Interference is a common and difficult problem in product testing. When the system circuit is disturbed, an interference wave with a certain frequency and a certain amplitude will be generated on the power line or signal line. As the display window of the product, the liquid crystal display module must be one of the interference performance windows.

For the system, the module is a pure input component, or a passive component, that is to say, the module has no ability to judge whether it is right or wrong when it receives a signal that satisfies the running sequence relationship. Wrong signals and data will generate wrong control word commands or display modes, resulting in wrong display effects.

The first task of removing interference is to find the source of interference or the location of interference effects, and then use effective methods to remove, weaken, or prevent, shield, and remedy.

EMI of liquid crystal display and solution

1. What should I do if a white screen/blue screen appears during work or interference testing?

This is because during the operation of the module, interference is applied to the power supply VDD or VSS of the module, or to the reset signal line of the module, causing the module to be reset. After reset, the internal registers of the module are initialized and the display is turned off.

Solution

① If the power line is disturbed, it is recommended to add a voltage stabilizing capacitor (10uF) and a filter capacitor (0.1uF/0.01uF) between the power line VDD and VSS near the module.

② If the reset signal line is disturbed, it is recommended to add a filter capacitor between the reset signal line and VSS near the module, and the capacitance should be 0.1uF or 0.01uF.

③ The selection of capacitance depends on the actual test results.

*Note: The so-called white screen or blue screen refers to the display module screen just after power on, only the backlight is on, and the contrast adjustment does not respond.

2. When working or interfering with the test, wrong characters or random dots (data error) are generated on the screen, which cannot be recovered. Only after power on again, the initialization register can be recovered.

This is because the control signal is disturbed, thereby modifying the register parameters. The general operation is to write display data, and the main working register parameters are not repeatedly written, so the above phenomenon occurs.

Solution

① If the transmission line is disturbed, use a magnetic ring, or tin foil or copper foil to shield the line; change the direction of the transmission line to avoid the interference environment; shorten the length of the transmission line; or add a transmission line driver to improve the driving ability and anti-interference ability.

② If the power line is disturbed, it is recommended to add a voltage stabilizing capacitor (10uF) and a filter capacitor (0.1uF/0.01uF) between the power line VDD and VSS near the module.

3. Work or interference test, if you can’t find the interference point or take circuit precautions, but still can’t eliminate the influence of interference, what should you do?

Periodically initialize the registers, do not use the RESET signal first, only operate the registers; if a crash occurs and cannot be recovered, you need to use the RESET signal to initialize, but this operation will cause a splash screen under normal display.

Solution

To ensure normal display without interference from initialization:

① The read register data can be used as the judgment basis for initialization, such as reading the display status word and special SRAM unit data.

② Use a negative display module to turn off the backlight when not working. At this time, it is not easy to observe the display content. When it is necessary to observe the display content, it is desirable to start the backlight. This time is used as the starting point of the reinitialization module, and it is not easy to be noticed.

4. Test the electrostatic interference of the product shell (especially the product panel), and the module has a white screen or random display.

Most of this interference interferes with the module circuit through the module's iron frame or glass.

Solution

① The iron frame of the module is connected to the earth.

② The iron frame of the module is connected to VSS.

③ The iron frame of the module is floating.

④ Add an insulating pad between the module iron frame and the metal case. The thicker the insulating pad, the greater the reduction in static electricity.

These four methods need to be tested in actual products to select an effective method.

5. There is no external source of interference, but white screen or disordered display may also appear.

This situation is also interference, that is, interference within the system, mainly caused by software program conflicts.

Solution

First, find out the law of interference.

If the following situations occur during the writing process of the module, it is easy to cause the module to crash, error display, etc.: an interrupt program (I/O addressing mode) is generated during the operation of the module, control signals are modified, data is modified and other wrong operations, This will cause the module to freeze or display errors.

The improvement is to turn off interrupt response operation during module operation.