Digital controller for a true multi-point touch surface useable in a computer system

• US 8,054,299 B2
• Filed: 01/08/2007
• Issued: 11/08/2011
• Est. Priority Date: 01/08/2007
• Status: Expired due to Fees

• Current Assignee: Apple Inc.
• Original Assignee: Apple Inc.
• Inventors: Krah, Christoph H.
• Primary Examiner(s): Shalwala; Bipin
• Assistant Examiner(s): CRAWLEY, KEITH L
• Application Number:
  US11/621,067
• Publication Number:
  US 20080165134A1
• Time in Patent Office: 1,765 Days
• Field of Search: 345173-174, 178/18.06, 178/19.03, 349/12
• US Class Current: 345/174
• CPC Class Codes: G06F 3/04166 Details of scanning methods, e.g. sampling time, grouping of sub areas or time sharing with display driving Synchronisation with the driving of the display or the backlighting unit to avoid interferences generated internally G06F3/04184
• CPC Class Codes: G06F 3/04182 Filtering of noise external to the device and not generated by digitiser components
• CPC Class Codes: G06F 3/044 by capacitive means
• CPC Class Codes: G06F 3/0446 using a grid-like structure of electrodes in at least two directions, e.g. using row and column electrodes

Abstract

An embodiment of an improved touch surface controller for controlling a multi-point touch surface such as a touch screen or a touch pad in a computer system is disclosed. The improved touch system controller detects signal capacitance from the pixels in the touch surface digitally, and therefore is easier and simpler to implement then previous approaches relying on analog detection and processing. Specifically, signal capacitance is measured by measuring a number of clock cycles taken to discharge or charge the input node of a comparator. Embodiments of the disclosed touch surface controller are particularly useful in the context of detecting multiple simultaneous touches on the touch surface.

Claims

1. A controller for a touch surface having a plurality of capacitive pixels, each capacitive pixel having a first electrode and a second electrode, different from the first electrode and defining a pixel capacitance therebetween, the controller comprising:

stimulation circuitry for applying a stimulation voltage having a first and second voltage level to only the first electrode of at least some of the plurality of capacitive pixels,

a plurality of detection circuits, one detection circuit corresponding to each channel defined as a group of capacitive pixels arranged along a given direction;

each detection circuit comprising;

input circuitry having an input node capable of being coupled to the second electrode of the at least some of the plurality of capacitive pixels for detecting the pixel capacitance of the at least some of the plurality of capacitive pixels, wherein a change in the pixel capacitance at a given pixel indicates proximity of a touch to that pixel;

charging and discharging circuitry for alternately charging the capacitive pixels to a reference voltage after the stimulus voltage of the first voltage level is applied to the first electrode of the capacitive pixels, and discharging the capacitive pixels to the reference voltage after the stimulus voltage of the second voltage level is applied to the first electrode of the capacitive pixel;

the charging and discharging circuitry generating a hold signal indicating whether a charging or discharging of the capacitive pixels is in progress; and

counting circuitry for digitally counting at least one of a charging time period to charge the capacitive pixels to the reference voltage and a discharging time period to discharge the capacitive pixels to the reference voltage, each of the charging time period and the discharging time period indicative of pixel capacitance of the at least some of the plurality of capacitive pixels;

wherein the hold signals from each channel are logically combined to indicate whether a charging or discharging of the capacitive pixels is in progress for any channel and wherein the stimulation circuitry is prevented from applying another stimulation voltage until the logically combined hold signals indicates that all channels have completed their respective charging and discharging.

2. The controller of claim 1, wherein the wherein the plurality of capacitive pixels are arranged in rows and columns, and wherein the input circuitry is coupled to each column, each column corresponding to one of the channels.

3. The controller of claim 2, wherein the charging and discharging time periods are independent of changes in stray capacitance due to touching of the touch surface.

4. The controller of claim 1, wherein the input circuitry comprises a comparator, and wherein the input node comprises a first input to the comparator, and wherein the reference voltage comprises a second input to the comparator.

5. The controller of claim 1, wherein the charging and discharging circuitry includes a current source and a current sink respectively and the counting circuitry digitally counts a time period to charge and discharge the capacitive pixels to the reference voltage.

