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- Overclocking a Sempron754 on an Asus K8S-MX
- Felger Carbon
May 11, 2005, 8:32 pm
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Yes, you can overclock a uATX board with built-in video. The K8S-MX
mobo allows 125% overclocking the CPU, which I've done on my
Sempron754 2600+, now running at 2.0GHz instead of its normal 1.6GHz.
I figured as long as AMD had pre-overclocked the CPU voltage (1.5V
instead of 1.35/1.4V) I might as well take advantage.
When overclocking a K8 with a locked multiplier (like mine) you have
to look out for the HT I/O bus and the DDR memory, whose clocks will
move in lockstep with the CPU frequency.
First, the HT: Standard on this SIS-based uATX is 800MHz. You have
two choices. I tried both and they both worked. The conservative
approach is to say that Asus knew what it was doing by limiting the HT
to 800MHz. So, back the HT setting to 600MHz, and when the CPU is
bumped up the max, the HT will be at a conservative 750 MHz. The
other thing is to realize that Asus is probably trying to protect
K8S-MX users from themselves. So they limited the HT to 800MHz so
that when the silly user overclocked to max, the HT would rise to
1GHz - which the SIS chip will run fine at. Like I said, I tried both
and they both worked.
The memory is not as simple. I assume everybody will start with
DDR3200, called "200" in the bios because that's the actual clock MHz
applied to the DDR sticks. A simplistic approach would be to set the
DDR clock to 166. That way, when you overclock to 80% of max, the
actual DDR clock will be back up to an even 200 again. To overclock
to max, you have the option of setting the DDR clock to 133 (boo!) or
pushing the DDR spec by allowing the 166 (166.667) setting to be
overclocked to 208.33MHz. If you take this choice, you can increase
the DDR voltage, normally 2.5V, to 2.6V.
That's all the problems, right? Wrong. The problem is, the mobo's
bios is too damn smart.
Say you initially run the unit with vanilla settings. No
overclocking. Then enter the bios, select manual CPU clock frequency
but leave the setting at the normal 200. Now you get to see the
default reading of the SPD roms on the DDR sticks; the CL, for
instance will probably be 3 (as mine was). Well, if you manually set
the DDR clock to 166, the mobo bios is smart enough to know that, at
166MHz, your DDR can operate at CL=2.5, and will make that adjustment!
<-- **big problem**
So when you then increase the CPU clock to 250, the DDR stick will be
at 208.33 and CL=2.5, which for a DDR stick that's nominally 200 and
CL=3 is very, very bad.
The reason you initially run the unit with vanilla settings, no
overclocking, is to determine the SPD values of the DDR parameters.
Then manually enter all of them, except 166 for the DDR clock. That
way, when you overclock the CPU to 250, the DDR stick will be at
208.33 and CL=3 (if that was the default). Don't forget to set the
DDR voltage to 2.6v.
Yes, fellow penurious overclockers, inexpensive uATX mobos _can_ be
overclocked. Even the ones made by Asus with bioses that are too
clever by half. ;-)
The Sempron754 2600+? No sweat. It could run lots faster, if the
mobo would allow it.
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