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I have a circuit that tests the continuity of all the wires in an A-B USB cable, by plugging both ends of the cable in female connectors on the PCB.

It is operated by a 3V3 tolerant MCU.

That creates a risk of damaging the chip by inadvertently plugging in a cable that was already plugged in a computer port, injecting 5V on the chip and breaking it (it already happened.)

I want to design a protection circuit that won't prevent current from passing from A to B, and B to A.

Simply putting a diode on the VCC pin of the B connector would prevent me from testing on both directions, however I would like to be able to do that.

Is there a possible way to prevent 5V from reaching the MCU, while still being able to pass a 3V3 signal? Maybe something like a diode that would only trigger over a certain threshold voltage?

enter image description here

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  • \$\begingroup\$ Where's the MCU power supply in your picture? How does the MCU drive the cable wires? How does the MCU connect to the other ends of the cable to monitor the signals? \$\endgroup\$
    – Andy aka
    Feb 2, 2021 at 16:18
  • \$\begingroup\$ The MCU is powered by a 3v3 regulated usb B input. It drives the tested cable with a walking 1's algorithm, testing all of the IO pins (each connected to a USB wire) as inputs and outputs. If the IO pin being set to 1 is read by its receiving input IO pin (if the input pin reads HIGH) then the wire conductivity is validated \$\endgroup\$
    – JCSB
    Feb 2, 2021 at 16:23
  • \$\begingroup\$ What is the maximum input current specification for the MCU's IO pins? Add series resistors to all vulnerable IO so that current does not exceed the specified max current when illegally fed with voltages via the red line in your picture. If you say "I don't know what you mean" then, as per my earlier comment, circuit schematics are required so that they can be red-inked. \$\endgroup\$
    – Andy aka
    Feb 2, 2021 at 16:31
  • \$\begingroup\$ Are you really trying to measure conductivity or are you just verifying connectivity? \$\endgroup\$ Feb 2, 2021 at 16:44
  • \$\begingroup\$ I am testing continuity, thank you I changed the name in the thread!! \$\endgroup\$
    – JCSB
    Feb 2, 2021 at 16:53

1 Answer 1

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switch to using drivers and receivers to put the signals on the cable

eg MAX3232: these are 25V tolerant, and I've accidentally connected them to USB ports without causing any damage to the USB port.

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