Interference is a common and very difficult problem in the whole product testing, when the system circuit is interfered with, in the power line or signal line will produce a certain frequency, a certain amplitude of interference wave. LCD module as the product display window, is bound to be one of the interference performance window.

For the system, the module is a pure input type component, or a passive component, which means that the module receives signals that meet the operating timing relationship are not able to judge right and wrong. Wrong signals and data can generate wrong control word instructions or display modes, resulting in wrong display effects.

The first job of removing interference is to find the source of interference or the location of the interference impact, and then to remove, weaken, or prevent, shield, remedy with effective methods.

LCD EMI and Solutions

1. Work or do interference test when there is a white screen / blue screen, how to do?

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

Solutions

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

(2) If the reset signal line is disturbed, it is recommended to add a filter capacitor between the reset signal line to VSS at the position close to the module, and the capacitor capacity is selected as 0.1uF or 0.01uF.

(3) The choice of capacitance value depends on the actual test results.

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

2. The work or interference test, the screen produces the wrong characters or messy dots (data error), can not be recovered, can only be restored after re-powering, initialize the register.

This is because interference is applied to the control signal, thus modifying the register parameters, the general operation is to write the display data, the main working register parameters are not repeatedly written, so the above phenomenon occurs.

Solutions

(1) If the transmission line is interfered with, use magnetic rings, or tin foil, copper foil to shield the line; change the direction of the transmission line to avoid 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 subject to interference, it is recommended to add a voltage regulator capacitor (10uF) and a filter capacitor (0.1uF/0.01uF) between VDD and VSS of the power line near the module.

3. Work or interference test, can not find the point of interference or take circuit prevention, still can not eliminate the impact of interference, what to do?

Regularly initialize the registers, do not use the RESET signal first, only operate the registers; if there is a crash that can not be recovered, you need to use the RESET signal to initialize, but this operation will cause the flicker screen phenomenon under the normal display.

Solutions

To ensure that the normal display is not disturbed by initialization:

(1) You can use the read register data as the judgment basis for initialization, such as reading the display status word and special SRAM cell data, etc.

(2) Use the negative display module to turn off the backlight when it is not working. At this time, it is not easy to observe the display content. When the display content needs to be observed, it is hoped that the backlight illumination will be activated, and this time is not easily noticed as the starting point for reinitializing the module.

4. The test product shell (especially the product panel) electrostatic interference, the module appears white screen or chaotic display phenomenon.

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

Solutions

(1) The module's iron frame connected to the earth.

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

(3) The iron frame of the module floats.

(4) Add an insulating pad between the iron frame of the module and the metal shell. The thicker the insulating pad, the greater the reduction of static electricity.

These four methods need to be tested in the actual product to choose an effective method.

5. There is no external source of interference, but there will also be a white screen or chaotic display phenomenon.

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

Solutions

First, find out the law of the appearance of interference.

If the following situations occur during the writing of the module, it is easy to cause the module to die, wrong display, etc.: the interrupt program (I/O addressing mode) is generated during the operation of the module, the control signal is modified, the data is modified and other wrong operations, which will cause the module to die or display errors.

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