CA1243376A - Method and apparatus for capacitive keyboard scanning - Google Patents
Method and apparatus for capacitive keyboard scanningInfo
- Publication number
- CA1243376A CA1243376A CA000498468A CA498468A CA1243376A CA 1243376 A CA1243376 A CA 1243376A CA 000498468 A CA000498468 A CA 000498468A CA 498468 A CA498468 A CA 498468A CA 1243376 A CA1243376 A CA 1243376A
- Authority
- CA
- Canada
- Prior art keywords
- key
- capacitors
- row
- node
- pulse
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
Links
Classifications
-
- G—PHYSICS
- G06—COMPUTING; CALCULATING OR COUNTING
- G06F—ELECTRIC DIGITAL DATA PROCESSING
- G06F3/00—Input arrangements for transferring data to be processed into a form capable of being handled by the computer; Output arrangements for transferring data from processing unit to output unit, e.g. interface arrangements
- G06F3/01—Input arrangements or combined input and output arrangements for interaction between user and computer
- G06F3/02—Input arrangements using manually operated switches, e.g. using keyboards or dials
-
- H—ELECTRICITY
- H03—ELECTRONIC CIRCUITRY
- H03K—PULSE TECHNIQUE
- H03K17/00—Electronic switching or gating, i.e. not by contact-making and –breaking
- H03K17/94—Electronic switching or gating, i.e. not by contact-making and –breaking characterised by the way in which the control signals are generated
- H03K17/965—Switches controlled by moving an element forming part of the switch
- H03K17/975—Switches controlled by moving an element forming part of the switch using a capacitive movable element
- H03K17/98—Switches controlled by moving an element forming part of the switch using a capacitive movable element having a plurality of control members, e.g. keyboard
-
- H—ELECTRICITY
- H03—ELECTRONIC CIRCUITRY
- H03K—PULSE TECHNIQUE
- H03K17/00—Electronic switching or gating, i.e. not by contact-making and –breaking
- H03K17/94—Electronic switching or gating, i.e. not by contact-making and –breaking characterised by the way in which the control signals are generated
- H03K17/96—Touch switches
- H03K17/962—Capacitive touch switches
- H03K17/9622—Capacitive touch switches using a plurality of detectors, e.g. keyboard
-
- H—ELECTRICITY
- H03—ELECTRONIC CIRCUITRY
- H03M—CODING; DECODING; CODE CONVERSION IN GENERAL
- H03M11/00—Coding in connection with keyboards or like devices, i.e. coding of the position of operated keys
- H03M11/20—Dynamic coding, i.e. by key scanning
-
- H—ELECTRICITY
- H03—ELECTRONIC CIRCUITRY
- H03K—PULSE TECHNIQUE
- H03K17/00—Electronic switching or gating, i.e. not by contact-making and –breaking
- H03K17/94—Electronic switching or gating, i.e. not by contact-making and –breaking characterised by the way in which the control signals are generated
- H03K17/96—Touch switches
- H03K2017/9602—Touch switches characterised by the type or shape of the sensing electrodes
- H03K2017/9604—Touch switches characterised by the type or shape of the sensing electrodes characterised by the number of electrodes
- H03K2017/9613—Touch switches characterised by the type or shape of the sensing electrodes characterised by the number of electrodes using two electrodes per touch switch
Abstract
Abstract A capacitive keypad (24) is scanned in such a manner that the capacitance value (C) of an individual key in a given row is multiplied by the sum of the relative capacitance values of all the other keys in the same row by means of an amplifier (32) to generate a combined pulse.
The combined pulse is then compared by a comparator (28) to a threshold voltage (Vref 3) to verify whether the individual key in question is in the closed or open position. The verification can be made independently of keypad-to-keypad variations in the capacitances of the keys, since the combined pulse value does not depend upon the absolute capacitance value of the keys, but only upon their relative values. Also disclosed is a method for scanning to determine the presence of multiple key closures. This involves comparing the capacitance of an interrogated key to not only the sum of the other capacitors of its row, but also to the sum of the capacitors of the other row, and columns and a comparison of the resulting value to that generated by previous scans to detect change.