6. The controller of claim 5, further comprising a state machine, other than the charging and discharging circuitry, for enabling and disabling the current source and current sink respectively.

7. The controller of claim 4, further comprising means for randomizing or varying the reference voltage to reduce noise susceptibility.

8. The controller of claim 1, further comprising means for randomizing or varying a frequency content of the stimulation voltage to reduce noise susceptibility.

9. A controller for a touch surface having a plurality of capacitive pixels arranged in rows and columns, each capacitive pixel having a first electrode and a second electrode, different from the first electrode, the controller comprising:

stimulation circuitry for applying a stimulation voltage to the first electrode of at least some of the plurality of capacitive pixels, the stimulation voltage having a periodic waveform and having a first and second voltage level;

a plurality of detection circuits, one detection circuit corresponding to each channel defined as a group of capacitive pixels arranged along a given direction;

each detection circuit comprising;

input circuitry having an input node coupled to the second electrode of the plurality of capacitive pixels for each column for detecting the capacitance of the capacitive pixels;

charging and discharging circuitry coupled to the input node for alternately charging the capacitive pixels to a reference voltage after the stimulus voltage of the first voltage level is applied to the first electrode of the capacitive pixels, and discharging the capacitive pixels to the reference voltage after the stimulus voltage of the second voltage level is applied to the first electrode of the capacitive pixel;

the charging and discharging circuitry generating a hold signal indicating whether a charging or discharging of the capacitive pixels is in progress; and

counting circuitry for digitally counting a charging time period to charge the input node to the reference voltage and a discharging time period to discharge the input node to the reference voltage, each of the charging and discharging time periods indicative of the capacitance of the capacitive pixels; and

wherein the controller further comprises;

processing circuitry for averaging the charging and discharging time periods over multiple charging and discharging cycles to obtain an average value indicative of the capacitance of the capacitive pixels; and

wherein the hold signals from each channel are logically combined to indicate whether a charging or discharging of the capacitive pixels is in progress for any channel and wherein the stimulation circuitry is prevented from applying another stimulation voltage until the logically combined hold signals indicates that all channels have completed their respective charging and discharging.

10. The controller of claim 9, wherein the charging and discharging time periods are independent of changes in a stray capacitance associated with each column due to touching of the touch surface.

11. The controller of claim 9, wherein the detection circuitry comprises a comparator, and wherein the input node comprises a first input to the comparator, and wherein the reference voltage comprises a second input to the comparator.

12. The controller of claim 9, wherein the charging and discharging circuitry includes a current source and a current sink respectively and the counting circuitry digitally counting a time period to charge and discharge the capacitive pixels to the reference voltage.

13. The controller of claim 12, further comprising a state machine, other than the charging and discharging circuitry, for enabling and disabling the current source and current sink respectively.

14. The controller of claim 11, further comprising means for randomizing or varying the reference voltage to reduce noise susceptibility.

15. The controller of claim 9, further comprising means for randomizing or varying a frequency content of the stimulation voltage to reduce noise susceptibility.

16. A method for operating a touch surface, the touch surface having a plurality of capacitive pixels, each capacitive pixel having a first electrode and a second electrode, different from the first electrode, the method comprising:

stimulating the first electrode of at least some of the plurality of capacitive pixels with a stimulation waveform having a first and second voltage level;

detecting the stimulation waveform at the second electrode of the at least some of the plurality of capacitive pixels at an input node of a detecting circuit;

alternately charging the input node to a reference voltage after stimulating the first electrode with the stimulation waveform having the first voltage level and discharging the input node to the reference voltage after stimulating the first electrode with the stimulation waveform having the second voltage level; and

digitally counting at least one of a charging time period to charge the input node to the reference voltage and a discharging time period to discharge the input node to the reference voltage, each of the charging and discharging time periods indicative of a capacitance of the capacitive pixels;

generating a hold signal indicating whether a charging or discharging of the capacitive pixels is in progress for any one of a plurality of channels, each channel defined by a group of capacitive pixels arranged in a given direction;

logically combining the hold signals for each channel to indicate whether a charging or discharging of the capacitive pixels is in progress for any channel; and

inhibiting another stimulation of the first electrodes until the logically combined hold signals indicates that all channels have completed their respective charging and discharging.

