activelow

Activelow (often written as active-low) is a term used in digital electronics to describe a signal or input that is considered asserted when its voltage is low, typically near 0 volts. The opposite concept is active-high, where assertion occurs at a higher voltage close to the supply. The two notions define how a control line is interpreted by digital circuits and software.

In practice, an active-low signal means that the device is enabled, reset, or otherwise activated when the line is pulled to a low level. Designations such as RESET_N, CS_N, or IRQ# commonly indicate active-low behavior, sometimes shown on schematics with a bar over the symbol or a small bubble on the input pin. The use of active-low signals is widespread due to historical and electrical considerations, including compatibility with open-collector or open-drain outputs and simple pull-up resistor schemes.

Electrical characteristics depend on the logic family. For TTL, a signal is treated as low below roughly 0.8 V and high above about 2 V, while CMOS devices use near-ground and near-supply levels. Active-low lines often rely on pull-up resistors to keep the line in a safe default high state when not asserted. Debouncing may be required for mechanical inputs, and some lines use edge-triggered behavior or level-sensitive conditions.

Advantages of active-low designs include a robust default state, easier interfacing with open-collector/drain hardware, and straightforward wiring to Vcc. Potential downsides involve inverted logic in larger circuits, which can cause confusion if not well documented.

See also: active-high, pull-up resistor, open-collector, logic levels, reset signal.