LCD debugging is the most prone to problems and solutions. What is the principle of capacitive touch screen?

Principle: This article puts aside the technical level of LINUX touch screen device driving principle. The touch screen uses an AD conversion module to convert the touch signal on the screen into a digital signal. The touch screen is commonly used as a four-wire resistor. When touched, the function module converts the analog signal into a digital signal, which is commonly referred to as AD conversion. In LINUX, these numbers are usually read by interrupts. When the screen is touched, the interrupt occurs. LINUX reads the converted number through the serial port or I2C, SPI or internal data channel, and then passes the value to the INPUT layer. When the TP enters sleep, it must turn off the CTP interrupt to prevent false triggering of the interrupt, resulting in an abnormality.

Why check? Cause A, the touch screen and the LCD display are two different physical devices. The pixels processed by the LCD, for example, the resolution we usually call is 600x800. Actually, the width of each line is 600 pixels and the height is 800 pixels. The data processed by the touch screen is the physical coordinates of the point, and the coordinates are through the touch screen. Collected by the controller. A certain conversion is required between the two. B, secondly, when installing the touch screen, there are inevitably certain errors, such as rotation and translation, which also need to be corrected. C, again, the material of the resistive touch screen itself is different and its parameters will change over time, so frequent corrections are needed (the capacitive touch screen requires only one calibration).

The more common verification program is TSLIB. After the TSLIB check, the application reads the value in the TSLIB and it is now accurately located. Correction principle: The calibration process of the touch screen is generally: display certain marks (such as "+") in several different positions of the screen in turn, and click the marks with the touch pen to complete the correction. If PT(x, y) represents a point on the touch screen and PL(x, y) represents a point on the LCD, the result of the correction is to obtain a transformation matrix M such that PL(x, y) = M·PT(x , y). Finally, assume that the coordinates of the three points of the LCD are (XL1, YL1), (XL2, YL2), (XL2, YL2), and the three points on the touch screen are (XT1, YT1), (XT2, YT2), (XT3). , YT3), then the two equations are:

Thus, the correction of the touch screen is actually solving the above equations, and obtaining six coefficients: A, B, C, D, E, F. The above equations can be solved according to Clem's law. After obtaining 6 coefficients, all the coordinates obtained through the touch screen will be brought into the formula (1) to obtain the coordinates represented by pixels on the LCD.

In fact, during the calibration, the coordinates of the touch screen of the acquired touch screen have a certain error, that is to say, several three sets of point coordinates are collected, and A, B, C, D, E, and F are respectively calculated, and the results are not the same. In the ts_calibrate of tslib, five sets of point coordinates are collected. For the specific code, see perform_calibraTIon() in ts_calibrate.c. In general, the more points you collect, the higher the accuracy of the correction. It is only redundant when collecting too many points, which has little effect on the accuracy of correction, but increases the calculation time.

Android coordinate conversion processing: This implementaTIon is a linear transformaTIon using 7 parameters

(a, b, c, d, e, f and s) to transform the device coordinates (Xd, Yd) into screen coordinates (Xs, Ys) using the following equaTIons:

s*Xs = a*Xd + b*Yd + c

s*Ys = d*Xd + e*Yd + f

Among them: Xs, Ys: LCD coordinates; Xd, Yd: touch screen coordinates.

When a qualified liquid crystal display device is used, it may sometimes malfunction due to unreasonable use, unsuitable conditions, unqualified accessories, or improper installation methods. The reasons and troubleshooting are as follows:

After a few hours or days of use, the electrode is discolored and appears black and brown "writing", and the liquid crystal cell generates bubbles so that it cannot be displayed. This is due to the fact that the DC component of the driving voltage is too large, causing electrochemical anti-blood. Check the circuit and replace the large DC component with a new liquid crystal display device. When the "writing" is just present, the liquid crystal display device can be heated to a temperature above the storage temperature. Even if the display surface of the liquid crystal display device is all black, the temperature rise is stopped, and after the natural cooling, the "writing" can be generally removed.

After the assembly, the pen segment that should not be displayed is also faintly displayed, so that it cannot be read. The reason may be:

(1) The leads are not clean. Wipe it off with a dry cloth.

(2) The weather is too hot and the glass surface is conductive. It can be restored after drying indoors.

(3) The common electrode or the segment electrode is suspended, and can be eliminated after reinstallation.

(4) The amplitude of the AC square wave is asymmetrical, which causes the cutoff to be unclear, and the square wave amplitude can be adjusted.

(5) The conductive rubber stripe is not square, non-parallel, and the insulation performance is poor. Replace the conductive rubber strip.

Poor contrast, or negative image, or display confusion, or all display, generally due to the back electrode floating, can be eliminated.

External disturbances may also cause display confusion and eliminate interference.

The decoder is normal, but all pixels are displayed. Generally, the back electrode is not connected, and the floating or back electrode is connected to the DC.

(1) The electrode lead is contaminated, resulting in poor assembly contact.

(2) Conductive rubber is contaminated, resulting in poor assembly contact.

The above two items only need to be cleaned and reassembled. Do not touch the cleaned parts with your hands during assembly.

(1) The edge of the glass is broken, and the outer lead conductive layer is scratched.

