Introduction to the Fundamentals of touch screen

Introduction of input class equipment

1, IO input and output, is a concept in the computer system. The main function of the computer is to obtain the data from the outside and then calculate and process the output data and output to the external (the computer can be regarded as the data processor). Computer and

External interaction is through IO. Each computer has a standard input and standard output.

2, the common input class equipment

Keyboard, mouse, touch screen, joystick, sensor, (the camera is not a typical input class device).

Second, touch screen introduction

1, the characteristics of the touch screen

(1) Touch screen and human relations are very close, especially capacitive touch screen.

(2) The touch screen and monitor are closely related.

(3) Typical applications: mobile phones, tablet computers, cash registers, industrial fields.

2, the classification of touch screen

(1) The common touch screen is divided into 2 kinds: resistive touch screen and capacitive touch screen. The early use of resistive touch screen, later invented the capacitive touch screen.

(2) The characteristics of these two are different, the interface is different, the programming method is different, the principle is different.

3, touch screen and display of the contact and difference

(1) First to understand: touch screen is a touch screen, to respond to human touch events; The display is the display, used to display. The current display is usually LCD.

(2) Why a lot of people confuse these two concepts, mainly because the touch screen and display on the general product are made together. The general outer layer is a touch screen, the touchscreen is transparent, very thin, the bottom is the display to display the image, flat

The image you see is displayed on the display and is visible through the touchscreen.

Three, the principle of resistive touch-screen

Resistive touch screen is actually a kind of sensor, although already used not much, but still have a lot of LCD module using resistive touch screen, this screen can use four, five, seven or eight lines to generate screen bias voltage, while

Read back the voltage of the touch point, here is the main example of the four line for illustration.

1, Film + glass (requires sharp hard objects click)

(1) The main points are thin and transparent. The hardness of the front panel is slightly weak, can be bent by hard objects, the rear menisci sclerosis Denaturalization is very high, will not bend.

(2) The front panel and the back panel in peacetime did not live, under the external pressure, the front panel occurred (local) deformation, at this point before and after the panel will be live. This is shown in the figure below:

2, ITO (conductive + transparent + uniform pressure drop)

(1)  Ito is a material, in fact, a coating, characterized by transparent, conductive, evenly applied. (such as the metal coating in the upper right image)

(2) glass and plastic are non-conductive, but after the coating of Ito has become conductive (while maintaining the original transparent characteristics).

(3) Ito not only conducts and has a resistor, so the intermediate coating of ITO is equivalent to a resistor being connected between the sides of the same layer. Because the equivalent resistance formed by ITO is evenly distributed throughout the plate

, where the voltage value at a certain point on the board is proportional to the position value of this point.

(4) Touch screen after operation, press after the coordinates are pressed, coordinates is actually location information, this position information and voltage is proportional to, and this point of voltage can be converted through AD. This is how the entire resistive touch screen works.

3, x/Y axis time-sharing ad conversion

(1) below to study how to get the voltage of this point

(2) on the first panel of the pair of electrodes to add voltage, and then on the other panel of one electrode and the first panel of the ground to measure. The test has no results when not pressed, but at the point where someone presses the 2 panel touch

, the contact causes the overall voltage value on the second panel to be equal to the voltage at the contact point, so the voltage measured at this time is the voltage value at the first panel of the contact.

(3) The above process in one direction can be measured in the direction of the coordinate value, after the removal of the voltage and then in the other direction of the electrode to add voltage, so the kabuki re-application, you can get the coordinates of another direction. This one touch event ends.

For example: we first add a voltage between x+ and X, when someone presses the touchscreen and a contact is formed in the corresponding position, then we measure the voltage between the y+ and GND (or Y-and GND).

In fact, the voltage value is the voltage at the point of contact, because the resistance is evenly distributed, so the position of the point in the X-direction can be calculated, the same as the measurement of the y-axis is the same reason.

4, the Resistance touch screen calibration

(1) The voltage value is proportional to the coordinate value, so it needs to be calibrated. Calibration is to calculate (0, 0) The voltage value of the coordinate point.

5, Resistive touch screen hardware interface

(1) For resistive touch screen, his hardware interface is divided into two main: one is the SOC built-in resistive touch screen controller, the other is an external special touch screen control chip, the touch pad sensor connected to this control chip, the chip

Internal Logic circuit or the built-in program code can calculate the contact coordinates according to the above principle and convert the digital amount to the host Soc via the I²c interface.

(2) and for the first interface requires the SOC's resistive touch screen controller can do the above tasks, and the need to convert the sensor analog to digital, so this will generally be associated with the ADC, and in the s5pv210 this

The SOC is actually integrating the ADC module with the touchscreen module, which is followed by a key analysis.

Four, the principle of capacitive touch-screen

1, human body current sensing

Using the body's current sensing phenomenon, a capacitor is formed between the finger and the screen, and the finger touches a tiny current that causes the current to flow on the 4 electrodes on the touchpad, and the controller calculates the ratio of the 4 current.

The coordinates of the touch point (this calculation involves ad conversion).

2, dedicated circuit computing coordinates (hardware interface)

(1) Resistive touch screen sensor itself is very simple, the calculation of coordinates is very simple, so you can use the SOC Resistive touch screen controller directly connected to the touchpad sensor, the SOC internal controller to complete the calculation of coordinates and

Ad conversion is not a problem, and is not a big burden on the SOC itself. Capacitive touchscreen, however, requires a single IC for coordinate calculations, so capacitive touch screens do not require the host SOC controller to be involved in the operation.

So this interface of capacitive touch screen is actually the second kind of hardware interface of resistive type, and capacitive touch screen can only be implemented as the second interface at present.

(2) Why is this design? The main reason is because the coordinate calculation of capacitive touch screen sensor is too complex, the ordinary programmer can not write the appropriate code to solve the problem, so in the capacitive touch screen in addition to the touchpad also attached an IC

Perform specialized coordinate point calculations and statistics. This IC is solely responsible for manipulating the touchpad to obtain touch operation information, and then communicates with the host Soc via a digital interface (typically I²c).

3. Multiple blocks support multi-touch

(1) Resistive touch screen does not support multi-touch, this is the principle of its own limitations, can not be changed can not be improved.

(2) Capacitive touch screen can support multi-touch (can also be single-touch). According to the previous capacitive touchscreen, a single capacitive touchscreen panel cannot support multi-touch, but a large touch panel can be divided into

Multiple small chunks, each of which is equivalent to a single, small capacitive touchscreen panel.

(3) Multiple blocks support multi-touch to make capacitive touch screen coordinate calculation complicated, but this complexity is absorbed by capacitive touch IC, or through the digital interface and host SOC Communication report touch information (touch points, each touch point of the sitting

Standard, etc.).

4, to provide access to the interface of I²c

(1) The entire capacitive touch screen consists of 2 parts: a touchpad sensor and a capacitive touch IC. Touch pad sensor is a physical device, capacitive touch IC generally do touch screen soft line (FPC, such as the chip on the soft line) above,

The capacitive touch IC is responsible for manipulating the touchpad, The number of touch points, touch coordinates and other information are obtained through ad conversion and analysis, and then the SOC is communicated with a specific digital interface. This digital interface is i²c.

(2) for our host SOC, capacitive touch screen is actually an i²c slave device. The host only needs to access the device via the I²c bus (from the device has its own specific slave address). From here, its

There is no difference between a real capacitive touch screen and other sensors (gsensor, etc.).

Reference: "Teacher Zhu Embedded Linux development \1.arm bare metal complete \1.15.arm bare Metal part 15th-touch screen touchscreen"

Introduction to the Fundamentals of touch screen