Touch detection techniques for capacitive touch sense systems

• US 8,040,142 B1
• Filed: 03/28/2007
• Issued: 10/18/2011
• Est. Priority Date: 03/31/2006
• Status: Expired due to Fees

• Current Assignee: Monterey Research, LLC (Vector Capital Corporation)
• Original Assignee: Cypress Semiconductor Corporation (Infineon Technologies AG)
• Inventors: Bokma, Louis W., Page, Andrew C., Seguine, Dennis R.
• Primary Examiner(s): NATALINI, JEFF WILLIAM
• Application Number:
  US11/729,818
• Time in Patent Office: 1,665 Days
• Field of Search: 324/658
• US Class Current: 324/658
• CPC Class Codes: G01R 27/2605 Measuring capacitance capacitive sensors G01D5/24
• CPC Class Codes: G06F 3/041 Digitisers, e.g. for touch screens or touch pads, characterised by the transducing means
• 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: H03K 17/955 using a capacitive detector
• CPC Class Codes: H03K 17/9622 using a plurality of detectors, e.g. keyboard
• CPC Class Codes: H03K 2217/94026 Automatic threshold calibration; e.g. threshold automatically adapts to ambient conditions or follows variation of input
• CPC Class Codes: H03K 2217/96066 Thumbwheel, potentiometer, scrollbar or slider simulation by touch switch
• CPC Class Codes: H03K 2217/960705 Safety of capacitive touch and proximity switches, e.g. increasing reliability, fail-safe

Abstract

A technique for recognizing and rejecting false activation events related to a capacitance sense interface includes measuring a capacitance value of a capacitance sensor within the capacitance sense interface to generate a measured capacitance value. The measured capacitance value is analyzed to determine a baseline capacitance value for the capacitance sensor. The baseline capacitance value may be updated based at least in part upon a weighted moving average of the measured capacitance value. The measured capacitance value may also be analyzed to determine whether the capacitance sensor was activated during a startup phase and to adjust the baseline capacitance value in response to determining that the capacitance sensor was activated during the startup phase.

Claims

1. A machine-readable storage medium that provides instructions that, if executed by a machine, will cause the machine to perform operations comprising:

measuring a capacitance value of a capacitance sensor within a capacitance sense interface to generate a measured capacitance value;

analyzing the measured capacitance value to determine a baseline capacitance value for the capacitance sensor;

updating the baseline capacitance value in real-time based at least in part upon a weighted moving average of the measured capacitance value;

analyzing the measured capacitance value to determine whether the measured capacitance value drops below the baseline capacitance value by a negative threshold value for a predetermined period of time, wherein the predetermined period of time is greater than a single clock cycle; and

updating the baseline capacitance value based on the measured capacitance value measured after the measured capacitance value dropped below the negative threshold value.

2. The machine-readable storage medium of claim 1, wherein analyzing the measured capacitance value to determine the baseline capacitance value comprises updating the weighted moving average every Nth interval of measuring the capacitance value, where N is an integer greater than one.

3. The machine-readable storage medium of claim 2, wherein analyzing the measured capacitance value to determine the baseline capacitance value further comprises holding the baseline capacitance value substantially steady during activation events of the capacitance sensor.

4. The machine-readable storage medium of claim 1, wherein the capacitance sense interface includes a plurality of capacitance sensors, the machine-readable storage medium further providing instructions that, if executed by the machine, will cause the machine to perform further operations, comprising:

scanning the plurality of capacitance sensors within the capacitance sense interface in sequence to measure capacitance values for the plurality of capacitance sensors to generate a plurality of measured capacitance values;

analyzing the plurality of measured capacitance values to determine a group baseline capacitance drift; and

updating baseline capacitance values for the plurality of capacitance sensors based on the group baseline capacitance drift.

5. The machine-readable storage medium of claim 1, further providing instructions that, if executed by the machine, will cause the machine to perform further operations, comprising:

monitoring a slope of the measured capacitance value; and

rejecting an activation of the capacitance sensor as an electrostatic discharge (“

ESD”

) event if conditions indicative of an ESD event are sensed, wherein conditions indicative of the ESD event include at least two of the following;

the slope of the measured capacitance value turning positive and having a first magnitude greater than a positive slope threshold,

the slope of the measured capacitance value turning negative having a second magnitude greater than a negative slope threshold, and

the measured capacitance value crossing an ESD threshold below the baseline capacitance value.

6. The machine-readable storage medium of claim 5, wherein rejecting the actuation of the capacitor sensor further comprises masking all actuations of the capacitance sensor for a period of time after sensing the conditions indicative of the ESD event.

