R680 GPU

The GPU powering the Radeon 3870 X2 is known as R680. R680 is built largely on the RV670 chip already in use on the Radeon 3870 – for instance, both chips feature 320 stream processors and are built on TSMC’s 55-nm manufacturing process. The key difference is that R680 runs at 825MHz, whereas the RV670 GPU used in the Radeon 3870 topped out at 775MHz. Here are the other key specs of R680:

AMD Radeon GPU Comparison
	ATI Radeon HD 3870 X2	ATI Radeon HD 3870
# of Transistors	1.3 billion	666 million
Manufacturing Process	55-nm	55-nm
Stream Processors	640	320
Texture Units	32	16
Render back-ends (ROPs)	32	16
GPU Clock Speed	825	775
Memory Configuration	900MHz GDDR3	1.125GHz GDDR4
DirectX	10.1	10.1
MSRP	$449	$219.99

As you can see, the Radeon 3870 X2 ships with GDDR3 memory modules that run 225MHz slower than the GDDR4 used on the Radeon 3870. However, the GDDR3 memory on the 3870 X2 runs at lower latency than the GDDR4 used on the 3870. The lower latency improves performance, helping to offset the difference in clock speed. In addition, GDDR3 is cheaper than GDDR4. This is important considering that the Radeon 3870 X2 ships with 1GB of memory. (Keep in mind that 512MB of memory is distributed to each GPU.)

Like RV670, R680 sports a 256-bit memory interface and supports DirectX 10.1. Paired to each GPU is 512MB of memory.

One interesting aspect of the Radeon 3870 X2 is the 48-lane PCIe 1.1 interconnect bridge which links both GPUs to each other. This chip sits between both GPUs, providing a bi-directional x16 lane link between both GPUs, and is the same bridge chip that was used previously on Radeon HD 2600 X2 cards. AMD initially planned to use an upcoming PCIe 2.0 interconnect bridge chip but ultimately settled on the 1.1 chip at the request of their board partners, who were eager to see the 3870 X2 card hit the market as quickly as possible. Most of today’s latest apps still don’t take full advantage of PCIe as it is, so it’s doubtful that a PCIe 2.0 interconnect bridge would have had much of an impact on performance.

The 3870 X2 Board

The PCB of the Radeon HD 3870 X2 board measures in at 10.5” in length. This is the same size as the GeForce 8800 GTX/Ultra PCB. Also like the GeForce 8800 GTX and Ultra, the board requires two power connections. On the right edge of the board you’ll find an 8-pin PCIe 2.0 power connector, while a second 6-pin PCIe connector is located just above the card’s fan. The board needs both power connectors in order to operate, although just like the 2900 XT, you don’t need a power supply with an 8-pin PCIe 2.0 power connector. The 8-pin connector is only required for overclocking.

In operation, the board runs surprisingly quiet. With dual GPUs running at over 800MHz and 1GB of memory, you’d assume that the board would run hot and the card’s fan would thus generate a lot of noise to keep everything cool, but fortunately this isn’t the case. At idle for instance, we noted temps in the 45-47 degree Celsius range, while load temps peaked at 67 degrees. Because the GPUs don’t get very toasty under load, the card’s fan doesn’t have to spin up to higher RPMs.

Once the Radeon 3870 X2 is installed within your system and the drivers are loaded, the board automatically operates in CrossFire mode. In fact, at this time there is no way to disable CrossFire. The 3870 X2 card is compatible with all motherboards, so you can operate the board in CrossFire mode regardless if your motherboard’s chipset actually supports CrossFire or not. We ran the card on an EVGA nForce 680i motherboard without any problems.

When you’re ready to upgrade for even more performance, two Radeon 3870 X2 cards can be linked together to support CrossFire X, AMD’s 4-Way CrossFire solution. Unfortunately CrossFire X drivers haven’t been released by AMD yet, but we’ve been told that they should be available sometime later this quarter.