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MOTHERBOARD IDIOSYNCRASIES
The Asus P5W DH Deluxe motherboard is a top-of-the-line
product, and is priced accordingly. It has been the favorite of
Conroe overclockers, and at the time of this writing (mid-September
2006)
has been on constant backorder for over a month due to this popularity.
It does, however, have some oddities, many of them related to
cooling and overclocking.
The most glaring problem is the shiny copper-colored covers on
the
north and south bridge heat sinks (the ones that say "Digital Home" and
"ASUS"). These must
be removed; they completely obstruct any airflow through what are
otherwise pretty good heat sinks. They are very easy to remove.
Second, the stock south bridge TIM is low quality and really
should
be replaced. This is fairly straightforward. With the board out of the
case, use needle-nose plyers to squeeze the push pins on the back of
the board, which allows them to slide through the holes in the
motherboard.
Then use a large flat screwdriver levered on the blue IDE connector to
pry up the south bridge heat sink. It will let go with a small "pop".
Pry off the south bridge heat
sink to replace its TIM (release the push pins first).
Here you see why this TIM should
be replaced.
Next, replace the TIM. As always when replacing TIM, clean the
chip and heat sink
thoroughly then apply the smallest amount of new TIM that will cover
the chip. The push pins snap through the board and hold the heat sink
firmly in place. When done, the heat sink looks much more competent:
South bridge heat sink with cover
removed.
If you plan to keep the stock north bridge heat sink, you
should
replace its TIM as well. The heat sink and related VRM radiator are
held onto the board with four push pins. Take care while cleaning the
heat sink not to damage the foam spacer that fits around the north
bridge chip.
There is a fully illustrated how-to for the above steps here.
The temperature readings on the P5W DH are very unusual. The
temperature reported by the supplied PC Probe 2 software, and by SpeedFan,
as "motherboard" is actually an on-chip sensor inside the south bridge.
The temperature reported as "CPU" is a sensor on the board near the CPU
socket, not the on-chip thermal diode or the DST (digital thermal
sensor,
a new feature in the Core 2). As a result, the reported motherboard
temperature is unusually high, and the reported CPU temperature is
exceptionally low. Also, there is a bug in Probe 2 (or perhaps the
BIOS) that causes it to occasionally report a CPU temperature of 256C.
There are two reasons why the on-board CPU temperature is well
below the actual CPU chip temperature. One is that the motherboard has
a large heavy copper layer around the CPU socket (called Stack Cool)
that draws heat away from the socket very well. The other is that the
conduction from the CPU die to the motherboard is quite poor, as shown
in this illustration from the data sheet:
Why a sensor in the socket can't
accurately report CPU temperature.
The IHS (integrated heat spreader) in the above picture is
nickel-plated copper, and is designed to transmit heat to the CPU heat
sink, which is mounted above it with an external layer of TIM in
between.
The on-chip CPU DST can be accessed with Core
Temp, RMClock,
or Everest
Ultimate Edition,
and is highly accurate. However, it is reported as a negative number
relative to the (secret) temperature at which automatic throttling
starts. Core Temp and Everest use 85C for this temperature, RMClock
uses 97C. In some sense, the absolute temperature is not relevant; any
good CPU cooling system will prevent throttling. All of these
temperatures are well below the 105C junction temperature that
semiconductors are designed to withstand.
The remaining issues with this board are with its BIOS. First,
very
early versions of the BIOS would not POST with Conroe CPUs. This was fixed
in release 0801 and all later releases.
Second, many Asus boards overclock the north bridge, a
feature called Hyper Path 3 in the BIOS. On the P5W DH, this is
done by tying the CPU to the 1066 strap and the north bridge to the 800
strap. This reduces the latency through the north bridge, but causes it
to run very hot, and also limits how fast the FSB can be run. To
successfully overclock this board, Hyper Path 3 needs to be disabled.
There is a detailed explanation of 975X overclocking here.
Third, it is not possible to change the CPU Vcore voltage
unless
Enhanced C1 and SpeedStep are disabled in the BIOS. This is made
explicit in later BIOS versions, but many people had trouble early on.
COOLING THE HARD DISKS AND
POWER SUPPLY
The P180 case design separates the power supply and
motherboard air
flow using two chambers. The hard disks can be mounted in either
chamber. The lower chamber has a fan in the middle that pulls air
across the hard disk cage and pushes it across the power supply. I put
my disks in the lower chamber.
I chose the Antec
Phantom 500
"semi-fanless" power supply for my system, knowing that its fan would
never come on as long as the case fan was running. The P500 is a
revision of the Phantom 350, which is completely passive.
This makes the P500 larger than a standard power supply, which requires
that the center fan be at most 25mm thick. The stock fan is 38mm thick,
and very noisy, so it was destined to be replaced by a Nexus.
To slow this fan down, I installed a temperature-sensitive NMT-3
fan controller from Noise
Magic.
This controller has an output ranging from 5V to 12V as the temperature
ranges from 28C to 42C when connected to a 12V source, and passes the
tachometer signal through to the motherboard. Regardless of
temperature, it outputs full voltage at startup, to get the fan
spinning. This is needed because Nexus fans do not start reliably below
6V. (I also put an NMT-3 on the top case fan.)
Here is what the lower chamber fan and its NMT-3 look like
mounted on the mid-chamber baffle, ready for installation.
Lower chamber fan, NMT-3
controller, and baffle.
The Phantom 500 derives most of its cooling from air flow
across its
top heat sink. To maximize the effectiveness of this cooling, that area
needs to be open, as well as the back part of the case. Consequently,
all spare power supply cables should be tucked under the supply.
Tuck unused power supply cables
in the bottom to keep the top open for cooling.
The power supply fan activates when the top heat sink reaches
51C.
However, the center fan keeps it well below this, even when spinning as
slowly as 400 RPM. The result is silence.
The case has two sliding panels that allow cables to go
between the
lower and upper chambers. The main one for the power cables has a
rounded notch that seals air flow quite well. The second one for the
disk cables does not. This is simply rectified with a notch so it can
completely isolate
the two chambers. Here's what the slider looks like after being notched:
A notch in the smaller slider
lets the SATA data and power cables through.
So with just one fan and some cable management, the bottom
third of the system is taken care of.
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