METHOD AND APPARATUS TO MEASURE SELF-CAPACITANCE USING A SINGLE PIN

• US 20110073383A1
• Filed: 09/25/2009
• Published: 03/31/2011
• Est. Priority Date: 09/25/2009
• Status: Active Grant

• Current Assignee: Atmel Corporation (Microchip Technology Incorporated)
• Original Assignee: Atmel Corporation (Microchip Technology Incorporated)
• Inventors: Simmons, Martin John
• Granted Patent: US 8,552,994 B2
• Time in Patent Office: Days
• US Class Current: 178/18.06
• CPC Class Codes: G01R 27/2605 Measuring capacitance capacitive sensors G01D5/24
• CPC Class Codes: H03K 17/962 Capacitive touch switches
• CPC Class Codes: H03K 2017/9606 using one electrode only per touch switch
• CPC Class Codes: H03K 2217/960725 Charge-transfer

Abstract

A method for measuring capacitance in a sensor device using an internal reference circuit element(s), and without implementing additional circuitry and devices external to the sensor device, is described. In some embodiments a method uses an output pin of the sensor device and an internal reference capacitor of the sensor device to identify a touch applied to a touch point or electrode coupled to the touch sensor. The method applies reference voltages to charge the reference capacitor and measure a signal received from an electrode, wherein the touch sensor controls switching within the touch sensor to apply the reference voltages to the reference capacitor.

Claims

1. A touch sensor system, comprising:

at least one input port to receive an input signal from at least one electrode, the at least one electrode having a coupling capacitance;

an Analog to Digital Converter (ADC) having an ADC input and an ADC output;

a capacitor coupled to the ADC input;

a multiplexer (MUX) having multiple MUX inputs and a MUX output that is coupled to the ACD input, wherein the at least one port is coupled to one of the multiple MUX inputs;

a first switch for coupling a first reference voltage to a first input of the multiple MUX inputs;

a switch controller to control the first switch to apply the first reference voltage to the first input of the MUX; and

a sensor controller to receive a first digital value from the ADC output after application of the first reference voltage, and to determine a change in the coupling capacitance of the at least one electrode as a function of the first digital value.

2. The touch sensor system of claim 1, further comprising:

a second switch coupling a second reference voltage to the first input of the multiple MUX inputs; and

a MUX controller to select the at least one electrode as an input to the ADC input,

wherein the switch controller is further to control the second switch to apply the second reference voltage to the first input to the MUX, and

the sensor controller is further to receive a second digital value after application of the second reference voltage, and

to determine a change in the coupling capacitance of the at least one electrode as a function of the first digital value and the second value.

3. The touch sensor system of claim 1, wherein the first reference voltage is opposite in polarity to the second reference voltage.

4. The touch sensor system of claim 1, wherein the sensor controller is further configured to evaluate the first and second electrode voltages against a threshold value to detect a touch at the at least one electrode.

5. A touch sensor apparatus, comprising:

a reference circuit to measure electrical changes at a touch sensor electrode;

a first switch to apply a first voltage signal to the reference circuit;

a second switch to apply a second voltage signal to the reference circuit; and

a measurement unit to measure a first change in electrical behavior of the touch sensor electrode after application of the first voltage signal, and to measure a second change in electrical behavior of the touch sensor electrode after application of the second voltage signal.

6. The touch sensor apparatus of claim 5, further comprising a controller to compare the first and second changes in electrical behavior of the touch sensor electrode to detect a touch to the touch sensor electrode.

7. The touch sensor apparatus of claim 6, wherein the reference circuit comprises:

a capacitor having one side coupled to an input of the measurement unit and an opposite side coupled to a reference voltage, wherein the first voltage signal and the second voltage signal are of opposite polarity with respect to the reference voltage.

8. The touch sensor apparatus of claim 7, wherein the touch sensor apparatus further comprises a port to receive an input signal from the touch sensor electrode, and wherein the measurement unit is configured to measure the input signal from the touch sensor electrode after application of the first voltage signal and after application of the second voltage signal.

9. A method, comprising:

controlling a first switch to charge a reference capacitor to a first voltage level, the reference capacitor coupled to an input of an Analog to Digital Converter (ADC);

receiving a first analog signal from a touch sensor electrode at the input of the ADC;

converting the first analog signal to a first digital value provided at an output of the ADC;

detecting a touch to the touch sensor device based on the first digital value.

10. The method of claim 9, further comprising

controlling a second switch to charge the reference capacitor to a second voltage level;

receiving a second analog signal from the touch sensor electrode at the input to the ADC;

converting the second analog signal to a second digital value at the output of the ADC; and

controlling a multiplexer (MUX) to select an input to the ADC.

11. The method of claim 9, wherein the reference capacitor is further coupled to a reference voltage, and the first voltage is opposite in polarity to the second voltage with respect to the reference voltage.

12. The method of claim 11, wherein the reference voltage is a function of the first voltage level.

13. The method of claim 12, wherein the second voltage level is a ground voltage.

14. The method of claim 9, wherein detecting a touch further comprises:

calculating a difference between the first and second digital values; and

comparing the difference to a threshold value.

15. The method of claim 9, wherein controlling the first switch to charge the reference capacitor to the first voltage level comprises closing the first switch and opening the second switch.

16. The method of claim 15, wherein controlling the second switch to charge the reference capacitor to the second voltage level comprises closing the second switch and opening the first switch.

17. The method of claim 16, wherein converting the first analog voltage to a first digital value at the output of the ADC comprises opening the first and second switches, and wherein converting the second analog voltage to a second digital value at the output of the ADC comprises opening the first and second switches.

18. A computer-readable medium comprising instructions which, when implemented by one or more machines, cause the one or more machines to:

control a first switch to charge a reference capacitor to a first voltage level, the reference capacitor coupled to an input of an Analog to Digital Converter (ADC);

receive a first analog signal from a touch sensor electrode at the input of the ADC;

convert the first analog signal to a first digital value provided at an output of the ADC;

control a second switch to charge the reference capacitor to a second voltage level;

receive a second analog signal from the touch sensor electrode at the input to the ADC;

convert the second analog signal to a second digital value at the output of the ADC; and

detect a touch to the touch sensor device based on the first and second digital values.

19. The computer-readable medium of claim 18, wherein to detect a touch is further configured to

calculate a difference between the first and second digital values;

and

compare the difference to a threshold value.

20. The computer-readable medium of claim 18, wherein to convert the first analog voltage to the first digital value at the output of the ADC is further to open the first and second switches, and wherein to convert the second analog voltage to the second digital value at the output of the ADC is further to open the first and second switches.