[hwg]

If I have a bit of time I could maybe do that. However, I'm far from an expert on overclocking. Just an EE who designs circuit boards and knows some of the general issues.

To expand a bit on my previous post, all chips are designed to work at some nominal temperature. Of course they work over a wide range of temps, but they are designed for best performance at a certain temp. Now, I don't design microprocessors, but I do use them in my designs. My guess is that the chip designers are trying for best timing margins at the temperatures the chip is most likely to see. Let's say the average CPU is going to be in a box at room temp (20 to 25C). You have a heat sink and fan, and the chip is generating heat, so its internal temperature is going to be higher, lets say 50C, just as an example. So the chip designer is going to design for max timing margins at 50C.

Now, when you overclock it, of course you have to dump out the extra heat you generate from running it faster, so that the chip does not overheat. So you use more exotic cooling. But I'm not sure you want to cool it TOO well. (My comment about dunking the thing in LN2 was kind of tounge-in-cheek...) If the chip is designed for best timing margins at an internal temp of 50C, and your fabulous cooling has it running at an internal temp of -50C, the chip just might hit the limit on its internal timing sooner than it would at 50C. My basic point is that colder is not always better. It is usually better up to a point, but beyond a certain point, you might be making things worse by going colder.

I have personally seen chips (not CPU's, but other digital devices) that work fine at room temp, but start to show flaky behavior at -5C. And that's -5C ambient, so the chip internal temp was probably above 0.

Generally speaking, semiconductors do better at cooler temps. They use less power and are able to run a little faster. But that only goes so far. If you take it too far out of its designed temperature range, it might get worse, especially at the hairy edge of 5GHz and beyond, where the timing margins are miniscule to begin with.

If I was going to dive into overclocking, I would probably do an experiment, where I used something like a TEC (thermo-electric cooler) with a control loop around it, to control the chip temperature as accurately as possible. Then I would do a bunch of test runs at different temperatures and plot Fmax vs. Temperature (the internal CPU temp). I would expect to see Fmax (the maximum CPU speed) peak somewhere in the neighborhood of 0 to 20C, and get worse on both the hot and cold ends. I've never done anything like that, so that's a pure guess. If anyone here has done that experiment, I'd be interested in seeing the results.

Once again, I'm no expert in overclocking. You guys are doing some very cool stuff (no pun intended). Good luck on all your projects.

hwg