Pull-up resistor

In electronic logic circuits, a pull-up resistor (PU) or pull-down resistor (PD) is a resistor used to ensure a known state for a signal. More specifically, a pull-up resistor or pull-down resistor ensures that a wire will have a high logic level or low logic level, respectively, in the absence of a driving signal. It is typically used in combination with components such as switches and transistors, which physically interrupt the connection of subsequent components to ground or to V_CC. A closed switch creates a direct connection to ground or V_CC, but without a PU or PD, when the switch is open, the rest of the circuit would be left floating (i.e. it would have an indeterminate voltage), which is generally undesirable.

For a switch that is used to connect a circuit to ground, a pull-up resistor (connected between the circuit and V_CC) ensures a well-defined voltage (i.e. V_CC) when the switch is open. For a switch that is used to connect a circuit to V_CC (e.g. if the switch is used to transmit a "high" signal when closed), a pull-down resistor connected between the circuit and ground ensures a well-defined ground voltage (i.e. logical low) across the remainder of the circuit when the switch is open.