This article includes a list of general references, but it lacks sufficient corresponding inline citations. (January 2015) |
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 VCC. Without such resistor, closing the switch creates a direct connection to ground or VCC; when the switch is open, a problem arises as the rest of the circuit would be left floating (i.e. it would have an indeterminate voltage).
For a switch that is used to connect a circuit to ground, a pull-up resistor (connected between the circuit and VCC) ensures a well-defined voltage (i.e. VCC, or logical high) when the switch is open. For a switch that is used to connect a circuit to VCC (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.