Electromagnetic interference of LCD display screen is a common and very difficult problem in the test of the whole product. When the system circuit interferes, interference waves of a certain frequency and certain amplitude are generated on the power line or signal line.
As a display window of the product, the LCD display module is certainly one of the performance Windows of interference. To the system, a module is a pure input component, or passive component, which is the ability of the module to receive any signal satisfying the operation timing relationship without judging right and wrong. The wrong signal and data will produce the wrong control word instruction or display pattern, resulting in the wrong display effect.
The primary work of eliminating THE ELECTROMAGNETIC interference of the LCD display screen is to find the interference source or the position of the influence of interference, and then to eliminate, weaken, or prevent, shield, remedy with effective methods.
LCD display electromagnetic interference and solutions
Q: Why does the LCD display screen appear white/blue when it is working or doing interference tests?
A: This is because, during the module’s work, interference is applied to the module’s power supply VDD or VSS, or is applied to the module’s RESET signal line, resulting in the module being RESET. The result of the reset is that the module internal registers were initialized.
The solution is: if the interference of THE LCD is applied to the power line, it is suggested to incorporate a voltage stabilizing capacitor (10uF) and a filter capacitor (0.1uf / 0.01UF) between the power line VDD and VSS closest to the module. If the interference is applied on the RESET signal line, it is recommended to incorporate a filter capacitor between the VSS at the position closest to the module. The capacitance shall be 0.1uF or 0.01UF. The choice of capacitance value depends on the actual test results.
Q: During work or interference test, the screen produces wrong characters or disorderly points (data error), and cannot be recovered, only the power can be recharged, initialization register can be restored.
A: This is because the interference is applied to the control signal so that the register parameters are modified, the general operation is to write and display data, there is no main work of the register parameters are written repeatedly, so the above phenomenon.
The solution is: if the interference is applied to the transmission line,
A: magnetic ring, or tin foil, copper thin as the line shielding;
B: Change the direction of the transmission line to avoid interference environment;
C: Shorten the length of the transmission line; Or add a transmission line driver, to improve the driving ability and anti-interference ability.
Q: What should I do if I can’t find the interference points or take circuit precautions while working or testing interference?
A: The timing of the register initialization, first do not use the RESET signal, only the register operation; If there is a crash that cannot be recovered, it needs to be initialized with the RESET signal, but this operation will cause a splash screen under the normal display. Therefore, in order to ensure that the normal display is not disturbed by the initialization
1）Reading register data can be used as the judgment basis for initialization, such as reading display status word, special SRAM unit data, etc.
2）Use the negative display module and turn off the backlight when not in operation. At this time, it is not easy to observe the display content; when the display content needs to be observed, it is expected to start the backlight and light up.
Q: Static interference test is carried out on the LCD product shell (especially the product panel), and the module appears white screen or disorderly display
A: Most of this interference occurs through the iron frame or glass of the module to interfere with the module circuit.
The ways to improve are:
1) The iron frame of the module is connected to the earth,
2) Iron frame VSS of the module
3) The iron frame of the module floats in the air
4) Add an insulation pad between the iron frame and the metal shell of the module. The thicker the insulation pad is, the greater the reduction of static electricity will be;
These four methods need to be tested in the actual product to choose the one that works.
Q: There is no external interference source, but there is also a white screen or random display phenomenon
A: This kind of situation also belongs to the interference, is the interference within the system, is mainly caused by the software program conflict. First of all find out the interference of law, the following opportunity if occurred in the writing process of the module, prone to crash, error display module: interrupt program are working in the module (I/O addressing mode) generated in the process of the control signal is modified, data is modified, such as the wrong operation, lead to module freezes or display an error.
An improvement is to turn off the interrupt response operation during the operation module.