On Thu, 6 Jul 2006, Joseph Harrison wrote:
... I had a 512MB PC2100 in the machine but it started
reporting that it was a 256MB with no errors, and when I ran
the memcheck soft it showed 512 available butonly 256 in use,
and when forced to run all 512 there were massive errors. It
ran for 6 weeks at 256 in use, but then quit and will not
boot or show bios, guess its dead.
Here is something that may or may not help with that board's
memory. The default dimm voltage is 2.8v, not 2.5v the specs
call for. The following may not apply to your specific board
revision however.
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<http://www.lostcircuits.com/motherboard/asus_a7m266/4.shtml>
V I/O and V I/O1
"V I/O usually refers to the 3.3V supplied to the DIMM slots,
here it is referring to the chipset (North Bridge) which, for
simplicity reasons, was originally designed to run at the same
voltage as the DIMMs (some other chipsets are running at 1.4 or
1.7 V). With the move to DDR and its lower operating voltage of
2.5 V (VDDQ), the voltage regulators needed to be redesigned to
provide both a 3.3V (adjustable) and a 2.5 V (adjustable).
Therefore, the new nomenclature calls V I/O the chipset voltage
and V I/O1 the memory voltage. The jumpers for each setting are
in the path of the AGP slot (V I/O) and above the IDE interface
(V I/O1). Interestingly, while ASUS adheres to the guidelines
for the V I/O voltage of 3.3 (scalable to 3.56) V, V I/O1 is
set slightly out of specs, that is, the base setting is 2.8V
instead of 2.5V with the option increasing it to 2.9V or
decreasing it to 2.7V. Most current DDR chips are based on a
0.17m die process, thus, the internal DIMM voltage is regulated
to some 1.8V and only the wave form of the output signal is
sharpened. This can speed up the DIMMs but also cause unwanted
effects like signal ringing, particularly at low load, that is
lower total system memory. On the other hand, some DIMMs still
require slightly higher voltage than others.
The drawback is that any excessive voltage can cause thermal
runaway or local heat buildup in the memory chips causing their
performance to deteriorate quite dramatically. Undocumented in
the manual is the option of completely removing the jumper from
the V I/O1 header which will bring down the voltage to 2.5V and
thus DDR specification. This point will later become extremely
important as I will show below."
<http://www.lostcircuits.com/motherboard/asus_a7m266/6.shtml>
Compatibility
"At the factory default setting, the A7M266 is somewhat picky
regarding memory. Some DDR DIMMs that would run 145 MHz on the
VIA Apollo Pro266 chipset based Shuttle AV30 were not happy at
all under overclocked conditions, some would not even run
continuous stress tests at 133 MHz. After testing 8 different
DDR modules, some kind of a trend became obvious, that is, all
Micron-chip based modules were running great. Most Samsung
chip-based DIMMs would boot into Windows at 133 MHz but fail
under load. Hyundai-chip based DIMMs would run stable up to 136
MHz but not clock higher. The main problem with this pattern is
that the 64 Mbit Micron chips used on the test samples will not
go into serial production, in addition, Micron/Crucial does not
sell PC2100 at this point."
The Cure
"By completely removing the jumper from the VI/O1 header, the
VIO1 is reduced to 2.5V and, thus within the DDR specs.
Interestingly, the Micron-based DIMMs lost some overclocking
capability (139 MHz stable). However, all other DIMMs were now
running 100% stable at stock FSB (133 MHz) and would overclock
quite a bit. Still the pattern of the ASUS A7M266 not liking
Samsung chips cannot be disregarded."
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