The combined pulse is then compared by a comparator (28) to a threshold voltage (Vref 3) to verify whether the individual key in question is in the closed or open position. The verification can be made independently of keypad-to-keypad variations in the capacitances of the keys, since the combined pulse value does not depend upon the absolute capacitance value of the keys, but only upon their relative values. Also disclosed is a method for scanning to determine the presence of multiple key closures. This involves comparing the capacitance of an interrogated key to not only the sum of the other capacitors of its row, but also to the sum of the capacitors of the other row, and columns and a comparison of the resulting value to that generated by previous scans to detect change.
Description
33~7~
METHOD AND APPARATUS FOR CAPACITIVE KEYsOARD SCANNING
Technical Field The invention relates to the scanning of keys of a capacitive keypad for detecting and verifying their switching states.
Back~round of the Invention Keyboards are essential in user interface equipment of various types, including calculators, typewriters, computer terminals, and the like. An electronic keyboard in its most elemental form is simply an array of switches which may be operated by the user to convert physical motion to electrical signals. The switches may take various forms. Make-or-break mechanical contact switches become degraded by mechanical wear and contamination and, additionally, have a tendency toward "contact bounce." Attempts to avoid these problems have led to the development of capacitive switches. These involve a capacitance having one plate which is movable by depression of the key. The resulting change in the capacitance value is sensed to verify the key depression.
A common way of sensing the capacitance value is to charge the capacitor by a current source to a reference voltage.
The required time for the charging can be used to determine whether the capacitance value is high or low by comparing it to some reference value. For example, the reference value may be a reference capacitor charged by a simi~ar current. The number of individual leads to the keys of a capacitive keyboard can be reduced by the use of key scanning techniques. These involve locating key closures by sensing the capacitance values of entire rows and columns of keys to identiey closed keys at their common intersections. Examples of some capacitive keyboards and scanning techniques Eor them are described in the following:
U.S. Pat. ~,211,915, issued Jul. 8, 1980 to . ~
~33~
METHOD AND APPARATUS FOR CAPACITIVE KEYsOARD SCANNING
Technical Field The invention relates to the scanning of keys of a capacitive keypad for detecting and verifying their switching states.
Back~round of the Invention Keyboards are essential in user interface equipment of various types, including calculators, typewriters, computer terminals, and the like. An electronic keyboard in its most elemental form is simply an array of switches which may be operated by the user to convert physical motion to electrical signals. The switches may take various forms. Make-or-break mechanical contact switches become degraded by mechanical wear and contamination and, additionally, have a tendency toward "contact bounce." Attempts to avoid these problems have led to the development of capacitive switches. These involve a capacitance having one plate which is movable by depression of the key. The resulting change in the capacitance value is sensed to verify the key depression.
A common way of sensing the capacitance value is to charge the capacitor by a current source to a reference voltage.
The required time for the charging can be used to determine whether the capacitance value is high or low by comparing it to some reference value. For example, the reference value may be a reference capacitor charged by a simi~ar current. The number of individual leads to the keys of a capacitive keyboard can be reduced by the use of key scanning techniques. These involve locating key closures by sensing the capacitance values of entire rows and columns of keys to identiey closed keys at their common intersections. Examples of some capacitive keyboards and scanning techniques Eor them are described in the following:
U.S. Pat. ~,211,915, issued Jul. 8, 1980 to . ~
~33~
2 --D.E~ Miller et al.
U.S. Pat. ~,305,135, issued Dec. 8~ 1981 to J.P. Dahl et. al.
U.S. Pat. ~,~05,917, issued Sept~ 20~ 1983 to T.Y. Chai U.S. Pat. ~,414,538, issued Nov. 8, 1983 to Schnizlein "Matrix Scanning Logic for a Capacitive Switching Keyb~ard," by John W. Volpe, in C~mputer Desi~n (USA) vol. 12, No. 1 8~-88, Jan. 1973 "Capacitive Keyboard FET Sense Circuit," by E.G.
