Deassert

Deassert is a term used in digital electronics and computer engineering to describe releasing a control signal from its asserted state, returning it to its inactive level. In practice, to assert a signal is to drive it to the active level to enable a function; to deassert is to return it to the inactive level or to release the line so that other circuitry may control it. The exact meaning depends on whether the signal is active-high or active-low.

For active-high signals, assertion typically means driving the line high, and deassertion means driving it low or letting it return to low through a pull-down path. For active-low signals, assertion means driving the line low, and deassertion means returning it to high (or releasing it to be pulled high). This distinction is important for understanding behavior of reset lines, enables, chip selects, and similar control signals.

Common contexts for deassertion include reset and enable controls. A deasserted reset releases a processor or peripheral from the reset state, allowing normal operation. A deasserted enable disables a functional block, preventing it from driving outputs or participating in operations. Deassertion can be synchronous, occurring on a clock edge, or asynchronous, occurring independently of the clock. Designers must account for timing and synchronization to avoid glitches, metastability, or unintended behavior during deassertion, especially in asynchronous circuits or across different clock domains.

In practice, deassertion is a fundamental concept in timing diagrams, hardware description languages, and data sheets, underpinning safe startup, controlled enablement, and predictable system behavior.