About Touch

Touch Screen

1. Working principle

TP is commonly used as a resistive touch screen for modules, and TP is essentially a resistive voltage divider. They are composed of two thin layers of resistance, which are separated by a very thin insulating layer, which usually exists in the form of plastic particles. When you touch the screen, the two thin layers of resistance will be deformed enough to make electrical connection between them. Then the software calculates the short circuit position of the two layers by detecting the voltage generated on the voltage divider, and finally determines the touch position. Detect X coordinate: add reference voltage between X and X, connect Y and Y together, measure Y and Y voltage, and then calculate the X coordinate of the touch point according to the partial voltage principle. Detect Y coordinate: add reference voltage between Y and Y, connect X and X together, measure X and X - voltage, and then calculate the Y coordinate of the touch point according to the partial voltage principle. The above process is automatically completed by the TP driver IC and program on the test board.

2. Common fault analysis:

1) No touch: touch does not respond A. TP FPC is disconnected. Use a multimeter to measure the resistance at the TP pad. Direct measurement of resistance between X and Y should be between 300 and 900 Ouyang. B. False welding and short connection. Use a multimeter to measure the resistance between the corresponding pins of the FPC gold finger on the main screen. C. Procedural issues 100%

2) Poor touch linearity. Left and right, up and down are offset. A. TP itself. B. Procedural issues. There is no calibration procedure, or the calibration procedure is not done well.

3) The touch points fall up, down, left and right. A. Procedural issues. B. Design issues. Leader definition error

                                                            Writing:  Drowen

                                                       February 17, 2023