Crabtree et al., in IBM Technical Disclosure Bulletin, Vol. 24, No. 5, Oct. 1981 One type of capacitive keyboard which is of particular interest commercially is the ~ilm, or membrane keypad, which is a laminated structure of two membranes which carry a printed circuit pattern separated by an insulating membrane con-taining a per~oration associated with each key area. The circuit patterns of the membranes include mutually opposed capacitance plates registered with the perforations. The pressing of one of the circuit pattern membranes into a perforation results in an increased capacitance for the opposed plates.
One troubling aspect of the manufacture of membrane keybads is that variations in the thickness dimensions of the perforated insulating membrane or in the thickness of other insulating layers which might be present between the opposed capacitance plates result in significant changes in the values of the capacitances from one keypad to the next. AS a result, it is necessary to adjust separately for each keypad the reference value to be used for veri~ying key closures. This adds considerably to the manuiacturing cost of the product.
Summary of the Inven ion In accordance with the novel method and apparatus of the present invention, a capacitive keyboard is scanned in a
U.S. Pat. ~,305,135, issued Dec. 8~ 1981 to J.P. Dahl et. al.
U.S. Pat. ~,~05,917, issued Sept~ 20~ 1983 to T.Y. Chai U.S. Pat. ~,414,538, issued Nov. 8, 1983 to Schnizlein "Matrix Scanning Logic for a Capacitive Switching Keyb~ard," by John W. Volpe, in C~mputer Desi~n (USA) vol. 12, No. 1 8~-88, Jan. 1973 "Capacitive Keyboard FET Sense Circuit," by E.G.
Crabtree et al., in IBM Technical Disclosure Bulletin, Vol. 24, No. 5, Oct. 1981 One type of capacitive keyboard which is of particular interest commercially is the ~ilm, or membrane keypad, which is a laminated structure of two membranes which carry a printed circuit pattern separated by an insulating membrane con-taining a per~oration associated with each key area. The circuit patterns of the membranes include mutually opposed capacitance plates registered with the perforations. The pressing of one of the circuit pattern membranes into a perforation results in an increased capacitance for the opposed plates.
One troubling aspect of the manufacture of membrane keybads is that variations in the thickness dimensions of the perforated insulating membrane or in the thickness of other insulating layers which might be present between the opposed capacitance plates result in significant changes in the values of the capacitances from one keypad to the next. AS a result, it is necessary to adjust separately for each keypad the reference value to be used for veri~ying key closures. This adds considerably to the manuiacturing cost of the product.
Summary of the Inven ion In accordance with the novel method and apparatus of the present invention, a capacitive keyboard is scanned in a
3~16 ~ 3 --manner in which the capacitance of a given key is compared to the capacitance of one or more other keys of the same keyboard. Verifica~ion of a key closure is determined by the relative capacitance values, rather than b2ing dependent upon a reference value. The relative capacitance values remain substantially the same from key-board to keyboard despite manufacturing variations, such as in the thickness of insulating layers for membrane keypads. In this way, the adjustment of a reference value for individual keyboards is made unnecessary Eor any variations in capacitances from one keyboard to the next~
In accordance with an aspect of the invention there is provided a method of determining the switched state of one of a plurality of key capacitors in rows and columns of a capacitive keyhoard, comprising the steps of:
applying an interrogating pulse to one side of the one key capacitor in one row and at the same time also to one side of at least one other key capacitor of the keyboard; then, sensing the resulting response pulse at the other side of the one key capacitor and multiplying it by tne resulting response pulse at the other side of the other key capacitors to obtain a combined pulse; and then, comparing the combined pulse to a reference threshold to generate a high-low output which indicates whether the one key is closed.
In accordance with another aspect of the invention there is provided a capacitive keyboard of the type having a plurality of key capacitors organized in an array of rows and columns and means for electronically scanning said capacitors to generate a high-low output indicative of the switching condition of the capacitors, the improved scan-ning means therein comprising: an operational amplifier having first and second input nodes and an output node, the first input node being connected to a first reference potential node; a comparator having first and second input nodes and an output node, the first input node being connected to the output node of the amplifier and the ~k33 - 3a -second input node being connected to a second reference potential node; first switching means for selectively connecting one side of any one of the capacitors of one of the rows alternately to an interrogating pulse source node and to the first input node of the comparator; second switching means for selectively connecting the other side of any one of the capacitors of the one row alternately to the second input node of the operational amplifier and to a supply voltage node; third switching means for periodically connecting the second input node of the amplifier to the output node of the amplifier; and control means having an `input node connected to the output node of the comparator for operating the first and second switching means.
