Method and circuitry for self testing of connectivity of touch screen panel

Abstract

A touch screen digitizing system includes a first resistive screen and a touch screen controller including an ADC and self-test circuitry having a driver switch coupled between a reference voltage and a first terminal of the first resistive screen, and a first test switch coupled between ground and a conductor connected to generate a first test voltage on the conductor indicative of connection resistance between the first resistive screen and the touch screen controller. Another test switch couples the test voltage to an input of the ADC. An output of the ADC is compared with a reference to determine whether the connection resistance is excessive. Connection resistance between a second resistive screen and the touch screen controller is measured similarly. Similar self-test circuitry operates to detect a short circuit between the first resistive screen and the second resistive screen.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram which is similar to FIG. 1 of prior art U.S. Pat. No. 6,246,394.

FIG. 2 is a simplified schematic diagram useful in explaining the basic structure and operation of the present invention.

FIG. 3 is a more detailed schematic diagram of a preferred embodiment of the present invention.

FIG. 4 is a flow chart of a self-test algorithm executed to test for excessive connection resistance in accordance with the embodiments of FIGS. 2 and 3.

FIG. 5 is a flow chart of a self-test algorithm executed to test for short circuits between resistive screens in accordance with the embodiments of FIGS. 2 and 3.

Claims

1. A touch screen digitizing system including a touch screen assembly including a first resistive screen with opposed first and second terminals and a second resistive screen with opposed third and fourth terminals, and a touch screen controller circuit including an analog-to-digital converter having first and second reference terminals, the improvement including self-test circuitry comprising: (a) a first driver switch coupled between a first reference voltage and the first terminal, and a first test switch coupled between a second reference voltage and a first conductor connected to the second terminal for energizing the first resistive screen to generate a first test voltage on the first conductor indicative of an amount of a first lumped connection resistance between corresponding connection pads connecting the first resistive screen and the touch screen controller circuit;(b) a second driver switch coupled between the first reference voltage and the third terminal, and a second test switch coupled between the second reference voltage and a second conductor connected to the fourth terminal for energizing the second resistive screen to generate a second test voltage on the second conductor indicative of an amount of a second lumped connection resistance between corresponding connection pads connecting the second resistive screen and the touch screen controller circuit;(c) a third test switch having a first terminal coupled to the first reference voltage and a second terminal coupled to the first reference terminal of the analog-to-digital converter, and a fourth test switch having a first terminal coupled to the second reference voltage and a second terminal coupled to the second reference terminal of the analog-to-digital converter;(d) a fifth test switch having a first terminal connected to the first conductor, a second terminal connected to the second conductor, and a third terminal coupled to a first input of the analog-to-digital converter for selectively coupling the first or second test voltage to the first input of the analog-to-digital converter; and(e) a comparator having a first input coupled to receive an output of the analog-to-digital converter representative of one of the first and second test voltages and a second input coupled to receive a first predetermined reference signal for producing a first pass/fail signal indicative of whether the first or second lumped connection resistance is excessive relative to a maximum resistance of the first resistive screen or a maximum resistance of the second resistive screen, respectively.

2. The touch screen digitizing system of claim 1 wherein if the second driver switch and the first test switch are both turned on, a third test voltage is generated on the first conductor indicative of whether or not there is a short circuit between the first and second resistive screens, and wherein the fifth test switch can be controlled to couple the third test voltage to the first input of the analog-to-digital converter, and wherein the second input of the comparator can receive a second predetermined reference signal and causes the comparator to produce a second pass/fail signal indicative of the presence of any short circuit between the first and second resistive screens.

3. The touch screen digitizing system of claim 1 wherein the self-test circuitry is included in the touch screen controller.

4. The touch screen digitizing system of claim 3 including a control circuit which operates to perform a self-test operation on the touch screen digitizing system while a normal touch screen digitizing operation is disabled.

5. The touch screen digitizing system of claim 4 wherein the control circuit includes register circuitry coupled to the output of the analog-to-digital converter for temporarily storing information representative of the one of the first and second test voltages, and wherein the register circuitry provides the information representative of the one of the first and second test voltages to the first input of the comparator.

6. The touch screen digitizing system of claim 5 wherein the register circuitry is coupled to an output of the comparator and stores the first pass/fail signal.

7. The touch screen digitizing system of claim 6 including a pass/fail output coupled an output of the touch screen controller.

8. The touch screen digitizing system of claim 5 wherein the register circuitry temporarily stores information representative of coordinates of a touch point during normal touch screen digitizing operation and provides the information representative of the coordinates of the touch point in serial-format to a coordinate data output of the touch screen controller.

