When people use display devices, they are not only affected by the earth’s magnetic field, but also by other devices and communication devices to a greater or lesser extent, which is also called electromagnetic interference. When these devices are working, they will become an electromagnetic source, forming a magnetic field around them and radiating electromagnetic waves outward. The size of the magnetic field and the intensity of the radiation are determined by the power of these devices.
The LCD display is a pure input component, or a passive component, for the entire system. The LCD display itself has no error correction function, that is, the LCD display module can receive any signal that satisfies the operation timing relationship without the ability to judge whether it is right or wrong. Wrong signals and wrong data will generate wrong control instructions or wrong display patterns, 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 it.
1. The LCD display has no display and there is no response when adjusting the contrast. This phenomenon is because during the operation of the entire machine, the power line or signal line of the LCD display 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 interference is applied to the power line, it is recommended to add a voltage stabilizing capacitor (10uF) and a filter capacitor (0.1uF or 0.01pF) between the power lines VDD and VSS closest to the LCD display module. If the interference is applied to the signal line, it is recommended to add a filter capacitor between the signal line closest to the LCD display and VSS, and the capacitor should be 0.1uF or 0.01uF. The selection of the above capacitor values needs to be determined according to the actual test results.
2. The screen produces wrong characters or random dots (data errors). Or the screen is shifted, upside down, etc. Sometimes it cannot be restored, and the screen can only be cleared and rewritten, or even powered on again to initialize the register to restore. This phenomenon is mostly caused by interference applied to 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, causing register parameters to be modified by mistake, display units to be written by mistake, etc. When the whole system is running, most programs only write data to the local display area, without writing 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; ② Change the direction of the transmission line to avoid the interference environment; ③ Shorten the length of the transmission line; ④ 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 (VSS) in the order of SCLK, then SDA, and then RSRESET.
Test and observe the improvement effect. If the interference signal comes from the system motherboard, the signal deformation can be seen from the LCD display terminal [ ]. This may be because the transmission line resistance between the MPU and the LCD display module is relatively large, and the interface driving ability of the MPU system is relatively weak, which makes it easy for the interference signal to invade. At this time, you can consider: ① Connect a small resistor in series on the transmission line to form a low-pass filter circuit with the input capacitor of the LCD display module end [ ] to eliminate the influence of interference; ② Add a transmission line driver to the system motherboard to improve the driving ability; ③ 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 is writing to the LCD display (/0 addressing mode), the system operation is interrupted, which may modify the control signal status of the LCD display module or the data to be written, resulting in the LCD display module setting error and crash or display error. The improvement method is to turn off the interrupt response function when the MPU calls the LCD display driver subroutine.
4. The interference point cannot be found or the circuit preventive measures cannot be taken, but the influence of interference cannot be eliminated.
At this time, software remediation solutions need to be considered. The simplest method is to initialize the registers regularly. First, do not use the RESET signal to reset, only rewrite the registers. Because the reset action will cause the normal display to flicker, and the display effect is not very good. If a freeze occurs and cannot be recovered, you can only use the RESET signal to force a reset, and then initialize the registers. 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 “off display” state, it is considered that the module is interfered with 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 “off display” state after initialization, it is necessary to use the RESET signal to force reset and initialize. If it is judged to be in the “on display” state, write a set of special data to several units of the display SRAM, and then read them back in turn to determine whether they are correct. If an error occurs, it is considered that the module is interfered with, call the initialization function, and refresh the data again.
5. Static electricity causes the LCD display module to have no display or display randomly: This is a common interference phenomenon. The interference pulse directly passes through the LCD display screen’s iron frame to affect the module’s circuit. Usually, 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, such a 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. ② 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. ③ 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 they all need to be tested in practice to select the appropriate method. If you want to order [Industrial Display Touch Screen], you can contact us at any time, and we can customize it for you professionally.