17. The method of claim 16, wherein the stimulation waveform is randomized in its frequency content.

18. The method of claim 16, wherein digitally counting the charging and discharging time periods comprises counting a number of clock cycles.

19. The method of claim 16, wherein the input node is coupled to a first input of a comparator, and wherein a second input of the comparator is coupled to the reference voltage.

20. The method of claim 19, wherein an output of the comparator is used together with a state machine to enable a current source and a current sink used to charge and discharge the input node.

21. A method for operating a multi-touch surface, the multi-touch surface having a plurality of capacitive pixels, each capacitive pixel having a first electrode and a second electrode, different from the first electrode, the method comprising:

stimulating the first electrode of at least some of the plurality of capacitive pixels with a stimulation waveform having a first and second voltage level;

receiving multiple touches at various ones of the capacitive pixels among the at least some of the plurality of capacitive pixels;

detecting a change in the capacitance at the second electrode of the various ones of the capacitive pixels, the detecting including;

alternately charging the second electrode to a reference voltage after stimulating the first electrode with the stimulation voltage of the first voltage level and discharging the second electrode to the reference voltage after stimulating the first electrode with the stimulation voltage of the second voltage level; and

digitally counting a charging time period to charge the second electrode to the reference voltage and a discharging time period to discharge the second electrode to the reference voltage, each of the charging and discharging time periods indicative of the capacitance of the capacitive pixels;

generating a hold signal indicating whether a charging or discharging of the capacitive pixels is in progress for any one of a plurality of channels, each channel defined by a group of capacitive pixels arranged in a given direction;

logically combining the hold signals for each channel to indicate whether a charging or discharging of the capacitive pixels is in progress for any channel; and

inhibiting another stimulation of the first electrodes until the logically combined hold signals indicates that all channels have completed their respective charging and discharging.

22. The method of claim 21, wherein detecting a change in the capacitance of the various ones of the capacitive pixels comprises stimulating the first electrodes with a square wave randomized in its frequency content.

23. The method of claim 21, wherein counting the charging and discharging time periods comprises counting a number of clock cycles necessary to charge and discharge the second electrode.

24. The method of claim 21, wherein detecting a change in the capacitance of the various capacitive pixels comprises comparison of a voltage on the second electrode with the reference voltage, and wherein the method further comprises:

averaging the charging and discharging time periods over multiple charging and discharging cycles to obtain an average value indicative of the capacitance of the capacitive pixels.

25. A touch sensitive electronic device comprising:

a touch screen including;

a plurality of first traces disposed in a first direction;

a plurality of second traces disposed transverse to and spaced apart from the plurality of first traces and forming a capacitive pixel at the intersection of each first and second trace, each capacitive pixel having a capacitance defined between the intersecting first and second trace; and

control circuitry including;

stimulation circuitry for generating a stimulus waveform having at least a first and second voltage level, the stimulus waveform applied to only the plurality of first traces or the plurality of second traces so as to apply the stimulus waveform to only one side of each of the capacitive pixels, the other side of each capacitive pixel coupled to ground through a stray capacitance;

a plurality of detection circuits, one detection circuit corresponding to each channel defined as a group of capacitive pixels arranged along a given direction;

each detection circuit comprising;

a comparator having an input node coupled to the other side of the capacitive pixel, the comparator having a reference input connected to receive a reference voltage;

a current source and a current sink connected to the input node to alternately charge and discharge the input node to the reference voltage;

the detection circuit operative to enable operation of the current source and current sink to charge and discharge the input node after a change in the stimulus waveform is applied to only the plurality of first traces or the plurality of second traces;

means for generating a hold signal indicating whether a charging or discharging of the capacitive pixels is in progress;

means for digitally counting at least one of a charging time period to charge the input node to the reference voltage and a discharging time period to discharge the input node to the reference voltage, each of the charging and discharging time periods indicative of the capacitance of the capacitive pixels; and

wherein the control circuitry further comprises;

means for logically combining the hold signals to indicate whether a charging or discharging of the capacitive pixels is in progress for any channel; and

means for preventing the stimulation circuitry from generating another stimulus waveform until the logically combined hold signals indicates that all channels have completed their respective charging and discharging.

