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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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  • EmmetBrown - Tuesday, March 08, 2011 - link

    Nice, but what about the Radeon HD 6450, 6570 and 6670?
    http://en.wikipedia.org/wiki/Comparison_of_ATI_Gra...

    Why they are available for OEM only? They looks interesting, especially the 6670, which with its 480 SP should be faster than the 5670 which has 400 SP and lower frequency. Do you plan to review them?
    Reply
  • Ryan Smith - Tuesday, March 08, 2011 - link

    As you note, they're OEM only. AMD will release them to the retail market eventually, but clearly they're not in a hurry. It's unlikely we'll review them until then, as OEM cards are difficult to come by. Reply
  • misfit410 - Tuesday, March 08, 2011 - link

    I have to ask, if you bring up the price and say that you might as well do two 6950's in SLI when this thing doubles the performance of the GTX580, I mean would it also not be the better solution than a GTX580 which is $500 while two 6950's can apparently double it for $550 being they can be found for $225 after rebates these days. Reply
  • Figaro56 - Tuesday, March 08, 2011 - link

    You sound a little confused. You can't run ATI cards in SLI, they run in what is called crossfire (or crossfirex which is the same thing). Two 6950's don't equal GTX580 in SLI. You need two HD 6970 cards in crossfire to nearly equal two GTX580 in SLI.

    In my opinion, why limit your performance with 2 HD 6950 cards, why not just bye the 2 HD 6970 cards and never have to second guess if you should have or not? But... That's just me. I have a job.
    Reply
  • silverblue - Tuesday, March 08, 2011 - link

    Totally unnecessary closing comment there, considering most people here do actually have jobs. Not everyone who has a job can afford such gear as there's more important things to spend money on. Reply
  • Thanny - Tuesday, March 08, 2011 - link

    You sound confused, too. He miswrote SLI, but you misunderstood his point entirely. He's saying that two 6950's are significantly faster than a single 580 for almost the same price. Reply
  • Loiosh - Tuesday, March 08, 2011 - link

    Hey guys, you forgot one other usage case that would necessitate this card: ATI+physx setup: http://www.shackpics.com/viewer.x?file=DumbVideoca...

    I'm currently running one and it requires a dual-GPU card. :/

    In my case I'm waiting for a watercooled version. BTW, you didn't say the release date for this?
    Reply
  • nanajuuyon - Wednesday, March 09, 2011 - link

    Funny after reading this review I went into town (Tokyo) to buy a new hard disk and saw this card for sale. So in Japan at least it is already on the market..... price was ridiculous though, 79,000YEN or $945 US..... I'm sure it will be available everywhere soon.

    Waterblocks on the other hand could be a couple month or so away I guess...
    Reply
  • Vinas - Tuesday, March 08, 2011 - link

    If you buy this you better have it on water. 'nuff said about all this tri slot cooler talk. Reply
  • JPForums - Tuesday, March 08, 2011 - link

    First off, nice article Ryan.
    Good data, relevant commentaries on said data, and conclusions.

    You mention in the article that you believe some of the shortcomings of the 6990 to be a lack of PCIe bandwidth. This got me thinking that perhaps it is a good time to revisit the effect of PCIe bandwidth on modern cards. Given the P67 only natively supports 16 lanes, I'm curious to see what effect it has on CF/SLI. It could make big difference in the recommended hardware for various levels of gaming systems.

    Typically, someone looking for a CF/SLI setup will get a board that supports more lanes. However, I have seen a situation where a friend built a budget i5 system and about 4 months later was in a position to acquire an HD5970 on the cheap (relatively speaking). Clearly, two HD5850s/HD5870s would have been an option.

    If newer cards are effectively PCIe bandwidth limited, then a 6990 may perform more closely to an HD6970 CF setup in such a system than it does in these graphs. This would be even more of a consideration at the high end if the rare boards with support for 4x8 lane (spaced) PCIe give you no real benefit over a more common 2x16 lane board (comparing 4 HD6970s to 2 HD6990s).
    Reply

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