ATI Radeon HD 2900 XT: PCB Design
Since the external memory bus of R600 is 512bits wide, they had to lay out and design a new PCB with appropriate specifications. Nevertheless, when ATI/AMD developers designed their Radeon HD 2900 XT they managed to stay within the dimensions of the previous generation family – Radeon X1800/X1900/X1950. The nуw generation Radeon doesn’t look like a monster at all, unlike GeForce 8800 GTX.
Because of the peculiar design of the cooling system, Radeon HD 2900 XT looks somewhat similar to GeForce 8800 GTS, differing, however, by ATI’s traditional red color. After years of experiments in this field, the leading graphics card makers seem to have finally come to the unified design for powerful graphics accelerators cooling. We are going to dwell on the Radeon HD 2900 XT cooling solution later in the corresponding section of the review, and in the meanwhile let’s take a closer look at the PCB layout, components location, etc. To give you a better view of the PCB design we removed the cooler:
As you can see, all electronic components on Radeon HD 2900 XT PCB are very close to one another, there are almost no empty spots anywhere, which is not surprising taking into account relatively modest size of the board, powerful voltage regulator circuitry and 512bit memory bus.
Radeon HD 2900 XT uses the so-called digital voltage regulator, like the one that we saw on the first Radeon X1950 Pro. However, this one is definitely intended to handle much higher powers: even the very first estimates suggest that R600 graphics core consumes more than 130W of power and is actually most likely to sit around 140-160W. Unlike the classical voltage regulator, the digital one boasts higher operational frequency (up to a few MHz) and uses almost no electrolytic capacitors. The heart of this circuitry is the two high-frequency digital PWM controllers - Volterra VT1115M – operating at up to 1.3MHz frequency. Power elements managed by controllers are not the traditional MOS transistors, but special controllers with digital interface. Judging by the number of inductors, the voltage regulator is designed as a seven-phase circuitry: one VT1115M can manage a few modules. In this case the upper chip manages three modules like that, while the lower chip – four. All 6 phases may be powering the graphics processor directly, while the seventh is one of the power sources for the memory, however, it can also be possible that there is a seven-phase GPU power circuitry and a separate voltage regulator for VDD and VDDQ.
External power is delivered to Radeon HD 2900 XT via two connectors, however, unlike the power connectors for GeForce 8800 GTX, one of Radeon’s is an 8-pin connector. It is not that much of a problem because it is backward compatible with the 6-pin response part, however if you use two six-pin cable plugs then some overclocking and temperature monitoring functions of Catalyst Control center will be unavailable. We cannot find an explanation of this circuitry design by AMD. As we know, although Nvidia GeForce 8800 GTX is pretty power-hungry overall, it loads both of its external power connectors more or less equally and at the same time not too heavily: each received about 40W. Even the recently announced GeForce 8800 Ultra uses six-pin connectors, although its PCB can accommodate an eight-pin connector instead one of the six-pin ones, just like the new AMD solution. In the meanwhile the maximum load each of the six-pin PCI Express connectors can take is defined as 75W. Together with the maximum load the same level slot is capable of bearing, we get the impressive number of 225W, which is more than enough for any graphics adapter. So, it looks like there is no objective need for an eight-pin power connector and shouldn’t be in the near future. Nevertheless, the Radeon HD 2900 XT owners who have PSUs with 6-pin connectors will have to put up with the absence of default overclocking and temperature monitoring features or use third party utilities, as there are no restrictions in this case.
The left side of the PCB where the GPU and memory chips are located is packed. You can see a lot of small components around the memory chips. This is actually not surprising because the small PCB size and 512bit memory bus require chips to be placed on both sides of the PCB, so the accompanying elements cannot be all shifted to the reverse side of the PCB, as they usually do it in designed with single-sided placement of the GDDR3 memory chips. As one chip features 32bit access bus, with 512bit overall memory bus and the total memory capacity of 512MB they had to use 16 chips 256Mbit each. Hynix HY5RS573225AFP-1 chips installed on Radeon HD 2900 XT are of that particular capacity, work at 2.2V voltage and support 1000MHz (2000MHz) frequency. Their actual frequency on Radeon HD 2900 XT is much lower and equals only 825MHz (1650MHz). This relatively low memory frequency means a few things actually, first of all, it ensures low heat dissipation, and secondly it guarantees high overclocking potential. But even without any overclocking, the bandwidth of the memory subsystem featuring 512bit bus and chips running at 825MHz (1650MHz) equals the impressive 105.6GB/s, which is much higher than by GeForce 8800 GTX (84.6GB/s) or GeForce 8800 GTS (64GB/s). These characteristics together with the enhanced ring bus controller give us some hope that the memory subsystem of the new Radeon HD 2900 XT will not turn into a bottleneck. It will most likely be the texturing processors and rasterizers that will impose limitations. As I have already said, memory chips are installed on both sides of the PCB, 8 chips on each side. The chips on the front side of the PCB are cooled by the primary cooler, while those on the reverse side – by the massive heat-spreader plate.
AMD R600 graphics processor, unlike Nvidia G80, uses open-die packaging, and the die itself is placed at 45 degree angle because of it large size. To protect the die edges from cracking, the packaging is enforced with a metal frame preventing the cooler base from shifting.
There are no zones with different clock frequencies in R600 and all graphics processor elements are running at 740MHz. The ability of AMD Shader processors to perform up to 5 operations per clock cycle unlike 2 only operations per clock by Nvidia processors allows them to make up for the lower working frequencies and for the fewer processors: R600 has 64, while G80 – 128.
There is an ATI Rage Theater 200 chip in the lower left corner of the PCB that came to replace the outdated Rage Theater that used to be on previous generation Radeon cards. When ATI/AMD was working on the new generation solution they decided not to give up VIVO support, unlike Nvidia. The new Theater 200 features more advanced and better quality 12-bit video ADCs and supports some technologies improving video encoding quality. The built-in Xilleon core inside R600 is responsible for analogue video out implementation, as it supports analogue video output in high resolution.
Radeon HD 2900 XT comes with pretty common connector configuration for high-end ATI/AMD graphics adapters. It features two dual-link DVI-I connectors and universal nine-pin VIVO/YPbPr connector that is incompatible with the standard S-Video cable. The lower DVI-I connector also supports multi-channel AC3 sound output via HDMI interface but requires a special ATI converter for that. Unfortunately, the sound stream can only be 16bit and the maximum sample rate is only 48kHz. At least these are the parameters set for the uncompressed sound stream in PCM format, that is why we cannot say that the new AMD solution is up to the High Definition Audio standards. Since starting with HDMI version 1.0 this standard supports 24-bit/192kHz sound playback, we suspect that the above described limitation is of mostly software nature and may be eliminated in the new driver versions later on.
R600 features hardware Composing Engine necessary for proper functioning of crossfire technology, therefore there are two standard connectors for flexible CrossFire bridges in the upper left corner of the PCB. These connectors allow uniting two Radeon HD 2900 XT cards into a dual-processor tandem that will ensure higher performance than a single one, of course.