Purpose of BAT54S

I have this snippet from a schematic, can anyone tell me the purpose of the diodes?

I think it is to clamp the input signal to prevent damage to the MCU input. The lower diode will conduct if the input voltage goes more negative then one doode forward voltage drop below ground. The upper diode will conduct if the input goes more positive than 3.3 volts plus one diode forward voltage drop.

Les.

The diodes are Schottky types which have a lower forward drop than junction diodes, to provide better over/under voltage protection.