CA1147061A - Self-optimizing touch pad sensor circuit - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

A plurality of capacitive touch pad sensors are multiplexed to the input of a common charge transfer analog-to-digital converter (CTAD) under the control of a control logic circuit. The no-touch digital output of each touch pad is stored in a memory. The touch pads are then sequentially addressed and their output level is digitized in the CTAD
and the digital output of each touch pad is compared to its no-touch value stored in the memory. If the digital value read is sufficiently different from the no-touch memorized value, a touch-detection signal is given. The no-touch digital reading in the memory is periodically updated, by a count of only one at most, if the updated no-touch value differs from the no-touch value in the memory.

Description

~ 9 Sl~lF-OPII~IZING ~OUCII l~An SlNSOR CIRCUIT

BACKGROUND OF THE INVENTION
This invention relates to a touch pad sensor circuit, and more specifically relates to a novel arrange-ment for multiplexing the readout of a plurality of capacitive touch pads using digital techniques.
Capacitive touch sensors are a well kno~n ~.eans for providing inputs to various devices including home appliances such as kitchen ranges which have touch pad devices arranged in a keyboard style array. The capacitive touch sensor is useful in this application since it isolates the user from system control circuits an~ voltages.
A typical capacitive touch sensor of the prior art includes a high voltage pulse generating device coup:Led to a capacitive touch plate which is, in turn, coupled to a receiver circuit. The touch plate is one electrode of a capacitor, isoLatedby a dielectric such as a glass plate.
When the user touches the glass plate, the capacitance value of the touch plate capacitor is changed since the capacitance to ground of the one electrode of the device is changed by the presence of the user. This capacitance change is small, but it will attenuate the pulse signal transferred by the capacitor from the pulse generator circuit to the receiver circuit. However, to provide reliable detection of the touch of the plate by a user, relatively high voltages should be generated by the pulse source, and sensitive detection circuits are needed which have stable, long-term operating characteristics.
A digital capacitance measuring circuit for digitiz-ing the output of a capacitive touch plate is described inour U.S. Patent 4,039,940~ dated August 2, 1977, entitled :.. ....: .. ... :: . ... . .
.,, ., .. . . , , . . . . : . : : :

