Pull-up resistors are resistors commonly used in logic circuits to help ensure a well-defined logical level at a pin under all conditions. It is used to prevent the logic state to be left as “floating”. The pull-up resistor solves this problem by pulling the value to a logical high state when switch S1 is open instead of leaving it floating.
Pull-up resistors are not a special kind of resistors, they are simple fixed-value resistors connected between the voltage supply and the appropriate pin, which results in defining the input or output voltage in the absence of a driving signal.
Pull-down resistors work in a similar manner as a pull-up resistor, except that they pull the pin to a logical low value. They are connected between ground and an appropriate pin on the device. Pull-down resistors must have a larger resistance than the impedance of the logic circuit, or it might pull the voltage down too much making the input voltage at the pin remain at a constant logical low value, with no regard to the switch position.
An example is a pull-down resistor used in a digital circuit. A pushbutton switch is connected between the supply voltage and a microcontroller pin. When the switch is closed, the microcontroller input is at a logical high value, but when the switch is open, the pull-down resistor will pull the input voltage down to ground (logical zero), preventing an undefined state.
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