Too much power to the people?

Does AMD have a problem on its hands with the power consumption of the new Radeon RX 480?

UPDATE (7/1/16): I have added a third page to this story that looks at the power consumption and power draw of the ASUS GeForce GTX 960 Strix card. This card was pointed out by many readers on our site and on reddit as having the same problem as the Radeon RX 480. As it turns out…not so much. Check it out!

UPDATE 2 (7/2/16): We have an official statement from AMD this morning.

As you know, we continuously tune our GPUs in order to maximize their performance within their given power envelopes and the speed of the memory interface, which in this case is an unprecedented 8Gbps for GDDR5. Recently, we identified select scenarios where the tuning of some RX 480 boards was not optimal. Fortunately, we can adjust the GPU's tuning via software in order to resolve this issue. We are already testing a driver that implements a fix, and we will provide an update to the community on our progress on Tuesday (July 5, 2016).

Honestly, that doesn't tell us much. And AMD appears to be deflecting slightly by using words like "some RX 480 boards". I don't believe this is limited to a subset of cards, or review samples only. AMD does indicate that the 8 Gbps memory on the 8GB variant might be partially to blame – which is an interesting correlation to test out later. The company does promise a fix for the problem via a driver update on Tuesday – we'll be sure to give that a test and see what changes are measured in both performance and in power consumption.

The launch of the AMD Radeon RX 480 has generally been considered a success. Our review of the new reference card shows impressive gains in architectural efficiency, improved positioning against NVIDIA’s competing parts in the same price range as well as VR-ready gaming performance starting at $199 for the 4GB model. AMD has every right to be proud of the new product and should have this lone position until the GeForce product line brings a Pascal card down into the same price category.

If you read carefully through my review, there was some interesting data that cropped up around the power consumption and delivery on the new RX 480. Looking at our power consumption numbers, measured directly from the card, not from the wall, it was using slightly more than the 150 watt TDP it was advertised as. This was done at 1920×1080 and tested in both Rise of the Tomb Raider and The Witcher 3.

When overclocked, the results were even higher, approaching the 200 watt mark in Rise of the Tomb Raider!

A portion of the review over at Tom’s Hardware produced similar results but detailed the power consumption from the motherboard PCI Express connection versus the power provided by the 6-pin PCIe power cable. There has been a considerable amount of discussion in the community about the amount of power the RX 480 draws through the motherboard, whether it is out of spec and what kind of impact it might have on the stability or life of the PC the RX 480 is installed in.

As it turns out, we have the ability to measure the exact same kind of data, albeit through a different method than Tom’s, and wanted to see if the result we saw broke down in the same way.

Our Testing Methods

This is a complex topic so it makes sense to detail the methodology of our advanced power testing capability up front.

How do we do it? Simple in theory but surprisingly difficult in practice, we are intercepting the power being sent through the PCI Express bus as well as the ATX power connectors before they go to the graphics card and are directly measuring power draw with a 10 kHz DAQ (data acquisition) device. A huge thanks goes to Allyn for getting the setup up and running. We built a PCI Express bridge that is tapped to measure both 12V and 3.3V power and built some Corsair power cables that measure the 12V coming through those as well.

The result is data that looks like this.

powertesting.jpg

What you are looking at here is the power measured from the GTX 1080. From time 0 to time 8 seconds or so, the system is idle, from 8 seconds to about 18 seconds Steam is starting up the title. From 18-26 seconds the game is at the menus, we load the game from 26-39 seconds and then we play through our benchmark run after that.

There are four lines drawn in the graph, the 12V and 3.3V results are from the PCI Express bus interface, while the one labeled PCIE is from the PCIE power connection from the power supply to the card. We have the ability to measure two power inputs there but because the GTX 1080 only uses a single 8-pin connector, there is only one shown here. Finally, the blue line is labeled total and is simply that: a total of the other measurements to get combined power draw and usage by the graphics card in question.

From this we can see a couple of interesting data points. First, the idle power of the GTX 1080 Founders Edition is only about 7.5 watts. Second, under a gaming load of Rise of the Tomb Raider, the card is pulling about 165-170 watts on average, though there are plenty of intermittent, spikes. Keep in mind we are sampling the power at 1000/s so this kind of behavior is more or less expected.

Different games and applications impose different loads on the GPU and can cause it to draw drastically different power. Even if a game runs slowly, it may not be drawing maximum power from the card if a certain system on the GPU (memory, shaders, ROPs) is bottlenecking other systems.