(2) The assembly pressure frame is not suitable.

The cause of the confusion may be: the back electrode is suspended, the drive is DC, the power supply fluctuates, the contact is poor, the battery is exhausted, and the like. Can be excluded for different reasons.

The function is turbulent, can't be adjusted, and the display is intermittent. The reason is that the power supply voltage is abnormal and the battery is exhausted. At this time, the battery needs to be replaced.

In order to find out the fault of the liquid crystal display device in use, the r×10kω resistance file of the pointer multimeter can be used for searching. This is a high-resistance file that can detect various on-off situations that affect the display. Because it has a DC voltage of 9 ~ 15v, it can drive the display of the liquid crystal display device, and judge whether the measuring device is normal from the display state. However, since the multimeter outputs a DC voltage, it is best not to drag it for a long time to avoid electrochemical reaction. You can use the following tips to reduce the DC damage effect, that is, hold a test pen in your hand, then hold the liquid crystal display back electrode with your fingers, and then use another test pen to detect the remaining electrode. At this time, the external power supply internal resistance will increase greatly, so the alkali Less damage to the DC component.

Another trick to detect using mains is also very practical. Take a pen line or ordinary wire, and wrap one end around the power cord of the desk lamp or other electrical appliances, about 2 to 5 winding. At this point, a weak AC voltage is induced in the wire. This induced voltage has a large internal resistance, and the AC induced voltage of 50hz is useless for general household appliances, but it is suitable for driving liquid crystal display devices. At this time, as long as the back electrode of the liquid crystal display device is pinched with a finger, and the end of the wire is touched with the outer lead of the segment electrode, the segment of the pixel can be displayed. It is very convenient to detect the quality of the liquid crystal display device by this method. However, since the current of the induced current is small, the voltage is still high, and therefore, in this way, it is found that the untouched pixels also appear crosstalk together. It is shown that this is because other external leads are suspended, and the crosstalk display disappears when the finger is used to touch the outer lead end of the crosstalk display.

1.smt: is the abbreviation of English "surface mount technology". Surface mount technology, which is a more traditional way of installation. The advantage is high reliability, the disadvantage is that the volume is large, the cost is high, and the miniaturization of lcm is limited.

2.cob: is the abbreviation of "chip on board" in English. That is, the chip is bonded on the pcb. Since the ic manufacturer is reducing the output of qfp (a kind of smt) package in the production of lcd control and related chips, the traditional smt mode in future products. Will be gradually replaced.

3.tab: is an abbreviation for "tape aotomated bonding" in English. That is, an anisotropic conductive adhesive connection method. The ic anisotropic conductive paste in the form of package tcp (tape carrier package) is fixed on lcd and pcb, respectively. This installation method can reduce the weight, volume, installation and reliability of lcm!

4.cog: is the abbreviation of "chip on glass" in English. That is, the chip is directly bonded to the glass. This installation method can greatly reduce the volume of the entire lcd module, and is easy to mass-produce, suitable for lcd for consumer electronic products, such as: mobile phones, pda and other portable electronic products. This type of installation will be the main connection between ic and lcd in the future, driven by ic manufacturers.

5.cof: is the abbreviation of "chip on film" in English. That is, the chip is mounted directly on the flexible pcb. This type of connection is highly integrated, and peripheral components can be mounted on the flexible pcb together with the ic. This is an emerging technology and is currently in trial production.

Here are some maintenance ideas based on our daily maintenance summary:

This is a fault that should be said to be very simple. There are two kinds of power supply and external power supply in the general LCD monitor extension.

Regardless of the power supply, its structure is much simpler than that of the crt display. The vulnerable parts are generally small components such as fuses, input inductors, switching transistors, and Zener diodes.

The rare fault is caused by the power supply of the motherboard cpu does not start, this part of the principle is also relatively simple, is through the key control board to the cpu, and then through the cpu output a control signal to drive the power conversion integrated circuit work.

This kind of problem is generally caused by high voltage abnormality, which is to protect the circuit. In this case, there is usually a display on the LCD screen. The method of viewing is “strabismus”.

The main point of maintenance is to compare the repair method.

Because the design of the high-voltage board of the current liquid crystal display is generally a symmetrical design, and the two sides may be substantially dead.

Generally, the old machine is prone to problems with the step-up transformer and the lamp tube. The protection circuit and process problems of the new machine are much more complicated.

This kind of problem is generally a malfunction of the LCD screen.

The bright line fault is generally a problem with the cable connecting the LCD body.

The dark line is generally the leakage of the body of the screen. The above two problems are basically the death penalty for the machine, and there is no maintenance value, because the price of a screen is too high.

This kind of problem is generally a problem with the driving voltage of the screen. If the driving circuit of the screen is on the motherboard, then it should be the fault of the motherboard. If the driving circuit of the screen is on the LCD screen, the screen should be replaced under normal circumstances. The risk is great.

Generally, it can be adjusted by entering the maintenance adjustment mode.

Interference: In different working modes, there may be some interference in the LCD display, most of which is normal, and a few are brought on the circuit; because the special production process of the liquid crystal display is caused only in the standard working mode. The detected problem can be considered a fault.