Picking a low-power CPU

CtrlAltDel · Post by **CtrlAltDel** » Thu Sep 02, 2004 11:49 am

I'm planning to build an HTPC box, and am having a few problems speccing it up. It's going to be replacing my Linux server, which is based on a VIA C3-933, which draws about 8W on max load, but being a Socket 370 means that the components on the motherboard are fairly low tech (no USB2 or Firewire or digital sound)

Would people go for something like a Pentium M or Athlon XP-M? Ideally I would go for a fully integrated motherboard, as it solves problems such as the vertical height taken up by add-in cards.

Given that the box is going to be running 24x7, mainly serving up email and web, with the occasional bit of TV (aided by a Hauppage PVR250), MP3 and DVD, the idle power drain needs to be as low as possible (and of course that'll govern how much noise it makes). What do you think?

POLIST8 · Post by **POLIST8** » Thu Sep 02, 2004 12:41 pm

You should try on an AMD 64m. They run pretty cool, especially the 35w ones. You can find tons of info about moving into this sector here:

http://forums.anandtech.com/messageview ... id=1321894

sthayashi · Post by **sthayashi** » Thu Sep 02, 2004 12:44 pm

I would recommend the XP-M. With the right Motherboard, you can use 8rdavcore to reduce voltage and power even further.

Tigr · Post by **Tigr** » Thu Sep 02, 2004 12:51 pm

I would not see a point in having a Athlon 64 for the job. This system requires more features on the motherboard than the CPU power, right? So I guess you could take the Athlon XP-Mobile and undervolt and underclock it. Mine is running passively cooled at 200x5=1GHz in a Abit NF7-S board. I am very happy with the setup and can recommend it. However, you probably need to look for a board with integrated video. Maybe Abit NF7-M with integrated GeForce MX would be the right one for the job?

Linus · Post by **Linus** » Thu Sep 02, 2004 2:01 pm

I recently put a Shuttle MN31N with an unlocked Athlon XP 1700+ into a Silverstone Lascala SST-LC02, with low power consumption as a prime consideration (worked out some of the details here). Unfortunately, 8rdavcore doesn't seem to work for adjusting anything but FSB on this motherboard, and I can't seem to get CPUMSR to change the multiplier or vcore, either.

I'm trying to decide which motherboard to try next, probably either the Biostar M7NCG 400 or the Abit NF7-M. The Biostar is cheaper and smaller (microATX), but the NF7 is on 8rdavcore's compatibility list. Neither has firewire, so I would need to buy a 1394 card and use up one of the case's two PCI slots.

By the way, the Silverstone case is great if you're looking for something low-profile - it uses riser cards to flip full-sized PCI cards horizontally. ATX motherboards just barely fit, and it only takes laptop optical drives.

jojo4u · Post by **jojo4u** » Thu Sep 02, 2004 2:37 pm

http://forums.silentpcreview.com/viewto ... ight=power schould give you some hints.

CtrlAltDel · Post by **CtrlAltDel** » Thu Sep 02, 2004 11:08 pm

So it seems the consensus is to fit an XP-M - are they just straight drop-ins into a Socket A motherboard, or is there something particular to look out for (apart from the ability to underclock, which is annoying as that rules out the wonderful A7N8X (which I already have for my desktop)) ?

Is it the chip that does all the clever power management, or is there circuitry required on the motherboard to do it?

sthayashi · Post by **sthayashi** » Thu Sep 02, 2004 11:21 pm

Actually, the A7N8X SHOULD have the ability to underclock, at least to a limited extent. You can/should be able control the multiplier value and/or the FSB, and the A7N8X SHOULD be able to handle the old 100 MHz FSB chips.

It just won't be able to undervolt. Though it's not clear to me how accurate it is in finding the stock voltage.

Anyways, I believe a motherboard has to be able to support 1.35 and/or 1.45 V to use the mobile processor. The reduced voltage and clock is how the chip can be used in a low power configuration. I have no idea if the A7N8X can handle that (probably not).

BTW, there is one caveat in going with XP-M for your needs. I don't think it will be able to play HD video well. I don't know for certain since I don't have an HD setup yet, but that's one problem I anticipate running into in the future

CtrlAltDel · Post by **CtrlAltDel** » Thu Sep 02, 2004 11:34 pm

sthayashi wrote:BTW, there is one caveat in going with XP-M for your needs. I don't think it will be able to play HD video well. I don't know for certain since I don't have an HD setup yet, but that's one problem I anticipate running into in the future

Don't worry, I won't be able to afford an HDTV for a while yet

Thanks for all the good advice!

Tigr · Post by **Tigr** » Fri Sep 03, 2004 12:06 pm

The Asus boards run with the default voltage of the CPU as the minimum. In other words, they detect the voltage that CPU requires and only allow to change it upwards. This is a major drawback with XP-M processors, because the bridges are configured differently for desktop and mobile CPUs. Therefore, the board detects the CPU as requiring 1.45V (IIRC) and that vecomes your minimum. At 1.45 V, the CPU runs hotter than it can. It is much better to find a motherboard that allows manual setting of voltage from at least as low as 1.1 V. The XP-M at 1.1 V is a much cooler CPU than at 1.45 V.

Regarding the HD video, I think the front bus speed there is much more important that the actual CPU Hz. Since you can run XP-M at 200 FSB, as opposed to 133/166 of the regular desktop chips, the CPU appears to be faster than it really is for the same clock speed. Of course, there is still a lower limit at which the CPU can process the stream, but at least you will not be hampered by the slower memory interface.