Dual-slope charging relaxation oscillator for measuring capacitance

• US 20080036473A1
• Filed: 08/09/2006
• Published: 02/14/2008
• Est. Priority Date: 08/09/2006
• Status: Abandoned Application

• Current Assignee: Cypress Semiconductor Corporation (Infineon Technologies AG)
• Original Assignee: Cypress Semiconductor Corporation (Infineon Technologies AG)
• Inventors: Jansson, Hakan K.
• Application Number:
  US11/502,267
• Publication Number:
  US 20080036473A1
• Time in Patent Office: Days
• US Class Current: 324/678
• CPC Class Codes: G01R 27/2605 Measuring capacitance capacitive sensors G01D5/24
• CPC Class Codes: G06F 3/03547 Touch pads, in which fingers can move on a surface
• CPC Class Codes: G06F 3/0362 with detection of 1D translations or rotations of an operating part of the device, e.g. scroll wheels, sliders, knobs, rollers or belts
• 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
• CPC Class Codes: H03K 17/962 Capacitive touch switches
• CPC Class Codes: H03K 2217/960715 Rc-timing; e.g. measurement of variation of charge time or discharge time of the sensor

Abstract

An apparatus and method for measuring a capacitance on the sensor element using two charge rates. The two charge rates may be two charging rates, or alternatively, two discharging rates for discharging the sensor element. Alternatively, both the two charging and discharging rates may be used to measure the capacitance. The method may be performed by charging a sensor element of a sensing device for a fixed time at the first charging rate, and charging the sensor element at the second charging rate to reach a threshold voltage after charging the sensor element for the fixed time. The method may also be performed by discharging the sensor element for a fixed time at the first discharging rate, and discharging the sensor element at the second discharging rate to reach a threshold voltage after discharging the sensor element for the fixed time.

Claims

1. A method, comprising:

providing a sensor element; and

measuring a capacitance on the sensor element using two charge rates.

2. The method of claim 1, wherein the two charge rates comprise a first charging rate and a second charging rate, and wherein measuring the capacitance comprises:

charging a sensor element of a sensing device for a fixed time at the first charging rate; and

charging the sensor element at the second charging rate to reach a threshold voltage after charging the sensor element for the fixed time.

3. The method of claim 2, further comprising discharging the measured capacitance using two discharging rates.

4. The method of claim 3, wherein the two discharging rates comprise a first discharging rate and a second discharging rate, and wherein discharging the measured capacitance comprises:

discharging the sensor element for a fixed time at the first discharging rate; and

discharging the sensor element at the second discharging rate to reach a threshold voltage after discharging the sensor element for the fixed time.

5. The method of claim 1, wherein the two charge rates comprise a first discharging rate and a second discharging rate, and wherein measuring the capacitance comprises:

discharging the sensor element for a fixed time at the first discharging rate; and

discharging the sensor element at the second discharging rate to reach a threshold voltage after discharging the sensor element for the fixed time.

6. The method of claim 1, wherein the fixed time is programmable.

7. The method of claim 1, wherein the threshold voltage is programmable.

8. The method of claim 1, wherein the two charge rates comprise a first charging rate and a second charging rate, and wherein the first and second charging rates are linear.

9. The method of claim 1, wherein the two charge rates comprise a first charging rate and a second charging rate, and wherein the first charging rate is exponential.

10. An apparatus, comprising:

a sensor element; and

a capacitance sensor coupled to the sensor element, wherein the capacitance sensor is operable to measure a capacitance on the sensor element using two charge rates.

11. The apparatus of claim 10, wherein the two charge rates comprise a first charging rate and a second charging rate, and wherein the capacitance sensor is operable to charge the sensor element for a fixed time at the first charging rate and to charge the sensor element at the second charging rate to reach a threshold voltage to measure the capacitance on the sensor element.

12. The apparatus of claim 10, wherein the two charge rates comprise a first discharging rate and a second discharging rate, and wherein the capacitance sensor is operable to discharge the sensor element for a fixed time at the first discharging rate and to discharge the sensor element at the second discharging rate to reach a threshold voltage to measure the capacitance on the sensor element.

13. The apparatus of claim 10, wherein the capacitance sensor comprises:

a controller circuit; and

a relaxation oscillator coupled to the controller circuit and the sensor element.

14. The apparatus of claim 13, wherein the controller circuit comprises:

a programmable timer coupled to the relaxation oscillator; and

a logic circuit coupled to the programmable timer and the relaxation oscillator.

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

a current source to provide a charging current to the sensor element;

a comparator coupled to the current source and the sensor element, wherein the comparator is operable to compare a voltage on the selected sensor element and the threshold voltage; and

a reset switch coupled to the comparator and current source, wherein the reset switch is operable to reset the charging current on the selected sensor element.

16. The apparatus of claim 15, wherein the capacitance sensor comprises a digital counter coupled to the relaxation oscillator, and wherein the digital counter is operable to count at least one of a frequency or a period of a relaxation oscillator output received from the relaxation oscillator.

17. The apparatus of claim 13, wherein the capacitance sensor resides in a processing device, wherein the sensor element and processing device are operable to detect a presence of a conductive object, and wherein the conductive object is at least one of a finger or a stylus.

18. An apparatus, comprising:

a sensor element; and

means for measuring a capacitance of the sensor element using two charge rates.

19. The apparatus of claim 18, wherein the two charge rates comprise a first charging rate and a second charging rate, and further comprising means for charging the sensor element at the first charging rate and at the second charging rate, wherein the first charging rate is different than the second charging rate.

20. The apparatus of claim 18, wherein the two charge rates comprise a first discharging rate and a second discharging rate, and further comprising means for discharging the sensor element at the first discharging rate and at the second discharging rate, wherein the first discharging rate is different than the second discharging rate.