Brief Description of ~t ~
FIGURE 1 is a schematic circuit diagram of a greatly simplified capacitive keypad arrangement in accordance with an embodiment of the present invention having only two keys.
FIGURE 2 is a simplified schematic circuit diagram of a capacitive keypad arrangement in accordance with another embodiment of the invention having several rows and columns of cross point connected keys and showing the circuitry associated therewith for the closure verification process.
Detailed DescriE~_n _ ~ ___ _ The circuit of the keypad 10 of Fig. 1 illustrates the na~ure of the key closure process in accordance with the invention. Only two keys are present in the keypad 10.
They are identified by their capacitances Cl and C2. The key C1 is connected between the inverting input port 12 of an operational amplifier 14 and a scanning pulse generator 16, which has its other side connected to ground potential.
The key C2 is connected in parallel with a zeroing switch S0 between the inverting input port 1? and the output port 22 of the amplifier 14. A voltage reference source 18 is connected between ground potential and the noninverting input port 20 of the amplifier 14. As the keypad 10 is '7~;
configured, the key C1 is scanned by a pulse from the pulse generator 16. The output pulse which will result at the output port 22 of the amplifier 1~ will have a magnitude of the inverse of the pulse from the pulse generator 16 times the ratio C1/C2. Thus it is seen that the magnitude of the reference voltage from the source 18 is not a significant factor in thls value. The magnitude of the scanning pulse from the generator can be readily made independent of ordinarily e~pected variations in power. The output of the amplifier 14, therefore, gives a capacitance ratio for the keys which can be compared to a reference value to determine whether the key C1 is closed. The reference value for the comparison can be the same for keypads with different values for their capacitances. So long as both the capacitances C1 and C2 are changed to the same degree, the ratio C1/C2 does not change. If it is now desired to scan the key C2, the keypad 10 must be reconfigured to effectively in-terchange the connections of C1 and C2, so that the capacitance of C2 is compared to that of C1. Such reconfiguration is readily accomplished by the provision of appropriate switches and controls.
The circuit of the ke~pad 24 of FIG. 2 illustrates in more detail the scanning process of a keypad in accordance with the invention. The keys of the keypad 24 are identified by their associated capacitances. These are designated by "C" followed by a two digit numher. The first digit indicates the row, and the second the column in which the key is located. A first side of all the capacitors in a particular column can be selectively connected to the source of an interrogation pulse of magnitude Vref1 or, alternatively, to the noninverting input 26 of a comparator 28 by khe action of a corresponding one of a set of toggle switches Sc1-Sc~ associated with the respective columns. A second side of all the capacitors in a particular row can be sel3ctl~ connaot~d ~o ~hs inv~tlng input ~ort 30 o~
~`
an operational amplifier 32 or, alternatively, to a positive supply voltage VDD by the action of a corresponding one of set of toggle switches SR1-SR4 associated with the respective rows. The noninverting input port 34 of the amplifier 32 is connected to a second reference voltage Vref2~ The output port 36 of the amplifier 32 is connected to the noninverting input port 26 of the comparator 28. The inverting input port 38 of the comparator 38 is connected to a third reference voltage Vref3. A zeroing switch S0 is connected between the invertinq input port 30 and the output port 36 of the amplifier 32 The switcheS Sc1~Sc4~ SR1 SR4, and S~ are operated by a controller 40.
They may take whatever form is most appropriate for the particular circuit implementation in question. For example, for an MOS circuit the switches are advantageously in the form of transmission gates. Controllers for operating the switches in whatever manner is suitable for their unction are Xnown to those in the art.
In the operation of the keypad 24, one capacitor o each row is compared with all the remaining capacitors of that same rown As the keypad 24 of FIG. 2 is configured, the value of C41 is being compared to the sum of the capacitors C42,C43,C44. The zeroing switch S0 is initially closed to bring the inverting input port 30 of the amplifier 32 to the same voltage as the output port 36.
