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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  • Figaro56 - Tuesday, March 08, 2011 - link

    2 HD 6970 Cards for $640? I don't think so! These cards are over $300 everywhere. I purchased 2 for $710 shipped and I thought that was a deal. Maybe reviews like yours here inflated the price and I purchased after the price adjustment. I have the same luck with gasoline on days I fill my tank. Reply
  • ViRGE - Tuesday, March 08, 2011 - link

    Looking at the Egg, there's 2 different 6970s at $320, which is probably where AT got $640 from.

    http://www.newegg.com/Product/Product.aspx?Item=N8...
    Reply
  • Figaro56 - Tuesday, March 08, 2011 - link

    All right, you got me there. I only buy XFX double lifetime warranty cards when I start spending this much on replacing my dual GPU solution.

    I seem to manage to actually re-sell my used video cards when I can offer then to a buyer with a lifetime warranty. XFX double lifetime warranty is not a sales gimic, it works. Heck, I would buy a used card if it had a lifetime warranty, it's kind of a no brainer given you actually want to buy that card int he first place.
    Reply
  • Arbie - Tuesday, March 08, 2011 - link

    Thanks for keeping the Crysis Warhead minimum FPS charts!! To me, Crysis/Warhead remains the defining game (and not only technically). I don't even look at the numbers on the other titles.

    Also of prime importance to me are the idle power and, to a slightly lesser extent, idle noise.

    Of course, like most people reading your review, I wouldn't be buying a 6990 even if it were silent. In fact, given that PC graphics requirements are apparently ramping down to console levels, I wonder how AMD/Nvidia are going to sell any significant number of cards above midrange. My HD 5770 will run everything at 1920x1200, though not always with all sliders maxed. However, I don't see much if any difference (in DX9) when I do enable 4xAA vs 2xAA etc. Certainly not enough to double the price of this $140 card.

    A nit on the Crysis Warhead minimum fps chart for 1920x1200 Frost Bench - Gamer Quality - Enthusiast Shaders + 4xAA: Your Dec 10 chart shows 6970CF at 66.2 fps but this Mar 11 chart shows 66.6. Can you believe anyone would actually notice this, much less comment on it? We are too absorbed in this tech stuff (ain't it grand...).
    Reply
  • strikeback03 - Tuesday, March 08, 2011 - link

    They did say the new drivers made a slight difference, that seems likely to be one of the configurations they retested Reply
  • morphologia - Tuesday, March 08, 2011 - link

    That isn't portrait orientation in the picture...it's landscape. Reply
  • taltamir - Tuesday, March 08, 2011 - link

    The card was measured at 77.3db in the article.
    1. At what distance was it measured?
    2. What is its db measurement 1 meter away?
    Reply
  • taltamir - Tuesday, March 08, 2011 - link

    I just looked it up, gold is worth 1430$/ounce right now.
    I highly doubt a watercooled 6990 will weigh half an ounce.
    Reply
  • ekrash - Tuesday, March 08, 2011 - link

    The performance bottleneck is also seen in nvidia's dual gpu offerings. Dual GPU cards operating in X16 PCIe slots must have their data lanes divided between the gpu's, so they are effectively operating at X8 data rates, not at X16 data rates. Whereas single gpu cards will utilize all X16 express lanes, and even then the PCIexpress standard may soon be obsoleted. I hope we can look forward to Intel's fiber optic technology effectively replacing all data bus signalling with 10GB fiber optic bus and peripheral device signalling which can simultaneously and independently utilize all of the different data protocols used for inter-device and system bus communications. Imagine soon AMD and Nvidia will be producing video cards with fiber-optic data buses which may change requirements for power supplied to present day PCI express slots and may change the standards in power supply manufacturing to require that additional power connector to a video card since the 75 watt PCIe slot will be obsolete.

    But ATI and Nvidia may also have to work with motherboard manufacturers to see if Intel's "Thunderbolt" fiber optic data buses can increase or freely throttle the video data bandwidth through its 10GB interface and would be tantamount to increasing data lanes from X16 to X32. It would be almost unlimited video bandwidth which far exceeds any bandwidth limitations vs availability that is needed today. Dual GPU's cannot promise the performance with the limitation of the PCIe X16 slot being divided to dual X8 channels, but it would be nice to see how they perform with unlimited bandwidth potential over a single 10GB fiber-optic. And that would change the battlefield between ATI-AMD and Nvidia.

    My 4870 X2's (Can run Quadfire) still rocks on enthusiast settings in Crysis and Warhead without any hiccups and I've not seen a slowdown of any sort on any level in Crysis.
    The price to performance ratio is declining and may affect my decision to purchase another dual GPU card, opting instead for single GPU card CF solutions that can utilize all X16 lanes by the GPU.

    BTW I did notice the lack of DATA on Crysis @1920x1200 with full enthusiast settings, so that data is missing from this review. Its Gamer plus enthusiast shaders.....not full enthusiast. As above the 4870 X2 runs full enthusiast settings, not one setting is scaled back, and not one hiccup....just smooth play throughout on a single 28" display.
    Reply
  • cmdrdredd - Tuesday, March 08, 2011 - link

    Why are we still using Crysis Warhead at "Gamer Quality"????? With cards like these why not turn everything maxed in game and then fidget with AA and the like? I don't get it. Reply

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