Radeon HD 4890
Being a revised RV770, the new graphics core RV790 is, however, not pin-compatible with its predecessor and made it necessary to develop a new PCB for the Radeon HD 4890 series. The PCB is of unified design, i.e. the same for both: the regular and OC versions of the new card.
The new products resemble the ordinary Radeon HD 4870, the family traits being perfectly visible even with the cooling system removed. There is indeed no reason for ATI to abandon its successful design solutions that can be reused to save time, resources and money. However, there are quite a number of significant differences, mostly concerning the GPU and memory power circuits. These parts of modern graphics cards are revised the most often.
So, the power circuitry of the Radeon HD 4890 has been reinforced to seven phases. The two top phases with single-channel PA0511 inductors and compact power drivers seem to be responsible for GDDR5 memory whereas the other five phases, with advanced power components and an assemblage of dual- and triple-channel PA1312NL and CPLA-3-50 inductors, power the graphics core. For comparison, the Radeon HD 4870 had a three-phase GPU voltage regulator with a possibility of expanding to four phases. The more complex power circuit is necessary as the Radeon HD 4890 is specified to have a peak power consumption of 190 watts.
The power circuit is controlled by two Volterra VT1165MF controllers. These chips allow to adjust the GPU voltage from software. It is for VT1165-based cards that EVGA has released its GPU Voltage Tuner program, so there seem to be no theoretical obstacles to creating a similar tool for the Radeon HD 4890.
Despite the increased capacity of the GPU voltage regulator, this Radeon HD 4890 OC series product carries a couple of ordinary 6-pin PCIe 1.0 connectors with a max load capacity of 75 watts. The bottom seat has two additional pins and can be used to install an optional 8-pin PCIe 2.0 connector. But to all appearances, the power consumption of the RV790 is not as high as we expect. We will check this supposition out in the appropriate section of the review.
PowerColor HD4890 Plus carries eight GDDR5 chips manufactured by Qimonda. They are placed in an L-shaped pattern around the GPU. Marked as IDGV1G-05A1F1C-40X, these chips have a capacity of 1 gigabit (32Mb x 32) and a voltage of 1.5V. The 40X suffix indicates a rated frequency of 1000 (4000) MHz but the company also offers faster GDDR5 memory that is able to work at frequencies up to 1250 (5000) MHz. The memory frequency of the PowerColor HD4890 Plus equals that of the reference card – 975 (3900) MHz. With a 256-bit memory bus this ensures a peak memory bandwidth of 124.8GBps. It would take a far more complex and expensive PCB design with a wider memory access bus to achieve a comparable bandwidth with GDDR3 memory. We also have to remind you that the memory controller has been slightly revised for RV790 and now supports burst read mode which was previously implemented in the RV710 and 730 cores.
Although the transistor count has increased but slightly in the RV790, from 956 to 959 millions, the die has become larger due to the ring of capacitors encircling the core. This solution helps eliminate parasitic noise from the signals and increase the frequency potential of the new chip. The GPU marking is still incomprehensible for the uninitiated. You can only learn the manufacture date from it. Here, it is the eighth week of the current year. It means that AMD has had working samples of RV790 since mid-February. The number 215 may refer to the chip’s frequency potential as in Nvidia’s GPU markings, but that’s just our supposition.
Like before, the die package is equipped with a protective metallic frame. As opposed to Nvidia, ATI still shuns the design with a heat-spreading cap. An extra thermal transition in the die – thermal interface – heat-spreader chain would not be good for a core which is already far from cool.
The latest version of CPU-Z knows the RV790 and even tells the die size correctly, but cannot report its revision and provides an incorrect transistor count. It is a Radeon HD 4890 OC, so the core frequency is increased to 900MHz. The ordinary (without “OC” in the name) version has nominal core frequency of 850MHz. We don’t think this difference is going to be significant in 3D applications because both versions have the same memory clock rate.
The new core has retained the old design with 10 SIMD units each of which incorporates 16 superscalar execution modules, 4 texture processors, a dedicated L1 cache, and control logic. Each execution module consists of five ALUs, one of which is capable of executing complex instructions, a branch control unit, and a set of general-purpose registers. The total amount of ALUs is 800, just as before. Four raster back-ends, equivalent to 16 classic ROPs, are responsible for rasterization operations including hardware FSAA resolve.
Radeon HD 4890 has the same interfaces as its predecessor. In its standard configuration it is equipped with two dual-link DVI-I ports supporting up to 2560x1600 resolutions and audio-over-HDMI (with adapter). A 7-pin mini-DIN port offers support for analog video output in S-Video, Composite and YPbPr formats. And finally, two CrossFire interfaces allow uniting up to four Radeon HD 4890 cards into a single CrossFireX subsystem.