Capacitive sensing device and method

• US 20090243632A1
• Filed: 03/23/2009
• Published: 10/01/2009
• Est. Priority Date: 03/27/2008
• Status: Active Grant

• Current Assignee: Renesas Electronics Corporation
• Original Assignee: NEC Electronics Corporation (Renesas Electronics Corporation)
• Inventors: Ozawa, Naofumi
• Granted Patent: US 8,125,232 B2
• Time in Patent Office: Days
• US Class Current: 324/679
• CPC Class Codes: G06F 3/04166 Details of scanning methods, e.g. sampling time, grouping of sub areas or time sharing with display driving Synchronisation with the driving of the display or the backlighting unit to avoid interferences generated internally G06F3/04184
• CPC Class Codes: G06F 3/044 by capacitive means
• 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

In a capacitive sensing device for detecting a change in capacitance of an electrode which is caused by contact with a human body, the improvement of determination speed may be enabled. M (m is an integer satisfying n≧m≧2) electrodes are selected from n (n is an integer equal to or larger than two) electrodes. Capacitors connected to the m electrodes are discharged in parallel during a predetermined period, and then charged. During the charging period, a potential of each of the m electrodes is compared with a reference potential. A difference between capacitance values of the capacitors connected to the m electrodes is determined based on a result obtained by the comparison between the m electrodes.

Claims

1. A capacitive sensing device comprising:

n (n is an integer equal to or larger than two) electrodes;

a selection section for selecting m (m is an integer satisfying n≧

m≧

2) electrodes from the n electrodes;

a charging and discharging control section coupled to the m electrodes, for controlling charging and discharging capacitors connected to the m electrodes to be performed in parallel;

a comparison section coupled to the m electrodes, for comparing a reference potential with a potential of each of the m electrodes during the charging performed by the charging and discharging control section, to generate comparison result signals associated with the m electrodes; and

a determination section coupled to the comparison section, to determine a difference between capacitance values of the capacitors coupled to the m electrodes based on the comparison result signals associated with the m electrodes.

2. The capacitive sensing device according to claim 1, wherein:

the comparison section comprises m comparators;

the determination section comprises m storage sections each corresponding to the m comparators;

the m storage sections store the comparison result signals obtained from the m comparators at a predetermined timing during the charging; and

the determination section determines the difference between the capacitance values of the capacitors coupled to the m electrodes based on the comparison result signals stored in the m storage sections.

3. The capacitive sensing device according to claim 2, wherein:

the determination section further comprises a measurement section;

the measurement section measures a time interval between changing points of the comparison result signals associated with the m comparators; and

the determination section determines the difference between the capacitance values of the capacitors connected to the m electrodes based on the measured time interval and the comparison result signals stored in the m storage sections.

4. The capacitive sensing device according to claim 2, wherein the predetermined timing is a timing at which any one of the comparison result signals from the m comparators changes.

5. The capacitive sensing device according to claim 3, wherein:

the determination section further comprises an exclusive OR calculation section;

the exclusive OR calculation section receives the comparison result signals from the m comparators;

the m storage sections store the m comparison result signals at a first changing point of an output from the exclusive OR calculation section after start of the charging; and

the measurement section measures a time interval between changing points of the output from the exclusive OR calculation section after the start of the charging.

6. The capacitive sensing device according to claim 2, wherein the predetermined timing is set as a time between a charging period required for charging up to the reference potential in a case where a human body is not in contact with the m electrodes and a charging period required for charging up to the reference potential in a case where the human body is in contact with at least one of the m electrodes.

7. The capacitive sensing device according to claim 6, wherein:

the determination section further comprises a measurement section; and

the m storage sections store the comparison result signals from the m comparators when a measurement time of the measurement section reaches a time indicated by the predetermined timing.

8. The capacitive sensing device according to claim 1, wherein:

the determination section comprises a measurement section;

the measurement section measures time intervals between start of the charging and m changing points of the comparison result signals; and

the determination section determines the difference between the capacitance values of the capacitors coupled to the m electrodes based on a difference between the measured time intervals.

9. The capacitive sensing device according to claim 8, wherein:

the comparison section comprises m comparators;

the determination section comprises m storage sections each corresponding to the m comparators;

the measurement section causes corresponding one of the m storage sections to store each of the measured time intervals associated with the m changing points of the comparison result signals; and

the determination section determines the difference between the capacitance values of the capacitors coupled to the m electrodes based on the measured time intervals stored in the m storage sections.

10. The capacitive sensing device according to claim 1, wherein the charging and discharging control section connects the m electrodes with a ground potential to discharge the capacitors connected to the m electrodes during a predetermined period, and connects the m electrodes with a power supply potential through resistors to charge the capacitors connected to the m electrodes.

11. The capacitive sensing device according to claim 1, wherein:

the determination section comprises a measurement section;

the measurement section measures a time interval between changing points of the comparison result signals associated with the m comparators; and

the determination section determines the difference between the capacitance values of the capacitors connected to the m electrodes based on the measured time interval.

12. A capacitive sensing method of sensing capacitance values of capacitors connected to n (n is an integer equal to or larger than two) electrodes, comprising:

selecting m (m is an integer satisfying n≧

m≧

2) electrodes from the n electrodes;

discharging capacitors connected to the m electrodes, in parallel for a predetermined period, and then charging the capacitors connected to the m electrodes;

comparing a reference potential with a potential of each of the m electrodes during the charging, to generate comparison results associated with the m electrodes; and

determining a difference between the capacitance values of the capacitors connected to the m electrodes based on the comparison results associated with the m electrodes,

wherein the selecting step, the charging and discharging step, the comparing step, and the determining step are performed n/m times.

13. The capacitive sensing method according to claim 12, wherein the determining step comprises obtaining the m comparison results at a predetermined timing during the charging, and determining the difference between the capacitance values of the capacitors connected to the m electrodes based on the obtained comparison results.

14. The capacitive sensing method according to claim 13, wherein the determining step further comprises determining the difference between the capacitance values of the capacitors connected to the m electrodes based on the obtained comparison results and a time interval between acquisition times of the comparison results associated with the m electrodes.

15. The capacitive sensing method according to claim 13, wherein the predetermined timing is set as a time between a charging period required for charging up to the reference potential in a case where a human body is not in contact with the m electrodes and a charging period required for charging up to the reference potential in a case where the human body is in contact with at least one of the m electrodes.

16. The capacitive sensing method according to claim 12, wherein the determining step comprises:

measuring time intervals between start of the charging and outputs of the comparison result signals; and

determining the difference between the capacitance values of the capacitors connected to the m electrodes based on a difference between the measured time intervals.

17. The capacitive sensing method according to claim 12, wherein the determining step comprises determining the difference between the capacitance values of the capacitors connected to the m electrodes based on a time interval between acquisition times of the comparison results associated with the m electrodes.