which CPU?

[florin]

I want to use either an Intel or an AMD processor, and reduce the frequency and/or the core voltage enough so that i can put a fanless heatsink on it.
Eliminating the fan is the main target.
Cheapness is another one, which rules out "mobile" CPUs.
Enough CPU power is another goal, which means i have to start with a 2+ GHz CPU and underclock it a bit, but keep it above 1GHz.

I'm looking at two main families of CPUs:
- AMD AXP AXDA 2.0 ... 2.4 GHz (Barton or Thoroughbred)
- Intel Celeron or Celeron-D at approx 2.4 GHz

Which CPUs are more appropriate for "underclocking"?
Will "underclocking" (less GHz, less core voltage) cool off the CPU enough to allow me to yank the fan out the heatsink?

Thanks,

[acaurora]

You have to consider the capabilities of the Celerons vs the P4s/Athlons. While the celerons are MUCH cheaper, they pretty much can't do heavy work, i.e., Doom 3. They can't really do anything other than email, internet, pretty much that's all. I am guessing from that assumption, that the celerons therefore use less power. p4s run cooler than athlons, according to what i've heard, and bartons are cooler than thorobreds. So... just to get that out there.

[Rusty075]

You'll should probably look seriously at the AMD CPU's. In terms of underclocking/undervolting they just have many more options available to them than the Intel CPU's do.

And you may want to re-examine your thinking about mobile CPU's being more expensive:

Mobile XP2400 @Newegg = $89. There's a chip that will run stock at 35watt, undervolt down to the single digit wattages, and is almost guaranteed to overclock into the 2.0-2.3Ghz range.

But I don't know if I would even worry about OC'ing it, if a 2.0-2.4 Cel-D is enough performance for you. The XP2400, at its stock speed of 1.8Ghz will outperform the 2.4Ghz Cel-D....all while producing less than half the heat..all for just $10 more than a 2.4Ghz Cel-D.

[silvervarg]

I'm looking at two main families of CPUs:
- AMD AXP AXDA 2.0 ... 2.4 GHz (Barton or Thoroughbred)
- Intel Celeron or Celeron-D at approx 2.4 GHz

The trick to keep high (or at least decent) overall speed is to keep the FSB fairly high. Since you will need quite a big reduction in speed the only reasonable option is to reduce the multiplier. This rules out all Intel desktop CPU's and all new AMD Throughbread and Barton desktop CPU's.

I suggest that you try to get hold of an old unlocked AMD (preferably a Barton) or buy a new AMD mobile Barton 2500+.

Keeping 1.0GHz (~XP 1500+ speed) is quite close to what you can manage with good convection cooling with a cheap Barton. Similar performance is possible with an Intel P4 clocked to 2.0GHz (with much lower FSB). The Intel solution will cost a lot more, since you probably need at least a 3.0C to start with.
~1.0GHz (7.5*133) is possible, but on the boarder of what can be cooled with convection, so you need to work a bit to get optimal convection cooling. If you can settle with something slower, like 6.0*133 you should be able to get things going with much less work.

[florin]

And you may want to re-examine your thinking about mobile CPU's being more expensive:

Mobile XP2400 @Newegg = $89. There's a chip that will run stock at 35watt, undervolt down to the single digit wattages, and is almost guaranteed to overclock into the 2.0-2.3Ghz range.

This is very neat! Now here's my question: that mobile Barton runs at a very low voltage (1.35V). I am looking for a very cheap motherboard, to minimize total cost, such as one of these.
Would there be any problem because of the low voltage? Are there special motherboards required for low-voltage CPUs? Or just pretty much any mobo will reconfigure itself properly?

More general, is there something on the CPU that tells the motherboard "thou shalt use such-and-such core voltage"?

[Nice Marmot]

If you look through anandtech.com's motherboard roundup reviews, they list min and max vcore (and stepping unit) for each board.

[greeef]

It's worth getting something like the NF7 or NF7-m with integreated video if your making a cheap system.

griff

[msm_zgok]

Nice Marmot, why even bother browsing all those pages at anandtech when SPCR has its own Undervoltable Motherboards list?

[florin]

And you may want to re-examine your thinking about mobile CPU's being more expensive:

Mobile XP2400 @Newegg = $89. There's a chip that will run stock at 35watt, undervolt down to the single digit wattages, and is almost guaranteed to overclock into the 2.0-2.3Ghz range.

This is very neat! Now here's my question: that mobile Barton runs at a very low voltage (1.35V). I am looking for a very cheap motherboard

Ok, i've found this one:

http://www.newegg.com/app/viewProductDe ... ory=BROWSE

It's a DFI AZ30-TL (KM266A chipset) motherboard. If you follow the Product Link on newegg.com, go to the manufacturer and download the manual, it is stated that it is "equipped with a switching voltage regulator that automatically detects 1.100V to 1.850V".

So, while not an undervoltable board per se, at least i know for sure that it supports the low 1.35V required by the mobile Barton. That should be enough, i think - if i lower the FSB frequency a bit, that should make the Barton cool enough to run without a fan.

...or would it?

[Rusty075]

What worries me about that particular board is the "automatically detects" portion of the description. It may see your mobile chip as just a desktop chip, and assign it the default desktop chip voltage....and not allow you to change it.

The total wattage of the CPU varies linearly with the speed, but with the square of the vcore.

An example: Take a 2500 barton. By default, in desktop configuration, it runs at 1833Mhz and 1.65v. In mobile form, it runs at 1833Mhz, but only 1.35v. The desktop 2500 puts out 68.3 watts of max heat, the mobile, 45 watts.

If you couldn't drop the vcore on the desktop chip, you would have to drop the FSB to 1225Mhz to get to the same wattage.

Undervolting doesn't reduce performance, underclocking does, so I'd be real sure that what I was buying would allow me undervolt.

Take a cruise through this long thread for lots more information on specific motherboards that people are using: Mobile Barton 2500 - owners and wannabe owners...

[Jordan]

I'v built a few 1.45V XP-M systems around various motherboards and all register the chip as a 500MHz CPU on 1.45V (1.575 for the Asus A7N8X). Go into your BIOS and see if you can maually adjust settings like VCore, FSB and Multi.

[silvervarg]

Florin, just as Rusty pointed out the "automatically detects" is not good enough. What you need to get is a board where you can manually select yout vcore in bios.
Check out SPCR Undervoltable Motherboard list.
If you don't like to take a big risk I suggest that you only buy a board that is on that list and that can undervolt down to 1.1V.

If you want to go cheap and fanless you have two options:
1. A board with integrated video.
2. Using a cheap low performance passive video card.

The problem is that since this is cheap solutions they will in stock form come with either an extremely small heatsink and a small fan on the video chip or possibly a small heatsink and no fan. If it comes with a small heatsink it may be designed to require some forced airflow in the case, and you don't want that.
So you better be prepared to change the heatsink on your video chip to be able to run passively. After lots of carefull consideration I picked the Abit NF7-M board (I signed up that board on SPCR undervoltable motherboard list). I put a Zalman 6000Cu cooler on the IGP northbridge chip, and it work great.