Touch detection techniques for capacitive touch sense systems
A technique for recognizing and rejecting false activation events related to a capacitance sense interface includes measuring a capacitance value of a capacitance sense element. The measured capacitance value is analyzed to determine a baseline capacitance value for the capacitance sensor. The capacitance sense interface monitors a rate of change of the measured capacitance values and rejects an activation of the capacitance sense element as a non-touch event when the rate of change of the measured capacitance values have a magnitude greater than a threshold level, indicative of a maximum rate of change of a touch event.
1. A method comprising:
measuring capacitance values of a capacitance sense element;
monitoring a rate of change of the measured capacitance values; and
rejecting an activation of the capacitance sense element as a non-touch event if conditions indicative of the non-touch event are sensed, wherein the conditions indicative of the non-touch event comprise the rate of change of the measured capacitance values having a magnitude greater than a threshold level, wherein the threshold level is indicative of a maximum rate of change of a touch event.
2. The method of claim 1 , further comprising analyzing the measured capacitance values to determine a baseline capacitance value for the capacitance sense element, wherein the analyzing comprises:
tracking the baseline capacitance value for the capacitance sense element;
calculating a weighted average based on a current capacitance value measured on the capacitance sense element and the capacitance values that were previously measured on the capacitance sense element; and
updating the baseline capacitance value with the weighted average.
3. The method of claim 2 , further comprising filtering the measured capacitance values using an infinite impulse response (IIR) filter before the calculating the weighted average.
4. The method of claim 1 , further comprising:
analyzing the measured capacitance values to determine a baseline capacitance value for the capacitance sense element;
accepting the activation of the capacitance sense element as a touch event; and
maintaining the baseline capacitance value when the activation of the capacitance sense element is accepted as the touch event to not skew the calculated weighted average.
5. The method of claim 1 , wherein the threshold level is a positive threshold and the maximum rate of change is a positive, maximum rate of change of the touch event.
6. The method of claim 1 , wherein the threshold level is a negative threshold and the maximum rate of change is a negative, maximum rate of change of the touch event.
7. The method of claim 1 , wherein the rejecting the activation comprises masking all activations of the capacitance sense element for a period after sensing the conditions indicative of the non-touch event.
8. The method of claim 1 , further comprising
determining if the non-touch event is an electrostatic discharge (ESD) event, wherein the determining comprises:
detecting that the non-touch event has conditions indicative of an ESD event, wherein the conditions indicative of the ESD event include at least one of the following:
the rate of change of the measured capacitance values turning positive and having a first magnitude greater than a positive threshold,
the rate of change of the measured capacitance values turning negative and having a second magnitude greater than a positive threshold, or
the measured capacitance values crossing an ESD threshold below a baseline capacitance value.
9. The method of claim 1 , further comprising:
analyzing the measure capacitance values to determine that the capacitance sense element has been activated when the measured capacitance values cross an activation threshold; and
analyzing the measured capacitance values to determine that the capacitance sense element has been deactivated when the measured capacitance values cross a deactivation threshold.
10. The method of claim 9 , further comprising analyzing the measured capacitance values to determine a baseline capacitance value for the capacitance sense element, wherein the 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 sense element.
11. The method of claim 1 , further comprising
updating a baseline capacitance value based at least in part upon a weighted moving average of the measured capacitance values.
12. The method of claim 1 , further comprising:
analyzing the measured capacitance values to determine a baseline capacitance value for the capacitance sense element; and
analyzing the measured capacitance values to determine whether the measured capacitance values are below the baseline capacitance value by a threshold value for a predetermined period; and
updating the baseline capacitance value based on the measured capacitance values measured after the measured capacitance values are below the baseline capacitance value by the threshold value.
13. A method comprising:
measuring capacitance values of a capacitance sense element;
monitoring a rate of change of the measured capacitance values; and
distinguishing between an activation of the capacitance sense element by a conductive object and a non-touch event when the rate of change of the measured capacitance values exceeds a magnitude greater than a threshold level, the threshold level indicative of a maximum rate of change of a touch event.
14. The method of claim 13 , further comprising analyzing the measured capacitance values to determine a baseline capacitance value for the capacitance sense element.
15. The method of claim 14 , wherein the analyzing comprises:
tracking the baseline capacitance value for the capacitance sense element;
filtering the measured capacitance values using an infinite impulse response (IIR) filter;
calculating a weighted average based on a current capacitance value measured on the capacitance sense element and the capacitance values that were previously measured on the capacitance sense element; and
updating the baseline capacitance value with the weighted average.
16. An apparatus comprising:
a capacitance measurement circuit to be coupled to a capacitance sense element, wherein the capacitance measurement circuit is configured to measure capacitance values of the capacitance sense element; and
event logic coupled to the capacitance measurement circuit, the event logic to measure a rate of change of the measured capacitance values and to distinguish between an activation of the capacitance sense element by a conductive object and a non-touch event when the rate of change of the measured capacitance values exceeds a magnitude greater than a threshold level, the threshold level indicative of a maximum rate of change of a touch event.
17. The apparatus of claim 16 , wherein the event logic is further configured to
determine if the non-touch event is an electrostatic discharge event (ESD) event when at least one of the following conditions is detected:
the rate of change of the measured capacitance values turning positive and having a first magnitude greater than a positive threshold,
the rate of change of the measured capacitance values turning negative and having a second magnitude greater than a positive threshold, or
the measured capacitance values crossing an ESD threshold below a baseline capacitance value.
18. The apparatus of claim 16 , further comprising hysteresis logic coupled to the event logic to determine whether capacitance sense element has been activated based on whether the measured capacitance values cross an activation threshold value and to determine whether the capacitance sense element has been deactivated based on whether the measured capacitance values cross a deactivation threshold.
19. The apparatus of claim 16 , further comprising baseline logic coupled to analyze the measured capacitance values to determine whether a baseline capacitance value associated with the capacitance sense element has drifted and to update the baseline capacitance value.