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The project I am working on uses a Beagle Bone Black (BBB) rev C, we are powering it externally through a cape according to the System Reference Manual's instructions.

The System Reference Manual makes it clear to not supply voltages to any pins before the SYS_RESET pin goes high (see here). The usual solution to this is to have any external circuitry be powered by the BBB's 5v or 3v pins. However, we cannot use this solution because we have a Teensy 3.2 that is running independent of the BBB and is connected to the BBB's UART2 pins (P9 header, pins 21 and 22). So we will be using a tri-state buffer to prevent voltage being applied to the pins when SYS_RESET is low, as discussed in this question.

I am worried that a tri-state buffer will interfere with the UART communication between the Teensy 3.2 and the Beagle Bone Black, or if a tri-state buffer is even needed on those UART2 pins.

TL;DR:
Is it possible to use a tri-state buffer in the middle of a UART line?

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Adding in some 10K series resistors should limit the current that flows through the BeagleBone's (or any other CMOS IC's) internal ESD clamp diodes.

I think you would be fine with tri-state buffers though. Or analog switches. I'd expect less signal degradation out of either of those than the resistor.

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  • \$\begingroup\$ I looked through the BBB rev C schematic and I couldn't find the ESD clamp diodes you mentioned? Is it within the AM335x itself? \$\endgroup\$
    – tzengia
    Commented Jul 9, 2020 at 22:49
  • \$\begingroup\$ @Expanding-Dev They are internal to ICs in general to prevent ESD damage during manufacturing. Not to be relied upon elsewhere because they aren't very powerful but series resistors limits the power they have to handle. \$\endgroup\$
    – DKNguyen
    Commented Jul 9, 2020 at 22:52
  • \$\begingroup\$ That makes more sense. Should the tri-state buffer be bi-directional for the UART? Or will it not matter? \$\endgroup\$
    – tzengia
    Commented Jul 9, 2020 at 22:55
  • \$\begingroup\$ Each line on a UART only goes one way so it shouldn't matter. I'm assuming you mean an actual UART, and not RS-232, RS-485, or RS-422. Even then, RS-485 is the only one of these where the lines are bidirectional. But RS-232 and RS-485 have other characteristics (like bipolar voltages or balanced transmission) so you do not want to stick something in the way. \$\endgroup\$
    – DKNguyen
    Commented Jul 9, 2020 at 23:27
  • \$\begingroup\$ yes, it's an actual UART. The BBB UART is connected to the Teensy 3.2's UART directly, no line drivers between them or anything like that. \$\endgroup\$
    – tzengia
    Commented Jul 10, 2020 at 2:51

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