Interference is a common and very tricky problem in the product machine test, when the system circuit is disturbed, in the power line or signal line to produce a certain frequency, a certain amplitude of the interference wave. Liquid crystal display LCD liquid crystal screen as a product display window, is bound to be one of the performance window of the interference.
LCD liquid crystal screen to the system, is a pure input type components, or passive components, both LCD liquid crystal screen to receive any signal to meet the operation of the timing relationship without the ability to judge right and wrong. Wrong signals and data will produce wrong control word command or display pattern, resulting in wrong display effect.
The first job of eliminating interference is to find the source of interference or the location of interference effects, and then to eliminate, attenuate, or prevent, shield, or remediate in an effective way.
LCD EMI and solutions
Q
What should I do if I get a white/blue screen while working or doing interference testing?
A
This is because the LCD is reset by interference applied to the power supply VDD or VSS of the LCD, or to the RESET signal line of the LCD during operation of the LCD. The reset results in initializing the LCD LCD internal registers and turning off the display.
cure
If the interference is applied to the power line, it is recommended to parallel a voltage regulator capacitor (10uF) and a filter capacitor (0.1uF/0.01uF) between the power lines VDD and VSS at the position closest to the LCD.
If the interference is applied to the RESET signal line, it is recommended that a filter capacitor with a capacitance of 0.1uF or 0.01uF be incorporated between the RESET signal line and VSS at the position closest to the LCD.
The choice of capacitance value needs to be based on the actual test results.
*Note: The so-called white screen or blue screen, refers to the LCD LCD screen as just power on the display, only the backlight is bright, adjust the contrast is no response.
Q
When working or interference test, the screen produces wrong characters or messy dots (data error), and cannot be recovered, and can only be recovered by re-powering up and initializing the registers.
A
This is because interference is applied to the control signals, causing register parameters to be modified, and the general operation is to write display data without repeating the writing of register parameters for the main work, so the above phenomenon occurs.
cure
If the interference is exerted on the transmission line, use magnetic ring, or tin paper, copper thin as the shielding of the line; change the direction of the transmission line to avoid the interference environment; shorten the length of the transmission line; or add the transmission line driver to improve the driving ability and anti-interference ability.
If the interference is applied to the power line, it is recommended to parallel a voltage regulator capacitor (10uF) and a filter capacitor (0.1uF/0.01uF) between the power lines VDD, VSS at the position closest to the LCD liquid crystal screen.
Q
What should I do if I can't find the interference point or take circuit prevention but still can't eliminate the influence of interference during work or interference testing?
A
Timing to initialize the registers, first of all, do not use the RESET signal, only the register operation; if there is a crash can not be recovered, it is necessary to use the RESET signal to initialize, but such an operation will produce a flashing screen phenomenon in the normal display.
cure
To ensure that the normal display is not disturbed by the initialization:
It is possible to read register data as the basis for initialization, such as reading display status words, special SRAM cell data, etc.
Use the negative display LCD liquid crystal screen to turn off the backlight when not in operation. At this time, it is not easy to observe the display content, when you need to observe the display content, you want to start the backlight to light up, and use this time as a re-initialization of the LCD LCD start point, which is not easy to be detected.
Q
Static interference test on the product shell (especially the product panel), LCD LCD screen appears white or chaotic display phenomena
A
Most of this interference is through the LCD LCD LCD iron frame or glass interference LCD LCD circuit.
cure
1) The iron frame of the LCD screen is connected to the earth.
2) The iron frame of LCD liquid crystal screen is connected to VSS.
3) The iron frame of the LCD liquid crystal screen is floating.
4) in the LCD LCD iron frame and metal shell with insulating pads, insulating pads, the thicker, the greater the abatement of static electricity.
These four methods need to be tested in real products to choose the effective method.
Q
There is no external source of interference, but there is also a white screen or chaotic display phenomenon
A
This situation is also classified as interference, which is internal to the system and is mainly due to program conflicts in the software.
cure
First identify the patterns in which disturbances occur.
If the following opportunities occur in the LCD LCD writing process, it is easy to produce LCD LCD crash, error display, etc.: interrupt program is operating the LCD LCD (I/O addressing mode) generated in the process of control signals are modified, data is modified and other errors in the operation, resulting in LCD LCD crash or display errors.
Improvement is made by turning off the interrupt response operation during operation of the LCD liquid crystal screen.