I create a board with two bus: UART and I2C. Now I am expanding the connection terminal, which has the following contacts: RX / TX / SDA / SCL / + 5V / GND.

The question is, do the I2C and UART buses affect each other? Should they be separated?

For example, UART / VCC-GND / I2C or UART / I2C / VCC-GND. What is the minimum distance I must observe? Tires are likely to be on the same layer.

  • \$\begingroup\$ A diagram would help us understand the question. \$\endgroup\$
    – Mattman944
    Commented May 19, 2019 at 9:12

1 Answer 1


Of course multiple signals on a cable always affect each other.You might rather want to know how this effect can be reduced and how long you can expand your cable at a maximum.

If you seperate these signals, you would have to provide each cable with a seperate GND wire. This can lead to another kind of problem: You will have a big GND loop where external noise can be coupled in.

If you want to use a single cable, you have least interference when you reduce the inductive loop areas between all signals. This could be done with a layout like this: TX / RX / GND / SDA / SCL / +5V. (GND should be in the middle of the cable and UART and I2C should be seperated).

I2C is not made to be extended because it has tight requirements for the capacitive load of the bus. Maximum extension length depends on the chosen speed of the I2C bus. For a rule of thumb, take maximum 1 meter cable length for 100 kBit speed, and 10 meter cable length for 10 kBit speed.

UART maximum cable length also depends on the UART speed and on the implementation (RS-232, RS-485, TTL,...). You must specify which type you are using.

  • 1
    \$\begingroup\$ Little Headsup: UART is no equivalent with RS-232/RS-485 which means UART defines how bits are sent but not the how (i.e. logic levels). So one could for example use CMOS 3.3V or TTL 5V levels for UART as well. But there is the possiblity to use differential and/or current based logic as well (e.g. LVDS), which can increase the resiliance against noise etc. \$\endgroup\$ Commented May 21, 2019 at 7:36
  • \$\begingroup\$ @ChristianB. Thanks, I updated my answer \$\endgroup\$ Commented May 21, 2019 at 12:21

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