~ 7~ RD-9689 C~PACITAN~ SENS0R, and assigned to the assignee of the present invention. In this patent, a charge transfer analog-to-digital converter (hereafter CTAD) circuit is shown which produces a digital output from the touch pad detector, which digital output changes from a no-touch value to a touch value in response to the touching of the sensor.
All touch pad systems to date, including the touch pad system described above in our U.~. Patent ~,039,940, have been hampered by a number of interrelated problems. Thus, the multiplexing limit of presently known touch pad systems is approximately two touch pads per sensor. In order to exceed two touch pads per sensor, extremely high drive voltages on the order of 100 to 200 volts, for example, are r~quired. Moreover, parasitic capacitive coupling between different drivers and the same sensor will cause differences in ~he voltage sensed for the different drivers for either touch or no-touch conditions.
Another of the problems is that in order to reliably `
sense the touch or no-touch condition, only about two drive lines can be associated with any given sensor. As many as three drive lines may be used for a given sensor but extreme care would be needed to balance the received signals from various drivers. This extreme care extends to touch pad layout and places undue layout restrictions on the placement and size of the touch pads.
A further problem is that in systems where there are a large number of touch pads, the necessity for having different threshold values for each sensor increases the system cost and requires a liarge number of trimming operations.
Variations in the characteristics of the individual touch pad panels have produced additional problems where these variations may be caused because of differences in the thickness of the glass and in the dielectric constant of the glass of the different tOUC]I pads. Variations of only a few ~2--,~ 4~ RD-9689 percent between touch pads can cause significant difficulty in making the touch or no-touch decision.
A -further source of variability is the aging of the touch pads, over a long period, and in environmental effects on the touch pad, such as buildup o-f grease and cooking material films on the glass panel which can be expected in household appliances.
In view of the above problems, good reliability can be obtained with a multiplexing level of one but the number of sensors required would mean additional circuit chips and a tremendous number of connections to the touch ` .
pad panel.
Multiplexing levels of two to three might be used with presently available technology to obtain reliable operation but these impose significanl; constraints on the appearance and user function aspects of the panel design.
BRIEF DESCRIPTION OF THE PRESENT INVE~TION
In accordance with the present invention, a plurality of capacitive touch pads is- associated with memory and logic elements in a ~anner *o allow a very high level of multiplexing to be obtained. The arrangement of the present invention is inherently tolerant to variations in the actual touch pad circuitry and in the touch pad driyer circuitry, and permits a minimum number of connections between the touch pad panel and the touch pad sensing electronics.
The present invention also accommodates long-term drift in circuit parameter values, and permits maximum latitude to the system designer in the placement and configuration of the touch pad.
In accordance with the invention, an analog-to-digital (A/D) conve~ter, preferably a CTAD,is provided to convert voltages associated with each of the touch pads into digital values. The CTAD converter may utilize a circuit identical to that disclosed in our U.S. Patent 4,039,940 RD ~ t3~ ~, 9 ~L~4 ~
di.s(ulsst-~c] ~Il)ovt,~. 'I'he di.g:it~ L readi~ ol>tained ~f'or the no tOUC]l co]1tlit.iol1 :for each touch pacl i.s stored iTI a memory. A '', control logic c:irc:u:it then cycles through all touch pads and compares the digital reading obtained from eacl1 of the tOIlCl1 ~
pads to the value of the no-touch reading for the respec1,ive ..
touch pad which has been stored in the memory. IY11en a .' significant departure from the no-touch condition is o~)tained, .:
in the proper direction, a touch indication is given for that particular touch ~ad and an appropriate control. functio]1 l.0 is initiated throu~h other contro1 circuitry. ,, Two different circuits are provided for determining ~hen a touch condition exists. In the first, a fixed value below the no-touc}1 value in memory is required to indicate a ,' tOUC]1 condit:ion. l'n the second arrange]llt-~l1t~ a fixed percentage of the no-touch value is de~ined, below which any received pulse is recognized as a touch condition. '' In accordance with an important feature of the ,:
' inventionl the touch pad sensor circuit is made to be self-optimi.zing by periodically operating all touch pads in an 20 optimizing mode whi.ch updates the no-toucll digital output in'.
the system mellZory. ~`he updated reatding, hol~ever, can be changed by only one count cluring any cycle of the optimizing mode to.prevent A transiellt sigTlal from causing any subst.ll1tial change in the no-touc}1 readings for any touch pad. The optimizing mode is interspersed with the normal touch or no-touch sensi.n~ mode as desired and as determinetl by the control logi.c means. Thus, the circuit hecomes inherently tolerant of all variations in the tOuc]l pad circuitry and is freed Eor the need for any maTIl.lal adjuslment of trimming of the circuit.
From the al)ove, orle object of the present inventio is to digitize the developed vol.tages associa~etl with each ~ f).,() Or a plur~ y of t:o~lch p~ s in a to~ch co11i.rol1el systeln;
to store l:h( no-~oucl~ d.i.gitl:l value fol- each tollch pad in a `:.
memory; and to compare the present digital reading for eac11 -touch pad with the no-to~lcl1 di~ital reading prev;.ously ~
stored to determine when a touch condition has occurled. n Anot11er object of this invention is to provide a novel method for continuous]y up1ating the no-~ouch vo1tage reading in a digital memory in such a way that these 1~eltlings are not significantly changed by short-term transients.
Stili another object of ti1is invention is to obtain an adjustment-free method for sensi.ng touch pad signals~ wh:ich are subject to a high level of`multiplexing.
Yet another object of this invention is to provide ....
a novel tOUC]I control s~ste~ l1ic}1 provi.des the designer with great freedom o:f layollt arld manufactllrit1g tolerance standards. :
These and other objects of this inventi.on wil]. become apparent upon consideration of the :Eollo~ing detailed desc1iption taken with the drawings BRIEF DESCRIPrION 0F 1`~iE DRAWIN(S ::
Figure l is a block di.agram of a first embodiment of the invention wherein a touch condition is indicated ~hen tl1e digital OUtpl~t of any of the touch pads is some fixed value below the no-touch value of the pad wl1ich was stored in the memory of the system.
I:igure 2 :is a block diagram of a secol1d embodiment of the invention wherei11 -ti1e tolicll co11~ition is recognizcd ..
when the digi~al o~lt~Ut of any of the touch pad; is a fi~ed .

percentage of the no-toucl1 vallle below the no-touct1 value stored 3) i.n ~he memory; a;;d .:
~ `:igures 3a to 3h are timing diagrams for Lhc various voltages an1 signals of Figllres l and 2.
I)lrAILII) I)L~CRIlTI()N OF TIIE INV~NI`I~N
l~eferring first to Figure 1 theIe is sho~.n in bloc~ diagrammatic form a processing circuit for processing --5-- .