Then the zeroing switch S0 is opened and a pulse of magnitude Vrefl below the reference voltage VDD
is applied to the first sides of all the capacitors of the first column. At this time the first sides of the capacitors of the remaining columns are connected to the noninverting input port 26 of the comparator 28. Also at this time, the other sides of the capacitors of the fourth row are connected to the inverting input port 30 of the - 6 - ~ 3t~
- amplifier 32, while the other sides of the capacitors of all the o~her rows are connec~ed ~o the positive supply voltage VDD. It is reasonable to assume the loop gain of the amplifier 32 to be sufficiently high to prevent the sum of the capacitances from the other sides of the capacitors of all the other rows rom adversely affecting its output. The output of the amplifier is a pulse of magnitude Vx above Vref2, which can be expressed as Vref2-Vref1 * C41/(C42~C43~C44)-The output Vx is then compared to a key closurethreshold reference voltage Vref3 by khe comparator 28 to the result in an output HL at the output port 42 of the comparator 28.
If the next key to be scanned is C42, then the switches sc1 and Sc2 are changed to their alternate positions and the pulsing process is repeated.
In this manner the entire keypad 24 can be scanned, and the condition of each key can be determined and verified by the reference voltage V~ef3 comparison with accuracy, regardless of mass variations in the values of the capacitances of the keys, since the variable parameter for de~ermining closure is independen~ of a reference voltage level.
While in the keypad 24 the key C41 of interest is compared to the other remaining keys in its row, it could also be compared to any one or more other keys on the keypad and still be within the scope of the invention if the comparison is accomplished by only a single scanning pulse for each key. However, comparison to the other keys of the row is advantageous in that it requires a minimum amount of switching for the scanning process.
A particularly advantageous logic sequence for scanning the keypad 24 is to continue to scan a key which has been detected as being in the closed position until it is detected as being open once again. The effects of ., ~ 3t~
errors resulting from noise or key bounce can be reduced by requiriny multiple successive detections of either the closed or open position of the key to be made before verification of that condition is considered established.
'rhe above scanning procedure for the keypad 24 is for detecting individual key closures, and does not detect multiple key closures. For detecting multiple key closures, the basic operation is similar to that described above, except that each key is compared to the additional row, then column keys. A detection in either comparison results in the continual scanning of the key until 3 successive scans verify the key closure. Once the key being interrogated is verified as being either open or closed, the keypad scan in resumed. At the -time of validation, the row-column information is compared to a second stage of information and this comparison is used to enable the generation of key closures independently. The keypad infor~ation is then transferred to the second stage for further comparisons.
Although the present invention is particularly advantageous for membrane keypads, it is applicable to any capacitive keyboard in which there are variations in the values of the key capacitances from one keyboard to the next.
In accordance with an aspect of the invention there is provided a method of determining the switched state of one of a plurality of key capacitors in rows and columns of a capacitive keyhoard, comprising the steps of:
applying an interrogating pulse to one side of the one key capacitor in one row and at the same time also to one side of at least one other key capacitor of the keyboard; then, sensing the resulting response pulse at the other side of the one key capacitor and multiplying it by tne resulting response pulse at the other side of the other key capacitors to obtain a combined pulse; and then, comparing the combined pulse to a reference threshold to generate a high-low output which indicates whether the one key is closed.
In accordance with another aspect of the invention there is provided a capacitive keyboard of the type having a plurality of key capacitors organized in an array of rows and columns and means for electronically scanning said capacitors to generate a high-low output indicative of the switching condition of the capacitors, the improved scan-ning means therein comprising: an operational amplifier having first and second input nodes and an output node, the first input node being connected to a first reference potential node; a comparator having first and second input nodes and an output node, the first input node being connected to the output node of the amplifier and the ~k33 - 3a -second input node being connected to a second reference potential node; first switching means for selectively connecting one side of any one of the capacitors of one of the rows alternately to an interrogating pulse source node and to the first input node of the comparator; second switching means for selectively connecting the other side of any one of the capacitors of the one row alternately to the second input node of the operational amplifier and to a supply voltage node; third switching means for periodically connecting the second input node of the amplifier to the output node of the amplifier; and control means having an `input node connected to the output node of the comparator for operating the first and second switching means.
