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How robust are the digital pins?


#1

I’m adding a low voltage shut-off P-Channel mosfet to my circuit to protect a 2 cell Lipoly battery pack.

The usual way to do this is to use an extra transistor to pull the gate from high (switch is off) to low (switch is on) - so you need 2 devices to make the high side switch.

What would happen to a UsbIzi if I did this:

  • Tie the gate to V+ (8.4V max) through a resistor. This is a weak pull-up and will hold the P switch in the off position.
  • Tie the gate directly to a Fez IO pin.
  • Make the Fez pin high impedance (input).
  • When ready, make it an output and pull it low. It will now switch on the MOSFET by pulling the gate to ground.

During the high impedance (input) state, the pin will “see” 8.4V. During the output state it will go to 0 volts and sink just a few micro amps.

I know the pins are 5V tolerant - but how high can one push this without damage? There should be some clamping diodes in there I think?

I know it is outside design specifications and “not recommended”, but what would realistically happen to the chip?