LCD Screen EMI Issues and Solutions
EMI, Electromagnetic Interference affects the functioning of electronic device, especially LCD screen. LCD display within a product is a pure "input" component, it passively accepts any input and shows them out. To eliminate EMI, first we need to identify the interference source and affecting parts. Then we can use corresponding method to reduce, eliminate or prevent electromagnetic radiation getting in to LCD display.
EMI on LCD and Solutions
Followings are several usual EMI scenario (Q&A):
Q: When LCD display is working or performing interference test, LCD screen turns blank, white or blue screen.(like when LCD display is first turned on, only back-light is ON. Changing contrast does nothing.)
A: This is most likely caused by interfering signal gets into power source VDD / VSS or RESET. If it is noise getting in to power source. Solution is to add regulating capacitor(10uF) and filtering capacitor(0.1uF/0.01uF) to VDD / VSS parallelly. In the case of interference on RESET, a filtering capacitor (0.1uF/0.01uF) can be added between RESET and VSS. Above capacitors' value should be chosen according to testing result.
Q: LCD screen is showing wrong characters or random dots. The only way to fix that is to reset the LCD display.
A: Normally, this happens because of noise getting into control signal, display data in register is changed. There are several potential solutions to this EMI issue.
1. Use choke coil, aluminum or copper foil to shield transmission lines.
2. Avoid intrusion noise by changing data lines' location.
3. Shorten transmission lines, or use bus transceiver.
Q: Can not locate interference point, or after applying interference prevention mechanism noise still exists on LCD screen.
A: Initialize and refresh display registers with correct data regularly. However, this operation will cause screen flicker if noise is not present. Alternately, use negative LCD (see LCD viewing mode), when observation is not needed, turns off LCD display back-light. When displaying is needed again, resets the LCD display module and turns on back-light. Thus, flickering is hard to detect.
Q: When performing static interference test on product or LCD front panel, LCD screen turns blank or shows random noise.
A: This type of interference is mostly transmit through LCD display's front glass or bezel. There are three ways to stop or reduce the transmitted static noise.
1. Ground the LCD display's bezel
2. Connect VSS with LCD module bezel
3. Add insulation layer between product case and LCD screen
Q: What are the hardware methods to suppress interference
A: 1. Install capacitor on power supply line or RESET
2. Put choke coil around power line, data bus or control lines
3. Connect serial resistor (around 10 Ohm) to power line
4. Put TVS diode on power supply line
5. Put capacitor on WR, E, /CS parallelly (100pF ~ 300pF ceramic CAP)
6. Enhance signal by using bus transceiver (such as 74LS245)