Hangzhou Xuhong Technology Co.,Ltd
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Electromagnetic interference (EMI) has always been a troublesome problem that affects the overall system operation of the product. When the system is subject to EMI, inter Electromagnetic interference (EMI) has always been a troublesome problem that affects the overall system operation of the product. ference waves of certain frequency and amplitude will be produced on the power cable or signal cables, thus affecting the normal operation of the whole product, which is shown by the LCD screen.
As we all know, TFT LCD module is a passive output module, which just accepts the input signal without any judgment. Therefore, the wrong signal and data will produce the wrong control instructions, which will lead to the wrong display effect and pattern. Therefore, how to solve the EMI problem of the TFT LCD module? The primary task is to find the source of interference, then eliminate and weaken, prevent and shield in the most effective way. Next we will analyze the TFT LCD module by electromagnetic interference of several cases, and the corresponding solutions.
1、 The LCD screen appears white or blue screen
There will be a white/blue screen situation when a TFT LCD module is working. The so-called white screen or blue screen means that the LCD module is only backlight on, and there is no response to contrast ratio adjustment. The reason is that during the operation of the whole machine, the EMI subjected on the power supply VDD or VSS of the LCD screen or on the RESET signal cable, lead to the LCD module being reset. The result is that the module's internal registers are initialized and the display module is turned off.
Solution:
If the interference is on the power cable, we can add a voltage regulator capacitor and a filter capacitor between VDD and VSS of the power cable which is closest to the LCD module.
If the interference is on the RESET signal cable, we will add a filter capacitor between the RESET signal cable which is closest to the LCD screen and the VSS, and the capacitor capacity is selected as 0.1uF or 0.01uF, and the choice of capacitance value needs to be based on the actual test effect
2、Wrong characters or data appear on the LCD screen
When the product is running, the LCD screen displays wrong characters or random dots (data error) and can’t resume, the only way is to re-power and initialize the register, because the register parameters are modified by the EMI of the control signal. The usual operation is to write the display data rather than repeatedly write the register parameters, so the above phenomenon occurs.
Solution:
If the EMI is on the transmission cable which is between the product MPU and the display screen, the following methods can be used:
(1) Make the shielding of cables by the magnetic ring , tin foil or copper thin;
(2) Change the direction of the transmission cable to avoid EMI;
(3) Shorten the length of transmission cable; Or add a transmission cable driver to improve the driving ability and anti-interference ability.
Please observe the display effect by testing to check whether it has improved or not.
Another case is EMI from the motherboard of the product, and the display screen will appear wrong characters. The reason may be that the resistance of transmission cable between the MPU and the LCD screen is too large, resulting in interference signals easy to invade. The solution is to connect a small resistance in series on the transmission cable and form a low-pass filter circuit with the input capacitance at the end of the display module, and then eliminate the influence of EMI.
3、Static interference of LCD screen
Due to the static interference of the LCD module shell and especially the LCD panel, the LCD screen will appear white screen or disorderly display phenomenon. This interference is mostly to interfere with the circuit of the LCD module through the metal frame of the module or the glass panel.
Solution:
(1) The metal frame of the LCD module should be ground connected.
(2) The metal frame of the LCD module is connected to the VSS, or set to be floating.
(3) The insulation pad is added between the metal frame of the LCD module and the metal shell. The thicker the insulation pad is, the greater the reduction of static electricity will be.
These three methods can be selected during test and observe the display improvement.
4、No any external interference source, but there is also a white screen or disorderly display phenomenon.
This situation also belongs to one kind of interference, which comes from the internal interference of the system, mainly caused by program conflict of the software. First of all, find out the rule of interference. If in the process of module writing, it is easy to occur module crash, error display, etc., it is necessary to consider the factors of interrupt program. The interrupt program will be generated in the process of MPU writing to the LCD screen, it may occur that the control signal is modified, the data is modified and other wrong operations, resulting in the display module crash or display error. The solution is to turn off the interrupt response function during the MPU's call to the LCD driver.
This paper analyzes the EMI of LCD screen and introduces the solution. Therefore, we need to find out the problem first, and then do a targeted test, and finally observe the display effect of the LCD screen whether it has improved. Of course, there are many cases of EMI, which requires our engineers to have enough experience to analyze and test.
AUO LCD:https://www.auo-lcd.com/products/auo-lcd-screen/
INNOLUX LCD:https://www.auo-lcd.com/products/innolux/
