Touch screen detection and diagnostics

• US 8,487,884 B2
• Filed: 06/24/2008
• Issued: 07/16/2013
• Est. Priority Date: 06/24/2008
• Status: Active Grant

• Current Assignee: NXP USA, Inc. (NXP Semiconductors NV)
• Original Assignee: Freescale Semiconductor, Inc. (NXP Semiconductors NV)
• Inventors: De Goes, Marcos Augusto, Olmos, Alfredo, Pietri, Stefano
• Primary Examiner(s): Mengistu, Amare
• Assistant Examiner(s): Sharifi-Tafreshi, Koosha
• Application Number:
  US12/145,004
• Publication Number:
  US 20090315835A1
• Time in Patent Office: 1,848 Days
• Field of Search: 345173-178, 178 1801- 1807
• US Class Current: 345/173
• CPC Class Codes: G06F 3/0416 Control or interface arrangements specially adapted for digitisers

Abstract

A method includes driving a current through a touch screen that is based on contact of the touch screen, generating a proportional second current, and detecting contact of the touch screen from the second current. Another method includes providing a touch screen with parallel plates, disabling contact detection when a plate voltage is lower than a threshold voltage, and enabling contact detection when the plate voltage is at least equal to the threshold voltage. A device includes a first node and a second node coupled to a touch screen, a third node, a first current mirror coupled to the second node and the third node configured to generate a current at the third node that is proportional to a second current at the second node, and a detection circuit that provides a signal based on the first current that indicates contact of the touch screen.

Claims

1. A method comprising:

driving a first current at a touch screen in response to a contact event to the touch screen, a magnitude of the first current based on a presence of the contact event to the touch screen;

generating a second current proportional to the first current via a first current mirror;

generating a voltage at a detection node based on the second current;

detecting the contact event based on the voltage at the detection node; and

determining whether the touch screen is shorted to a reference voltage based on whether the voltage at the detection node changes in response to a presence of the contact event at the touch screen.

2. The method of claim 1, further comprising generating the first current via a second current mirror and a reference current.

3. The method of claim 1, further comprising:

determining whether the touch screen is in one of a persistent open-circuit state or a persistent closed-circuit state based on whether the voltage at the detection node changes in response to a presence of the contact event at the touch screen.

4. The method of claim 1, wherein the touch screen comprises a pair of parallel conductive plates, the method further comprising:

disabling detection of the contact event responsive to a select plate of the pair of parallel conductive plates having a voltage lower than a threshold voltage; and

enabling detection of the contact event responsive to the select plate having a voltage at least equal to the threshold voltage.

16. The method of claim 1, wherein the first current is a direct current.

5. A method comprising:

providing a touch screen comprising a pair of parallel conductive plates;

disabling detection of a contact event to the touch screen responsive to a select plate of the pair of parallel conductive plates having a voltage lower than a threshold voltage, wherein the contact event results in an electrical contact between the pair of parallel conductive plates; and

enabling detection of a contact event to the touch screen responsive to the select plate having a voltage at least equal to the threshold voltage.

6. The method of claim 5, further comprising:

driving a current to the touch screen, the current having a magnitude based on whether a contact event is present at the touch screen, wherein the current is a direct current;

generating a voltage at a detection node based on the current; and

in response to enabling detection of a contact event, detecting a contact event based on the voltage at the detection node.

7. The method of claim 6, further comprising:

determining whether the touch screen is shorted to a supply voltage based on whether the voltage at the detection node changes in response to a presence of the contact event at the touch screen.

8. The method of claim 6, further comprising:

determining whether the touch screen is in one of a persistent open-circuit state or a persistent closed-circuit state based on whether the voltage at the detection node changes in response to a presence of the contact event at the touch screen.

9. A device comprising:

a first node adapted to be coupled to an interface of a touch screen;

a second node adapted to be coupled to the interface of the touch screen;

a third node;

a first current mirror coupled to the second node and the third node, the first current mirror configured to generate a first current at the third node that is proportional to a second current flowing through the touch screen from the first node to the second node; and

a detection circuit adapted to provide a signal based on the first current, the signal indicating whether a contact event is present at the touch screen.

10. The device of claim 9, further comprising:

a second current mirror coupled to the first node, the second current mirror configured to generate the second current at the first node based on a reference current.

11. The device of claim 10, further comprising:

a third current mirror coupled to the third node, the third current mirror configured to generate the first current at the third node in conjunction with the first current mirror and based on the reference current.

12. The device of claim 9, wherein the touch screen comprises a pair of parallel capacitive plates, the device further comprising:

a pre-charge circuit adapted to pre-charge the pair of parallel capacitive plates based on the second current.

13. The device of claim 12, further comprising:

a false touch detect circuit adapted to;

configure the first logic to disable detection of a contact event to the touch screen responsive to a select plate of the pair of parallel conductive plates having a voltage lower than a threshold voltage; and

configure the first logic to enable detection of a contact event to the touch screen responsive to the select plate having a voltage at least equal to the threshold voltage.

14. The device of claim 9, further comprising:

diagnostic logic adapted to determine whether the touch screen is shorted to a supply voltage based on whether a voltage at the third node changes in response to a presence of a contact event at the touch screen.

15. The device of claim 9, further comprising:

diagnostic logic adapted to determine whether the touch screen is in one of a persistent open-circuit state or a persistent closed-circuit state based on whether a voltage at the third node changes in response to a presence of a contact event at the touch screen.

17. The device of claim 9, wherein the first current is a direct current.