RD-96$9 the outputs of four touch pads TPl, TP2, TP3 and TP4. While ~.
four touch pads are shown in Figure l, it should be under~
stood that any desired number of touch pads could have been , ~ho~n. ~ t Each of the touch pads of Figure 1 consists of a '7., flat plate, shown as fla~ plates lO to 13 for touch pads TPl to TP4, respectively, where each of the flat plates -~
represents the co~non electrodes of a pair of coupling~'.
capacitors Ccl and Cc2 (shown only Eor TP3, in the interests of ~implicity).This common electrode of the coupl.ing capacitors may .
be coupled to ground, as during a touch condition, so that ~,~
the capacitance-to~-ground of the touch pads changes when an 'i~:!
operator physically touches the common electrode (or an insulator 1' suppor~ed by the common electrode~ with his finger.
Touch pads TPl and TP2 are connected to driver :
line Do b~ the coupling capacitors C'cl~ and are similarly ;3'~
coupled to output lines SO and Sl, respectively, by the coupling "' capacitor Cc2. Additional touch pacls of an extended array could have been connected to the line Do. Line Dl is sia~ rl.
coupled to touch pads TP3 and TP4 and these touch pads are , `~
also capacitively coupled to the output lines SO and Sl, respectively. Again, additional touch pads could have been coupled to the line ~l and to other outpu~ lines.
Lines Dl and Do are output lines of a driver means `.
14 which produces output voltage pulses on lines Dl and D
(Figures 3b and 3c, respectively) which may be relatively :
low-voltage pulses of the order, for example, of lO volts as directed by a control logic means 15, as will be described -.
more fully hereinafter.
The output lines SO and Sl are connected to the input of a suitable analog-to-digital converter 16. The signal on each line SO or Sl (~igures 3d and 3e, respectively) or any other line coming into the analog-to-digital convelter means, as selected by t}le control logic means 15, is converted from an analog value to its di~,ital value by A/D converter ]6.

~ .L RD-9689 ,~
In a preferred embodlment o~ the invention, the A/D converter 16 i5 of the CTAD type shown in our U.S. Patent ~,039,940.
The ou~put 16a o the CTAD eircuit 16 i8 applied to `.
a count input 17a of a CTAD counter 17. The A/D converter 16 also produces a "conve~sion done" output 16b (Figure 3f~
when it is has completed the conversion of the signal on line 1 SO or ~
A window-preset-word input circuit 18 is also .~.
ap~lied to a data input 17b o~ the counter 17 to establish a preselected, fixed count in counter 17 prior to counter 17 1 3 receiving a conversion count from A/D converter means, for :
a purpo~e hereinbelow more fully described. It should be ;1 under8tood that a single "window" count may be permanently ,'l~t wired into the window circuit (as implementing block 18 by .,.
permanent wiring at the data input of a resetable counter ,`1;
used for clrcuit 17), and that a manually or electrically .. ¦~
selectable fixed count can be implemented by means of swI,ches 1~
and the like at the counter data inputs in manner ~!' known to the art~
A digital memory 19 which may be a semiconductor '1l:
memory or any other desired type of memory is provided to ~.^
store the no-touch digital output reading of each of the ~-touch pads TPl to TP4 as will be described hereinafter. The ~i memory 19 is controlled by the control logic means 15 and -, can read out or receive data, via output 19b. or input '~.
19c, r,espectively, with respect to up/down counter` i-,.
: 20. The up/down counter 20 and the counter 17 are each connected to inputs of comparator 21 as shown. The comparator 21 has output lines 22 and 23, which, respectively, go high when the count in counter 20 exceeds or is less than 1 ' ~' ~7~
the count in counter 17.
The original processing circuit includes AND gates 24 and 25 which are connected to the up and down inputs, re-spectively, of the up/down counter 20 as well as AND gates 26 and 27 which are connected to the preset and clear input terminals 17c and 17d respectively of counter 17 as shown.
One input of AND gate 26 includes an inverter 29 connected to the optimized output line of the control logic means 15.
Finally, an AND gate 30 is provided which has an output indicating a touch condition at whichever touch pad TPl to TO4 has been addressed by the control logic means 15 at the time the touch-detected output is produced.
The opera-tion of Figure 1 is now described with reference being made to the timing diagrams of Figure 3a to 3h. As shown in Figure 3a, the control logic means 15 produces a reset pulse at the beginning of each time interval tl, t2, t3 and t4. Each of these pulses is produced at the same time that the memory means 19 is addressed for the no-touch readings stored in the memory for one of the touch pads TPl to TP4, respectively. Thus, during the time interval tl, the no-touch reading for touch pad TPl is being addressed and the processing of the circuit is concerned with the condition of the touch pad TPl. In a similar manner, during intervals t2, t3 and t4, the conditions of touch pads TP2, TP3 and TP4, respectively, are involved.
Considering first the cycle involving touch pad TPl, the cycle begins with a reset pulse (Figure 3a) at reset output 15a at the beginning of cycle tl, which pulse turns on the transistors 31 and 32 to connect lines S