7. The machine-readable storage medium of claim 1, further providing instructions that, if executed by the machine, will cause the machine to perform further operations, comprising:

determining that the capacitance sensor has been activated when the measured capacitance value crosses an activation threshold; and

determining that the capacitance sensor has been deactivated when the measured capacitance value crosses a deactivation threshold, wherein the activation threshold, the deactivation threshold, and the baseline capacitance value are different values.

8. The machine-readable storage medium of claim 1, wherein the capacitance sense interface includes a plurality of capacitance sensors, the machine-readable storage medium further providing instructions that, if executed by the machine, will cause the machine to perform further operations, comprising:

measuring capacitance values for the plurality of capacitance sensors to generate a plurality of measured capacitance values;

analyzing the plurality of measured capacitance values; and

mapping the plurality of measured capacitance values onto a selectable number of virtual capacitance sensors via interpolation.

9. An apparatus, comprising:

a capacitance sense interface including a capacitance sensor having a baseline capacitance value associated with the capacitance sensor;

a capacitance measurement circuit coupled to the capacitance sense interface to sense a measured capacitance value of the capacitance sensor and output a signal indicative of the measured capacitance value; and

startup logic coupled to analyze the signal to determine whether the capacitance sensor was activated during a startup phase and to adjust the baseline capacitance value in response to determining that the capacitance sensor was activated during the startup phase, wherein the startup logic determines whether the measured capacitance value crosses a negative threshold value for a predetermined period of time, wherein the predetermined period of time is greater than a single clock cycle.

10. The apparatus of claim 9, further comprising baseline logic coupled to analyze the signal to determine whether the baseline capacitance value associated with the capacitance sensor has drifted and to update the baseline capacitance value using a weighted moving average.

11. The apparatus of claim 9, further comprising hysteresis logic coupled to analyze the signal to determine whether the capacitance sensor has been activated based on whether the measured capacitance value crosses an activation threshold value and to determine whether the capacitance sensor has been deactivated based on whether the measured capacitance value crosses a deactivation threshold value, wherein the activation threshold value, the deactivation threshold value, and the baseline value are different values.

12. The apparatus of claim 9, further comprising electrostatic discharge (“

ESD”

) logic coupled to analyze the signal to determine whether an ESD event has occurred on the capacitance sensor and to reject the ESD event as a false activation of the capacitance sensor.

13. The apparatus of claim 12, wherein the ESD logic comprises:

means for determining a slope or rate of change of the measured capacitance value;

means for determining whether the slope is a positive slope greater than a positive threshold slope followed by a negative slope having a magnitude greater than a negative threshold slope; and

means for determining whether the measured capacitance value crosses an ESD threshold.

14. A method, comprising:

measuring a capacitance value of a capacitance sensor within a capacitance sense interface to generate a measured capacitance value;

analyzing the measured capacitance value to determine a baseline capacitance value for the capacitance sensor;

updating the baseline capacitance value based at least in part upon a weighted moving average of the measured capacitance value;

analyzing the measured capacitance value to determine whether the measured capacitance value drops below the baseline capacitance value by a negative threshold value for a predetermined period of time, wherein the predetermined period of time is greater than a single clock cycle; and

updating the baseline capacitance value based on the measured capacitance value measured after the measured capacitance value dropped below the negative threshold value.

15. The method of claim 14, wherein analyzing the measured capacitance value to determine the baseline capacitance value further comprises holding the baseline capacitance value substantially steady during activation events of the capacitance sensor.

16. The method of claim 14, further comprising:

monitoring a slope of the measured capacitance value; and

rejecting an activation of the capacitance sensor as an electrostatic discharge (“

ESD”

) event if conditions indicative of an ESD event are sensed, wherein conditions indicative of the ESD event include at least two of the following;

the slope of the measured capacitance value turning positive and having a first magnitude greater than a positive slope threshold,

the slope of the measured capacitance value turning negative having a second magnitude greater than a negative slope threshold, and

the measured capacitance value crossing an ESD threshold below the baseline capacitance value.

17. The method of claim 16, wherein rejecting the actuation of the capacitor sensor further comprises masking all actuations of the capacitance sensor for a period of time after sensing the conditions indicative of the ESD event.

18. The method of claim 14, further comprising

determining that the capacitance sensor has been activated when the measured capacitance value crosses an activation threshold;

and

determining that the capacitance sensor has been deactivated when the measured capacitance value crosses a deactivation threshold, wherein the activation threshold, the deactivation threshold, and the baseline capacitance value are different values.