Noise handling in capacitive touch sensors

• US 8,054,090 B2
• Filed: 10/22/2008
• Issued: 11/08/2011
• Est. Priority Date: 10/22/2008
• Status: Active Grant

• Current Assignee: Neodrón Limited (Atlantic IP Services Limited)
• Original Assignee: Atmel Corporation (Microchip Technology Incorporated)
• Inventors: Philipp, Harald, Pickett, Daniel, Yilmaz, Esat
• Primary Examiner(s): Nguyen, Vincent Q
• Application Number:
  US12/255,998
• Publication Number:
  US 20100097078A1
• Time in Patent Office: 1,112 Days
• Field of Search: 324/684, 324/678
• US Class Current: 324/684
• CPC Class Codes: G01R 27/2605 Measuring capacitance capacitive sensors G01D5/24
• CPC Class Codes: G01R 29/26 Measuring noise figure; Measuring signal-to-noise ratio
• CPC Class Codes: G06F 3/04184 Synchronisation with the driving of the display or the backlighting unit to avoid interferences generated internally
• CPC Class Codes: G06F 3/0443 using a single layer of sensing electrodes
• CPC Class Codes: G06F 3/0446 using a grid-like structure of electrodes in at least two directions, e.g. using row and column electrodes

Abstract

In a capacitive sensor of the type having X electrodes which are driven and Y electrodes that are used as sense channels connected to charge measurement capacitors, signal measurements are made conventionally by driving the X electrodes to transfer successive packets of charge to the charge measurement capacitors. However, an additional noise measurement is made by emulating or mimicking the signal measurement, but without driving the X electrodes. The packets of charge transferred to the charge accumulation capacitor are then indicative of noise induced on the XY sensing nodes. These noise measurements can be used to configure post-processing of the signal measurements.

Claims

1. A method comprising:

grounding a first plate of a charge-accumulation capacitor;

injecting, through a resistor coupled to a voltage source, a predetermined amount of charge onto a charge-measurement capacitor;

transferring an amount of charge accumulated on a second plate of the charge-accumulation capacitor to a first plate of the charge-measurement capacitor, the charge having accumulated on the second plate of the charge-accumulation capacitor due at least in part to noise; and

determining, through a measured voltage across the charge-measurement capacitor, the amount of charge.

2. The method of claim 1, further comprising calculating a noise factor to configure a filter based on the determined amount of charge.

3. The method of claim 1, further comprising if the calculated noise factor is above a specified threshold value, then applying the configured filter to perform a numerical process on a measured signal.

16. The method of claim 1, wherein injecting the predetermined amount of charge further comprises coupling the second plate of the charge-measurement capacitor to the voltage source for a predetermined amount of time.

17. The method of claim 1, wherein determining the amount of charge further comprises measuring an amount of time to reduce the measured voltage across the charge-measurement capacitor to ground.

18. The method of claim 17, wherein determining the amount of time further comprises counting a number of clock periods for the measured voltage across the charge-measurement capacitor to become ground.

4. An apparatus comprising:

a charge-measurement capacitor having a second plate coupled to a first plate of a charge-accumulation capacitor; and

one or more non-transitory computer-readable storage media embodying logic that is operable when executed to;

ground the first plate of the charge-accumulation capacitor;

inject a predetermined amount of charge onto the charge-measurement capacitor through a resistor coupled to a voltage source;

transfer an amount of charge accumulated on a second plate of the charge-accumulation capacitor to a first plate of the charge-measurement-capacitor, the charge having accumulated on the second plate of the charge-accumulations capacitor due at least in part to noise; and

determine, through a measured voltage across the charge-measurement capacitor, the amount of charge.

5. The apparatus of claim 4, wherein the logic is further operable when executed to calculate a noise factor to configure a filter based on the determined amount of charge due to noise.

6. The apparatus of claim 5, wherein the logic when executed is further operable to apply the configured filter to perform a numerical process on a measured signal, if the calculated noise factor is above a specified value.

7. The apparatus of claim 4, wherein injecting the predetermined amount of charge further comprises coupling the second plate of the charge-measurement capacitor to the voltage source for a predetermined amount of time.

8. The apparatus of claim 4, wherein the logic is further operable when executed to determine the amount of charge by measuring an amount of time to reduce the measured voltage across the charge-measurement capacitor to ground.

9. The apparatus of claim 8, wherein the logic is further operable when executed to determine the amount of time by counting a number of clock periods for the measured voltage across the charge-measurement capacitor to become ground.

10. A non-transitory computer-readable storage media embodying logic that is operable when executed to:

ground a first plate of a charge-accumulation capacitor;

inject, through a resistor coupled to a voltage source, a predetermined amount of charge onto a charge-measurement capacitor;

transfer an amount of charge on a second plate of the charge-accumulation capacitor to a first plate of the charge-measurement capacitor, the charge having accumulated on the second plate of the charge-accumulation capacitor at least in part due to noise; and

determine, through a measured voltage across the charge-measurement capacitor, the amount of charge.

11. The media of claim 10, wherein the logic is further operable when executed to calculate a noise factor to configure a filter based on the determined amount of charge.

12. The media of claim 11, wherein the logic is further operable when executed to apply the configured filter to a numerical process on a measured signal, if the calculated noise factor is above a specified value.

13. The media of claim 10, wherein injecting the predetermined amount of charge further comprises coupling the second plate of the charge-measurement capacitor to the voltage source for a predetermined amount of time.

14. The media of claim 10, wherein the logic is further operable when executed to determine the amount of charge by measuring an amount of time to reduce the measured voltage across the charge-measurement capacitor to ground.

15. The media of claim 14, wherein the logic is further operable when executed to determine the amount of time by counting a number of clock periods for the measured voltage across the charge-measurement capacitor to become ground.