~ 1 ~ 7~ RD-9689 and Sl to ground during the duration of the pulse. At the same time, the reset pulse will either clear or preset counter 17, depending upon the o~her inputs to gates 26 and 27 as will be disclosed more fully hereinafter. The reset pulse also fills the up/down counter 20 with the contents of the appropriate memory location of memory 19 containing the digital value o the lat3t no-touch reading of touch pad TPl. Note that the control logic 15 will supply the proper address to the address input l9a of ~ memory 19, corresponding to the particular touch pad being measured. Memory 19 outputs, at l9b, the stored no-touch value for the particular touch pad, for transfer to up/down counter 20.
When the reset pulse o~ Fi.gure 3a in interval tl ends, the appropriate driver line Do or Dl is turned on by t~e control logic means 15, as shown in Figures 3b and 3c, respectively Thus, when touch pad TPl is being measured, line Do is turned on. Note that line Do is also turned on in the next period t2 when touch pad TP2 is being measured.
Line Dl i8 turned on when touch pads TP3 an~ TP4 are being measured in time intervals t3 and t4, respectively.
The AtD converter 16 then measures the voltage on the appropriate sensed line SO or Sl (Figures 3d and 3e, respectively) as addressed by the control logic to correspond to the particular touch pad being measured. The counter, prior to receipt of the reading from A/D converter 16, has had a "window" count preset therein by the action of turning on the preset input 17c (via AND gate 26 whenever ~he RESET and OPTIMIZE outputs of control logic 15 are respec~ively present and not presentj i.e. respectively _g_ ~ ~7 ~ ~1 RD 968 a logic one and a logic zero in a positive logic system).
The resultant reading from the A/~ converter 16 i.s then counted in the counter 17, in additlon to the preset "wlndow"
count already stored in çounter 17.
When the voltage conversion in converter 16 is compl~ted, as indicated by a conversion-done signal (Figure 3f) which is applied to gates 24, 25 and 30, a comparison is made in the comparator 21 of the contents of counter 17 which i5 the measured value of (a) the touch pad output plus the preset "window" value, if any such value has been previously ' selected, and (b) the contents o counter 20 received from memory 19 which is the last no-touch value of the touch pad being measured. If the counter 17 count (signal A) is less than the count of counter ~0 (signal B), a signal (B ~A out7~ut) is applied to comparat:or output line 22.
This output, along with the conversion done output from counter 16 and the output from inverter 29 (since there is not an`"optini~e" output), will trip the touch-detect gate 30 indicating that a touch condition has been detected for the touch pad under evaluation. An output from the touch-detect gate 30 during interval tl identifies that touch pad TPl has been touched, thereby to cause the operation of some suitable control circuit (not shown). Thus, as illustrated for TPl, in Figure 3d, the no-touch output NT has a magnitude exceeding the magnitude of the touch otuput T for the particular touch pad then being interrogated. The difference between the no-touch and touch outputs (i.e. NT count - T count) must be established at some non-zero value3 taking into account variations of the signal on the output line te.g. SO) due to noise and other transient signals. Thus a lower ~ t ~ 'tt ij~ t ~?~ 7~ tJ~ `;j fi ~

