sticky bit
A fault in a digital circuit, where a signal does not change as expected, but remains always in one state.
sticky bit: a signal trapped in one state
A sticky bit is a failure mode in digital circuits where a signal line becomes locked at either logic high or logic low, regardless of what the driving logic should be producing. The affected line fails to toggle between states, remaining constant even when the circuit receives commands to change it. This is distinct from a normal signal that switches between 0 and 1 as the circuit operates; a sticky bit simply will not switch.
The most common cause is a short circuit between the signal line and either the positive supply rail (pulling it high) or ground (pulling it low). A stuck-high condition typically results from a short to VCC or VDD, while a stuck-low condition results from a short to ground. These shorts can originate from manufacturing defects like incomplete etching during PCB fabrication, solder bridges, or corroded traces. In older DIP and through-hole designs, a bent lead touching an adjacent conductor can also cause the fault. In modern surface-mount boards and integrated circuits, manufacturing contamination or probe damage during testing can introduce the defect.
Sticky bits are trivial to detect during board-level testing or IC characterization: the faulty line simply will not respond to stimulation. A test pattern that expects the signal to toggle will fail immediately. The fault affects any logic that depends on that signal, since downstream circuits receive only a constant value instead of the intended data or control signal. If the stuck line is a clock, address bit, or control signal, the entire system may be non-functional. If it is a data bit in a bus, only that bit position will report corrupt values.
The term 'sticky' refers to the signal sticking or adhering to one logic level, as if friction is preventing it from moving. It contrasts with intermittent faults, where a signal occasionally glitches but recovers, or open-circuit faults, where the signal floats to an undefined level. Sticky bits appear in failure analysis reports for both discrete circuit boards and semiconductor dies, and they are among the most straightforward electrical faults to diagnose and localize.