Brief Description of ~t ~
FIGURE 1 is a schematic circuit diagram of a greatly simplified capacitive keypad arrangement in accordance with an embodiment of the present invention having only two keys.
FIGURE 2 is a simplified schematic circuit diagram of a capacitive keypad arrangement in accordance with another embodiment of the invention having several rows and columns of cross point connected keys and showing the circuitry associated therewith for the closure verification process.
Detailed DescriE~_n _ ~ ___ _ The circuit of the keypad 10 of Fig. 1 illustrates the na~ure of the key closure process in accordance with the invention. Only two keys are present in the keypad 10.
They are identified by their capacitances Cl and C2. The key C1 is connected between the inverting input port 12 of an operational amplifier 14 and a scanning pulse generator 16, which has its other side connected to ground potential.
The key C2 is connected in parallel with a zeroing switch S0 between the inverting input port 1? and the output port 22 of the amplifier 14. A voltage reference source 18 is connected between ground potential and the noninverting input port 20 of the amplifier 14. As the keypad 10 is '7~;
configured, the key C1 is scanned by a pulse from the pulse generator 16. The output pulse which will result at the output port 22 of the amplifier 1~ will have a magnitude of the inverse of the pulse from the pulse generator 16 times the ratio C1/C2. Thus it is seen that the magnitude of the reference voltage from the source 18 is not a significant factor in thls value. The magnitude of the scanning pulse from the generator can be readily made independent of ordinarily e~pected variations in power. The output of the amplifier 14, therefore, gives a capacitance ratio for the keys which can be compared to a reference value to determine whether the key C1 is closed. The reference value for the comparison can be the same for keypads with different values for their capacitances. So long as both the capacitances C1 and C2 are changed to the same degree, the ratio C1/C2 does not change. If it is now desired to scan the key C2, the keypad 10 must be reconfigured to effectively in-terchange the connections of C1 and C2, so that the capacitance of C2 is compared to that of C1. Such reconfiguration is readily accomplished by the provision of appropriate switches and controls.
The circuit of the ke~pad 24 of FIG. 2 illustrates in more detail the scanning process of a keypad in accordance with the invention. The keys of the keypad 24 are identified by their associated capacitances. These are designated by "C" followed by a two digit numher. The first digit indicates the row, and the second the column in which the key is located. A first side of all the capacitors in a particular column can be selectively connected to the source of an interrogation pulse of magnitude Vref1 or, alternatively, to the noninverting input 26 of a comparator 28 by khe action of a corresponding one of a set of toggle switches Sc1-Sc~ associated with the respective columns. A second side of all the capacitors in a particular row can be sel3ctl~ connaot~d ~o ~hs inv~tlng input ~ort 30 o~
~`
an operational amplifier 32 or, alternatively, to a positive supply voltage VDD by the action of a corresponding one of set of toggle switches SR1-SR4 associated with the respective rows. The noninverting input port 34 of the amplifier 32 is connected to a second reference voltage Vref2~ The output port 36 of the amplifier 32 is connected to the noninverting input port 26 of the comparator 28. The inverting input port 38 of the comparator 38 is connected to a third reference voltage Vref3. A zeroing switch S0 is connected between the invertinq input port 30 and the output port 36 of the amplifier 32 The switcheS Sc1~Sc4~ SR1 SR4, and S~ are operated by a controller 40.
They may take whatever form is most appropriate for the particular circuit implementation in question. For example, for an MOS circuit the switches are advantageously in the form of transmission gates. Controllers for operating the switches in whatever manner is suitable for their unction are Xnown to those in the art.
In the operation of the keypad 24, one capacitor o each row is compared with all the remaining capacitors of that same rown As the keypad 24 of FIG. 2 is configured, the value of C41 is being compared to the sum of the capacitors C42,C43,C44. The zeroing switch S0 is initially closed to bring the inverting input port 30 of the amplifier 32 to the same voltage as the output port 36.
