Pull-up resistor

Common card · No. 272643

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 conjunction with components such as switches, transistors and connectors, that physically or electrically interrupt the connection of other components to a low impedance logic-level source, such as ground, positive supply voltage (VCC), or an actively-driven logic circuit output and thus cause the inputs of those components to float (i.e. to have an indeterminate voltage) — a condition which can lead to unpredictable and potentially damaging circuit behavior. For example, in the case of a switch which, when closed, connects a circuit to ground or positive supply voltage, without a PU or PD, when the switch is open, the circuit would be left floating. In...

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