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On the PCB we are currently designing we have two isolated grounds, one for the sensitive electronics (including a Jetson Orin NX SoM), and one for the outside-facing connections.

The difficult part is to manage the signals that have to pass between referenced to ground A (Jetson side) and referenced to ground B (connector side).

For unidirectional signals (like UART), this is easily done with a digital isolator (or an optocoupler).

For I2C signals, I managed to find some dedicated ICs, like the ADuM1250.

However, I have one digital GPIO for "future use", and I'm stuck about how to get bidirectional communication with isolation.

The hard constraints :

  • on side A, the signal is 1.8 V logic level (I can add a level shifter if needed)
  • on side B, the signal is 5 V logic
  • the Jetson on side A can source/sink 1 mA max
  • I want side B to accept <= 1 mA if side B is the input, and to provide at least 0.5 mA if it's the output
  • support communication at at least 1 kb/s
  • isolation of at least 400 V DC for 5 minutes, transients of at least 800 V

Nice to have:

  • compact design (so if possible, avoid extra level shifters)
  • for side B as output : have "nice" low levels (i.e. < 0.5 V)
  • higher communication speeds (up to 1 Mb/s)
  • B side output capable of sourcing/sinking more current (> 10 mA)

Do you have any idea how I could build such bidirectional communication with isolation?

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  • \$\begingroup\$ Why not use another ADUm1250 or similar device? \$\endgroup\$
    – Andy aka
    Feb 20, 2023 at 15:48
  • \$\begingroup\$ Does "future use" mean that the hardware must support dynamic reconfiguration? Can't you just wait until you know, and then solder an isolator with the correct number of signals going in each direction? \$\endgroup\$
    – CL.
    Feb 20, 2023 at 15:50
  • \$\begingroup\$ @Andyaka For using a ADuM1250 or similar, there are a few cons. First, I haven't been able to find any that works with 1.8V logic (and on at least one side the low voltage is really ugly (V_out_low is about 0.9V), making it hard to work even with 3.3V logic) : so I would at least probably need to add a level shifter. And second, the "up" state is rather weakly driven (just through the pull-up), while I would prefer a stonger drive (ie able to source more current, and avoid voltage drops when using current). But it is still good enough to meet the hard requirements. So I keep it as a backup. \$\endgroup\$
    – Sandro
    Feb 20, 2023 at 16:12
  • \$\begingroup\$ @CL : yes, we want dynamic reconfigure (we will probably need the pin as input during calibration (and once calibrated we can't reopen the device without loosing calibration), and we want to be able to provide customers a custom signal (in or out) if they want customization. We might go for one input pin and one output pin if there is no other solution, but otherwise, I would rather keep a single pin, even if it means increasing cost by 20$ \$\endgroup\$
    – Sandro
    Feb 20, 2023 at 16:18

2 Answers 2

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However, I have one digital GPIO for "future use", and I'm stuck about how to get bidirectional communication with isolation.

just put the applicable power and ground next to it and in the future install a suitable isolator for whatever purpose the pin gets

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  • \$\begingroup\$ It might have been a solution if future use only involved one direction. It is however quite likely that we will need this pin as input for calibration, so this stratégy would prevent to use it as output once calibration is done \$\endgroup\$
    – Sandro
    Feb 21, 2023 at 21:13
  • \$\begingroup\$ there's several different ways to get bidirectional isolation, they all have different drawbacks, you simply need to pick the right one. \$\endgroup\$ Feb 22, 2023 at 0:36
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This is possible.

You need three signals through a digital isolator: a direction signal, and one each for A→B and B→A. Add three-state buffers (74xx1G125/126) or SPDT switches (74xx1G3157) to disable the signals in the wrong direction.

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  • \$\begingroup\$ Thanks a lot. It's not very compact (if I'm not mistaken, I will need 4 buffers, 2 non-gates and 3 channels of digital isolators), and requires an additional GPIO to set direction, but otherwise I think it will it will match all other nice to haves (and requirements). So if someone comes up with a more compact solution, it would be nice, otherwise, I will try to squeeze this onto the PCB \$\endgroup\$
    – Sandro
    Feb 20, 2023 at 16:30

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