Clarification on Display Interface Ferrite Beads in MB1246 vs MB1331 Evaluation Boards

[MFawzy]

Hi,

I am designing a custom board based on the STM32H753, and as part of the design process I am reviewing the MB1246 and MB1331 evaluation board schematics for reference.

While studying the display interface, I noticed the following discrepancy:

• On the MB1331 board, the display interface pins include 470 Ω @ 100 MHz ferrite beads.

• On the MB1246 board, these ferrite beads are not present.

Since I am planning to use the STM32H753 with the same 7" display used on the MB1331 board, I would like clarification on:

1. What was the rationale behind adding ferrite beads on the MB1331 display interface while they were not used on MB1246?

2. Are the ferrite beads primarily intended for EMI/EMC performance, or are they required for signal integrity or noise filtering specific to the 7" display?

3. For a custom board design, would you recommend including the same ferrite beads as on MB1331, or are they optional depending on system-level EMC requirements?

Understanding this difference will help me ensure my design follows ST’s best practices and avoids potential display interface issues.

Thank you for your assistance.

Mohammed

[CYANG.1]

Hi,

I am not sure why beads are on each signal. I guess because there is a long cable between MB1331 and LCD board, if the cable is not well shielded, it may have SI issue during board certification.

[MFawzy]

Is there a way to technically confirm the purpose of those beads? Also, as I mentioned, my custom board will be based on the STM32H753 but will use the same 7" display as the STM32H7B3I-EVAL (‘LCD ROCKTEK RK070ER9427-CT’). Could you advise whether I should include the beads in my design or not?

Thanks,

[CYANG.1]

Hi,

As I didn't validate this LCD, I can not give you a conclusion. If possible, I suggest you keep a 0402 footprint on the board with 0ohm resistor at beginning. If there are no function and no EMC issue, you can remove the footprint on next PCB version. If there are issues, you still can put beads on your board to try to solve them.