Energy Micro claims “most energy-efficient MCU”

EDN Europe, 21 Oct 2009

Energy Micro’s EFM32 Gecko family of microcontrollers is based on the ARM Cortex-M3 core; matched with a set of peripherals tuned for energy-efficiency, the result is an MCU design that betters other chips vying for the same market niche, by a factor of four in energy consumption, its designers claim. That is, in battery-powered, low-duty-cycle applications, you might expect to achieve several times the battery life that other chips will deliver. Energy Micro is based in Oslo, Norway and is one of a number of companies that have sprung up in that location to deliver low-power processor and RF designs; CEO Geir Førre co-founded Chipcon and ran it for some time after its sale to Texas Instruments, before founding Energy Micro as a start-up.

Many of the advantages that the company claims for the EFM (“energy-friendly microcontroller”)32G arise from careful design of the analogue and mixed-signal aspects of the chip. Running example code (a prime number search algorithm) from flash memory, the chip uses under 180 μA/MHz. There are five levels of standby, ranging from 900 nA while running real-time clock, power-on reset, brown-out detector with full RAM and CPU state retention, to under 20 nA (from 2V) in the deepest-sleep mode. Noting that energy consumed from the battery is the product of power and time, the EFM32G’s designers reduced its demands by running at lower currents, and by taking less time to carry out typical tasks; by achieving a shorter time to wake to full operating speed, and shorter run-times to carry out typical tasks before shutting down again. There are, Førre says, no fundamentally new design techniques; “just careful applications of classical techniques such as clock gating, plus a very efficient interface to flash memory.” The wake-up time to full operation is under 2 μsec; one technique that assists this figure is that the chip has RC oscillators that start operation, with almost instantaneous stable clock generation, automatically switching to crystal oscillators when their output stabilises. Analogue design techniques also contribute to the low standby current, the use of a floating decoupling capacitor while in power-down mode being one example.
You can also carry out some operations without waking the core at all, under control of a state-machine-like function. Using a separate bus called the peripheral reflex system, you can, for example, use a timer to invoke an A/D conversion and store the results in RAM without waking the core. The ADC itself, an 8-channel block, runs 12-bit conversions at 1 Msample/sec for 200 μA; low data-rate, low-resolution tasks (6-bits and 1 ksamples/sec) use just 500 nA. There is an integrated LCD controller that will directly drive a 4x40 segment display with features such as blinking and up to 16 frames of animation, for a current drain of under 1 μA (not including the on-chip voltage boost function that will give you enough drive voltage for good contrast if you are operating the chip close to its lower supply limit, and core voltage, of 1.8V.) A low-energy UART operates from a 32-kHz clock to give full 9600 Baud data for 100 nA current consumption.

A development kit features a host board that provides full energy measurements (of the power the MCU is using), plus a plug-in breadboard panel on which you can develop circuitry; it will sell for $299, with the chips costing $1.55 (100,000). As an implementation of the Cortex-M3, the standard ARM tool chain applies – initially, Energy Micro will supply Keil and IAR compilers, the free version that come with the development kit being limited to 32k of flash. Energy Micro will offer 22 different EFM32G configurations over the next few months, in packages including (initially) QFN64 and BGA112, adding QFN32 and QFP100 later. The EFM32G operates from a single supply rail of between 1.8 and 3.8V. The operating temperature range is –40C to +85C. The microcontrollers provide up to 128 kbytes of Flash memory and up to 16 kbytes of RAM.