Overclocking in theory..

[Sampsa]

Hi guys,

All of us know that better cooling on your CPU and raising the operating voltage allows you to raise processor's clock frequency which makes it run faster. All this is called overclocking.

I wonder how many of us really know what happens when we cool down the silicon as cold as possible and why does this allow us to run the CPU faster.

Why I started this thread is because many IT sites have recently wrote news about our liquid helium experiment and many people have posted comments like: "Why does is need to be that cold? Isn't keeping the temperature at 0C enough?". Some people have even tried to explain the theory behind all of this but so far I haven't seen one very good and definitive answer. How deep we have to go? Atom level?

Please post if you have a good knowledge about this topic or know good articles, documents or books about topics related to overclocking or methods / laws of physics. Maybe we can combine a simple but still fairly in-depth answer what is overclocking so that everyone can easily understand why we are cooling our CPUs to extremely cold temperatures.

Let me know what you think?

[filmbot]

I've always thought that cold temperatures was just a mean to an end. The colder the object, the more volts it could tolerate. I know thats over simplifying it but whatever. Did you guys have anymore success using helium over nitrogen with the relatively same volts? I'm not educated enough to go in depth, though I am trying to go through the steps to do so, but that's always just been my outlook.

[Johnny Bravo]

In fairness Sampsa I'm more impressed that the AMD still functioned as a cpu at those temperatures! A few things to note:

• Lower temperatures = lower resistivity. Lower resistance allows for more current to pass along a given cross section of copper
• Switching Frequencies. When a transistor switches there are losses involved in the form of heat etc so as you up the frequency you up the heat load from switching and heat = increase in resistance resulting in more losses
• Exotic materials properties at cryogenic temperatures. I've no idea what AMD are currently using in their silicon lithography processes these days but the electrical behavior of materials at cryogenic temperatures can be remarkably different from that at room temperature.

Another thing is as we increase these frequencies into the GHz the usual digital on/off square wave basically turns into an analogue wave as the ability to charge/discharge the capacitive load fast enough to get a straight edge just cannot to achieved. I'd say the circuits ability to distinguish this level change will also dictate an upper limit to frequencies.

[Sampsa]

Johnny, great start. I'm sure I will be able to get some details from straight from AMD and I hope we get some comments from Intel side.

[Sampsa]

Did you guys have anymore success using helium over nitrogen with the relatively same volts?

Yes, with same vcore switching from LN2 to LH helped to raise the clock frequency 150 - 200 MHz. So just getting the silicon colder helps.

[SF3D]

Johnnys answer was awesome:

# Lower temperatures = lower resistivity. Lower resistance allows for more current to pass along a given cross section of copper
# Switching Frequencies. When a transistor switches there are losses involved in the form of heat etc so as you up the frequency you up the heat load from switching and heat = increase in resistance resulting in more losses.

Less resistance allow more current and more clear signal. Higher switching speed allow to do more operations / calculations in certain time (higher clockspeed).

This is the simple answer, but I would like to hear really good and in depth answer from some guru.. like DRwho or some engineer who works with these parts.

[fiveprime]

I've always wondered if (excluding cold bugs) if there is not a best temperature at which to overclock a given processor at a given voltage. I know typically the colder the better but maybe there is a sweet spot, this may be something worth investigating with these AMD chips.

[TheKarmakazi]

A very cool idea. Im no engineer or guru by any means, but I also thought that extreme cold limited electron migration.

I've always wondered if (excluding cold bugs) if there is not a best temperature at which to overclock a given processor at a given voltage. I know typically the colder the better but maybe there is a sweet spot, this may be something worth investigating with these AMD chips.

AMD sweetspot is 1°K

[Johnny Bravo]

I work in a team at the materials lab here in the uni aand at our meeting today brought this up as an end discussion here's what we decided:

• limiting factors of making a transistor switch faster basically boil down to your drive strength and the capacitance.

