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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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  • Figaro56 - Tuesday, March 08, 2011 - link

    I have 2 XFX HD 5870 cards for sale. I have a double lifetime warranty on these so you get the use of the second lifetime warranty on these. Interested? They are very great performers I can vouch for that. I am use to upgrading my GPU on an annual basis so I am upgrading to 2 HD 6970. $230 each. Reply
  • Thanny - Tuesday, March 08, 2011 - link

    Ignoring the inappropriateness of advertising here, I submit:

    http://www.newegg.com/Product/Product.aspx?Item=N8...

    Why would someone pay you $230 for a used product that can be obtained new at $190?
    Reply
  • fausto412 - Tuesday, March 08, 2011 - link

    I kinda wanted to see a chart with the most common gaming resolution...and can we benchmark with a Q9550 just for comparison? i would love to know if i'm holding back a video card by not going i5 or i7 and by how much. Reply
  • jabber - Tuesday, March 08, 2011 - link

    If you can afford a 6990 why would you be bothering using it with a Q9550 at 1680x1050. Hence why it isnt part of this review.

    This review is to show how it works for the intended market/customer.

    As I said before, this card isnt for folks like you (or me for that matter). Sorry.
    Reply
  • 7Enigma - Tuesday, March 08, 2011 - link

    The most common gaming resolution for this card is the one Ryan tested. It is pointless to test at a lower resolution other than possibly true 24" (1920X1200). And even at that res this card is really not needed. Reply
  • Figaro56 - Tuesday, March 08, 2011 - link

    BOYA to both of those resolutions. You should be playing your games at 2560x1600. Now that's what I'm talkin about! You'd be saying hell ya. Reply
  • Jorgisven - Tuesday, March 08, 2011 - link

    It seems we're getting into the Pentium IV trap, a bit. Big, hot, power-hungry, noisy chips...personally, I'm going to pass on this generation of GPUs. I'm waiting for a revolution in either manufacturing or coding. It's all well and good to have a fast computer for getting what you need done in minimal, but at the risk of the box taking flight because the fans are now of jet engine proportion in speed and power, I'd rather not be able to hear my fans over my headphones...or risk my cat getting sucked into the intake. Reply
  • jabber - Tuesday, March 08, 2011 - link

    Well we've kinda got what we asked for. We've all gamely been buying more and more powerful graphics cards with regards to brute force rendering power.

    We've shown we love buying 750w+ power supplies with multiple GPU connectors, buying SLI and Xfire setups galore.

    So the GPU corps think we love nothing more than just piling on more and more power and wattage to solve the situation.

    It works both ways.

    What we should have been doing was challenging AMD and Nvidia to develop smarter rendering techniques. Had either of them developed PowerVR to the state we are in today we would be in a far better place. Chances are the most power hungry card we'd have today would be 5770 level.

    We need something more efficient like PowerVR to take us to the next level.

    Less brute force and more finesse.
    Reply
  • therealnickdanger - Tuesday, March 08, 2011 - link

    Are you waiting to update your test system until the SATA port issue is corrected? Seems to me that anyone wanting to buy this card would also be using an overclocked 2600K... According to the Bench numbers, the 2600K offers roughly 30% more frames than the 920, depending on the game. That indicates to me that your test system is insufficient to properly test this card.

    Granted, since the vast majority of displays are fixed at 60Hz, fps counts beyond that don't really matter, but I have to wonder what impact this would have on folks with 120Hz-native LCDs. That extra 30% could make the difference.

    ... just sayin'. :)
    Reply
  • Ryan Smith - Tuesday, March 08, 2011 - link

    At this point we're waiting on SNB-E. SNB is very nice, but for a GPU testbed the lack of PCIe bandwidth is an issue. Reply

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