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Once Again The Card They Beg You To Overclock

One of the 5970’s unique attributes was that while at default clocks and voltages it was designed to meet a 300W TDP, it was designed for much more. AMD’s design called for it to be able to handle 400W, the amount of power needed to operate the card as if it were a true dual-GPU 5870. In practice this fell a bit short due to VRM temperatures, but for most games this was a workable solution.

In AMD’s case it has paid off well enough that with the 6990 they are returning with the same philosophy, differing only in implementation details.  AMD’s engineers have gone and built a card that can run its GPU at 6970-like GPU clocks (880MHz), you just have to do some overclocking to get there. And while AMD’s legal department will tell you that no overclock is guaranteed and that doing so voids any warranty, the design and the binning of GPUs virtually ensures every card can hit 6970 core clocks.

AMD refers to the 6990 as a 450W card. At default clocks it has a rated TDP of 375W but the cooler itself is designed to take 450W, which is why AMD went with so many design changes such as the dual-exhaust system and the exotic thermal compound. The result is that the card can generally keep itself cool at 6970 speeds, and in fact does a better job of this than the 5970 did at 5870 speeds. The catch here is that you will need sufficient cooling to deal with the heat the card dumps in to the case, 225W+ to be precise. Thus while the 6990 is already a card with specialized cooling requirements, the 6990 when overclocked is even more so. With FurMark our numbers point to our card drawing more than 500W, so 6990 overclocking is not for the faint of heart.

With the 5970 AMD enabled overclocking by producing a quick & dirty utility to bump the card’s voltage up to 5870 voltages, which then could be used with Overdrive to achieve the desired clocks. This certainly worked but it wasn’t smooth and it wasn’t consistent - not every vendor used AMD’s utility (particularly if they had their own in-house overclocking utility), and if you did use AMD’s utility then you had to set the voltage and do overclocking on every boot. AMD is not about to include voltage controls in the Catalyst Overdrive controls, so they’ve gone for a better way.


The 5970's ATI Overvolt Tool

Do you recall the BIOS selection switch on the 6900 series cards? On those cards, it was to allow users to safely flash new BIOSes to their cards while having a fallback BIOS to work from. The 6990 takes this concept and repurposes it to fit the 6990’s unique overclocking needs. The switch is still there, but instead of identical BIOSes the switch controls which performance BIOS is used. Position 2, the default position, is a write-protected BIOS that runs the 6990 at its default core clock of 830MHz and default core voltage of 1.12v. Position 1 is a write-enabled BIOS that runs the 6990 at the same core speeds and voltages as the 6970: 880MHz core clock and 1.175v core voltage; meanwhile memory clocks remain unchanged at sub-6970 speeds of 5GHz. AMD calls it the AUSUM switch (Antilles Unlocking Switch for Uber Mode); ignore the name, focus on the fact that the switch is what controls the core voltage on the 6990.


6950/6970 BIOS Switch

From a usability standpoint, the benefit of using the BIOS switch for this is that it’s much more consistent across vendors and it doesn’t require any software interaction. Just flip the switch and you’re done. However we would still count on seeing some vendors taking things a step further and offering fine-tuned voltage control for the card.

Along with the increase in the core clock and the voltage, AMD’s documentation also lists the PowerTune limit as being increased for uber mode. AMD tells us that the limit here is 450W (540W with +20% PT), however in our testing we were unable to hit that limit. Every test up to and including FurMark ran unthrottled, and we peg power consumption there at over 500W. If indeed there isn’t a PowerTune limit this is good news for extreme overclockers, but it means if you use uber mode PowerTune won’t be there to save your bacon if you push too hard.

Radeon HD 6990 BIOS Switch
Position Core Clock/Voltage PowerTune Limit Write-Protected
1 880Mhz/1.175v None No
2 (Default) 830MHz/1.12v 375W Yes

As far as additional overclocking is concerned we did not push our sample beyond uber clockspeeds. In uber mode we were already hitting GPU temperatures of 94C in Furmark, which is as high as we’re willing to go. Better cooling of course would allow easier overclocking, and with a an overdrive limit of 1.2GHz in uber mode, the card should vanish in a puff of smoke well before Overdrive becomes a limit.