• The drive strength we can adjust with the Vcc aka vcore

• The capacitance goes down with transistor size so the smaller the process the theoretically faster the transistor can be charged/discharged

When temperature was brought in it proved quite interesting. From a bulk view the lower the temperature the less electrons will be shooting off in random directions colliding and generally not flowing smoothly (resistance). Again the faster the switch time the bigger the current spike and the greater the thermal losses for that switch so excess heat will be an issue, one that cooling would take care of. Also we can get technical an invoke some transistor equations to relate kT to drive strength but it's a bit obscure to directly relate it to increasing the frequency. I'll hopefully get the equation rewritten in terms of temperature soonish

[Kean]

Johnny has right.
Some more:
The main guiltys are noises. When bus or transistor is working faster then normal operation speeds, there is much more noises.
There is something like heat noise. It happens when temp. are high, and it is lower when we lower temps.
Eg. when we adjust some voltage we are trying to avoid this noises, because there is higher potential, and heat noises causes by heat (because of current and resistance which generate heat) are relatively lower, and then devices can understand where is logical 1 and 0 (when noises are present, its difficult to recognise where is 1 and 0.

[Johnny Bravo]

Johnny has right.
Some more:
The main guiltys are noises. When bus or transistor is working faster then normal operation speeds, there is much more noises.
There is something like heat noise. It happens when temp. are high, and it is lower when we lower temps.
Eg. when we adjust some voltage we are trying to avoid this noises, because there is higher potential, and heat noises causes by heat (because of current and resistance which generate heat) are relatively lower, and then devices can understand where is logical 1 and 0 (when noises are present, its difficult to recognise where is 1 and 0.

Thermal noise - that was the term I was looking for!

good catch Kean

[V2-V3]

A few years ago i wrote up a wiki on overclocking thermodynamics and electrical properties due to the massive amount of questions from my 8GHz project seen over here: http://overclockingwiki.org/index.ph...iquid_Nitrogen

WorldRecord Overclocking 101

Xelmon wrote: "I could see voltage limiting, because mobos can only do so much.

The heat, @ -15C though, I really cant see it. Care to explain?"
[edit] Temperature

-15c only feels cold in relation to our normal body temperature, the real point at which something is cold is absolute zero or -273C anything above that is simply heat. and most materials begin to show signs of superconducting properties around absolute zero. superconducting is the ability to transmit large amounts of power at very high voltage with out the risk of burning up the lines or having any migration of the electricity (jumping out of the cable into near by objects)

so the colder you get a CPU in theory the more voltage you can pump into it with minimal damage to the circuits.

for example if you were to apply 1.9 Volts to a CPU (lets say a P4 631) with stock cooling the CPU would overheat very fast by overcoming the thermal capacity of the stock cooler. your CPU has a device in it (a thermal couple or diode) that will sense when the core temperature exceeds its safe zone (about +100C) this will tell the CPU to shut down in order to prevent fatal damage.

So why not put a really big copper cooler of dewm on it and pump 1.9 Volts through her?

If you manage to keep the CPU below its thermal threshold using water cooling or Air (lets say we have her at 1.9 Volts and its a nice 30C) an evil bugger known as Electron Migration will come into play.
[edit] Electron Migration

is something Chip manufacturers have been battling since the dawn of time. When you have a very small and very complex electrical component such as a CPU both heat and voltage are directly responsible for instability and damage to the component.

When we look a the very inner working of a CPU it is similar to the traces of a motherboard but on a Scale of (lets stick with the 631) 65nm meaning the transistors that open and close billions of times per second in effect computing lines of code (so you can have your Uber Interweb and... stuff) are actually "65 Nano Meters" wide! this means really really small. well because they are so small there is also not much distance between the "traces" and transistors.
[edit] Thermal Electrical Resistance

When you heat something up you are in effect adding to (in this case) the electrical resistance (meaning electricity has to push harder to get to the other side) so when we think of a CPU if you were at a theoretical (lets forget "minimal kinetic energy" for a moment) absolute zero (-273c) electricity would have no problem moving through the "maze of traces" in a CPU and if we were at say +100C electricity would have to work a lot harder to make it through the "maze".

