What is the difference between digital and analog input pins declared on microcontroller? In which situations we use analog and in which digital input pins? What will happen if we put an digital signal into into analog input pin, and vice versa?
The difference is that a digital pin is only able to detect whether there is a voltage present on a pin or not, while an analog pin is connected to an internal Analog to Digital Converter (ADC) and can measure the actual voltage on the pin.
A digital pin is suited to detect if a button has been pressed, while an analog pin might for example measure the position of a potentiometer.
You can easily apply a digital signal to an analog pin. But be aware that detection of an analog value via ADC is normally slower than reading out a digital pin. What you get is, depending on the value, a quite low or a quite high value, but nothing in-between.
If you apply an analog signal to a digital pin, however, things are not so easy. Depending on the circuitry used for reading out that pin, it may happen that you have a CMOS situation where both transistors are conducting, a situation which is conntected to high losses of power.