A while back I wrote a few notes on building a small home server box. Specifically that I had chosen this neat Morex case and this VIA EK Corefusion motherboard.
Here in southern Europe the summer has been in full swing, and it got hotter by the day. A few weekends back I thought I’d do a quick check on temperatures on this system to make sure it wasn’t about to burst into flames.
Logged in and ran ‘sensors’. Yikes! CPU at 77 degrees C, and the chassis temp only a couple of degrees lower.
Went over to the box and touched it. Very warm. Listened. No sound of a fan. Now the motherboard is fanless (in the 800MHz version I chose for this very reason) but the Morex case has a single internal fan which I was pretty sure I had wired in to the system/case fan header on the board. Peering in the side with a torch I confirmed that it most definitely wasn’t turning.
Investigation
Power off the box (very inconvenient as it really is a busy little thing) and take it to the workbench. Off with the lid. Poke around. Looks OK. Almost no dust in it at all (which was odd – most of my systems have half a sheep inside them.) Also checked the fan was still wired to the header.
The system runs completely headless, so connect up a spare screen and keyboard and power it on. Notice that the fan spins for the first few seconds of power, then the BIOS cuts it off. So in to the BIOS to see what it thinks is going on. Interesting…
VIA Corefusion BIOS Settings
The BIOS allows temperature monitoring to be simply enabled or disabled for CPU and/or chassis. If either goes over the threshold, the corresponding header is powered. And upon looking I see that the threshold it fixed at… 80 degrees. Wow. That’s flippin’ hot.
Each 80 setting also comes with what they call a “tolerance” number, of 0 up to 3 degrees. What the hell is that?
Anyway, I pack it all up again and set the threshold On and the tolerance to ‘2’. The system then creeps up to about 70 degrees after a few minutes (and we’re talking an ambient temperature of about 28 degrees for starters) And no fan.
Which header? CPU versus chassis temp
Having monitored the system for a while, I see the pattern of CPU temperature versus chassis temperature. Rather reasonable when one thinks about it: CPU temperature of course ramps up and down very quickly indeed in direct response to utilisation. Chassis temperature moves more slowly, both up and down. So you can have a chassis AND CPU temp of (say) 60, then the CPU peaks up to 75 for a couple of minutes. During this time the chassis temp might only pop up a degree or two. And when the CPU utilisation drops to nothing, the CPU temp can even drop below chassis temp for a little while. Of course the longer the CPU stays at a higher temperature, the closer the chassis temperature approximates to it. In the longer term they will be in constant proportion. But in the short to middle term they are not.
Hence the choice to change the single case fan from the chassis header to the CPU header. Let it respond to the CPU threshold being breached rather than the chassis temperature.
So what’s wrong? Nothing at all
System in the high 70’s (and that’s Celsius, note) and no fan? BIOS appears to only trigger at 80? And guess what? That’s absolutely right! If I cranked up the CPU to 82 degrees (the fixed threshold of 80 plus the mystical “tolerance value” which I set at 2) the fan kicks in. For the first time in its life. And it keeps spinning until we drop to the chilly depths of 77 degrees.
So we now know two things: the mystery “tolerance” value is actually the hysteresis offset, to stop the fan turning on and off rapidly when the threshold is breached.
The other thing we learn is that these VIA boards are as tough as old boots. Hot, very very hot, old boots. This fanless motherboard simply does not require cooling until we hit at least 80 degrees C. Which is amazing. And a tribute to the designers of this stuff. It’s robust beyond belief. I love it.
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