Sapphire Radeon HD 6990 Reviewccokeman - March 13, 2011
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Much like the HD 5970, the HD 6990 uses two GPU cores attached to a single PCB to deliver on the "King of the Hill" performance target that AMD is aiming for. Two Cayman XT GPU cores power this beast code named "Antilles" using the same architecture as the HD 6970. The Sapphire HD 6990 is a dual slot card much like the rest of the HD 6XXX and 5XXX family of GPUs from AMD. It should be no surprise that with the amount of hardware on board, this card is a full 12 inches in length, the same as the last generation HD 5970. For basic specs there are a total of 3072 Stream Processors clocked at 830/880MHz depending on whether or not you use the AUSUM switch feature, 192 Texture units, 64 ROPs and a massive 4GB of GDDR5 memory running through a 2 x 256bit bus. The GDDR5 memory is clocked at 1250MHz. What does all this add up to in terms of performance? If you can hold on a few seconds more, we're going to find out. The front side of the card has a decal from Sapphire but other than that, the card looks the same as a reference card. The front side of the card is dominated by the centrally located radial fan replacing the blower style fan used on past designs. The back side of the card has a back plate that is used to both stiffen the card and work as a heat sink for the eight memory modules on the back side of the PCB.
Connectivity options have changed since the HD 5970 with the HD 6990 coming with a single Dual Link DVI port and a total of four Mini Display Port 1.2 ports. This configuration is optimized for use with an Eyefinity setup with up to five monitors right now with the current technology. To get a full six monitors, you will need to use DisplayPort 1.2 equipped monitors that support daisy chaining or a not yet available Multi Stream Transport hub. With the change in connectivity, the card now has more room to breath and exhaust the heat from the front GPU heat sink. The rear of the card is open to allow the airflow from the rear heat sink to exhaust out from under the shroud only this time, that thermal load will be dumped into your case. This means that not only do you need a large chassis to install this card in you need one that has quite a bit of airflow to quickly exchange the air in the chassis as it heats up. Curious as to the temperatures coming out of the front and rear exhaust ports on the HD 6990 I pulled out my handy Kestral Airflow tracker and used its temperature monitoring capabilities to measure the temperature of the airstream. The front port sent air out at 44 degrees Celsius while the rear was similar at 41C. Don't put an HDD behind the card or it may just get the heat treatment.
The little yellow sticker that is strategically placed over the BIOS recovery switch implores you to read the manual before removing the sticker. On the HD 6970 and HD 6950 this switch is nothing more than a means to recover the VGA BIOS if it is somehow corrupted. Enterprising enthusiasts have done what we do when presented with an opportunity and figured out that an HD 6970 BIOS could be flashed to the HD 6950 to unlock the cores and deliver HD 6970 performance for the cost of an HD 6950. PowerColor has gotten in the game already and produced a card with the appropriate BIOSes already flashed. On the HD 6990, this switch is called the AUSUM (Antilles Unlocking Switch for Uber Mode) dual BIOS switch - and who says manufacturers don't have a sense of humor! This switch bumps up the GPU core voltage and clock speeds to 1.175v and 880MHz from 1.125v and 830MHz. The first position supports factory clocks and voltages while the second position is for the overclocking enthusiast. The decal is a clue that moving the switch to the higher clocks and voltages is not supported by any warranty. The statement in the press deck from AMD reads " AMD’s product warranty does not cover damages caused by overclocking,even when overclocking is enabled via AMD software and/or the Dual-BIOS Function on the AMD Radeon™ HD 6990". Kind of makes you wish they made us work for it rather than putting it there, daring us to use it. The switch is slightly recessed so the AUSUM mode cannot be accidentally enabled and subsequently voiding your warranty.
The Sapphire HD 6990 supports Quad CrossfireX with one other HD 6990 and most likely three way CrossfireX by adding an HD 6970 to the mix. The power profile of this card dictates that it is connected to dual 8 pin PCIe power connections to supply a total of 375 watts of power to the card. The HD 6990 is capable of using up to 450 watts using AMD's Power Tune technology when the AUSUM switch is pushed over to the hardware overclocking mode so a large enthusiast style power supply is going to be a requirement not an option.
AMD suggests that the card not be taken apart due to the Phase Change TIM that sets and creates a bond with heat to deliver an 8% improvement in thermal performance. Suggestions are just that so apart it came. The backplate is metal and acts as a passive heat sink for the memory modules on the back of the PCB. Pulling the HD 6990 apart requires pulling the back plate and heat sink brackets then gently prying the card apart. A little heat from a hair dryer helps by warming up the TIM to gain some separation. Then to get into the heat sink plate on the front of the card requires a series of screws to be removed. Underneath is the centrally located radial fan mounted on the finned aluminum plate. The HD 6990 differs from the HD 5970 tremendously here as there are two Vapor Chamber equipped heat sinks on opposite sides of the fan instead of a single large assembly as used on the HD 5970. The TIM indeed had hardened up due to my testing but was not as difficult to remove as some of the TIM applications I have seen over the past year.
One of the major flaws with the HD 5970 was that it was susceptible to throttling when the VRM circuit was overheated. In some cases running at stock speeds. To eliminate that as a concern, AMD have put the next generation digital programmable Volterra regulators in the center of the PCB to feed each core and its associated memory. These new devices are said to be more efficient, run cooler and have a higher current capacity than the previous design. THE PLX Bridge chip looks to be the same one used on the AMD HD 5970 and is again carrying the AMD logo on it.
The Cayman XT cores are built on a 40nm process at TSMC with 2.64 Billion transistors all in a die sized at 389mm2. The two cores are equipped with a total of 3072 stream processors, 192 texture units, 64 color ROPS and 256 Z/Stencil ROPs. The core is clocked at 830MHz as delivered from the manufacturer but can be bumped up to 880MHz with the AUSUM switch to deliver a higher level of performance. The GDDR5 memory used on this HD 6990 from Sapphire is made by Hynix and is rated for operation at 1250MHz and runs through a 2 x 356 bit bus. While these are the base specifications, they all add up to a significant increase in compute performance and deliver between 5.1 and 5.40 TFLOPS of compute performance with the memory delivering 320Gb/s of bandwidth.
On paper the HD 6990 looks like a killer card that should easily topple the HD 5970 from its perch as the fastest single video card.