Hardware Reference
In-Depth Information
tem) on or off via software. It is sometimes known as the
soft-power feature
. PS_ON is
most evident when you use it with an operating system (OS) such as Windows that sup-
ports the Advanced Power Management (APM) or Advanced Configuration and Power
Interface (ACPI) specification. When you shut down a PC from the Start menu, Windows
automatically turns off the computer after it completes the OS shutdown sequence. A sys-
tem without this feature only displays a message that it's safe or ready for you to shut
down the computer manually.
The Power Good Signal
In addition to supplying electrical power to run the system, the power supply ensures that
the system does not run unless the voltages supplied are sufficient to operate the system
properly. In other words, the power supply actually prevents the computer from starting
up or operating until all the power supply voltages are within the proper ranges.
The power supply completes internal checks and tests before allowing the system to start.
If the tests are successful, the power supply sends a special signal to the motherboard
called Power_Good. This signal must be continuously present for the system to run.
Therefore, when the AC voltage dips and the power supply can't maintain outputs within
regulation tolerance, the Power_Good signal is withdrawn (goes low) and forces the sys-
tem to reset. The system does not restart until the Power_Good signal returns.
The
Power_Good
signal (sometimes called
Power_OK
or
PWR_OK
) is a +5V (nominal)
activehighsignal(withavariationfrom+2.4Vthrough+6.0Vgenerallybeingconsidered
acceptable) that is supplied to the motherboard when the power supply has passed its in-
ternal self-tests and the output voltages have stabilized. This typically takes place any-
wherefrom100msto500ms(0.1-0.5seconds)afteryouturnonthepowersupplyswitch.
The power supply then sends the Power_Good signal to the motherboard, where the pro-
cessor timer chip that controls the reset line to the processor receives it.
In the absence of Power_Good, the timer chip holds the reset line on the processor, which
preventsthesystemfromrunningunderbadorunstablepowerconditions.Whenthetimer
chip receives the Power_Good signal, it releases the reset and the processor begins ex-
ecuting whatever code is at address FFFF0h (occupied by the motherboard ROM).
If the power supply can't maintain proper outputs (such as when a brownout occurs),
the Power_Good signal is withdrawn and the processor is automatically reset. When the
power output returns to its proper levels, the power supply regenerates the Power_Good
signal and the system again begins operation (as if you had just powered on). By with-
drawing Power_Good before the output voltages fall out of regulation, the system never
sees the bad power because it is stopped quickly (reset) rather than being allowed to op-
