Android capacitive screen (I): Basic Principles of Capacitive Screen

Keywords: Android capacitive screen TP Ito
Platform information:
Kernel: linux2.6/linux3.0
System: Android/android4.0
Platform: s5pv310 (Samsung exynos 4210)

Author: xubin341719 (You are welcome to reprint it. Please note the author)

Android capacitive screen (I): Basic Principles of Capacitive Screen

Android capacitive screen (2): basic concepts of driver debugging

Android capacitive screen (III): Driver debugging-Driver Analysis

I. Working Principle of Capacitive Screen

In general, the working principle of the touch screen is to report the coordinates, values of the X axis and Y axis. Previously, we analyzed the resistive touch screen, which uses ADC to detect and calculate the X and Y axis coordinate values. Next we will analyze the working principle of capacitive touch screen, see how it detects and computes the values of X and Y coordinates.

Unlike the resistive touch screen, capacitive touch screen does not rely on its fingers to create or change the voltage value to detect coordinates. Capacitive screens work through any charged object including human skin. A capacitive touch screen is made up of materials such as alloys or bismuth oxide (ITO). The charge is stored in a micro electrostatic mesh that is thinner than the hair. When a finger clicks the screen, it will absorb a small amount of current from the contact point, resulting in the pressure drop of the corner electrode, the use of sensor body weak current to achieve the purpose of touch. (This is why you don't respond when you touch the screen with a glove.) You can clearly describe how the capacitive screen works.

Ii. Composition of Capacitive screen Module

Touch screen:That is, the transparent part of our hand touch operation;

Touch IC:When the capacitive screen is touched, the coordinates of the contact point must be resolved through this chip for computation and processing.

1. There are two types of capacitive touch screens:

(1) surface capacitive:The surface capacitive uses a sensor located in four corners and a thin film uniformly distributed throughout the surface. It has a common ITO layer and a metal border. When a hand points to the screen, discharge the charge from the Panel and sense the four corners of the screen. No complicated Ito pattern is required;

(2) injection capacitor:Using one or more well-designed Ito with an eclipse candle, these Ito layers form multiple horizontal and vertical Electrodes through decay, using an independent chip with the sensing function at the same time in a line/column staggered. Currently, tablet computers, mobile phones, and vehicles use multiple injection capacitors. Therefore, our subsequent analysis shows the composition of the injection capacitor.

Axis coordinate sensor matrix of the projected capacitor: the rows and columns separated by the Axis coordinate sensor units, the X axis slide Y axis slide is implemented with two cross slide bars to detect the capacitance variation of each sensor unit. (Medium capacitor, actually transparent)

2. capacitive touch screen resolution, number of channels;

As shown in, the accuracy and resolution of the transparent electrode Capacitive screen on the X and Y axes are related to the number of channels on the X and Y axes. The more channels, the higher the resolution.

3. capacitive touch screen structure classification:

(1) single-layer Ito

Advantages: low cost, high transmission rate,

Disadvantage: poor anti-interference ability

(2) Dual-sided dual-layer Ito

Advantages: good performance and High Yield

Disadvantage: High Cost

(3) Dual-sided single-layer Ito

Advantages: good performance, strong antistatic Ability

Disadvantage: poor anti-interference ability

3. capacitive touch screen classification and working principle

(1) self-built capacitive touch screen

CP-Parasitic Capacitance

Increase the parasitic capacitance when the finger is touched: CP '= CP/cfinger

Detects changes in parasitic capacitance and determines the position of the finger touch.

(2) capacitive touch screen

CM-coupling capacitor

The coupling capacitance decreases during finger touch, detects the variation of coupling capacitance, and determines the position of the finger touch.

4. Why are there ghost points and how to eliminate them?

1. Why are there ghost points?

When a finger is pressed, there is only one intersection in the X and Y axes. When both fingers are pressed at the same time, there will be four intersections, as shown in. What we don't expect is the ghost point.

2. How to eliminate ghost points

(1) time-sharing method: Time-based multi-point touch, assuming that multi-point touch is performed at a time, the operation is separated by a sequent millisecond;

(2) Partition Method: Physically split the entire touch screen into several areas by judging the touch to enter the corresponding area, so as to split the ghost points into real points.