ZBasic Forum

An application calls for momentary ~+18v through 100k to a processor input pin. The classic protective solution calls for a clamp diode from the pin to Vcc, and clamp diodes are, I believe, already provided on the processor.

Assuming that and Vcc=5v, the pin would never exceed ~+5.4v; the clamp diode would draw current through the 100k if the voltage applied to the resistor exceeds that. So, for +18v, the current into the pin would be (18v-5.4v)/100k= ~126uA.

I believe that should be safe. Is it?

GTBecker wrote:So, for +18v, the current into the pin would be (18-5.6)/100k= ~124uA. I believe that should be safe. Is it?

My reading of the datasheet indicates that that current should be safe. There is no explicit specification like "maximum protection diode current" but there is "DC Current per I/O Pin ... 40.0mA" in the "Absolute Maximum Ratings" section.

The problem would be if the clamping diode fails open. That is why I've suggested using an external clamping diode.