The GPU die, manufactured on 40nm tech process, is the same size (255 sq. millimeters) and has the same transistor count (1.7 billion) as the GPUs of Radeon HD 68xx series products. Well, that’s because we’ve got a Barts LE chip here.
The GPU incorporates 800 shader processors, i.e. 160 processors less than in the Radeon HD 6850. The number of texture-mapping units is 40 (the HD 6850 has 48 TMUs). It is the raster operators that have been cut down most severely. There are only 16 ROPs here as opposed to the HD 6850's 32. AMD wants to make up for such a dramatic hardware modification by increasing the clock rate to 840 MHz, which is 65 MHz higher than the HD 6850's. The GPU voltage of our sample is 1.175 volts but we are not sure that off-the-shelf samples of this card are going to have the same voltage. The GPU’s voltage and clock rate are reduced to 0.9 volts and 100 MHz, respectively, in 2D applications.
The Radeon HD 6790 comes with 1 gigabyte of GDDR5 memory in 16 BGA-packaged chips installed on the face side of the PCB.
These are H5GQ1H24AFR T2C chips from Hynix. They have a voltage of 1.5 volts and a rated frequency of 5000 MHz. The graphics card’s memory frequency is actually 4200 MHz, so there are good overclocking opportunities. The frequency and voltage of the memory chips are reduced to 600 MHz and 1.35 volts in 2D mode. Most importantly, the memory bus is 256 bits wide, just like in the Radeon HD 68xx series, so the Radeon HD 6790 should have an advantage over the Radeon HD 5770 at high resolutions. Of course, this will only make a practical point if the card is able to deliver a playable frame rate at such resolutions.
The GPU-Z utility has the following to report about the Radeon HD 6790 graphics card:
Now let’s examine the card’s cooling system.
It represents a classic design consisting of a small aluminum heatsink with heat pipes for the GPU, a heatsink for the memory chips and power circuit components, a blower and a casing.
The heatsink on the Barts LE chip has three copper heat pipes, two of which are 6 millimeters in diameter and the third one is 8 millimeters in diameter. The pipes go out of the copper base which is 2 millimeters thick at its thinnest point.
There are slim aluminum fins soldered to the pipes. The fins are additionally soldered to the copper base. The heatsink isn't large but perhaps that’s quite enough for a Radeon HD 6790. We’ll check its efficiency out shortly.
The cooler is equipped with a 9.6W FD9238H12S blower from NTK (HK) Limited.
The blower’s speed is varied automatically within a range of 1200 to 4500 RPM.
In order to check out how hot the Radeon HD 6790 and other graphics cards included into this test session are, I ran the benchmark from Aliens vs. Predator (2010) in five cycles with maximum graphics quality settings (at 1920x1080 with 16x AF but without FSAA). It is at these settings that the GPUs had their highest temperatures. I used MSI Afterburner 2.2.0 beta 2 and GPU-Z version 0.5.3 as monitoring tools. Every test was carried out in a closed system case at an ambient temperature of 25°C.
The following diagrams show the temperature of the card with its fan controlled automatically (on the left) and at the maximum speed of the fan (on the right):
As you can see, the GPU is as hot as 76°C in the automatic mode, the fan working at up to 2040 RPM. This is not very loud, yet the card made itself heard in my rather quiet system case. At the maximum speed of the fan the Radeon HD 6790 is no hotter than 60°C but its fan is unbearably loud.
Unfortunately, my sample of Radeon HD 6790 wasn’t good in terms of overclocking. I could only increase the GPU clock rate to 910 MHz (+8.3%) and the memory clock rate to 4600 MHz (+9.5%) without having any stability issues or visual artifacts.
Hopefully, off-the-shelf samples of this card will have higher frequency potential.
Here’s the temperature of my overclocked Radeon HD 6790:
The top GPU temperature is now 80°C while the fan speed is 2170 RPM.