Setting a discharge rate and a charge rate of a relaxation oscillator circuit

• US 20080179112A1
• Filed: 01/30/2007
• Published: 07/31/2008
• Est. Priority Date: 01/30/2007
• Status: Active Grant

• Current Assignee: Monterey Research, LLC (Vector Capital Corporation)
• Original Assignee: Cypress Semiconductor Corporation (Infineon Technologies AG)
• Inventors: Peng, Tao, Qin, Zheng
• Granted Patent: US 8,058,937 B2
• Time in Patent Office: Days
• US Class Current: 178/18.06
• CPC Class Codes: G06F 2203/04111 Cross over in capacitive digitiser, i.e. details of structures for connecting electrodes of the sensing pattern where the connections cross each other, e.g. bridge structures comprising an insulating layer, or vias through substrate
• CPC Class Codes: G06F 3/03547 Touch pads, in which fingers can move on a surface
• CPC Class Codes: G06F 3/038 Control and interface arrangements therefor, e.g. drivers or device-embedded control circuitry
• CPC Class Codes: G06F 3/0445 using two or more layers of sensing electrodes, e.g. using two layers of electrodes separated by a dielectric layer
• 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

An apparatus and method for setting a ratio of a discharge rate to a charge rate for measuring a capacitance on a sensor element of a sensing device. The apparatus may include a sensor element of a sensing device, a relaxation oscillator having a first and a second programmable current source, and a ratio decoder to receive a ratio of a discharge rate to a charge rate, and to set the first and second programmable current sources based on the received ratio.

Claims

1. A method, comprising:

providing a sensor element of a sensing device; and

setting a ratio of a discharge rate to a charge rate for measuring a capacitance on the sensor element.

2. The method of claim 1, wherein setting the ratio comprises:

setting the charging rate for introducing a charge on the sensor element of the sensing device; and

setting the discharging rate for removing the charge on the sensor element.

3. The method of claim 1, further comprising measuring the capacitance on the sensor element to detect a presence of a conductive object on the sensing device.

4. The method of claim 3, wherein measuring the capacitance comprises:

introducing a charge on the sensor element at the charging rate; and

removing the charge on the sensor element at the discharging rate.

5. The method of claim 4, wherein measuring the capacitance further comprises:

comparing a voltage on the sensor element and a first reference voltage;

comparing the voltage on the sensor element and a second reference voltage;

switching from introducing the charge on the sensor element to removing the charge on the sensor element when the voltage on the sensor element is equal to or greater than the first reference voltage; and

switching from removing the charge to introducing the charge on the sensor element when the voltage on the sensor element is equal to or less than the second reference voltage.

6. An apparatus, comprising:

a sensor element of a sensing device;

a relaxation oscillator, coupled to the sensor element, having a first programmable current source and a second programmable current source; and

a ratio decoder coupled to the first and second programmable current sources, wherein the ratio decoder is configured to receive a ratio of a discharge rate to a charge rate, and to set the first and second programmable current sources based on the received ratio.

7. The apparatus of claim 6, wherein the relaxation oscillator further comprises:

a first comparator coupled to one end of the sensor element for a first input and to a first voltage reference for a second input; and

a second comparator coupled to the one end of the sensor element for a first input and to a second voltage reference for a second input.

8. The apparatus of claim 7, wherein the relaxation oscillator further comprises:

a first switch coupled between the first programmable current source and the sensor element, wherein the first switch is controlled by an output of the first comparator; and

a second switch coupled between the second programmable current source and the sensor element, wherein the second switch is controlled by an output of the second comparator.

9. The apparatus of claim 8, further comprising a digital counter coupled to at least one of an output of the first comparator and an output of the second comparator, wherein the digital counter is configured to count at least one of a frequency or a period of the at least one of the output of the first comparator and the output of the second comparator.

10. The apparatus of claim 9, wherein the relaxation oscillator and the ratio decoder reside in a processing device, wherein the processing device is configured to measure a capacitance on the sensor element.

11. The apparatus of claim 10, wherein the processing device is configured to detect a presence of a conductive object on the sensing device.

12. The apparatus of claim 6, wherein the relaxation oscillator is configured to place a charge on the sensor element until a voltage on the sensor element is equal to or greater than a first threshold and to decrease the voltage on the sensor element until the voltage on the sensor element is equal to or less than a second threshold.

13. The apparatus of claim 12, wherein the relaxation oscillator comprises:

a first voltage supply coupled to the relaxation oscillator to provide a first voltage reference as the first threshold; and

a second voltage supply coupled to the relaxation oscillator to provide a second voltage reference as the second threshold.

14. The apparatus of claim 6, wherein first programmable current source is configured to introduce a charge on the sensor element based on the set charging rate, and the second programmable current source is configured to decrease the charge on the sensor element based on the set discharging rate.

15. The apparatus of claim 14, wherein the relaxation oscillator comprise:

a first switch coupled between the first programmable current source and the sensor element; and

a second switch coupled between the second programmable current source and the sensor element, and wherein the first switch is opened and the second reset switch is closed when a voltage on the sensor element is equal to or greater than a first reference voltage, and wherein the first switch is closed and the second reset switch is opened when the voltage on the sensor element is equal to or less than a second reference voltage.

16. The apparatus of claim 6, wherein the relaxation oscillator further comprises a selection circuit coupled to the sensor element and one or more additional sensor elements, and wherein the selection circuit is configured to select either the sensor element or one of the one or more additional sensor elements to measure a capacitance on the selected sensor element.

17. The apparatus of claim 6, wherein the ratio of the discharge rate to the charge rate is approximately a 1 to 1 ratio.

18. The apparatus of claim 6, wherein the ratio of the discharge rate to the charge rate is approximately 1.6 to 1 ratio.

19. An apparatus, comprising:

a sensor element of a sensing device; and

means for setting a ratio of a discharge rate to a charge rate of a relaxation oscillator to measure a capacitance on the sensor element.

20. The apparatus of claim 19, further comprising means for swinging a voltage on the sensor element between two reference voltage levels.

21. The method of claim 19, further comprising means for balancing a cycle-to-cycle variation in the capacitance caused by noise.

22. The method of claim 19, further comprising means for reducing power consumption of the sensing device.