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How Qualcomm's S4 Krait processors take on NVIDIA Tegra 3, TI OMAP5

News by Michael Oryl on Tuesday March 06, 2012.

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I met with Qualcomm at Mobile World Congress last week to talk about the current state of its line of mobile processors, just as I did with TI to talk about OMAP5.

Like TI, Qualcomm is moving to an updated processor architecture for its latest family of Snapdragon S4 processors. While not being pure ARM Cortex-A15 compatible processors, Qualcomm does admit that the new Krait cores in the S4 line are A15-like, just as the Scorpion cores in the S3 were based on ARM Cortex-A9 type specs. Qualcomm licenses ARM's technology so that it can use it to build its own integrated chipset solutions that are compatible, but not exact copies of ARM's reference designs. This allows Qualcomm to do some things a bit differently.

One of the differences that Qualcomm likes to tout is the ability of its dual-core and quad-core Krait processors to run asymmetrically, with each core running at a different clock speed. Unlike NVIDIA's Tegra 3 and Tegra 2 processors, or TI's OMAP family of processors, the individual Krait cores in a Snapdragon S4 don't all have to be running at the same speeds.

Such an approach makes sense given the lack of proper multi-threading support we see in Android applications today. And even when a smartphone or tablet truly is taking care of multiple tasks at the same time, how often is it that all such tasks are equally CPU intensive?

The answer is "not very often."

Consider editing an HD video on your phone (intensive) while Twitter updates in the background (non-intensive). On a traditional ARM system that offers only synchronized processor cores, such as those offered by TI, NVIDIA, and others, you might have both cores running at 1.5GHz, due to the demands of video editing, even though Twitter could get by with a much more power-frugal 400MHz speed.

On a Qualcomm processor, you can do just that. In fact, the company showed a demo of a tablet running on a quad-core S4 Pro chipset (4 Krait cores) with each running at different speeds, depending on the need. Some intensive HD video tasks had a core running at near full speed, drawing heavily from the device's battery, while paused videos idled along at much more pedestrian clock speeds that sipped power.

This is where asynchronous architectures like Qualcomm's shine.

NVIDIA, for its part, has tried to address that with a power efficient "companion core" in the quad-core Tegra 3. The companion core is the "PLUS-1" in the company's "4-PLUS-1" marketing efforts. That lone, power efficient core is used during all those times when not much is going on, such as the aforementioned background Twitter update. On top of that, a Tegra 3 processor can turn off the companion core and use anywhere from 1 to 4 of its main cores at a time, allowing it to scale up as needed - even if all cores have to run at the same speed. It's not asymmetric, but it's better than having to run all 4 cores all the time.

But as I mentioned in the TI OMAP5 article, ARM Cortex-A15 based processors are faster and more efficient than the A9 cores used by NVIDIA and others. And in the case of Qualcomm's S4 Krait cores, part of that efficiency comes in reduced heat output. Even though it won't make your phone run faster, heat output from your device's CPU is still wasting battery power. Qualcomm has gone to great lengths to increase the thermal efficiency of its processors, which, in turn, makes them more energy efficient overall.

The company showed me a infrared photograph of three devices taken at differing time intervals. In the photo we could see that the smartphone running Qualcomm's processor remained cool even after 15 minutes of high-speed processing. A TI OMAP4 processor in a Motorola DROID RAZR ran hotter across the timed shots, but nowhere nearly as hot as what appeared to be a Tegra-based device.

Qualcomm Snapdragon S4 Krait architecture and thermal efficiency
Qualcomm Snapdragon S4 Krait architecture and thermal efficiency

And as we established, heat matters when it comes to battery efficiency. It also matters when it comes to user comfort. I can tell you from personal experience that a Motorola DROID RAZR runs hot. I expect Tegra 3 devices like HTC's One X and LG's Optimus 4X HD to run even hotter under load.

Part of that thermal efficiency comes from Qualcomm's modern 28nm process that it builds its chips with, which compares very favorably with NVIDIA's dated 40nm process.

Qualcomm brings more to the table, though. It's integrated Adreno graphics co-processors are very fast, and the new Adreno 320 GPU in the recently announced S4 Pro line of quad-core chipsets offers four times as much graphics processing power than the already impressive Adreno 225 GPU used today.

Qualcomm says that it has about 120 devices in the pipeline that use its chipsets, and that it's going to be offering its 4G LTE(INFO) Gobi branded chips in new device types this year. Qualcomm might be a bit late getting into the quad-core market, but the company believes it already offers processors that can stand toe to toe with the best that Texas Instruments and NVIDIA can offer today.

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About the author

Michael Oryl
Michael is the Philadelphia based owner and former editor-in-chief of MobileBurn.com. You can follow him on Twitter as @MichaelOryl

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