pulluppulldownnätverk

A pull-up/pull-down network is a fundamental concept in digital electronics, referring to the use of resistors connected to a signal line to ensure it has a defined logic level when no active signal is driving it. This prevents the signal from "floating" in an indeterminate state, which can lead to unpredictable behavior and errors in digital circuits.

A pull-up resistor connects the signal line to the positive power supply (Vcc or Vdd). When the driving device is not actively pulling the line low, the pull-up resistor ensures the line is held at a high logic level. Conversely, a pull-down resistor connects the signal line to ground (GND). When the driving device is not actively pulling the line high, the pull-down resistor ensures the line is held at a low logic level.

These networks are commonly used with open-drain or open-collector outputs. These types of outputs can only actively drive a signal to one logic level (typically low). The pull-up or pull-down resistor provides the necessary pull to the other logic level when the output is not active. For example, a button connected to a microcontroller input might use a pull-up resistor. When the button is not pressed, the pull-up keeps the input high. Pressing the button connects the input to ground, pulling it low.

Internal pull-up and pull-down resistors are often integrated into microcontrollers and other digital integrated circuits, simplifying circuit design and reducing component count. The choice between a pull-up and pull-down depends on the desired default logic state of the signal.