Radeon HD 6990 Overdrive Limits

Of course all of this talk of overclocking cannot be held without saying something about power consumption. With 2 8pin PCIe power sockets the 6990 is already drawing the full 150W per 8pin line the PCIe specification calls for; uber mode exceeds this, potentially by quite a bit. AMD has engineered the 6990 to pull most excess power from the PCIe power sockets and not the slot itself (since the slot is the weakest link), so a notably overbuilt power supply would be necessary. AMD hasn’t provided any official guidance here, but a well-built power supply offering 20A (240W) per 8pin line with an independent rail for each line would seem to be the minimum to get away with uber mode.

Ultimately however, as we’ll see the 6990OC doesn’t have nearly as large a performance bump to it as the 5970OC did. Thanks to the much higher default clocks, the 6990OC’s core clock is only 6% faster and the memory clock is the same, versus 17% faster on the core clock and 20% faster on the memory clock for the 5970. As a result you get much better performance out of the box, but unlike the 5970 flipping the magic switch doesn’t significantly increase the card’s performance this time around. So unlike the 5970 if you want to significantly improve performance over stock, you’ll have to do some equally significant custom overclocking on the 6990.

Finally, in a close examination of a minor detail, unlike on the 6950/6970 it’s clear that AMD doesn’t intend for this switch to be easily accessible. The switch on the 6990 is slightly recessed, not by enough to make it hard to hit but enough that you’ll never accidentally hit it. Flipping the switch would need to be a conscientious action, which makes sense given the fact that doing so would void the card’s warranty.

Update: After publication of this article there's been some slight confusion on the matter of the AWSUM switch and the warranty. AMD's official guidance is that overclocking the card voids the warranty, which means that AWSUM/uber mode is warranty breaking. Technically speaking just flipping the switch doesn't break the warranty - it's operating the card that does - but retail cards will come with a sticker over the switch warning users of the potential danger of overclocking and that it violates the warranty. So breaking the sticker to flip the switch will for all practical purposes violate the warranty. Specific policies may differ by partner, however.

Meet The 6990, Cont PCI-Express Compliance: Does It Even Matter?
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  • nafhan - Tuesday, March 08, 2011 - link

    I generally buy cards in the $100-$200 range. Power usage has gone up a bit while performance has increased exponentially over the last 10 years. Reply
  • LtGoonRush - Tuesday, March 08, 2011 - link

    I'm disappointed at the choices AMD made with the cooler. The noise levels are truly intolerable, it seems like it would have made more sense to go with a triple-slot card that would be more capable of handling the heat without painful levels of noise. It'll be interesting to see how the aftermarket cooler vendors like Arctic Cooling and Thermalright handle this. Reply
  • Ryan Smith - Tuesday, March 08, 2011 - link

    There's actually a good reason for that. I don't believe I mentioned this in the article, but AMD is STRONGLY suggesting not to put a card next to the 6990. It moves so much air that another card blocking its airflow would run the significant risk of killing it.

    What does this have to do with triple-slot coolers? By leaving a space open, it's already taking up 3 spaces. If the cooler itself takes up 3 spaces, those 3 spaces + 1 open space is now 4 spaces. You'd be hard pressed to find a suitable ATX board and case that could house a pair of these cards in Crossfire if you needed 8 open spaces. Triple slot coolers are effectively the kryptonite for SLI/CF, which is why NVIDIA isn't in favor of them either (but that's a story for another time).
    Reply
  • arkcom - Tuesday, March 08, 2011 - link

    2.5 slot cooler. That would guarantee at least half a slot is left for airspace. Reply
  • Quidam67 - Tuesday, March 08, 2011 - link

    if it means a quieter card then that might have been a compromise worth making. Also, 2.5 would stop people from making the il-advised choice of using the slot next to the card, thus possibly killing it! Reply
  • strikeback03 - Tuesday, March 08, 2011 - link

    With the height of a triple slot card maybe they could mount the fan on an angle to prevent blocking it off. Reply
  • kilkennycat - Tuesday, March 08, 2011 - link

    Triple-slot coolers... no need!!

