Electronic – the function of a diode connected to a GPIO


I'm reading a schematic (done by a former employee) with a microcontroller, and I came across this circuit:

Sample circuit

What is the function of D1?

Also, I think that R2 isn't necessary, because the control input come from another GPIO that always has 1 or 0 as its output.

Besides this one, there is another block of this same circuit but connected to the RESET pin of the microcontroller. This part of the circuit is used to flash the microcontroller via ISP.

R1 = 47k
R2,R3 = 10k
D1 = 1N4148

Best Answer

It's there to protect the B-E junction of Q1 from reverse breakdown.

For positive input voltages, the B-E junction of Q1 will conduct, and the voltage at the base will be limited to about +0.65V. As long as R1 is sized appropriately to limit the current, fairly arbitrary positive voltages can be applied to the input.

D1 provides a similar path for negative input currents, guaranteeing that the base voltage never goes below -0.65V for the same range of negative voltages.