With electricity being the lazy creature it is, its always looking for the easy way out. (remember those two words Electron Migration?) If two traces are very close to each other in the core of a CPU and one is in essence "Closer to the maze exit" and Mr. electricity is working hard to run through at +100C he will simply cheat and migrate (Jump) over the insulator (wall) to the other trace this causes the CPU to destabilize resulting in a flawed computation ending in a blue screen or shutdown.

Imagine there are hundreds of millions of transistors in a CPU core that is about 1x1cm x 1mm thick and when the migration starts there are usually thousands jumping over causing fatal errors and extreme instability.

The damage electron migration can cause is the death of your CPU or a permanent return of corrupted information. This happens when a Higher voltage trace jumps into a near by lower voltage zone due to its lower electrical resistance and the result is a fried portion of the chip rendering it useless...
[edit] Hardware

If we want to say make our CPU (631) run at 8GHZ we have to take into account every thing listed above and a few more things like our budget Sad !st off we know we need to keep it cool, so we get a nice Team NexGen Rev 2.5 Copper Liquid Nitrogen Pot and a frosty 160L Dewar of LN2 from the local A-L compressed gas company. we solve the problem of Water Condensation with nice Neoprene tape and heating elements. We have a good ole NexGen Full modded ASUS COMMANDO board capable of supplying more voltage than needed. we have a CPU that is a good steeping and can take us to 8GHZ and the Micron D9GKX that can handle at least 1200MHZ at 4-4-4-12 @ 2.7V we have a nice PSU that can push 800W of clean power. a crappy GPU that is PCI (less stress on the FSB) and pulls minimal power from the board.

so now we know why we need the CPU cold but we don't know why we need the extra voltage
[edit] Voltage

The CPU speed is a Frequency! measured in Hz, or kHz (kilohertz, 10^3 Hz), MHz (megahertz, 10^6 Hz), GHz (gigahertz, 10^9 Hz) & THz (terahertz, 10^12 Hz) (thanks wiki). (Hertz not the car place, but rather cycles per second) now in order to have a CPU operate at a frequency and be stable enough for use we have to match a voltage (voltage is acts as the stabilizer for the CPU speed) to carry the signal through the CPU and successfully compute and output information. Intel does a good job of this when developing faster chips they spend a great deal of resources finding the optimal voltage for a set frequency by testing possible environmental temperatures (home, office, server room....) so if we are to increase the frequency of a CPU we need to in crease the voltage to stabilize the faster signal but with the increase in voltage we have an increase of heat in the CPU and heat will destabilize the CPU with electron migration. when you double the Voltage of a CPU you quadruple the heat generated.

This is where the LN2 comes into play, If we want a speed of 8GHz or 266% of the stock 3GHz we need a lot of voltage. and to prevent the CPU from heating up and causing electron migration we cool it to about -150c (LN2 is -196c) this allows the CPU to operate with less electrical resistance and hopefully prevent migration when running such a high voltage like 1.9 Volts.


The colder you can get the CPU the more voltage it can take allowing a higher clock frequency.

[V2-V3]

A very cool idea. Im no engineer or guru by any means, but I also thought that extreme cold limited electron migration.



AMD sweetspot is 1°K

below 17K silicon starts to become a superconductor, So i would say to have an effective cooling solution/overclock around 17K would be prime

[Gautam]

I wrote this on another forum a while ago, an extremely rough conceptual overview relating temperatures and voltage to the rise and fall times of transistors charging and discharging the parasitic capacitances at a given node. The standard model is to approximate a transistor as a resistor and a capacitor. This resistivity is inversely proportional to electronic mobilty, D*q/KT, where D and K are physical constant, q is electronic charge and T is of course temperature. So, in a nutshell, lower temperatures -> lower resistivity -> quicker charging/discharging of capacitances -> lesser delay in producing a "1" or "0" -> higher stable frequency.

