Proximity/touch detector and calibration circuit

• US 20020196066A1
• Filed: 06/20/2001
• Published: 12/26/2002
• Est. Priority Date: 06/20/2001
• Status: Active Grant

• Current Assignee: Apple Inc.
• Original Assignee: Apple Computer Incorporated (Apple Inc.)
• Inventors: Krah, Christoph H., Cappels, Richard D.
• Granted Patent: US 6,583,676 B2
• Time in Patent Office: Days
• US Class Current: 327/291
• CPC Class Codes: H03K 17/962 Capacitive touch switches
• CPC Class Codes: H03K 2217/94026 Automatic threshold calibration; e.g. threshold automatically adapts to ambient conditions or follows variation of input

Abstract

A calibration circuit and method for a proximity/touch detector allow automatic calibration to the proximity/touch detector components, chassis affects, and ambient conditions such that initial factory calibration and periodic manual calibration are not needed. The calibration circuit switches a capacitance into the input capacitance of a Schmitt trigger free running oscillator to change the output frequency of the oscillator. A capacitive sensor forms part of the input capacitance. The change in frequency simulates the frequency shift associated with the difference in input capacitance generated when an object, such as a finger, is touching the capacitive sensor and when the capacitive sensor is free from contact with the object.

Claims

1. An apparatus, comprising:

a calibration circuit comprising;

a transistor and a first capacitor coupled in parallel;

a second capacitor coupled in series with the first capacitor and the transistor;

a capacitive sensor coupled in series with the second capacitor, the first capacitor and the transistor;

an oscillator coupled in parallel with the sensing electrode, the second capacitor, the first capacitor and the transistor; and

a microprocessor coupled to the transistor to turn the transistor “

on”

to short the first capacitor to simulate an increase in input capacitance to the oscillator that would accompany an object contacting a capacitive sensor.

2. The apparatus of claim 1 wherein the microprocessor is coupled to an output of the oscillator to measure the frequency output of the oscillator when the transistor is turned “

on.”

3. The apparatus of claim 1 wherein the microprocessor comprises logic coupled to compare the frequency output of the oscillator when the transistor is turned “

on”

to the frequency output of the oscillator when the transistor is turned “

off.”

4. The apparatus of claim 1 wherein the microprocessor comprises logic coupled to measure a frequency shift associated with the frequency output of the oscillator when the transistor is turned “

on” and

the frequency output of the oscillator when the transistor is turned “

off.”

5. The apparatus of claim 1 wherein the microprocessor comprises logic coupled to determine that a frequency shift associated with the frequency output of the oscillator when the transistor is turned “

on” and

the frequency output of the oscillator when the transistor is turned “

off”

as going from a condition of the capacitive sensor being free from contact to a condition of the an object being in contact with the capacitive sensor.

6. The apparatus of claim 1, wherein the oscillator comprises:

a Schmitt trigger; and

a feedback resistor coupled from an output of the Schmitt trigger to an input of the Schmitt trigger.

7. The apparatus of claim 1, further comprising a resistor coupled between the collector of the transistor and a supply voltage to reverse biases the collector-base junction of the transistor when the transistor is turned “

off.”

8. The apparatus of claim 1, further comprising a resistor coupled between the collector of the transistor and a supply voltage to reduce the capacitance of the collector when the transistor is turned “

off.”

9. The apparatus of claim 1, further comprising a resistor coupled between the collector of the transistor and a supply voltage to modify the impedance seen by the oscillator when the transistor is turned “

off.”

10. An apparatus, comprising:

a calibration circuit to modify input capacitance of an oscillator to simulate a differential input capacitance associated with a capacitance generated when an object is in contact with a capacitive sensor and a capacitance generated when the capacitive sensor is free from contact with an object.

11. The apparatus of claim 10 wherein the calibration circuit comprises

a switch coupled in parallel to an input capacitance and in series with a capacitive sensor;

an oscillator having an input coupled in parallel with the switch; and

logic coupled to close the switch to short the first capacitor to simulate the differential input capacitance to the oscillator that would accompany an object contacting the capacitive sensor.

12. A system, comprising:

an sensing electrode;

an oscillator whose input is coupled to the sensing electrode;

a microprocessor whose input is couple to an output of the oscillator; and

a calibration circuit whose input is coupled to a microprocessor output and whose output is coupled to the oscillator input wherein the calibration circuit is to modify input capacitance of an oscillator to simulate a differential input capacitance associated with a capacitance generated when an object is in contact with a capacitive sensor and a capacitance generated when the capacitive sensor is free from contact with an object.