Hardware Reference
In-Depth Information
however, most newer fans use the lower 100mA figure. Note that, although most fans in desktop
systems run on +12V, portable systems can use fans that run on +5V or even +3.3V.
Systems with modern form factors based on the ATX or BTX standards include another special
signal. This feature, called
PS_ON
, can turn the power supply (and thus the system) 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 supports 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 system 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withdrawn (goes low) and forces the system to reset.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) active high
signal (with a variation from +2.4V through +6.0V generally being considered acceptable) that is
supplied to the motherboard when the power supply has passed its internal self-tests and the output
voltages have stabilized. This typically takes place anywhere from 100ms to 500ms (0.1-0.5
seconds) after you turn on the power supply switch. The power supply then sends the Power_Good
signal to the motherboard, where the processor 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 prevents
the system from running under bad or unstable power conditions. When the timer chip receives the
Power_Good signal, it releases the reset and the processor begins executing 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 withdrawing 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 operate using unstable or improper power levels, which
can cause memory parity errors and other problems.
Note
You can use the Power_Good feature as a method of implementing a reset switch for the PC.
The Power_Good line is wired to the clock generator circuit, which controls the clock and