~47~9~ RD-9689 limit NT' is established as a number of counts below which limit the touch pad output mu~t fall prior to a valid touch condition (at magnitude T) being establlshed. The difference W between the expected no-touch magnitude NT and the lower no-touch limit magnitude (i.e. NT-NTI~ is the "window"; this "window" is a preselected fixed-offset coun~ preloaded from circuit 18 into counter 17 to bias the count in a direction opposite that of a touch output to assure that transients and other undesired signals do not cause a lower, touch count (T) when the associated touch pad has actually not been activated.
In a similar manner, the output of a touch-detect signal during intervals t2, t3 or t4 will indicatethat touch pads TP2, TP3 or TP4, respectively, have been touched, as shown in Figure 3h.
During the last part of any cycle associated with any particular touch pad, the contents of the up/down counter 20, which is the no-touch value o~ the particular touch pad, is rewritten into the memory 19, as indicated in Figure 3g.
This process then continues sequentially to constantly monitor whether or not a touch has been registered on any o~
the touch pads TPl to TP4 as well as any other additional touch pads w~ich might be added to the system.
In accordance with an important feature of the invention, the system of Figure 1 is systematically operated in an optimi~ation mode after some given number of measure-ment modes of operation. During the system optimization mode, the memory l9 will be updated with the latest values of the no-touch condition ~or the touch pads TPl to TP4.

~ 1 ~ 7~ RD-9689 When the optimization mode is entered, the control logic cirucit 15 produces an output signal on the "optimize"
line 15b and the operation of the circuit is ldentical to its normal operation with two exceptions. The first is that during the reset period, the counter 17 is not preset to a "window" value (as the output of gate 26 does not activate counter input 17c), but instead the counter is cleared to a zero value. That is, no offset is applied to counter 17. This is because the two inputs of gate 27 are both high with the optimize and reset signal, thereby activating the clear input 17d of counter 17. Secondly, when the conversion is completed by converter 16, a comparison is made between the contents of the up/down counter 20 (which is the past no-touch value) and the contents of the CTA~ counter 17 which is the present no-touch value.
I~ the two counts are equal, then no action is taken. However, if the present measured value is greater than the past value, then an output signal is applied to line 23 to trip gate 24 and apply an input signal to the up input of counter 20 to bump counter 20 up by one count.
This revised value is thereafter routed to the data input l9c o~ memory 19 and is then rewritten into memory 19 responsive to a WRITE signal from control logic 15 to memory 19.
Similarly, if the present no-touch count is lower than the count in counter 20, then line 22 is activated to trip gate 25, 25, thereby ap~lying an input signal to the down terminal of counter 20 bu~.~ing the counter down by one count. Again, this altered value is rewritten into memory 19 at the end of the optimized cycle.
By permitting a change of only one count in the ~ ~7 ~ RD-9689 up/down counter 20 in any given cycle, the circuit accommodates the conditions where a large voltage transient may have occurred during optimization or the optimiza~ion mode was entered simultaneously with a uSeT touch condition occurring on th,e particular pad. This condition will then be corrected during the next optimization mode.
From the abo~e, it will be seen that the novel arrangement of Figure 1 permits the multiplexing of a large number of touch pads while requiring relatively few con-nections to the touch pad system and accommodating wide , varia~ions in the touch pad circuits. The novel circuit also permits ~he continuous updating of the no-touch condition for each of the touch pads, ~hich updating is not signifi-cantly changed by short-term transients so that the circuit is self-optimizing.
Figure 2 shows a second embodiment of the inventlon whcrein the CTAD counter 17 is preset with a fixed percentage of the no-touch value of any one of the touch pads TPl th~ough TP4. Thus, the circuit differs from' Figure 1 essentially in that the AlD counter 17 in Figure 1 was preset with a fixed percentage value. The circuit of Figure 2 and its operation are otherwise identical to that of Figure 1 and similar components have been given similar identifying numerals.
In Figure 2 an inverter 40 has been added which brings a signal from memory output 19b to a shift input 17e ~of the coun~er 17. This will enable the shift of bits in the counter 17 to the right in order to divide by two, four, eight or the like, thereby to preset the desired percentage of the no-touch value which is to be read out of the counter 17 ~ 7 ~ D-9689 during the comparison operation. Thus, to apply a fixed "wlndow" of (100/2N)%, ~he previously stored no-touch value i6 applied to counter 17 and is shifted N times to the right, in the coun~eri an ~-right shift is equivalent to a division by 2N. Illustratively, if N=2, the stored value is shifted right twice, whereby 100/2N% or(25%) of the no-touch value is preset into coun~er 17; a touch condition now occurs only if the touch pad output value is below ` 75Z~ (i.e. lOO - 100/2N)% o the no-touch value.
: Although a preferred embodiment of this invention has been described, many variations and modifications will now be apparent to those skillPd in the art, and it is therefore preferred that the instant invention be limited not by the specific disclosure herein but only by the appended claims.

