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I'm designing a PCB to use a 4.3" LCD TFT display. The LCD has a 24-bit RGB parallel interface. The design needs to be EMC compliant.

My question is, what are the design requirements I should pay attention to when routing the board?

The LCD will be running at 12 MHz, and the resolution is 480x272. The PCB spec is 2 layer 1 Oz copper board. I have identified that the trace impedance is required to be 50 Ω. I'll try to keep the trace length as short as possible (less than 50 mm).

These are the questions that I have so far:

  1. Do I need termination resistors on the RGB data lines? How do I calculate the required resistor values?

  2. Do I need to length-match the data lines with respect to the clock signal?

Are there any other requirements that I should consider in my design?

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  • \$\begingroup\$ How did you determine you need 50 ohm trace impedance, and did you verify if the trace geometry is such that it can be realized on your 2 layer PCB? \$\endgroup\$
    – Justme
    Commented Aug 8, 2022 at 5:30
  • \$\begingroup\$ @Justme I found the requirment on a design guideline document. Yes, I did calculate the impedance and found it to be within 10% of 50 Ohms. \$\endgroup\$
    – ADGAN
    Commented Aug 8, 2022 at 5:35
  • \$\begingroup\$ What design document? For the LCD? If you know the LCD model and have links to datasheet and documents then add them in the question. I am just asking, if you do 24 parallel tracks of 50 ohm impedance, how wide the track will be and what distance the tracks need to have between each other, so how wide the PCB must be, so is it physically realizable. \$\endgroup\$
    – Justme
    Commented Aug 8, 2022 at 5:51
  • \$\begingroup\$ @Justme The LCD datasheet doesn't have any routing guidelines. I was searching on this topic and found this document. [link]docs.toradex.com/102492-layout-design-guide.pdf. (Pg 37). The traces are 0.254mm wide and trace seperation to 0.203mm. \$\endgroup\$
    – ADGAN
    Commented Aug 8, 2022 at 6:27
  • \$\begingroup\$ No that trace geometry does not end up with 50 ohm impedance with a standard 1.6mm two layer PCB. \$\endgroup\$
    – Justme
    Commented Aug 8, 2022 at 7:51

1 Answer 1

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The PCB spec is 2 layer 1 OZ copper board.

Well, if you manage to get 50 Ohm trace for this LCD on a 2-layer board with meaningful trace widths please find me :) I hope you are aware of the fact that the PCB thickness should be very low. If you are to use a 1.6mm-thick board it'll be impractical.

The traces are 0.254mm wide and trace seperation to 0.203mm.

I should remind that these suggested widths and separations are for 4- or higher layer boards.

Do I need termination resistors on the RGB data lines? How do I calculate the required resistor value?

Hard to tell. You must worry more about the rise and fall times instead of termination. You may need to add series resistors to dampen the ringing caused by fast slew rates. You can still place resistors for termination on the PCB but don't populate at the first run, then after some investigation you can decide if a termination is needed. But I'm pretty sure that you'll need series resistances.

Do I need to length match the data lines with respect to the clock signal?

You're not working on differential pairs but still, it's good to match the lengths.

I also want to add that there's a statement on p.37 of the datasheet:

The requirements below can be greatly relaxed if lower resolutions such as VGA 640x480 are used.

But still, for EMC purpose, don't exceed the suggested maximum trace length (100 mm).

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  • \$\begingroup\$ Thanks for your reply. I thought the W/H<1 requirement is only valid for differential microstrip impedances not for single-ended microstrip impedances. [link] everythingrf.com/rf-calculators/microstrip-impedance-calculator. Am I correct to use the microstrip impedance calculator since RGB signals are not differential pairs? \$\endgroup\$
    – ADGAN
    Commented Aug 9, 2022 at 1:56
  • \$\begingroup\$ @ADGAN oh yeah, you are right. I missed that part. I'm removing the respective section from my answer. Thanks for pointing out. \$\endgroup\$
    – Rohat Kılıç
    Commented Aug 9, 2022 at 6:48

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