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 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 such cases, a pull-up or pull-down resistor ensures stable, reliable, and safe operation of the circuit.

Pull-up and pull-down resistors are implemented in various ways. Often they are provided as discrete devices, mounted on the same circuit board as the logic devices that use them. Many microcontrollers and FPGAs provide internal, programmable PU and PD resistors for their logic input pins to reduce the need for external components.

This article is issued from Wikipedia. The text is available under Creative Commons Attribution-Share Alike 4.0 unless otherwise noted. Additional terms may apply for the media files.