Hardware Reference
In-Depth Information
erate 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 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 in-
cluded with this topic.
See
“
Parity and ECC
,”
p.
345
(
Chapter 6
,
“
Memory
”).
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 prop-
erly 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,youlikelyhaveaproblemwiththePower_Goodtiming.Youshouldinstallanew,
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
problemsareoftentheresultofimproperPower_Goodsignaltiming.Acommonexample
iswhenyoureplaceamotherboardinasystemandthenfindthatthesystemintermittently
fails to start properly when you turn on the power. This can be difficult to diagnose, es-
pecially 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
