Understanding A GPIO Input Protection Circuit

Official Hardware Mega Due
1

Hi. I’m designing circuitry that will allow a DUE to conduct a number of point-to-point continuity tests in a load prior to power being applied to the load. One commonly applied method illustrated below uses a pair of stacked schottky diodes to limit voltage at the input to between -0.3V and +3.6V regardless of whether or not S1 is closed (applying +28V at R1).

Intuitively, closing S1 looks risky. Should the DUE’s regulator be expected to maintain +3.3V when S1 is closed? I’d appreciate some advice.

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In the Due spec sheet the only related data I see is " DC Current per I/O Pin: 8 mA"
which would make R1 more like 3k. I imagine 4.7k would be OK to use.
For better protection use an external 3.3v regulator for the protection diode cathode, instead of the Due internal 3.3v supply.

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What has 28V to do with continuity measurements on unpowered circuitry?

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The continuity tests are conducted while the load is unpowered. 28V is applied to the system to operate it.

5

About clamping diodes.
Be careful when you dump current onto a pin or onto the power rail of the MCU.
The regulated 3V3 supply of the Due is supply-only, and can't handle dump current in excess of what the power rail already draws.
Example: If the MCU draws 30mA, then dumping more than 30mA on the rail will increase VCC.
Leo..

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