9. The touch screen digitizing system of claim 3 wherein the first driver switch and the second driver switch include N-channel transistors and wherein the first test switch and the second test switch include P-channel transistors.

10. The touch screen digitizing system of claim 9 wherein the third test switch, fourth test switch, and fifth test switch each include a double-throw, single-poll switch.

11. The touch screen digitizing system of claim 3 wherein the first predetermined reference signal is selectable by a user from a plurality of reference signals stored in a reference signal memory.

12. The touch screen digitizing system of claim 3 wherein the comparator is a digital comparator, wherein the first input is a multi-bit digital signal and wherein the second input is a multi-bit digital signal.

13. The touch screen digitizing system of claim 12 wherein the first pass/fail signal produced by the comparator represents a pass condition if the one of the first and second test voltages exceeds the first predetermined reference signal.

14. The touch screen digitizing system of claim 4 wherein the control circuit automatically begins normal touch screen digitizing operation after a value of the first pass/fail signal indicative of a pass condition has been generated.

15. The touch screen digitizing system of claim 4 wherein the control circuit automatically enters an idle condition which prevents normal touch screen digitizing operation after a value of the first pass/fail signal indicative of a fail condition has been generated.

16. A method of self-testing a touch screen digitizing system including a touch screen assembly including a first resistive screen with opposed first and second terminals and a second resistive screen with opposed third and fourth terminals, and a touch screen controller circuit including an analog-to-digital converter having first and second reference terminals, the method comprising: (a) closing a first driver switch coupled between a first reference voltage and the first terminal, and closing a first test switch coupled between a second reference voltage and a first conductor connected to the second terminal to energize the first resistive screen to generate a first test voltage on the first conductor indicative of an amount of a first lumped connection resistance between corresponding connection pads connecting the first resistive screen and the touch screen controller circuit;(b) coupling the first reference terminal of the analog-to-digital converter to the first reference voltage and coupling the second reference terminal of the analog-to-digital converter to the second reference voltage;(c) selectively coupling the first test voltage to an input of the analog-to-digital converter; and(d) comparing an output signal produced by the analog-to-digital converter representative of the first test voltage with a first predetermined reference signal by means of a comparator to produce a first pass/fail signal indicative of whether the first lumped connection resistance is excessive relative to a maximum resistance of the first resistive screen.

17. The method of claim 16 including (e) closing a second driver switch coupled between the first reference voltage and the third terminal, and closing a second test switch coupled between the second reference voltage and a second conductor connected to the fourth terminal to energize the second resistive screen to generate a second test voltage on the second conductor indicative of an amount of a second lumped connection resistance between corresponding connection pads connecting the second resistive screen and the touch screen controller circuit;(f) selectively coupling the second test voltage to the input of the analog-to-digital converter; and(g) comparing another output signal produced by the analog-to-digital converter representative of the second test voltage with the first predetermined reference signal by means of the comparator to produce a second pass/fail signal indicative of whether the second lumped connection resistance is excessive relative to a maximum resistance of the second resistive screen.

18. The method of claim 17 including performing steps (e) through (g) only if the first pass/fail signal produced in step (d) indicates a pass condition.

19. A touch screen digitizing system including a touch screen assembly including a first resistive screen with opposed first and second terminals and a second resistive screen with opposed third and fourth terminals, and a touch screen controller circuit including an analog-to-digital converter having first and second reference terminals, the improvement including self-test circuitry comprising: (a) a first driver switch coupled between a first reference voltage and the first terminal, and a first test switch coupled between a second reference voltage and a first conductor connected to the second terminal to generate a test voltage on the first conductor indicative of the presence of any short circuit connection between the first and second resistive screens;(b) means for coupling the first reference voltage to the first reference terminal of the analog-to-digital converter, and means for coupling the second reference voltage to the second reference terminal of the analog-to-digital converter;(c) a second test switch having a first terminal connected to the first conductor and a second terminal coupled to an input of the analog-to-digital converter for coupling the test voltage to the input of the analog-to-digital converter; and(d) a comparator having a first input coupled to receive an output of the analog-to-digital converter representative of the test voltage and a second input coupled to receive a predetermined reference signal for producing a pass/fail signal indicative of whether or not a short circuit exists between the first and second resistive screens.

20. The touch screen digitizing system of claim 19 wherein the first input of the comparator receives a logical complement of the output of the analog-to-digital converter representative of the test voltage.