One interesting note on our data compared to what Tom’s Hardware presents – we are using a second order low pass filter to smooth out the data to make it more readable and more indicative of how power draw is handled by the components on the PCB. Tom’s story reported “maximum” power draw at 300 watts for the RX 480 and while that is technically accurate, those figures represent instantaneous power draw. That is interesting data in some circumstances, and may actually indicate other potential issues with excessively noisy power circuitry, but to us, it makes more sense to sample data at a high rate (10 kHz) but to filter it and present it more readable way that better meshes with the continuous power delivery capabilities of the system.

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Image source: E2E Texas Instruments

An example of instantaneous voltage spikes on power supply phase changes

Some gamers have expressed concern over that “maximum” power draw of 300 watts on the RX 480 that Tom’s Hardware reported. While that power measurement is technically accurate, it doesn’t represent the continuous power draw of the hardware. Instead, that measure is a result of a high frequency data acquisition system that may take a reading at the exact moment that a power phase on the card switches. Any DC switching power supply that is riding close to a certain power level is going to exceed that on the leading edges of phase switches for some minute amount of time. This is another reason why our low pass filter on power data can help represent real-world power consumption accurately. That doesn’t mean the spikes they measure are not a potential cause for concern, that’s just not what we are focused on with our testing.

Setting up the Specification

Understanding complex specifications like PCI Express can be difficult, even for those of us working on hardware evaluation every day. Doing some digging, we were able to find a table that breaks things down for us.

pcie-power-rail-2-0.png

We are dealing with high power PCI Express devices so we are only directly concerned with the far right column of data. For a rated 75 watt PCI Express slot, power consumption and current draw is broken down into two categories: +12V and +3.3V. The +3.3V line has a voltage tolerance of +/- 9% (3.03V – 3.597V) and has a 3A maximum current draw. Taking the voltage at the nominal 3.3V level, that results in a maximum power draw of 9.9 watts.

The +12V rail has a tolerance of +/- 8% (11.04V – 12.96V) and a maximum current draw of 5.5A, resulting in peak +12V power draw of 66 watts. The total for both +12V and +3.3V rails is 75.9 watts but noting from footer 4 at the bottom of the graph, the total should never exceed 75 watts, with either rail not extending past their current draw maximums.

Diving into the current

Let’s take a look at the data generated through our power testing and step through the information, piece by piece, so we can all understand what is going on. The graphs built by LabVIEW SignalExpress have a habit of switching around the colors of data points, so pay attention to the keys for each image.

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Rise of the Tomb Raider (1080p) power draw, RX 480, Click to Enlarge

This graph shows Rise of the Tomb Raider running at 1080p. The yellow line up top is the total combined power consumption (in watts) calculated by adding up the power (12V and 3.3V) from the motherboard PCIe slot and the 6-pin PCIe power cable (12V). The line is hovering right at 150 watts, though we definitely see some spiking above that to 160 watts with an odd hit above 165 watts.

There is a nearly even split between the power draw of the 6-pin power connector and the motherboard PCIe connection. The blue line shows slightly higher power draw of the PCIe power cable (which is forgivable, as PSU 6-pin and 8-pin supplies are generally over-built) while the white line is the wattage drawn from the motherboard directly.

Below that is the red line for 3.3V power (only around 4-5 watts generally) and the green line (not used, only when the GPU has two 6/8-pin power connections).

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Rise of the Tomb Raider (1080p) power draw, RX 480, Click to Enlarge

In this shot, we are using the same data but zooming on a section towards the beginning. It is easier to see our power consumption results, with the highest spike on total power nearly reaching the 170-watt mark. Keep in mind this is NOT with any kind of overclocking applied – everything is running at stock here. The blue line hits 85 watts and the white line (motherboard power) hits nearly 80 watts. PCI Express specifications state that the +12V power output through a motherboard connection shouldn’t exceed 66 watts (actually it is based on current, more on that later). Clearly, the RX 480 is beyond the edge of these limits but not to a degree where we would be concerned.

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The Witcher 3 (1080p) power draw, RX 480, Click to Enlarge

The second game I tested before the controversy blew up was The Witcher 3, and in my testing this was a bigger draw on power than Rise of the Tomb Raider. When playing the game at 1080p it was averaging 155+ watts towards the end of the benchmark run and spiking to nearly 165 watts in a couple of instances.

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The Witcher 3 (1080p) power draw, RX 480, Click to Enlarge

Zooming in a bit on the data we get more detail on the individual power draw from the motherboard and the PCIe 6-pin cable. The white line of the MB +12V power is going over 75 watts, but not dramatically so, while the +3.3V power is hovering just under 5 watts, for a total of ~80 watts. Power over the 6-pin connector goes above 80 watts here as well.

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