26. The electronic device as recited in claim 25 wherein the detection circuitry comprises a state machine.

28. The electronic device as recited in claim 25 further comprising means for randomizing or varying the sink or source currents to reduce noise susceptibility.

29. The electronic device as recited in claim 25 further comprising means for randomizing or varying the reference voltage to reduce noise susceptibility.

27. A touch sensitive electronic device comprising:

a touch screen including;

a plurality of first traces disposed in a first direction;

a plurality of second traces disposed transverse to and spaced apart from the plurality of first traces and forming a capacitive pixel at the intersection of each first and second trace, each capacitive pixel having a capacitance defined between the intersecting first and second trace; and

control circuitry including;

stimulation circuitry for generating a stimulus waveform having at least a first and second voltage level, the stimulus waveform applied to only the plurality of first traces or the plurality of second traces so as to apply the stimulus waveform to only one side of each of the capacitive pixels, the other side of each capacitive pixel coupled to ground through a stray capacitance;

detection circuitry including a comparator having an input node coupled to the other side of the capacitive pixel, the comparator having a reference input connected to receive a reference voltage;

a current source and a current sink connected to the input node to alternately charge and discharge the input node to the reference voltage;

the detection circuitry operative to enable operation of the current source and current sink to charge and discharge the input node after a change in the stimulus waveform is applied to only the plurality of first traces or the plurality of second traces; and

the detection circuitry including means for digitally counting at least one of a charging time period to charge the input node to the reference voltage and a discharging time period to discharge the input node to the reference voltage, each of the charging and discharging time periods indicative of the capacitance of the capacitive pixels;

wherein the means for digitally counting determines a time period according to the formula;

Δ

T=Δ

Vstim*(Cstray*Csig)/[(Cstray+Csig)*Isink]

where,

Δ

T is the time for charging or discharging,

Δ

Vstim is the change in the stimulus waveform,

Cstray is the stray capacitance,

Csig is the capacitance of the capacitive pixel, and

Isink is the current of the current source or current sink.

30. A touch sensitive electronic device comprising:

a touch screen including;

a plurality of first traces disposed in a first direction;

a plurality of second traces disposed transverse to and spaced apart from the plurality of first traces and forming a capacitive pixel at the intersection of each first and second trace, each capacitive pixel having a capacitance defined between the intersecting first and second trace; and

control circuitry including;

stimulation circuitry for generating a stimulus waveform, the stimulus waveform, applied to only the plurality of first traces or the plurality of second traces so as to apply the stimulus waveform to only one side of each of the capacitive pixels, the other side of each capacitive pixel coupled to ground through a stray capacitance;

a plurality of detection circuits, one detection circuit corresponding to each channel defined as a group of capacitive pixels arranged along a given direction;

each detection circuit comprising;

a comparator having an input node coupled to the other side of the capacitive pixel, the comparator having a reference input connected to receive a reference voltage;

a current source and a current sink connected to the input node to alternately charge and discharge the input node to the reference voltage;

the detection circuit operative to enable operation of the current source and current sink to charge and discharge the input node after a change in the stimulus waveform is applied to only the plurality of first traces or the plurality of second traces; and

means for generating a hold signal indicating whether a charging or discharging of the capacitive pixels is in progress; and

a digital timing circuit operatively coupled to the output of the comparator for digitally at least one of a charging time period to charge the input node to the reference voltage and a discharging time period to discharge the input node to the reference voltage, each of the charging and discharging time periods indicative of the capacitance of the capacitive pixels; and

wherein the control circuitry further comprises;

means for logically combining the hold signals to indicate whether a charging or discharging of the capacitive pixels is in progress for any channel; and

means for preventing the stimulation circuitry from generating another stimulus waveform until the logically combined hold signals indicates that all channels have completed their respective charging and discharging.

31. The electronic device as recited in claim 30 further comprising means for randomizing or varying the sink or source currents to reduce noise susceptibility.

32. The electronic device as recited in claim 31 further comprising means for randomizing or varying the reference voltage to reduce noise susceptibility.

33. The electronic device as recited in claim 30 further comprising means for randomizing or varying the reference voltage to reduce noise susceptibility.