Radeon HD 6870: PCB Design and Cooling System
The new Radeon HD generation is much different from the older one. Instead of curvy lines and rounded-off edges, we now see a stern design with sharp corners. We wouldn’t say the new design of the cooler casing can affect anything in terms of performance, but it really makes it impossible to confuse a Radeon HD 6870 with a Radeon HD 5870 or HD 5850, especially as the new card is a couple of centimeters longer than its predecessor.
As opposed to the Radeon HD 5870, the Radeon HD 6870 lacks a metallic heat-spreading back-plate on the reverse side of the PCB. This part of the card is overall quite unexciting and lacks any interesting design features save for one CrossFire slots instead of two on the Radeon HD 5800 series. The most important things are hidden inside, of course. When we took the cooler off, we saw the following:
The first thing to catch our eye is the nonstandard layout of the power circuit. The four-phase GPU voltage regulator is located not in its usual place at the back of the PCB, but at the front, right next to the DVI, HDMI and DisplayPort connectors. It is based on integrated packs that combine MOSFETs and drivers. This unusual layout may help improve the cooling of the power components, but we must confess it’s the first time we’ve ever seen such a solution.
The GPU voltage regulator is based on a CHL8214 controller from CHiL Semiconductor. Such controllers are rather rare on modern graphics cards. We’ve only seen one on the Nvidia GeForce GTX 480. According to its product brief, the CHL8214 is a senior model in its series.
A small uP6122 chip from uPI Semiconductor is responsible for powering the graphics memory. It is located together with its accompanying chips in its traditional place near the external power connectors. Both connectors are of the 6-pin variety and have a load capacity of 75 watts. Considering the simplified design of the Barts processor compared to the RV870, they should be enough to power up this Radeon HD 6870 despite its higher GPU voltage (1.175 volts). The developer had to increase the latter parameter to make the GPU stable at a clock rate of 900 MHz. The PCB design does not provide for 8-pin power connectors with higher load capacity.
The Radeon HD 5870 used to come with memory chips from Samsung Semiconductor whereas the Radeon HD 6870 is equipped with H5GQ1H24AFR chips from Hynix. They have a capacity of 1 Gb (32 Mb x 32) and a rated voltage of 1.5 volts. The T2C suffix in the marking indicates their rated frequency of 1250 (5000) MHz. There are eight such chips on the PCB for a total of 1024 megabytes. The 256-bit memory bus and memory frequency of 1050 (4200) MHz provide a peak memory bandwidth of 134.4 GBps which is almost the same as that of the GeForce GTX 470. The Radeon HD 6870 is unlikely to be let down by a lack of memory bandwidth.
The Barts die has a queer rectangular shape and is much smaller than the RV870. Like in every other solution from AMD, there is no heat-spreading cap in the GPU design. The metallic plate on the die package is the only protection. For the first time in the history of the Radeon family the die lacks an ATI logo. There is an AMD logo instead because AMD has made a decision (a very questionable decision, in our opinion) to give up the ATI brand. The rest of the marking is as incomprehensible as before. The only thing you can learn from it is the manufacturing date of this batch of dies. Here, it is the 36th week of 2010. It means that AMD already had large quantities of Barts cores capable of working at 900 MHz in mid-September.
The GPU-Z utility version 0.4.7 supports the Barts and correctly identifies its configuration, except for the revision number. The lack of a checkmark in the OpenCL checkbox is due to the fact that we used an ordinary version of the AMD Catalyst driver rather than the APP Edition which adds OpenCL support. GPU-Z doesn’t report the number of texture-mapping units but we can tell you that the Radeon HD 6870 has 56 of them. Another popular utility, MSI Afterburner, can identify the new Radeon HD cards as well, but its 2.0.0 version cannot control their GPU voltage. The diagnostics panel makes it clear that the GPU frequency lowers from 900 to 100 MHz in the power-saving mode while the memory frequency drops to 300 (1200) MHz. This should make the new card highly economical at low loads.
As we’ve said above, the new Radeon HD family features unprecedented connectivity options. Indeed, there are as many as five connectors at the card’s mounting bracket: two DVI-I ports, two mini-DisplayPorts, and one HDMI. Judging by the marking, you can only use the bottom DVI-I to establish an analog connection by means of an appropriate adapter. The DisplayPorts support DP++ mode. That is, they can output DVI signal via a passive adapter. As described in the theoretical part of this review, the Radeon HD 6800 supports any monitor configurations. It has only one CrossFire connector, so it looks like you cannot combine more than two Radeon HD 6800s into a single graphics subsystem. This feature must be reserved for the more advanced Radeon HD 6900 series.
The cooling system hasn’t changed much and is free from revolutionary innovations. The memory chips and power components are cooled by an aluminum plate which has thermal pads for contact. The GPU is cooled by an aluminum heatsink with a copper base.
The heatsink is rather small but has as many as three heat pipes, two of which are 8 millimeters in diameter. It is fastened to the PCB with four spring-loaded screws and an X-shaped stiff back-plate. A layer of dark-gray thermal grease is applied in the place of contact with the GPU. As you can see in the photo, the cooler’s casing is designed in such a way as to drive some of the fan’s airflow towards the side panel of the system case because the vent grid in the card’s mounting plate is not large. We wouldn’t say that this cooler makes a strong impression, but the Barts is simpler than the Cypress and is supposed to dissipate less heat. So, it should be satisfied with that cooler notwithstanding the increased voltage. The noise factor is the only issue left to be discussed.