Then the zeroing switch S0 is opened and a pulse of magnitude Vrefl below the reference voltage VDD
is applied to the first sides of all the capacitors of the first column. At this time the first sides of the capacitors of the remaining columns are connected to the noninverting input port 26 of the comparator 28. Also at this time, the other sides of the capacitors of the fourth row are connected to the inverting input port 30 of the - 6 - ~ 3t~
- amplifier 32, while the other sides of the capacitors of all the o~her rows are connec~ed ~o the positive supply voltage VDD. It is reasonable to assume the loop gain of the amplifier 32 to be sufficiently high to prevent the sum of the capacitances from the other sides of the capacitors of all the other rows rom adversely affecting its output. The output of the amplifier is a pulse of magnitude Vx above Vref2, which can be expressed as Vref2-Vref1 * C41/(C42~C43~C44)-The output Vx is then compared to a key closurethreshold reference voltage Vref3 by khe comparator 28 to the result in an output HL at the output port 42 of the comparator 28.
If the next key to be scanned is C42, then the switches sc1 and Sc2 are changed to their alternate positions and the pulsing process is repeated.
In this manner the entire keypad 24 can be scanned, and the condition of each key can be determined and verified by the reference voltage V~ef3 comparison with accuracy, regardless of mass variations in the values of the capacitances of the keys, since the variable parameter for de~ermining closure is independen~ of a reference voltage level.
While in the keypad 24 the key C41 of interest is compared to the other remaining keys in its row, it could also be compared to any one or more other keys on the keypad and still be within the scope of the invention if the comparison is accomplished by only a single scanning pulse for each key. However, comparison to the other keys of the row is advantageous in that it requires a minimum amount of switching for the scanning process.
A particularly advantageous logic sequence for scanning the keypad 24 is to continue to scan a key which has been detected as being in the closed position until it is detected as being open once again. The effects of ., ~ 3t~
errors resulting from noise or key bounce can be reduced by requiriny multiple successive detections of either the closed or open position of the key to be made before verification of that condition is considered established.
'rhe above scanning procedure for the keypad 24 is for detecting individual key closures, and does not detect multiple key closures. For detecting multiple key closures, the basic operation is similar to that described above, except that each key is compared to the additional row, then column keys. A detection in either comparison results in the continual scanning of the key until 3 successive scans verify the key closure. Once the key being interrogated is verified as being either open or closed, the keypad scan in resumed. At the -time of validation, the row-column information is compared to a second stage of information and this comparison is used to enable the generation of key closures independently. The keypad infor~ation is then transferred to the second stage for further comparisons.
Although the present invention is particularly advantageous for membrane keypads, it is applicable to any capacitive keyboard in which there are variations in the values of the key capacitances from one keyboard to the next.
Claims (11)
1. A method of determining the switched state of one of a plurality of key capacitors in rows and columns of a capacitive keyboard, comprising the steps of:
applying an interrogating pulse to one side of the one key capacitor in one row and at the same time also to one side of at least one other key capacitor of the keyboard; then, sensing the resulting response pulse at the other side of the one key capacitor and multiplying it by the resulting response pulse at the other side of the other key capacitors to obtain a combined pulse; and then, comparing the combined pulse to a reference threshold to generate a high-low output which indicates whether the one key is closed
applying an interrogating pulse to one side of the one key capacitor in one row and at the same time also to one side of at least one other key capacitor of the keyboard; then, sensing the resulting response pulse at the other side of the one key capacitor and multiplying it by the resulting response pulse at the other side of the other key capacitors to obtain a combined pulse; and then, comparing the combined pulse to a reference threshold to generate a high-low output which indicates whether the one key is closed
2. The method defined in claim 1 wherein the interrogating pulse is applied to all the other key capacitors in the one row, said resulting response pulse at the other side of the one key capacitor being multiplied by the resulting response pulse at the other side of all the other key capacitors in the one row.
3. The method defined in claim 2, comprising also the step of comparing the combined pulse resulting from the detection of the one key of the one row to the combined pulses of the detection of the other rows and columns.
4. The method defined in claim 3 wherein the additional step includes comparing the high-low output resulting from the detection of the one key of the one row to the high-low output resulting from the detection of all the keys in another row.
