Introduction
As we all know, resistors play a important role in limiting current in the circuit. Among then, pull-up resistors and pull-down resistors are often mentioned and frequently used in electronics. The pull-up is to clamp the uncertain signal to a high logical level through a resistor, which acts as a current limiter; while the pull-down resistor clamps the uncertain signal to a low logical level. Because there are only two states of high level and low level in digital circuits, it is uncertain at the initial stage of digital signals.

1 Why pull-down and pull-up resistor?
Pull-up and pull-down resistors are often applied when interfacing a switch or some other input with a microcontroller or other digital gates. That is, in the initial stage of digital circuit power-on, because the high logical level and low level of the output state are uncertain, in order to make the circuit state normally, a pull-up resistor or pull-down resistor is needed to stabilize the uncertain circuit state. The low logical level is connected to GND inside the IC, and the high level is connected to the super resistance inside the IC.