Analog elimination of ungrounded conductive objects in capacitive sensing
In a method for performing a touch determination with a capacitive sensor, a self capacitance measurement of a capacitive sensor is initiated, wherein at the same time a mutual capacitance measurement including the capacitive sensor is performed. Such a method can be performed such that the self capacitance measurement and the mutual capacitance measurement differentially cancel with ungrounded conductive objects approaching or touching the capacitive sensor and additively combine for grounded objects approaching or touching the capacitive sensor.
1. A method for performing a touch determination with a capacitive sensor comprising a first electrode and a second electrode;
initiating a self capacitance measurement of the first electrode and performing in parallel a mutual capacitance measurement including the first and second electrode, wherein after a start and before an end of the self capacitance measurement a pulse is fed to the second electrode, wherein a measurement circuit is designed such that grounding a sensor capacitance of the capacitive sensor has an opposite influence on self-capacitance measurement with respect to the mutual capacitance measurement and un-grounding the sensor capacitance has an equal influence on self-capacitance measurement and the mutual capacitance measurement.
2. The method according to claim 1 , wherein the pulse starts after the sensor has been set to a high impedance state during the self capacitance measurement.
3. The method according to claim 2 , wherein the second electrode is formed by a shield or guard electrode arranged in proximity to the first electrode to provide a capacitive coupling.
4. The method according to claim 1 , wherein the self capacitance measurement is a capacitive voltage divider measurement.
5. The method according to claim 1 , wherein the self capacitance measurement is a charge time measurement.
6. The method according to claim 1 , further comprising a calibration method, wherein the calibration method is performed before performing a touch determination, the calibration method comprising:
performing an individual self capacitance measurement and storing a first measurement value;
performing an individual mutual capacitance measurement and storing a second measurement value; and
calculating a scale factor from said first and second measurement values;
wherein the method for performing a touch determination includes applying said scale factor to said self capacitance or said mutual capacitance measurement.
7. A method for performing a touch determination with a capacitive sensor comprising a first electrode and a second electrode capacitively coupled with the first electrode:
charging the first electrode to a first level and a sampling capacitor to a second level;
coupling the first electrode with the sampling capacitor while the capacitive sensor is set to high impedance and starting a pulse fed to the second electrode;
after a settling phase determining a first settled voltage level at the sampling capacitor;
thereafter charging the first electrode to the second level and the sampling capacitor to the first level;
coupling the first electrode with the sampling capacitor while the sensor is set to high impedance, wherein the pulse is terminated after coupling of the first electrode with the sampling capacitor;
after a settling phase determining a second settled voltage level at the sampling capacitor, wherein a measurement circuit is designed such that grounding a sensor capacitance of the capacitive sensor has an opposite influence on self-capacitance measurement with respect to the mutual capacitance measurement and un-grounding the sensor capacitance has an equal influence on self-capacitance measurement and the mutual capacitance measurement.
8. The method according to claim 7 , wherein the first level is a predetermined voltage VDD and the second voltage level is a ground level VSS.
9. The method according to claim 7 , wherein the second level is a predetermined voltage VDD and the first voltage level is a ground level VSS.
10. The method according to claim 7 , wherein the pulse has a voltage level of VDD and begins after expiration of a predetermined time period which starts when coupling the first electrode with the sampling capacitor.
11. A microcontroller for performing a touch determination with a capacitive sensor comprising a first electrode and a second electrode capacitively coupled with the first electrode, comprising:
a capacitive measurement circuit comprising a sampling capacitor and associated analog-to-digital converter, a plurality of switches and an I/O (Input/Output) port configurable to perform a self capacitance measurement of the first electrode and a mutual capacitance measurement of a capacitor formed by the first and second electrode; wherein the microcontroller is programmable:
to control the plurality of switches, the sampling capacitor, and the associated analog-to-digital converter to initiate the self capacitance measurement of the first electrode coupled with the capacitive measurement circuit and to perform in parallel a mutual capacitance measurement including the first and second electrode, wherein after a start and before an end of the self capacitance measurement the microcontroller controls the I/O port to feed a pulse to the second electrode, wherein the capacitive measurement circuit is designed such that grounding a sensor capacitance of the capacitive sensor has an opposite influence on self-capacitance measurement with respect to the mutual capacitance measurement and un-grounding the sensor capacitance has an equal influence on self-capacitance measurement and the mutual capacitance measurement.
12. The microcontroller according to claim 11 , wherein the pulse is started after the plurality of switches are controlled to switch the capacitive sensor in a high impedance state during the self capacitance measurement.
