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ATI Radeon HD 4870

PCB Design and Specifications

This section will make you amends for our not having discussed the design of the Radeon HD 4870 before. The PCB of the senior model is as large as the PCB of the Radeon HD 4850 but differs in wiring and component layout.

The card is largely covered with a massive dual-slot cooling system which makes the 4870 look not unlike the Radeon HD 2900. The card is shorter than the GeForce GTX 280, though. This is good because you won’t have problems installing your Radeon HD 4870 into small system cases. All the technical details we may be interested in are to be found under the cooler.

The power circuit is more sophisticated than on the junior model. This is logical since the GPU has a higher clock rate and higher power consumption. Instead of a single two-phase regulator, there is a three-phase one with an option of reinforcement by adding a fourth phase. This option would be redundant for a RV770 clocked at 750MHz, but may come in handy when designing PCBs for ATI’s next GPU.

The elements at the top edge of the PCB, particularly the two switching transistors and Pulse PA0511 inductors, constitute a two-phase voltage regulator for the memory chips. The whole power circuit deployed on the Radeon HD 4870 is based on a Volterra VT1165MF PWM-controller that had been used on both ATI’s (e.g. Radeon HD 3870 X2) and Nvidia’s (GeForce GTX 200) products. The voltage regulators work at a high frequency, so there are almost no electrolytic capacitors in the circuit. Such regulators are often referred to as “digital” but it is only the operating frequency that distinguishes them from their “analog” counterparts.

The card is equipped with two standard 6-pin PCI Express 1.0 power connectors that have a max load capacity of 75 watts. Considering the 55nm tech process and modest appetite of the RV770 chip, this should be quite enough. There is no need for an 8-pin PCI Express 2.0 connector which is rated for loads up to 150 watts.

We guess the most interesting thing about the Radeon HD 4870 is its GDDR5 memory. As opposed to GDDR3/4, GDDR5 can send or receive two 32-bit words per clock cycle, which doubles its effective frequency. Thus, GDDR5 chips working at a real frequency of 900MHz have an effective frequency of 3600MHz and appropriate bandwidth. GDDR5 has less strict requirements to the wiring than the previous memory types. Particularly, the routes do not have to be all the same length. This makes the PCB simpler and the graphics card, cheaper to make.

The Radeon HD 4870 carries eight Qimonda IDGV51-05A1F1C-40X chips with a capacity of 512Mb each (16Mb x 32). GDDR5 being not so particular about the wiring, the L-shaped positioning of the chips is okay. It helps simplify the PCB wiring even more in comparison with semicircular positioning.

The memory chips work at a voltage of 1.5V and have a rated frequency of 1GHz. For GDDR5 memory, this means an effective frequency of 4GHz. The memory frequency of the reference Radeon HD 4870 is set lower at 900 (3600) MHz, providing a memory bandwidth of 115.2GBps with a 256-bit memory bus. The results of gaming tests suggest that the Radeon HD 4850, let alone the senior model, does not feel any lack of memory bandwidth. Unfortunately, the total capacity of the local memory bank of both Radeon HD 4800 models is only 512 megabytes whereas a number of modern games can use more memory, especially at high resolutions. ATI’s solutions are inferior to the GeForce GTX 200 in this respect but the release of the Radeon HD 4870 should restore the balance.

The GPU marking is rather vague but it is clear that the memory controller is reprogrammed to support GDDR5. Many sources refer to this version of the chip as RV770 XT, which is not indicated by the marking yet helps distinguish versions of the RV770 chip set up for different graphics memory types. The GPU is clocked at 750MHz and incorporates 800 ALUs (grouped into 160 superscalar execution units), 40 texture processors and 16 render back-ends. The metallic frame on the packaging protects the die against chipping.

The left part of the PCB, near the connectors, is not very interesting. Most of it is just covered with a screen. There are no seats for additional chips because none is necessary. The DisplayPort interface is supported by the RV770 while analog video input is outdated. The card has two dual-link DVI-I ports (with support for resolutions up to 2560x1600), a 7-pin universal mini-DIN port, and two CrossFireX connectors for building multi-GPU subsystems out of as many as four Radeon HD 4870 cards. Of course, you’d need a mainboard with so many PCI Express x16 slots and a system case that can accommodate four dual-slot graphics cards. The audio-over-HDMI feature is supported by means of an appropriate adapter from ATI.

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