Claims (9)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A touch pad sensor circuit comprising, in combination:
a plurality of drive lines;
a plurality of sense lines;
a plurality of capacitive touch pads, each having an input coupled to one of said drive lines and an output coupled to one of said sense lines, each touch pad being coupled between a different combination of drive and sense lines from any other touch pad;
driver circuit means for providing a drive signal to a selected one of said plurality of drive lines;
an A/D converter circuit:;
means for selectively coupling each one of said plurality of sense lines to said A/O converter circuit;
control logic means coupled to said driver circuit means for determining the sequence in which each of said drive lines is selected to receive said drive signal in sequential manner; said control logic means also for causing said coupling means to couple sequential ones of said sense lines to said A/D converter circuit;
a counter connected to said A/D converter circuit for producing a count of the digital output thereof;
memory means coupled to said control logic means for storing a digital value representative of the magnitude of an output signal of each of said touch pads in a no-touch condition without contact thereof by an outside influence with said touch pad;
up/down counter means for receiving the no-touch digital value stored in said memory means for the one touch pad then coupled between a drive line having said driving signal thereon and said A/D converter circuit; and comparator circuit means for comparing the digital output count of said counter and the no-touch digital value in said up/down counter means to produce an output signal when the count of the A/D converter circuit differs from the count in said up/down counter means by more than a preselected amount.

2. The touch pad sensor circuit of claim 1, further including means for detecting a change in the no-touch digital value of the output of the one touch pad then coupled between a drive line having a drive signal thereon and said A/D converter circuit; and means connected to said memory means for changing the no-touch digital value stored therein for said one touch pad, then coupled between the driven drive line and said A/D converter circuit, to a new no-touch digital value higher or lower than the no-touch digital value previously stored in said memory means for that touch pad.

3. The touch pad sensor circuit of claim 2, wherein said control logic means provides an optimize signal;
said comparator means provides a different one of a pair of output signals if the output of said one touch pad then coupling said driven drive line to said A/D converter circuit is respectively higher or lower than the no-touch digital value stored in said memory means for that particular sensor; and further including means for changing the count in said up/down counter means responsive to one of said comparator means output signals being generated.

4. The touch pad sensor circuit of claim 3, wherein the count in said up/down counter means is changed by a maximum of one count for each optimize signal provided by said control logic means.

5. The touch pad sensor circuit of claim 1, wherein said A/D converter circuit is a CTAD converter.

6. The touch pad sensor circuit of claim 1 further including means for preloading an offset count into said counter to establish a detection window.

7. The touch pad sensor circuit of claim 1 further including means for loading the no-touch digital value, for the driven touch pad then coupled to said A/D
converter circuit, from said memory means into said counter and for subsequently shifting the count in said counter by N
places, where N is an integer, to establish an offset equal to (100/2 N)% of the no-touch digital value previously stored in said memory means for the driven touch pad then being coupled to said A/D converter circuit.

8. A touch pad sensor comprising, in combination:
at least one capacitive touch pad;
driver circuit means for providing a drive signal to said capacitive touch pad;
An A/D converter circuit coupled to said driver circuit means through said capacitive touch pad;
a first counter connected to said A/D circuit for producing a digital count representative of an output of said touch pad in one of touch and no-touch conditions;
memory means for storing a last previous no-touch digital count representative of a last previous output of said touch pad;
an up/down counter receiving said last previous no-touch digital count from said memory means;
comparator means for comparing the counts in said first counter and in said up/down counter;
circuit means connected between said comparator means and said up/down counter for increasing or decreasing the count in said up/down counter when said up/down counter contains a digital count received from said memory means which digital count is lower or higher respectively than the count in said first counter; and means for reading the last count in said up/down counter back into said memory means.

9. The touch pad sensor of claim 8, wherein the count in said up/down counter is changed by a maximum of one count in any given comparison between the counts in said up/down counter.