Power Good Signal
Encyclopedia
In addition to the voltage
s and currents that a computer
needs to operate, power supplies also provide a signal called the Power Good signal, sometimes written as Power_OK or Power_Good or it can be distinguished by its gray color. Its purpose is to tell the computer all is well with the power supply and that the computer can continue to operate normally. If the Power-Good signal is not present at startup, the CPU is held in reset state. If a Power-Good signal goes down during operation, the CPU will shut down. The Power-Good signal prevents the computer from attempting to operate on improper voltages and damaging itself.
The ATX
specification defines the Power-Good signal as a +5 volt (V) signal generated in the power supply when it has passed its internal self-tests and the outputs have stabilized. This normally takes between 0.1 and 0.5 seconds after the power supply is switched on. The signal is then sent to the motherboard
, where it is received by the processor timer chip that controls the reset line to the processor.
In the absence of the Power-Good signal, the timer chip continuously resets the processor, which prevents the computer from running under bad or unstable power conditions. When the timer chip receives the Power-Good signal, it stops resetting the processor, and then the processor executes whatever code is at address FFFF:0000 (usually the ROM BIOS).
If the power supply cannot maintain proper outputs, such as when a brownout
occurs, the Power-Good signal is withdrawn, and the processor is automatically reset. When proper output is restored, the Power-Good signal is regenerated, and the computer again begins operation. By withdrawing the Power-Good signal, the computer never receives the bad power because it is stopped quickly (reset) rather than allowed to operate on unstable or improper power levels, which can cause parity errors and other problems.
Cheaper and/or lower quality power supplies do not follow the ATX specification of a separate monitoring circuit; they instead wire the power good output to one of the 5 V lines. This means the processor will never reset given bad power unless the 5 V line drops low enough to turn off the trigger, which could be too low for proper operation.
, that a power supply takes to become fully ready. According to Cool Max, manufacturer of an LCD power supply tester model PS-228, power good values are often considered abnormal if detected lower than 100ms or higher than 900ms.
Voltage
Voltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...
s and currents that a computer
Computer
A computer is a programmable machine designed to sequentially and automatically carry out a sequence of arithmetic or logical operations. The particular sequence of operations can be changed readily, allowing the computer to solve more than one kind of problem...
needs to operate, power supplies also provide a signal called the Power Good signal, sometimes written as Power_OK or Power_Good or it can be distinguished by its gray color. Its purpose is to tell the computer all is well with the power supply and that the computer can continue to operate normally. If the Power-Good signal is not present at startup, the CPU is held in reset state. If a Power-Good signal goes down during operation, the CPU will shut down. The Power-Good signal prevents the computer from attempting to operate on improper voltages and damaging itself.
The ATX
ATX
ATX is a motherboard form factor specification developed by Intel in 1995 to improve on previous de facto standards like the AT form factor. It was the first big change in computer case, motherboard, and power supply design in many years, improving standardization and interchangeability of parts...
specification defines the Power-Good signal as a +5 volt (V) signal generated in the power supply when it has passed its internal self-tests and the outputs have stabilized. This normally takes between 0.1 and 0.5 seconds after the power supply is switched on. The signal is then sent to the motherboard
Motherboard
In personal computers, a motherboard is the central printed circuit board in many modern computers and holds many of the crucial components of the system, providing connectors for other peripherals. The motherboard is sometimes alternatively known as the mainboard, system board, or, on Apple...
, where it is received by the processor timer chip that controls the reset line to the processor.
In the absence of the Power-Good signal, the timer chip continuously resets the processor, which prevents the computer from running under bad or unstable power conditions. When the timer chip receives the Power-Good signal, it stops resetting the processor, and then the processor executes whatever code is at address FFFF:0000 (usually the ROM BIOS).
If the power supply cannot maintain proper outputs, such as when a brownout
Brownout (electricity)
A brownout is an intentional drop in voltage in an electrical power supply system used for load reduction in an emergency. The reduction lasts for minutes or hours, as opposed to short-term voltage sag or dip. The term brownout comes from the dimming experienced by lighting when the voltage sags...
occurs, the Power-Good signal is withdrawn, and the processor is automatically reset. When proper output is restored, the Power-Good signal is regenerated, and the computer again begins operation. By withdrawing the Power-Good signal, the computer never receives the bad power because it is stopped quickly (reset) rather than allowed to operate on unstable or improper power levels, which can cause parity errors and other problems.
Cheaper and/or lower quality power supplies do not follow the ATX specification of a separate monitoring circuit; they instead wire the power good output to one of the 5 V lines. This means the processor will never reset given bad power unless the 5 V line drops low enough to turn off the trigger, which could be too low for proper operation.
PG Values
Power good value is based on the delay in msMillisecond
A millisecond is a thousandth of a second.10 milliseconds are called a centisecond....
, that a power supply takes to become fully ready. According to Cool Max, manufacturer of an LCD power supply tester model PS-228, power good values are often considered abnormal if detected lower than 100ms or higher than 900ms.