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I am wondering at what distance UART lines begin to interfere with each other? And can I expect the UART lines below to function properly? The PCB board below has 6 UART lines running alongside one another (the purple traces).

Some info about the PCB below:

  1. The purple traces are approximately 1mm apart 0.25mm lines. Except when red UART lines run directly above purple lines. A ground plane is in-between though.
  2. 4 layer PCB top layer, inner1, inner2, and bottom are signal (red), ground, signal(purple), power planes respectively.
  3. All planes are filled with a ground layer around the traces.
  4. The UART lines are at 1.8V.

Circuit

A close-up close-up

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  • \$\begingroup\$ Considering that TX and RX are often run right next to each other in the same cable for several meters, I don't think you need to worry about it on a PCB. If you wanted to run at 1MHz or something it could be trouble. But at 115200 and below you should be fine. \$\endgroup\$
    – user57037
    Commented Apr 15, 2021 at 3:22
  • \$\begingroup\$ It's probably not an issue here, but running signal lines above a power (rather than ground) plane does significantly increase crosstalk in most cases. The exception being when the signal lines are able to use the power plane as the return path. If you're concerned, but sensitive traces as close as possible to a ground layer. \$\endgroup\$ Commented Apr 17, 2021 at 0:18

2 Answers 2

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4x spacing over a ground plane should be no problem.

I normally use a spacing of three times the trace width for normal signal traces.

Especially for UART signals that are relatively low speed there will not be an issue as most UARTS have noise rejection logic where they sample the signal three times so any coupling would be rejected anyway.

Clock and reset signals to registers are the most sensitive as any transient can permanently alter the sate of a register.

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At 1 mm distance there will be no cross talk:

  1. UART currents are really low

  2. Electric and magnetic fields won't vary much in time because UART lines voltage value is constant most of the time. Current vary only when the line goes from 0 to 1 and vice versa.

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