What is the use of clamping diode at the pin of a controller

regards,
A

 

Are you talking about adding an external discrete diode, or are you asking about the internal clamping diodes that are intrinsic in every CMOS input pin structure?

 

I am asking about the external diode

 

The purpose of a clamp is to restrict the voltage, that can be applied to a pin, to a convenient range. It does this by presenting a lower imepdance to voltage at an input pin than the impedance of the pin itself. As we know from Ohms law if current has two paths it can choose from then most of the current will pick the path with the lowest impedance.