activehigh

Active high is a term used in digital electronics to describe a signal or control that is considered asserted when it assumes a high voltage level (logic 1). This is in contrast to active-low signals, which are asserted when the signal is at a low level (logic 0). Whether a given pin or line is treated as active high depends on the circuit design and labeling conventions.

In practice, an active-high signal enables or triggers a function when it is high. For example, a microcontroller output connected to an LED with the LED’s anode tied to the pin and the cathode through a resistor to ground will turn the LED on when the pin goes high. Similarly, a device’s enable or reset line might be described as active-high if asserting a high level enables the device or resets it accordingly. Documentation and schematics may label such signals as ACTIVE_HIGH or active_high to indicate this behavior.

Implementation considerations can include how the line is biased and how worst-case stability is achieved. Active-high signals may use pull-down resistors to define a known low state when undriven, whereas active-low lines often use pull-up resistors. The choice between active-high and active-low can affect noise immunity, wiring simplicity, and compatibility with open-collector or open-drain interfaces, where participations of multiple devices on a shared line differ based on the asserted polarity.

In software contexts, an active-high parameter or flag typically means the feature is enabled when the value is true or set to 1. Understanding whether a control is active-high or active-low is essential when interfacing hardware, documenting behavior, and ensuring correct initialization and operation.

See also: active-low.