Solutions to electromagnetic interference problems on LCD screens
When people use display devices, they are not only affected by the earth’s magnetic field, but also to some extent by other devices and communication devices, which is also called electromagnetic interference. When these devices are working, they will become electromagnetic sources, form a surrounding magnetic field, and radiate electromagnetic waves outward. The size and radiation intensity of the magnetic field are determined by the power of these devices.
The LCD screen is a pure input component, or a passive component, for the entire system. The LCD screen itself has no error correction function, that is, the LCD display module can receive any signal that meets the operation timing relationship without being able to judge whether it is right or wrong. Wrong signals and wrong data will produce wrong control instructions or wrong display modes, resulting in wrong display effects. The first task of eliminating interference is to find the source of interference and the location where the interference occurs, and then use effective methods to eliminate, weaken, shield and remedy the interference.
1. The LCD screen has no display and no response when adjusting the contrast. This phenomenon is because during the entire machine operation, the power line or signal line of the LCD screen is subject to electromagnetic interference, generating interference pulses, causing the LCD display module to be reset. The result of the reset is to initialize the internal registers of the module and turn off the display at the same time. The recommended solution is: If the power line is interfered with, it is recommended to add a voltage stabilizing capacitor (10UF) and a filter capacitor (0.1UF or 0.01PF) between the power line VDS and VSS closest to the LCD module. If the signal line is interfered with, it is recommended to add a filter capacitor between the signal line closest to the LCD and VSS, and the capacitor should be 0.1uf or 0.01uf. The selection of the above capacitor value needs to be determined based on the actual test results.
2. The screen produces wrong characters or random dots (data errors). Or the screen is moved, inverted, etc. Sometimes it cannot be restored, and the screen can only be cleared and rewritten, or even restarted to initialize the register to restore. This phenomenon is mainly caused by interference with control signals such as /WR signal, /RD signal or E signal or /CS signal. Interference signals are more likely to produce incorrect waveforms on these signal lines, resulting in incorrect modification of register parameters, incorrect writing of display units, etc. When the entire system is running, most programs only write data to the local display area, without writing to other addresses or repeating some registers that are only set during initialization, so the above phenomenon occurs. Assuming that the interference signal is applied from space to the transmission line between the MPU and the LCD display, it is recommended that:
① Use a magnetic ring or tin foil or copper foil as a shield for the transmission line; change the direction of the transmission line to avoid the interference environment; c shorten the length of the transmission line; in the key signal line of the parallel interface mode: first add a small capacitor of 100~300pf to the ground (VSS) in the order of /WR (RD) signal or E signal, then /CS signal, and then RS signal. Serial interface mode: first add a small capacitor of 100~300pf to the ground in the order of SCLK, then self-disablement, and then reset.
Test and observe the improvement effect. If the interference signal comes from the system motherboard, the deformation of the signal can be seen from the LCD display [] end. This may be because the transmission line resistance between the microprocessor and the LCD display module is relatively large, and the interface driving capability of the microprocessor system is relatively weak, making it easy for interference signals to invade. At this point, you can consider: ① Connect a small series resistor on the transmission line to the input capacitor at the end of the LCD display module to form a low-pass filter circuit [] to eliminate the influence of interference; add a transmission line driver on the system motherboard to improve the driving capability; j Use Schmitt circuit to shape the signal, etc.
3. There is no interference source, but there will be no display or chaotic display
This situation is also classified as interference, but it is an internal interference of the system, mainly due to software program conflicts. At this time, the first thing to consider is the interrupt program. When the MPU writes to the LCD display (/0 addressing mode), the system operation is interrupted, which may change the control signal state of the LCD display module or the data to be written, resulting in LCD display module setting errors and crashes or display errors. The improvement method is to turn off the interrupt response function when the MPU calls the LCD display driver subroutine.
4. It is impossible to find the interference point or take circuit precautions, but the influence of interference cannot be eliminated.
At present, software remediation solutions need to be considered. The simplest way is to initialize the registers regularly. First, do not use the reset signal to reset, just rewrite the registers. Because the reset action will make the normal display flicker, and the display effect is not very good. If freezing occurs and cannot be recovered, you can only use the reset signal to force a reset and then initialize the register. In order to ensure that the normal display is not disturbed by the initialization and to repair the influence of the interference in the shortest time, it is recommended to read the “status word” of the LCD display module as the basis for initialization. When it is judged that the module is running in the “display off” state, it is considered that the module is disturbed and there is no display, so the initialization function is called to restart the module and turn on the display. If the module is still in the “display off” state after initialization, it is necessary to use the reset signal to force reset and initialization. If it is judged to be in the “display” state, write a set of special data to several cells of the display SRAM, and then read them in turn to determine whether they are correct. If an error occurs, it is considered that the module is disturbed, call the initialization function, and refresh the data again. It is necessary to use the reset signal to force reset and initialization. If it is judged to be in the “display” state, write a set of special data to several cells of the display SRAM, and then read them in turn to determine whether they are correct. If an error occurs, it is considered that the module is disturbed, call the initialization function, and refresh the data again. It is necessary to use the reset signal to force reset and initialization. If it is judged to be in the “display” state, write a set of special data to several cells of the display SRAM, and then read them in sequence to determine whether they are correct. If an error occurs, it is considered that the module is disturbed, call the initialization function, and refresh the data again.
5. Static electricity causes the LCD display module to have no display or random display: This is a common interference phenomenon. The interference pulse passes directly through the iron frame of the LCD screen and affects the circuit of the module. Generally, the LCD display module does not want the iron frame to float and become a static charge accumulation surface, so it is connected to VSS. However, this connection makes it easy for external interference to directly affect the VSS line through the iron frame. There are three ways to connect the iron frame: ① Use an insulating pad to isolate the module iron frame from the metal panel of the system. The thicker the insulating pad, the greater the reduction of static electricity. -The iron frame of the LCD module is connected to the metal shell, and the metal shell is connected to the ground. At this time, it may be necessary to disconnect the iron frame from the VSS in the module. The iron frame of the module is connected to VSS, and the ground of the metal shell is connected to VSS. These three connection methods are related to the system chassis structure and ground processing, and need to be tested in practice to select the appropriate method. If you want to order (industrial display), you can contact us at any time, we can customize it for you professionally. They all need to be tested in practice to choose the appropriate method. If you want to order (industrial display), you can contact us at any time, we can customize it for you professionally. They all need to be tested in practice to choose the appropriate method. If you want to order (industrial display), you can contact us at any time, we can customize it for you professionally.
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