Severely underclocking/undervolting a Thoroughbred XP2000+

chrispitude · Post by **chrispitude** » Sun Apr 13, 2003 2:46 pm

This should be a pretty interesting read for most of you.

I am building a video deinterlacing/scaling HTPC (Home Theater PC) machine. The machine uses the ATech Fabrication Heatsink Case, which features small size, excellent airflow management, and an audio component look. It will feature Immersive Inc.'s Holo3DGraph Deinterlacer/Scaler card, with a MP1-modified fanless Radeon 9000 for optimal video output to the ceiling-mounted projector.

You can see a photo album of my construction progress here. I have pictures specifically of the ATech Fabrication case here.

In a dedicated home theater environment, the name of the game is "quiet." I am trying to eliminate as many fans as possible. I chose the Abit NF7-S motherboard because of its ability to undervolt all the way down to 1.1V Vcore. With a 0.13um Thoroughbred XP2000+, I've gotten the machine to run at 933MHz (133MHz x 7.0) with a CPU core voltage of only 1.175v. I can run CPU burn tests indefinitely with no errors. At these settings, the CPU generates so little heat that it takes several minutes for the retail HSF to get warm if I unplug the fan! This should work out very well when I switch to a fanless Thermalright SLK800 heatsink, with some air directors added so the case exhaust fans pull air through the heatsink.

However, I'm having one fairly serious issue which I can't figure out. The machine posts fine if I reset, or if I turn the machine on and off with the ATX power button. However, if I physically unplug the power cord and plug it back in, the machine fails to post when I turn it on. Through further testing, I have determined that the machine will only post after power removal if the CPU core voltage is set to 1.325v or greater.

I'm not quite sure why there would be a difference between unplugging the machine and simply turning it off via the power button. Sure, an ATX power supply provides standby power, but why should this affect the machine's power-up?

- Chris

o0Dreadite0o · Post by **o0Dreadite0o** » Sun Apr 13, 2003 3:52 pm

I have no idea about any of the problems your having (ive never done any undervolting) but why dont you run at the highest FSB and the lowest multiplier you can get? All the bandwidth you can get at that low mhz would be helpful, and that NF7 can take some high FSB's.

Just my thoughts. =P

Asmordean · Post by **Asmordean** » Wed Apr 16, 2003 5:58 pm

Sounds like you might have a weak CMOS battery. Try sticking a new one in there. They are usually CR2032

chrispitude · Post by **chrispitude** » Sat Apr 19, 2003 6:59 am

Thank you for both of your replies. The FSB performance isn't as much of a concern with this machine as heat generation, as the PC will be in a shelving enclosure where heat buildup is a concern. I had a second NF7-S motherboard from a second machine I was building, which I swapped into this machine. I had the same problem - machine would only cold start with a Vcore of 1.325 or greater. This makes me not suspect the battery, because as long as the Vcore is high enough, the machine can be unplugged and all the settings are preserved.

I really do suspect this is just a bug or limitation in Abit's cold start power-up sequence, which is really unfortunate. The thing was rock-solid stable at 1.175v, and generated virtually no heat even without a CPU fan.

Thanks to everyone for their help!

- Chris

powergyoza · Post by **powergyoza** » Sat Apr 19, 2003 12:55 pm

Just a quick question. What's the need to be able to cold-cold-start the HTPC?

I agree that any cpu that can run at 1.175 should be able to cold-cold-boot at the same voltage.

BTW - That's a nice looking amp you have in your pic gallery? I keep forgetting really how big those transformers get in the hi-fi world.

deebass · Post by **deebass** » Thu Apr 24, 2003 10:34 am

On older Abit motherboards, VIA based chipset, you can boot up at 1.1V. I've tested KR7A-Raid and KX333-R
With a 1700+ JIUHB I've got similar figures. At speeds of 1Ghz and 1.2V I can run it fanless, but I need a little airflow in the case.
What is more impressive is that I can run it at its stock speed with only 1.25V, and overclocks up to XP2000+ (those motherboards can't set the multiplier higher than 12.5x) with only 1.4V.