powergood

Powergood, commonly abbreviated as PG, is a control signal generated by a computer’s power supply to indicate that the main output rails have reached stable, acceptable voltages and that the system can safely begin operation.

In PC power architectures such as AT and ATX, the power supply asserts Power Good after the +5V, +3.3V, and +12V rails have settled within their specified tolerances and after a short validation delay. The PG signal is delivered to the motherboard, typically as a digital high level or via an open-collector line that the motherboard pulls up. When PG is asserted, the motherboard releases the processor reset and allows the system to start initializing. If PG is not asserted, or if it drops, the system remains reset or powered down.

PG serves as a protective and coordinating mechanism. It ensures that unstable rails do not cause improper operation during startup, and it responds to faults by deasserting the signal. Conditions that can cause PG to fail include under-voltage, over-voltage, short circuits, or loss of one or more rails. The exact thresholds and timing depend on the standard in use, but a typical requirement is that rails be within tolerance for a brief interval before PG is considered valid.

In modern hardware documentation, Power Good is referenced as part of the power-on sequence and reset logic. It is an essential signal for reliable startup, proper initialization of components, and safe shutdown coordination.