    However, if one is even contemplating Crossfire or SLI then a triple-slot space between the PCIe X16 SOCKETS for a pair of high-power 2-slot-cooler graphics cards with "open-fan" cooling (like the 6990) is recommended to avoid one card being fried by lack of air. This socket-spacing allows a one-slot clear air-space for the "rear" card's intake fan to "breathe". (Obviously, one must not plug any other card into any motherboard socket present in this slot)

    In the case of a pair of 6990 (or a pair of nVidia's upcoming dual-GPU card), a minimum one-slot air-space between cards becomes MANDATORY, unless custom water or cryo cooling is installed.

    Very few current Crossfire/SLI-compatible motherboards have triple-slot (or more) spaces between the two PCIe X16 connectors while simultaneously also having genuine X16 data-paths to both connectors. That socket spacing is becoming more common with high-end Sandy-Bridge motherboards, but functionality may still may be constrained by X8 PCIe data-paths at the primary pair of X16 connectors.

    To even attempt to satisfy the data demands of a pair of 6990 Cross-Fire with a SINGLE physical CPU, you really do need a X58 motherboard and a Gulftown Corei7 990x processor, or maybe a Corei7 970 heavily overclocked. For X58 motherboards with triple-spaced PCIe sockets properly suitable for Crossfire or SLI , you need to look at the Asrock X58 "Extreme" series of motherboards. These do indeed allow full X16 data-paths to the two primary PCIe X16 "triple-spaced" sockets.

    Many ATX motherboards have a third "so-called" PCIe X16 socket in the "slot7" position. However, this slot is always incapable of a genuine X16 pairing with either of the other two "X16" sockets, Anyway this "slot 7" location will not allow any more than a two-slot wide card when the motherboard is installed in a PC "tower" -- an open-fan graphics card will have no proper ventilation here, as it comes right up against either the power-supply (if bottom-loaded) or the bottom-plate of the case.
    Reply
  • Spazweasel - Tuesday, March 08, 2011 - link

    Exactly. For people who are going to do quad-Crossfire with these, you pretty much have to add the cost of a liquid cooling system to the price of the cards, and it's going to have to be a pretty studly liquid cooler too. Of course, the kind of person who "needs" (funny, using that word!) two of these is also probably the kind of person who would do the work to implement a liquid cooling system, so that may be less of an issue than it otherwise might be.

    So, here's the question (more rhetorical than anything else). For a given ultra-high-end gaming goal, say, Crysis @ max settings, 60fps @ 3x 2500x1600 monitors (something that would require quad Crossfire 69xx or 3-way SLI 580), with a targeted max temperature and noise level... which is the cheaper solution by the time you take into account cooling, case, high-end motherboard, the cards themselves? That's the cost-comparison that needs to be made, not just the cost of the cards themselves.
    Reply
  • tzhu07 - Tuesday, March 08, 2011 - link

    Before anyone thinks of buying this card stock, you should really go out and get a sense of what that kind of noise level is like. Unless you have a pair of high quality expensive noise-cancelling earbuds and you're playing games at a loud volume, you're going to constantly hear the fan.

    $700 isn't the real price. Add on some aftermarket cooling and that's how much you're going to spend.

    Don't wake the neighbors...
    Reply
  • Spivonious - Tuesday, March 08, 2011 - link

    70dB is the maximum volume before risk of hearing loss, according to the EPA. http://www.epa.gov/history/topics/noise/01.htm

    Seriously, AMD, it's time to look at getting more performance per Watt.
    Reply

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