5. The method defined in claim 4 wherein the other row is adjacent the one row.
6. The method defined in claim 5 comprising scanning the one key at least three times in succession to verify its closure.
7. The method defined in claim 6 comprising scanning the one key at least three times in succession to verify its return to the open condition.
8. The method defined in claim 7 wherein the comparing of the additional step is to a high-low output stored as a result of the detection of all the keys of all the rows and columns.
9. A capacitive keyboard of the type having a plurality of key capacitors organized in an array of rows and columns and means for electronically scanning said capacitors to generate a high-low output indicative of the switching condition of the capacitors, the improved scanning means therein comprising:
an operational amplifier having first and second input nodes and an output node, the first input node being connected to a first reference potential node;
a comparator having first and second input nodes and an output node, the first input node being connected to the output node of the amplifier and the second input node being connected to a second reference potential node;
first switching means for selectively connecting one side of any one of the capacitors of one of the rows alternately to an interrogating pulse source node and to the first input node of the comparator;
second switching means for selectively connecting the other side of any one of the capacitors of the one row alternately to the second input node of the operational amplifier and to a supply voltage node;
third switching means for periodically connecting the second input node of the amplifier to the output node of the amplifier; and control means having an input node connected to the output node of the comparator for operating the first and second switching means.
an operational amplifier having first and second input nodes and an output node, the first input node being connected to a first reference potential node;
a comparator having first and second input nodes and an output node, the first input node being connected to the output node of the amplifier and the second input node being connected to a second reference potential node;
first switching means for selectively connecting one side of any one of the capacitors of one of the rows alternately to an interrogating pulse source node and to the first input node of the comparator;
second switching means for selectively connecting the other side of any one of the capacitors of the one row alternately to the second input node of the operational amplifier and to a supply voltage node;
third switching means for periodically connecting the second input node of the amplifier to the output node of the amplifier; and control means having an input node connected to the output node of the comparator for operating the first and second switching means.
10. The apparatus defined in claim 9 wherein the capacitors are cross-point connected.
11. The apparatus defined in claim 10 wherein the first switching means selectively connects the one side of all the capacitors in a given column to the pulse source node or to the first input node of the comparator, and wherein the second switching means selectively connects the other side of all the capacitors in a given row to the second input node of the amplifier or to the supply voltage node.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US06/685,856 US4651133A (en) | 1984-12-24 | 1984-12-24 | Method and apparatus for capacitive keyboard scanning |
| US685,856 | 1984-12-24 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1243376A true CA1243376A (en) | 1988-10-18 |
Family
ID=24753958
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000498468A Expired CA1243376A (en) | 1984-12-24 | 1985-12-23 | Method and apparatus for capacitive keyboard scanning |
Country Status (6)
| Country | Link |
|---|---|
| US (1) | US4651133A (en) |
| EP (1) | EP0187021A3 (en) |
| JP (1) | JPS61203716A (en) |
| KR (1) | KR940001107B1 (en) |
| CA (1) | CA1243376A (en) |
| ES (1) | ES8705639A1 (en) |
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-
1984
- 1984-12-24 US US06/685,856 patent/US4651133A/en not_active Expired - Lifetime
-
1985
- 1985-12-18 EP EP85309230A patent/EP0187021A3/en not_active Ceased
- 1985-12-23 ES ES550355A patent/ES8705639A1/en not_active Expired
- 1985-12-23 CA CA000498468A patent/CA1243376A/en not_active Expired
- 1985-12-23 KR KR1019850009717A patent/KR940001107B1/en not_active IP Right Cessation
- 1985-12-24 JP JP60289505A patent/JPS61203716A/en active Granted
Also Published As
| Publication number | Publication date |
|---|---|
| EP0187021A2 (en) | 1986-07-09 |
| EP0187021A3 (en) | 1987-06-24 |
| JPH0556690B2 (en) | 1993-08-20 |
| KR940001107B1 (en) | 1994-02-14 |
| JPS61203716A (en) | 1986-09-09 |
| ES8705639A1 (en) | 1987-05-01 |
| KR860005277A (en) | 1986-07-21 |
| US4651133A (en) | 1987-03-17 |
| ES550355A0 (en) | 1987-05-01 |
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