Method and apparatus to measure self-capacitance using a single pin

• US 8,552,994 B2
• Filed: 09/25/2009
• Issued: 10/08/2013
• 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
• Primary Examiner(s): Chow, Van
• Application Number:
  US12/567,473
• Publication Number:
  US 20110073383A1
• Time in Patent Office: 1,474 Days
• Field of Search: 345173-179
• US Class Current: 345/173
• 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 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 ADC input, wherein the at least one input 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 second switch for coupling a second reference voltage to the first input of the multiple MUX inputs;

a switch controller operable to;

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

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

a sensor controller operable to;

receive a first digital value from the ADC output after application of the first reference voltage;

receive a second digital value from the ADC output after application of the second reference voltage;

determine, based at least upon the first and second digital values, an amount of mains interference;

remove the determined amount of mains interference from the first digital value; and

determine a change in the 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 MUX controller operable to select the at least one electrode as an input to the ADC input; and

wherein the sensor controller is further operable to determine a change in the capacitance of the at least one electrode as a function of the first digital value and the second digital 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 digital values against a threshold value to detect a touch at the at least one electrode.

5. 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;

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

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;

determining, based at least upon the first and second digital values, an amount of mains interference;

removing the determined amount of mains interference from the first digital value;

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

6. The method of claim 5, further comprising

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

7. The method of claim 5, wherein the reference capacitor is further coupled to a reference voltage, and the first voltage level is opposite in polarity to the second voltage level with respect to the reference voltage.

8. The method of claim 7, wherein the reference voltage is a function of the first voltage level.

9. The method of claim 5, wherein the second voltage level is a ground voltage.

10. The method of claim 5, wherein detecting a touch further comprises:

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

comparing the difference to a threshold value.

11. The method of claim 5, 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.

12. The method of claim 5, 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.

13. The method of claim 5, wherein converting the first analog signal to a first digital value at the output of the ADC comprises opening the first and second switches, and wherein converting the second analog signal to a second digital value at the output of the ADC comprises opening the first and second switches.

14. A non-transitory 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;

determine, based at least upon the first and second digital values, an amount of mains interference;

remove the determined amount of mains interference from the first digital value;

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

15. The non-transitory computer-readable medium of claim 14, wherein to detect a touch is further configured to:

calculate a difference between the digital value and second digital value; and

compare the difference to a threshold value.

16. The non-transitory computer-readable medium of claim 14, 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

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.