[TheKarmakazi]

Great info V2-V3 & Gautam !

and I was j/k about the 1°K. Team Finland did have their cpu at a frosty 31°K with the LHe though.

[Gautam]

The damage electron migration can cause is the death of your CPU or a permanent return of corrupted information. This happens when a Higher voltage trace jumps into a near by lower voltage zone due to its lower electrical resistance and the result is a fried portion of the chip rendering it useless...

IMHO electromigration is almost completely irrelevant to the discussion at hand. Sure, colder temperatures keep the transistor gates intact for longer, but that has little to do with why it allows them to switch faster. Though, semiconductor physics is my weak spot.

[cadaveca]

Tighter lattice due to shirnkage from the cold? Just simply lowers resistance, or what?

[Expat GriZ]

This might be slightly off-topic and I apologize in advance, I completley agree with the preceeding posts but how about the power coming from wall outlet?? What kind of power are we feeding our gear in the first place?? I used to live in Perth Oztralia and the old power grid was horrendous, spiking up and down, (215V daytime, 245-257V @ night) Being a bit of an audiophile I installed 2 dedicated 20A lines run on their own phase (old 3 phases house) and the improvement in sound quality was very noticable, even to my Ozzie pals that thought that much stereo gear was crazy in the first place. We always listened at night. Zero background noise, deep open sound stage.. imagery like no tomorrow, etc. etc. Then I bought a swanky power conditioner and took it to another level as well. I'm now living in Belgium, power supply is more stable but noisy. I now have 2 25 Amp dedicated lines for the stereo and I'm thinking of yanking out my Power Conditioner and plugging the mother board into it and start with nice, clean unrestricted power. It's just like putting better fuel in your car!! This should eliminated even more potential scatter, jitter, etc. and allow for better current to the board. I think I've even seen a few power conditioners, stabilizers in some of the high end overclocking pictures.

[Johnny Bravo]

.... This resistivity is inversely proportional to electronic mobilty, D*q/KT, where D and K are physical constant, q is electronic charge and T is of course temperature....

Yep that's the equation I was looking for Gautam

[Gautam]

Yes...there are a couple of long equations which most computer/electrical engineering students are forced to become intimate with for the different modes of operation of a transistor. (I would rather not recount them here, but you can probably find them by looking up "unified model" if you really wish) Suffice it to say, the important ones in question are directly proportional to electron mobility.

[SNiiPE_DoGG]

wow guys this is an xtremely interesting thread, this is definitely the reason why I chose to switch to EE a week ago and I cant wait to get to some higher level classes, thats for sure.

[Gautam]

You won't be able to wait till they're over once you get to them.

[Wiggy McShades]

wow guys this is an xtremely interesting thread, this is definitely the reason why I chose to swtich to EE a week ago and I cant wait to get to some higher level classes, thats for sure.

soo troy NY does that mean you go to RPI?



on a more relevant note one of my professors actually brought this stuff up in class asking if anyone had heard about the cpu being run under liquid helium. He was actually curious about how it ran at such a low temperature. Now i wish i had asked him more about it.

[SNiiPE_DoGG]

soo troy NY does that mean you go to RPI?



on a more relevant note one of my professors actually brought this stuff up in class asking if anyone had heard about the cpu being run under liquid helium. He was actually curious about how it ran at such a low temperature. Now i wish i had asked him more about it.

yes I am currently a freshman @ RPI next semester is when it gets interesting though.... right now I'm just taking chem and phys 1100 :\ I know the subjects completely and fully but my HS sucked so bad I couldn't take the AP's to credit out.

[Wiggy McShades]

yes I am currently a freshman @ RPI next semester is when it gets interesting though.... right now I'm just taking chem and phys 1100 :\ I know the subjects completely and fully but my HS sucked so bad I couldn't take the AP's to credit out.

I almost went there. I'm at BU computer systems engineering ...... EE is for girls