1. Introduction to input equipment

1. IO input and output is a concept in computer systems. The main function of the computer is to obtain data from the outside and then perform calculations and processing to obtain the output data and output it to the outside (the computer can be regarded as a data processor). Computer and external interaction is through IO. Every computer has a standard input and standard output.

2. Common input devices

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

 

Two, touch screen introduction

1. Features of touch screen

(1) Touch screens are closely related to people, especially capacitive touch screens.

(2) The touch screen and the display are closely related.

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

2. Classification of touch screens

(1) Common touch screens are divided into two types: resistive touch screens and capacitive touch screens. In the early days, a resistive touch screen was used, and then a capacitive touch screen was invented.

(2) These two types have different characteristics, different interfaces, different programming methods, and different principles.

3. The connection and difference between touch screen and display screen

(1) First of all, it must be clear: the touch screen is a touch screen, used to respond to human touch events; the display screen is a display screen, used to display. The display screens used today are generally LCDs.

(2) Why many people confuse these two concepts, mainly because the touch screen and the display screen are made together on the general product. Generally, the outer layer is a layer of touch screen, the touch screen is transparent and very thin; the bottom is the display screen It is used to display images, and the images that are usually seen are displayed on the display screen and can be seen by people through the touch screen.

 

 

Third, the principle of resistive touch screen

Resistive touch screen is actually a kind of sensor. Although it is not used much, there are still many LCD modules that use resistive touch screen. This kind of screen can use four-wire, five-wire, seven-wire or eight-wire to produce screen bias. Set the voltage and read back the voltage of the touch point at the same time. Here we mainly take four wires as an example.

1. Film + glass (need to click on sharp and hard objects)

(1) The main point is thin and transparent. The front panel is slightly harder and can be bent by hard objects, while the rear panel is very hard and will not bend.

(2) The front panel and the rear panel are not held together at ordinary times. When pressed by an external force, the front panel undergoes (partial) deformation, at which point the front and rear panels will be held together. As shown in the left picture below:

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2. ITO (conductive + transparent + uniform voltage drop)

(1) ITO is a kind of material, in fact a kind of paint, which is characterized by transparency, conductivity, and uniform application. (Such as the metal coating in the right picture above)

(2) Originally, glass and plastic are non-conductive, but after coating with ITO, they become conductive (while still maintaining the original transparent characteristics).

(3) ITO is not only conductive but also resistive, so evenly smearing ITO in the middle is equivalent to connecting a resistor between the two sides of the same layer. Because the equivalent resistance formed by ITO is evenly distributed across the board, the voltage value at a certain point on the board is proportional to the position value of this point.

(4) After the touch screen is operated, what is needed after pressing is the pressed coordinates. The coordinates are actually position information. This position information is proportional to the voltage, and the voltage at this point can be obtained through AD conversion. This is the working principle of the entire resistive touch screen.

3. X/Y axis time-sharing AD conversion

(1) Next, we will study how to get the voltage of the pressed point

(2) Apply a voltage to a pair of electrodes on the first panel, and then measure between an electrode on the other panel and the ground of the first panel. The test has no results when it is not pressed, but when someone presses it, the two panels touch at the point where it is pressed. The contact will cause the overall voltage value on the second panel to be equal to the voltage value at the contact point, so the measurement is done at this time The voltage obtained is the voltage value of the contact on the first panel.

(3) Perform the above process once in one direction to measure the coordinate value of that direction. After the process is completed, remove the voltage and apply voltage to the electrode in the other direction, so repeat the trick to get the coordinate in the other direction. . So far, the touch event is over. For example, as shown in the figure below: we first add a voltage between X+ and X-, when someone presses the touch screen, a contact will be formed at the corresponding position, then we will measure Y+ and GND (or Y -And GND), then the voltage value obtained is actually the voltage value at the contact point. Because the resistance is evenly distributed, the position of the point in the x direction can be calculated; the same is true for measuring the Y axis. The truth.

