What’s not quite MCU, and not quite SoC?

There has been a lot of railing lately about how we don’t have quite the right chips for the upcoming wave of wearables. Chips one would drop in a smartphone are often overkill and overpowered, burning through electrons too quickly. Chips one would use for a simple control task generally lack peripherals and performance, offsetting their low power advantage.

One area we can feel this pressure building is in the Arduino community. Authentic Arduino boards have been Atmel AVR based, hosting 8-bit microcontrollers in a low power, easy-to-use, and easy-to-program environment. The popularity of Arduino shields for I/O expansion has driven a comber of compatible boards tossing 16- and 32-bit MCUs, and even some notable low-end SoCs, into the mix. Of course, straying into SoC territory introduces other popular form factors as well.

As with most technology with a sizable following, Arduino is now being forced to grow, as much due to its own success as due to external competition. Having mastered the basics of 8-bit, many makers are now stretching their capability and taking on more ambitious goals. At the same time, the pinout popularized by the Arduino UNO remains in vogue, and changing too many things at once is bad form.

Last week’s debut of the Arduino Zero, and its first real world appearance at Maker Faire Bay Area 2014 over the weekend, marked the launch of the industrial-strength Atmel SAM D21 microcontroller with its ARM Cortex-M0+ core into the hearts and minds of makers. This isn’t the first time a 32-bit ARM MCU has been seen in the Arduino neighborhood, but the endorsement with official Atmel-based hardware is a big step for the community.

A closer look reveals our budding trend of interest. The Atmel SAM D family is built for traditional control tasks, sans on-chip wireless and opting instead for ultra-low power and a mostly MCU-like peripheral mix. Revealed in the block diagram – note, on the Arduino Zero not all these features make it off the board – is a very SoC-like chip architecture, scaled down for power.

The Cortex-M0+ processor core in the SAM D21 connects to three AHB bus segments plus memory ports via a high speed bus matrix, effectively separating traffic for memory, DMA and USB, power and clock management, and mixed signal I/O. One might wonder out loud what “high speed” means given a 48 MHz core frequency and the mix of peripherals shown, but let’s keep the context in mind.

Commenting on the current mania of smartphone SoCs being crammed into wearables with less than stellar results so far, I made a statement a few days ago I’ll stand by:

A typical smartphone chip is 1W – for many wearables, that will be at least one and maybe two orders of magnitude off.

The SAM D21 weighs in at less than 70uA/MHz, meaning fully clocked it pulls somewhere around 3.3mW – in the range of interest. Within that envelope, it can run full speed USB 2.0, unimpeded by other interrupts and without external components in “device” mode. An integrated I[SUP]2[/SUP]S block brings full-rate audio streaming, and the obligatory Atmel integrated peripheral touch controller enables user interfacing. Also on chip are a flexible serial block, a multi-channel A/D, comparators, a D/A, plus a spate of timers.

Strictly speaking, the SAM D21 is still an MCU, demarcated primarily in terms of on-chip flash and RAM and no external memory. By retaining ultra-low power behavior while segmenting buses and rethinking integration for mixed signal, USB, audio, and touch, Atmel has taken a first step into the space between control MCUs and smartphone SoCs – with an eye on what wearables actually need to succeed. It’s a smart move to get the Arduino community on board and find out where this can go.

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