Hardware Reference
In-Depth Information
reset lines to the microprocessor. When you ground the Power_Good line with a switch, the
timer chip and related circuitry reset the processor. The result is a full hardware reset of the
system. Instructions for making and installing a reset switch can be found in the section
“Making and Installing a Reset Switch” in the Technical Reference portion of the DVD
included with this topic.
See “
Parity and ECC
,”
p.
357
.
On pre-ATX systems, the Power_Good connection is made via connector P8-1 (P8 pin 1) from the
power supply to the motherboard. ATX, BTX, and later systems use pin 8 of the 20/24-pin main
power connector, which is usually a gray wire.
A properly designed power supply delays the arrival of the Power_Good signal until all the voltages
stabilize upon turning on the system. Poorly designed power supplies, which are found in many low-
cost systems, often do not delay the Power_Good signal properly and enable the processor to start too
soon. (The normal Power_Good delay is 0.1-0.5 seconds.) Improper Power_Good timing also
causes CMOS memory corruption in some systems.
Note
If you find that a system consistently fails to boot up properly the first time you turn on the
switch, but that it subsequently boots up if you press the reset or Ctrl+Alt+Delete warm boot
command, you likely have a problem with the Power_Good timing. You should install a new,
higher-quality power supply and see whether that solves the problem.
Some cheaper power supplies do not have proper Power_Good circuitry and might just tie any +5V
line to that signal. Some motherboards are more sensitive to an improperly designed or improperly
functioning Power_Good signal than others. Intermittent startup problems are often the result of
improper Power_Good signal timing. A common example is when you replace a motherboard in a
system and then find that the system intermittently fails to start properly when you turn on the power.
This can be difficult to diagnose, especially for the inexperienced technician, because the problem
appears to be caused by the new motherboard. Although it seems as though the new motherboard is
defective, it usually turns out that the power supply is poorly designed. It either can't produce stable
enough power to properly operate the new board, or it has an improperly wired or timed
Power_Good signal (which is more likely). In these situations, replacing the supply with a higher-
quality unit, in addition to the new motherboard, is the proper solution.
Power Supply Form Factors
The shape and general physical layout of a component is called the
form factor
. Items that share a
form factor are generally interchangeable, at least as far as their sizes and fits are concerned. When
designing a PC, the engineers can choose to use one of the popular standard PSU form factors, or they
can elect to build their own custom design. Choosing the former means that a virtually inexhaustible
supply of inexpensive replacement parts will be available in a variety of quality and power output
levels. Going the custom route means additional time and expense for development. In addition, the
power supply is unique to the system and generally available for replacement only from the original
manufacturer. This precludes any upgrades as well, such as installing higher-output replacement
