Digital Controller for a True Multi-point Touch Surface Useable in a Computer System

• US 20080165134A1
• Filed: 01/08/2007
• Published: 07/10/2008
• Est. Priority Date: 01/08/2007
• Status: Active Grant

• Current Assignee: Apple Inc.
• Original Assignee: Apple Computer Incorporated (Apple Inc.)
• Inventors: Krah, Christoph H.
• Granted Patent: US 8,054,299 B2
• Time in Patent Office: Days
• US Class Current: 345/173
• 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 multi-touch surface having a plurality of capacitive pixels, comprising:

input circuitry for detecting the capacitance of the capacitive pixels, wherein a change in the capacitance at a given pixel indicates proximity of a touch to that pixel,

wherein the input circuitry detects a change in charge on a stray capacitance due to the change in capacitance at the pixel by digitally counting a time period to charge or discharge the change in charge on the stray capacitance at an input node to the input circuitry, wherein the change in charge on the stray capacitance is caused by a change in a stimulus voltage applied to the capacitive pixel.

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.

3. The controller of claim 2, wherein the time period is independent of changes in the stray capacitance.

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 a reference voltage comprises a second input to the comparator.

5. The controller of claim 1, wherein counting the time period comprises counting a number of clock cycles.

6. The controller of claim 5, wherein a change in the number of clock cycles scales linearly with a change in the capacitance of the capacitive pixels.

7. The controller of claim 1, wherein the controller further comprises a waveform generator for stimulating the capacitive pixels.

8. The controller of claim 7, wherein the waveform generator randomizes the frequency content of the stimulus waveforms so as to reduce noise.

9. A controller for a multi-touch surface having a plurality of capacitive pixels arranged in rows and columns, comprising:

detection circuitry for detecting the capacitance of the capacitive pixels, wherein the detection circuitry is coupled to each column at an input node;

circuitry for charging and/or discharging the capacitance at each column; and

counting circuitry for digitally counting a time period to charge or discharge the capacitance at the input node.

10. The controller of claim 9, wherein the time period is independent of changes in a stray capacitance associated with each column.

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 a reference voltage comprises a second input to the comparator.

12. The controller of claim 9, wherein counting the time period comprise counting a number of clock cycles.

13. The controller of claim 12, wherein the number of clock cycles scales linearly with the capacitance.

14. The controller of claim 9, wherein the controller further comprises a waveform generator for stimulating the capacitive pixels.

15. The controller of claim 14, wherein the waveform generator randomizes the frequency content of the stimulus waveforms so as to reduce noise.

16. A method for operating a multi-touch surface, the multi-touch surface having a plurality of capacitive pixels, the method comprising:

stimulating at least one of the capacitive pixels with a stimulation waveform at a first side of the capacitive pixel;

detecting the stimulation waveform at a second side of the capacitive pixel at an input node of a comparator;

charging and/or discharging the input node; and

timing the charging and/or discharging of the input node to digitally determine a change in the capacitance of the capacitive pixel due to a touch to the multi-touch surface.

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

18. The method of claim 16, wherein the input node is charged and/or discharged using a current source and/or a current sink.

19. The method of claim 16, wherein timing the charging and/or discharging of the input node comprises counting a number of clock cycles.

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

21. The method of claim 20, wherein an output of the comparator is used to enable a current source and/or a current sink used to charge and/or discharge the input node.

22. A method for operating a multi-touch surface, the multi-touch surface having a plurality of capacitive pixels, the method comprising:

receiving multiple touches at various of the capacitive pixels;

detecting a change in the capacitance of the various capacitive pixels; and

converting the detected change in capacitance to a count value, wherein the count value is indicative of a change in the pixel capacitance.

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

24. The method of claim 22, wherein the change in the pixel capacitance is measured using a current source and/or a current sink.

25. The method of claim 22, wherein converting the detected change in capacitance to a count value comprises counting a number of clock cycles necessary to charge and/or discharge the change in pixel capacitance.

26. The method of claim 22, wherein detecting a change in the capacitance of the various capacitive pixels comprises comparison to a reference voltage.