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Closer Look at ATI Radeon HD 3870 X2

PCB Design and Specifications

When two GPUs are installed on one graphics card, the wiring of an appropriate PCB proves to be a daunting task. The engineering folk from the former ATI Technologies have coped with the job brilliantly, fitting an advanced power circuit, both graphics cores, a switch chip, and two sets of memory chips into one PCB the size of an Nvidia GeForce 8800 GTX.

The ATI Radeon HD 3870 X2 seems to be longer, yet it is just an illusion provoked by the long casing of the cooler. The two graphics cards have the same dimensions with a length of 27 centimeters. Thus, the ATI Radeon HD 3870 X2 is one of the longest consumer-class graphics cards and may not fit into some system cases.

The cooler hides the most interesting parts of the card, so we had to take it off to scrutinize our sample of Radeon HD 3870 X2. An odd thing about the PCB is not that it is very complex and carries a lot of components, but that it is not populated uniformly. The back of the PCB is almost empty on both sides. It doesn’t seem to be reserved for further reinforcement of the power circuit as there are no seats for additional load-bearing elements here. Instead, there are a seat for some controller and seats for connectors. We don’t know what this part of the PCB was meant for, but its emptiness indicates at least a theoretical possibility that the ATI Radeon HD 3870 X2 could be made shorter with no loss in functionality.

The high component density made the developer use a more expensive digital power circuit working at a higher frequency but not requiring large electrolytic capacitors. Such a circuit was first deployed on ATI’s Radeon X1950 Pro and we can see the familiar elements here: the power supply of each GPU is managed by a dual-phase circuit with a Volterra VT1165MF controller. Each circuit can be reinforced by increasing the number of phases to three for the left GPU and four for the right GPU. There is a dedicated voltage regulator for each set of memory chips.

We know that the reference Radeon HD 3870 consumes up to 80W of power. Considering that the GPUs of the Radeon HD 3870 X2 are clocked at a higher frequency, the total peak power draw of the new card should be 170-180W. That’s why it is equipped with two power connectors. One of these connectors is an 8-pin PCI Express 2.0 one, with a load capacity of 150W. The card works well even if you use two 6-pin PCI Express 1.0 plugs. Like with the Radeon HD 2900, the plugging of a 6-pin cable into the card’s 8-pin connector only makes it impossible to overclock the card with the standard options of Catalyst. The connectors are placed somewhat inconveniently, far from each other and facing downward. It may be difficult to connect the power cables if the card is already installed in the system case.

A PEX8547 chip from PLX Technology is the card’s PCI Express switch. It has three PCI Express x16 ports complying with the PCI Express 1.1 specification, so the new card doesn’t support PCI Express 2.0 mode. This chip was developed specifically for high-performance graphics hardware. It features low latencies and Peer-to-Peer mode in which the graphics cores can communicate with each other directly, bypassing the controller. It’s similar to AMD’s 580X and 790 North Bridges.

Coming in PBGA packaging, the switch is rather large. It occupies more room on the card than the chip that used to be installed on the GeForce 7950 GX2. The PEX8547 has a power draw of 5W.

Two RV670 chips are placed closely to the right and left of the switch. There is nothing extraordinary about them except that their frequency is increased from 775MHz (the GPU frequency of the Radeon HD 3870) to 825MHz. It is a rather vague parameter, by the way, since ATI’s Radeon HD processors have 26 different frequency zones. The chips do not have protective frames around the dies. Theoretically, the Radeon HD 3870 X2 has got such tremendous resources as to challenge the Nvidia GeForce 8800 Ultra even. The new card features a total of 640 ALUs grouped into 128 computing units with 5 ALUs in each. It also has eight large texture processors and eight rasterization processors (ATI calls them render back-ends).

The texture processors and render back-ends have a complex architecture that was described in our reviews of the ATI Radeon HD 2900 XT and ATI Radeon HD 3800. Still, the ATI Radeon HD 3870 X2 can be said to have roughly 32 TMUs and 32 ROPs in the traditional terms. While the amount of ROPs is not a critical parameter in modern applications, the small number of TMUs has always been a bottleneck of the ATI Radeon HD architecture. The Radeon HD 3870 X2 is the first solution from AMD/ATI to be free from it.

The pure computing power the Radeon HD 3870 X2 can provide on MADD operations (Multiply-Add) is estimated by the developer at over 1 teraflop. That’s an impressive number but the Radeon HD architecture depends heavily on the shader code compiler. Added to that, the new card also depends on the efficiency of CrossFire technology. Like single RV670-based graphics cards, the ATI Radeon HD 3870 X2 supports PowerPlay, the power management technology from ATI.

As opposed to the Radeon HD 3870, the Radeon HD 3870 X2 uses cheaper GDDR3 memory. Each graphics core is surrounded with eight Samsung K4J52324QE-BJ1A chips, four on the face side of the PCB and four on the reverse side of it. These 512Mb chips (16Mb x 32) have a voltage of 1.9V for all the circuitry and a rated frequency of 1000 (2000) MHz. They are clocked at 900 (1800) MHz on the Radeon HD 3870 X2. The dense component mounting must have made it impossible to place the memory chips in the most optimal way and their frequency was reduced a little to ensure stable operation. The card features a total of 1024MB of memory, but only 512MB are available for 3D applications as is the case with every dual-GPU system.

The reference version of Radeon HD 3870 X2 is equipped with two dual-link DVI-I ports with support for the audio-over-HDMI feature. It also has a standard 7-pin mini-DIN connector for analog video output in Composite, S-Video and YPbPr formats. The single difference from the single-chip Radeon HD 3800 is that the card has only one CrossFire connector because the second channel is used by the card itself for its GPUs to communicate with one another. It means that CrossFire configurations with more than two Radeon HD 3870 X2 cards (and four Radeon HD 3800 GPUs) are unlikely to be supported.

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