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I'm designing a board with a control port. The control port allows an external controller to reboot the system by pulling the BRD_RESET low, effectively restarting both regulators.

However it is possible to use an external DC power supply (5-12V) instead of the USB bus voltage provided by the control port.

In this case I could fry my external controller (5V system) because the BRD_RESET is pulled high to the external supply voltage (5-12V) by R1.

How can I protect my external controller? I need to limit the voltage it's seeing on BRD_RESET to VBUS and at the same time allow it to pull down BRD_RESET to GND

enter image description here

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I suggest using diodes to clamp the voltage. In the configuration bellow, the diodes will clamp as soon as the Input signal goes over 5V+Vf.

What this means is that the maximum voltage you will have at the reset port will be typically 5.7V. Excess voltage will be blocked by the diodes.

This solution needs you to have a 5V source on your design that can provide the clamping reference.

schematic

simulate this circuit – Schematic created using CircuitLab

You might also want to look into TVS diodes, you have more info on that here. These diodes will clamp to their rated voltage without the need of a power supply to fix that value.

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