13. A system comprising a microcontroller according to claim 12 , comprising said first electrode connected with the microcontroller through a first port and said second electrode arranged in proximity to the first electrode and connected with the microcontroller through a second port.
14. The microcontroller according to claim 11 , wherein the capacitive measurement circuit comprises a capacitive voltage divider measurement circuit.
15. The microcontroller according to claim 14 , wherein the capacitive voltage divider measurement circuit comprises:
a first switch unit coupled between an external pin and the sampling capacitor and operable to charge the first electrode to a first or second voltage level or to couple the first electrode with the sampling capacitor;
a second switch unit coupled with the sampling capacitor and operable to charge the sampling capacitor to either said first or second voltage level;
wherein the analog to digital converter is operable to be coupled with the sampling capacitor; and
a control unit controlling said first and second switch unit.
16. The microcontroller according to claim 15 , further comprising a second external pin coupled with the I/O port which is configurable to operate as an output port, wherein the control unit controls the output port generate said pulse.
17. A system comprising a microcontroller according to claim 16 , comprising said capacitive sensor connected with the microcontroller through said first external pin and said second external pin.
18. The microcontroller according to claim 11 , wherein the capacitive measurement circuit comprises a charge time measurement, wherein the microcontroller is configured to control a self capacitance measurement with said charge time measurement unit, wherein the charge time measurement unit is connected with a first external pin which is connected with the capacitive sensor.
19. The microcontroller according to claim 18 , further comprising a second external pin coupled with the I/O port which is configurable to operate as an output port, wherein the microcontroller is configured to control the output port to generate said pulse.
20. A system comprising a microcontroller according to claim 19 , comprising said capacitive sensor connected with the microcontroller through said first external pin and said second external pin.
21. The microcontroller according to claim 11 , wherein the microcontroller is configured to perform a calibration before performing a touch determination, wherein the microcontroller controls:
an individual self capacitance measurement and stores a first measurement value;
an individual mutual capacitance measurement and stores a second measurement value; and
wherein the microcontroller is further configured to calculate a scale factor from said first and second measurement values;
wherein for performing a touch determination the microcontroller is configured to apply said scale factor to said self capacitance or said mutual capacitance measurement.
22. The microcontroller according to claim 21 , wherein the scale factor changes charging levels during said self capacitance measurement or a voltage level during said mutual capacitance measurement.
23. A method for performing a touch determination with a capacitive sensor comprising a first electrode and a second electrode, comprising:
initiating a self capacitance measurement of the first electrode,
initiating a mutual capacitance measurement including the first electrode and the second electrode, wherein the second electrode receives a pulse after the capacitive sensor has been switched into high impedance during the self capacitance measurement and wherein the pulse is terminated before an end of the self capacitance measurement;
performing a scaling of either an output value of the self capacitance measurement or the mutual capacitance measurement; and
combining the output values of the self capacitance measurement and the mutual capacitance measurement, wherein a measurement circuit is designed such that grounding a sensor capacitance of the capacitive sensor has an opposite influence on self-capacitance measurement with respect to the mutual capacitance measurement and un-grounding the sensor capacitance has an equal influence on self-capacitance measurement and the mutual capacitance measurement.
24. The method according to claim 23 , wherein combining the output values comprises adding the output values.
25. The method according to claim 23 , wherein a shield or guard electrode is arranged in proximity to the capacitive sensor to provide a capacitive coupling.
26. The method according to claim 23 , wherein the self capacitance measurement is a capacitive voltage divider measurement.
27. The method according to claim 23 , wherein the self capacitance measurement is a charge time measurement.
28. The method according to claim 23 , further comprising a calibration method to determine a scaling factor, wherein the calibration method is performed before performing a touch determination, the calibration method comprising:
performing an individual self capacitance measurement and storing a first measurement value;
performing an individual mutual capacitance measurement and storing a second measurement value; and
calculating a scaling factor from said first and second measurement values.
29. A method for performing a touch determination with a capacitive sensor, comprising:
initiating a self capacitance measurement of a capacitive sensor in parallel with a mutual capacitance measurement including the capacitive sensor such that the self capacitance measurement and the mutual capacitance measurement differentially cancel with ungrounded conductive objects approaching or touching said capacitive sensor and additively combine for grounded objects approaching or touching said capacitive sensor, wherein a measurement circuit is designed such that grounding a sensor capacitance of the capacitive sensor has an opposite influence on self-capacitance measurement with respect to the mutual capacitance measurement and un-grounding the sensor capacitance has an equal influence on self-capacitance measurement and the mutual capacitance measurement, wherein the mutual capacitance measurement comprises feeding a pulse capacitively into said capacitive sensor.