 

4. Calibration of resistive touch screen

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

5. Hardware interface of resistive touch screen

(1) For resistive touch screens, his hardware interfaces are mainly divided into two types: one is the built-in resistive touch screen controller of the SoC, and the other is an external special touch screen control chip, which combines the touch panel sensor with this control The chip is connected, this chip

The internal logic circuit or built-in program code can calculate the contact coordinates according to the above-mentioned principle and convert them into digital quantities and send them to the host Soc through the I2C interface.

(2) For the first interface, Soc's resistive touch screen controller needs to be able to complete the above-mentioned tasks by itself, and it needs to convert the analog quantity of the sensor into a digital quantity, so this is generally linked with the ADC, and in The s5pv210 SoC actually integrates the ADC module and the touch screen module, and will focus on the analysis later.

 

Fourth, the principle of capacitive touch screen

1. Human body current induction

Using the human body current induction phenomenon, a capacitance is formed between the finger and the screen. When the finger is touched, a tiny current is drawn. This current will cause current to flow on the four electrodes on the touch panel. The controller calculates the ratio of these four currents. The coordinates of the touch point can be calculated (AD conversion is involved in this calculation process).

 

2. Dedicated circuit to calculate coordinates (hardware interface)

(1) The principle of the resistive touch screen sensor itself is very simple, and the calculation of coordinates is also very simple, so it can be directly connected to the touch panel sensor through the resistive touch screen controller of SoC, and the controller inside Soc completes the calculation of the coordinates. There is no problem with AD conversion, and it is not a big burden for SoC itself. But the capacitive touch screen is different. The capacitive touch screen needs to bring an IC for coordinate calculation. Therefore, the capacitive touch screen does not need the host SoC controller to participate in the work. Therefore, this interface of the capacitive touch screen is actually the second type of hardware interface of the resistive type mentioned above, and the capacitive touch screen can only be implemented as the second type of interface at present.

(2) Why is it designed like this? The main reason is that the coordinate calculation of the capacitive touch screen sensor is too complicated, and ordinary programmers cannot write appropriate codes to solve this problem. Therefore, in addition to the touch panel, an IC is added to the capacitive touch screen for special coordinate point calculation and statistics. This IC is fully responsible for manipulating the touch panel to obtain touch operation information, and then communicating with the host SoC through a digital interface (usually I2C).

3. Multiple blocks support multi-touch

(1) The resistive touch screen does not support multi-point touch, which is limited by its own principle and cannot be changed and cannot be improved.

(2) The capacitive touch screen can support multi-point touch (or single-point touch). According to the principle of capacitive touch screen mentioned earlier, a single capacitive touch screen panel cannot support multi-touch, but a large touch panel can be divided into multiple small blocks, each block is equivalent to an independent small capacitor Touch screen panel.

(3) Multiple blocks support multi-touch which makes the capacitive touch screen coordinate calculation more complicated, but this complexity is absorbed by the capacitive touch IC. The touch information is reported through the digital interface and the host SoC communication (the number of touch points, each touch point). Coordinates, etc.).

4. Provide I2C access interface to the outside

(1) The entire capacitive touch screen consists of two parts: the touch panel sensor and the capacitive touch IC. The touch panel sensor is a physical device. The capacitive touch IC is generally implemented on the flexible flat cable (FPC, such as the chip on the soft flat cable in the figure below) of the touch screen. The capacitive touch IC is responsible for controlling the touch panel, and it is obtained through AD conversion and analysis. Touch the number of touch points, touch coordinates and other information, and then communicate with the SoC through a specific digital interface. This digital interface is I2C.

(2) For our host SoC, the capacitive touch screen is actually an I2C slave device. The host only needs to access the slave device through the I2C bus (the slave device has its own specific slave device address). From this point of view, there is actually no difference between a capacitive touch screen and other sensors (gsensor, etc.).