Pull-up resistor

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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.^[1] 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. Without such resistor, closing the switch creates a direct connection to ground or V_CC; 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, or logical high) when the switch is open. For a switch that is used to connect a circuit to V_CC (e.g. if the switch or button is used to transmit a "high" signal), 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.