Touch sense interface circuit
View Patent ↗A touch sensing interface ( 101 ) includes a noise touch sensing circuit ( 110 ) that utilizes electromagnetic (EM) noise in a touch panel ( 102 ) to determine whether a pad of the touch panel is touched, such as by a finger of a user. The noise touch sensing circuit detects noise in a first pad ( 104 ) and in a second pad ( 105 ) of the touch panel, compares power of the noise in the first pad to power of noise in the second pad, and outputs a signal ( 132 ) indicative of the first pad being touched when power of noise in the first pad exceeds power of noise in the second pad by a pre-selected amount. The touch sensing interface may include a capacitive touch sensing circuit ( 120 ) and a controller ( 130 ) that selectively actuates the noise touch sensing circuit and the capacitive touch sensing circuit.
1. A noise touch sensing circuit for detecting electromagnetic (EM) noise at pads of a touch panel, the noise touch sensing circuit comprising:
a first comparator with hysteresis having an input node coupled to a first pad of a touch panel, the first comparator with hysteresis outputting a first signal responsive to magnitude of EM noise at the first pad;
a second comparator with hysteresis having an input node coupled to a second pad of the touch panel, the second comparator with hysteresis outputting a second signal responsive to magnitude of EM noise at the second pad;
a first counter for counting toggling of the first signal;
a second counter for counting toggling of the second signal; and
a threshold controller, coupled to the first comparator with hysteresis and to the second comparator with hysteresis, wherein the threshold controller:
sets a positive threshold (V p1 ) and a negative threshold (V m1 ) of the first comparator in response to a count outputted by the first counter,
sets a positive threshold (V p2 ) and a negative threshold (V m2 ) of the second comparator in response to a count outputted by the second counter, and
outputs a noise touch sensing signal in response to a difference between the count of the first counter and the count of the second counter.
2. The noise touch sensing circuit of claim 1 , wherein the threshold controller outputs a noise touch sensing signal when the count outputted by the first counter is greater than the count outputted by the second counter by a pre-selected amount.
3. The noise touch sensing circuit of claim 1 , wherein the first comparator with hysteresis includes a first Schmitt trigger and wherein the second comparator with hysteresis includes a second Schmitt trigger.
4. The noise touch sensing circuit of claim 3 , wherein the first comparator with hysteresis detects magnitude of a voltage attributable to EM noise at the first pad, and wherein the second comparator with hysteresis detects magnitude of a voltage attributable to EM noise at the second pad.
5. The noise touch sensing circuit of claim 4 , including:
circuitry for maintaining a constant DC voltage at the input node of the first comparator with hysteresis; and
circuitry for maintaining a constant DC voltage at the input node of the second comparator with hysteresis.
6. The noise touch sensing circuit of claim 1 , wherein the threshold controller:
increases the positive threshold (V p1 ) and decreases the negative threshold (V m1 ) of the first comparator in response to an increase in the count outputted by the first counter, and
increases the positive threshold (V p2 ) and decreases the negative threshold (V m2 ) of the second comparator in response to an increase in the count outputted by the second counter.
7. The noise touch sensing circuit of claim 6 , wherein the threshold controller:
increases the positive threshold (V p1 ) and decreases the negative threshold (V m1 ) by a same factor over a previous setting of the respective threshold, and
increases the positive threshold (V p2 ) and decreases the negative threshold (V m2 ) by a same factor over a previous setting of the respective threshold.
8. The noise touch sensing circuit of claim 7 , wherein the threshold controller outputs a noise touch sensing signal when the count of the first counter is greater than the count of the second counter by at least 3.
9. A method with a touch panel, the touch panel including a plurality of pads, comprising:
measuring power of electromagnetic (EM) noise in a first pad; and measuring power of EM noise in a second pad;
comparing the power of EM noise measured in the first pad to the power of EM noise measured in the second pad; and
providing an indication of a touch occurring at the first pad when the power of the EM noise measured in the first pad is a pre-selected amount greater than the power of the EM noise measured in the second pad,
wherein the step of measuring power of EM noise in a first pad includes:
setting a positive threshold of a first comparator with hysteresis, coupled to the first pad, to a low value and setting a negative threshold of the first comparator with hysteresis to a high value;
in response to an output signal of the first comparator with hysteresis, changing thresholds by increasing the positive threshold and decreasing the negative threshold of the first comparator with hysteresis;
for each change of thresholds, calculating a difference between the positive threshold and the negative threshold; and
storing, as pad 1 V pm , a maximum difference between the positive threshold and the negative threshold that resulted in toggling of the output signal of the first comparator with hysteresis.
10. The method of claim 9 , wherein the step of measuring power of EM noise in a second pad includes:
setting a positive threshold of a second comparator with hysteresis, coupled to the second pad, to a low value and setting a negative threshold of the second comparator with hysteresis to a high value;
in response to an output signal of the second comparator with hysteresis, changing thresholds by increasing the positive threshold and simultaneously decreasing the negative threshold of the second comparator with hysteresis;
for each change of thresholds, calculating a difference between the positive threshold and the negative threshold; and
storing, as pad 2 V pm , a maximum difference between the positive threshold and the negative threshold that resulted in toggling of the output signal of the second comparator with hysteresis.
11. The method of claim 10 , in which the step of measuring power of EM noise in a first pad and the step of measuring power of EM noise in a second pad are performed during a same time interval, and in which the step of comparing the power of EM noise in the first pad to the power of EM noise in the second pad includes calculating pad 1 V pm minus pad 2 V pm .
12. The method of claim 11 , in which the step of outputting a signal indicative of a touch occurring at the first pad includes determining whether pad 1 V pm is a pre-selected amount greater than pad 2 V pm .
13. The method of claim 9 , in which
the step of measuring power of EM noise in the second pad includes measuring power of EM noise in a plurality of neighboring pads, wherein the plurality of neighboring pads constitute pads that are closest to the first pad;
the step of comparing includes comparing the power of EM noise measured in the first pad to an average of power of EM noise measured in each of the plurality of neighboring pads; and
the step of providing includes providing an indication of a touch occurring at the first pad when the power of the EM noise measured in the first pad is a pre-selected amount greater than the average of the power of the EM noise measured in each of the plurality of neighboring pads.