different frequencies and receives the corresponding sensed capacitance values from the first traces Y 1 ˜Y n  and the second traces X 1 ˜X m , and then acquires a capacitance offset of each of the first traces Y 1 ˜Y n  and the second traces X 1 ˜X m . 
     Step S 41 : The control unit  23  further performs the ADC calibration procedure and a subsequent self-capacitance scanning procedure with respect to the sensing unit  10  under the self-capacitance scan mode, and sequentially outputs the driving signal to the first traces Y 1 ˜Y n  and the second traces X 1 ˜X m . 
     Step S 42 : The driving and receiving unit  21   a ,  22   a  determines a driving and sampling frequency according to the capacitance offset of a corresponding driven first trace Y 1 ˜Y n  or a corresponding second trace X 1 ˜X m  when the driving unit  21  is controlled to output the driving signal, and stores the driving and sampling frequency in the first driving and receiving unit  21   a  or the second driving and receiving unit  22   a.    
     Step S 43 : The first driving and receiving unit  21   a  or the second driving and receiving unit  22   a  senses capacitive signals of the first traces Y 1 ˜Y n  or the second traces X 1 ˜X m  with the driving and sampling frequencies, converts the sensed capacitive signals into sensed capacitance values, and outputs the sensed capacitance values to the control unit  23 . 
     As regular touch devices are rectangular in shape, the first traces Y 1 ˜Y n  aligned along the first-axis direction (longitudinal side) are more prone to the issue of different RC loads arising from the lengths of the connection wires L than the second traces X 1 ˜X m  aligned along the second-axis direction (lateral side), only the first traces Y 1 ˜Y n  may be scanned during the foregoing pre-scanning procedure to acquire the capacitance offsets of the first traces Y 1 ˜Y n . Thus, in the following self-capacitance scanning procedure, the first driving and receiving unit  21   a  performs the self-capacitance scanning procedure with respect to the first traces Y 1 ˜Y n  with the driving and sampling frequencies determined according to the corresponding driving and sampling frequencies of the first traces Y 1 ˜Y n , and the second driving and receiving unit  22   a  performs the self-capacitance scanning procedure with respect to the second traces X 1 ˜X m  with the preset fixed frequency or the preset different frequencies. 
     There are several approaches for the control unit  23  to determine current sampling frequencies for the receiving unit  22  according to the corresponding capacitance offsets of the first traces. 
     Approach I: The control unit  23  first configures a lowest first sampling frequency reference value corresponding to a highest capacitance offset so that the capacitance offsets of the first traces progressively decreasing in magnitude correspond to the respective sampling frequencies progressively increasing from the lowest first sampling frequency reference value. Alternatively, the control unit  23  first configures a highest second sampling frequency reference value corresponding to a lowest capacitance offset so that the capacitance offsets of the first traces progressively increasing in magnitude correspond to the respective sampling frequencies progressively decreasing from the highest first sampling frequency reference value. Thus, a range of the sampling frequencies can be determined according to the corresponding capacitance offsets of the driven first traces. 
     Approach II: The control unit can set up a lookup table. The lookup table contains various capacitance offsets and corresponding sampling frequencies. A sampling frequency can be mapped in the lookup table by referring to the capacitance offset of a corresponding driven first trace in the lookup table. 
     From the foregoing first to third embodiments, irrespective of the applications of the mutual-capacitance scan circuit, the self-capacitance and mutual-capacitance scan circuit and the self-capacitance circuit, the present invention always performs a pre-scanning procedure with respect to the sensing unit  10  to acquire the capacitance offsets of the first traces and the second traces or either one of the first traces and the second traces, configures a sampling frequency according to a corresponding capacitance offset upon subsequent scanning, and scans the sensing unit  10  with the sampling frequencies. Hence, a higher sampling frequency is configured to correspond to a lower RC load of a sensor trace in the subsequent scanning, and a lower sampling frequency is configured to a higher RC load of a sensor trace. The present invention can not only acquire the sensed capacitance values more accurate than those acquired by using a fixed sampling frequency, but also can automatically adjust the sampling frequencies. In comparison with manual measurements for RC loads of sensor traces and suitable sampling frequencies, the present invention is simpler and more time-saving and provides a higher coordinate report rate of a touch object. 
     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 function of the invention, the disclosure is illustrative only. Changes may be made in detail, 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.