Under deconstruction

Low cost ARM computers

by Ketil Malde; June 15, 2012

The ARM CPU is one of the oldest RISC architectures, dating back to the Acorn. Although MIPS and PowerPC still hold some territory among embedded systems, ARM is by all indications the most ubiquitous architecture for general computing. Until the $25 Raspberry PI, ARM computers tended to cost as much as a low end PC, and thus they were mostly for enthusiasts. With prices well below the $100 mark, a large market for low power devices seems to be emerging.

The ones I’ve looked at comes with either an ARM11 (which confusingly enough is an incarnation of the ARMv6 architecture) or a Cortex-A (ARMv7) CPU. The latter is about twice as fast at the same clock speeds, and tends to be clocked 20-50% higher as well. For many applications (e.g. HTPC), GPU is important, and here things get even more complicated. The current generation consists of the Mali 400MP (used in Samsung Galaxy SII/III), nVidia’s Tegra 3 (HTC One X), and the PowerVR SGX543 (iPhone 4S). From the benchmarks, these appear to be roughly equivalent in performance.

So, let’s see what we’ve got:

Raspberry PI

The Rπ is a $25 or $35 device, sporting a Broadcom BCM2835 System-on-Chip. This SoC contains an ARM1176JZFS (ARMv6) core at 700Mhz. It is said to include a floating-point-unit, which makes me wonder if the others don’t?

The GPU is a Videocore 4 GPU. It advertises 1Gpixel/s, 1.5Gtexel/s, and 24 GFLOPs, I’ve seen other numbers that the Mali 400MP does 10GFLOPs at 300MHz, so this looks pretty strong. This seems a bit marketing-oriented, but also indicates substantially better graphics performance for the Rπ. At any rate, it should be able to do 1080p at 30 FPS and decode H.264 video, so it should be sufficient for most HTPC needs.

It has USB, HDMI, RCA video, and audio outputs, and the extra $10 for the B model buys you ethernet and an extra USB port. It boots from the SDcard. If you want wifi, you’ll need a dongle. Finally, it has a set of GPIO pins, so you can program it to blink leds, listen to button presses, or open your garage door.

VIA APC 8750

The APC contains a VIA WonderMedia ARM 11 (also ARMv6) SoC running at 800MHz, and it is equipped with 512MB RAM and 2GB flash. The form-factor is Neo-ITX, which means that it will fit into existing PC cases. Connectivity is good, with VGA, HDMI, 4xUSB 2.0, a microSD slot, 10/100 Eth, audio, and mic connectors - but no wifi. Apparently it doesn’t boot from SD, which means you might be able to brick it by flashing it with a broken bootloader. Also, the ads say 720p video only, but the SoC claims to support 1080p. At any rate, it looks like a rather weak option for graphics.

Rikomagic MK802 and Mele A1000

The AllWinner A10 SoC contains a single-core 1GHz Cortex 8 (and thus ARMv7) CPU paired with a Mali 400MP GPU. CPU performance can thus be expected to be a substantial improvement over the ARM11-based solutions, with decent if not quite cutting-edge graphics. In addition to performance, Ubuntu officially supports ARMv7 (but not v6), and it also advertises 2160P video - four times the pixels of normal full HD video. Good luck finding a matching display.

The Rikomagic MK802 is a tiny “thumb drive” form factor. It comes with 512 or 1024MB RAM, 4G flash in addition to the SD card slot (which it boots from), and contains wifi, HDMI, and USB 2.0. The link above claims 1.5GHz and Mali 400, but Ali Express says 1GHz, and perhaps more importantly, that the GPU is “OpenGL ES2.0 (AMD Z430) / OpenVG1.1(AMD Z160)”. Now, as far as I can tell, the AMD Z430 is also known as Adreno 200, and is an old GPU with about 10% of the Mali’s performance. It doesn’t seem to make much sense, since the Mali is claimed to be built into the SoC, but I wonder what this is supposed to mean.

The Mele A1000 is the flexible alternative. It, too, comes with a case, but it’s of the more usual set-top-box type, and the benefit is more flexibility, adding SATA, 4xUSB 2.0, VGA, and digital audio out to the MK802s outputs.

You can buy either at ~$74 (with an extra $10 to get the 1GB RAM MK802), and although it’s the most expensive options, it’s still quite cheap: the similar form-factor Cotton Candy costs $200, although it has a dual-core Cortex A9 and 1GB. Also, it looks like a bare A10 board is in the works. This could be an interesting competitor to the Rπ.

Summary

So, to sum up: for a cheap board with GPIO, and if you don’t mind acquiring a case separately, get the Raspberry PI. If you want to use an existing PC case, the VIA board might work out, but otherwise the extra $20 for the Mele A1000 is probably a better deal since you get a box, more CPU and GPU power, and built-in wifi. If you value discression over connectivity, the MK802 looks pretty good.

At less than $75 (and certainly at $25), I think a barrier has been broken. If you’re moderately enthusiastic, you can pick one of these just for fun. Just like apps and smartphones, enthusiast will come up with new and interesting ways to use these in all kinds of settings. But currently, the most interesting aspect these have in common is availability. Specifically, that there is none: they all seem to be out of stock. Another important challenge is software availability. The CPU is perhaps simple enough (although I’m told the Linux ARM tree is rather complicated), but the GPUs are a mess of proprietary, closed technology. There is an Open Source driver (called Lima) for the Mali GPUs, but it’s unclear how well it works, and all the sites I’ve looked at were at most able to bring up a rather shaky Ubuntu or Android. These two issues must be addressed, but as soon as they are, I think there is a pretty sizable market out there for whomever manages to churn out the devices in sufficient numbers, and manages to get good - preferably open source - drivers letting people take advantage of GPUs and peripherals.

Follow-ups

Discussion on Google+

There’s also a table over at socialcompare.com that lists the features of available low-cost ARM boards.

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