When it comes to applications including the Internet of Things (IoT), consumer, industrial, medical, and other battery-powered devices — e.g., fire alarms, healthcare, medical, wearable, and devices placed in rural, agriculture, offshore, and other remote areas — ultra-low-power consumption is the name of the game.
MCU manufacturers are constantly competing with each other to offer the lowest power consumption possible. The latest ultra-low-power offering comes from the folks at Atmel, who have just announced their SMART SAM L21 — an ARM Cortex-M0+ based family of MCUs that boast power consumption down to 35µA/MHz in active mode and 200nA in sleep mode — which is said to “Extend battery life from years to decades.”
In the case of the EEMBC ULPBench the industry-standard benchmark for low power when it comes to MCUs, the SAM L21 achieves a score of 185 (higher is better), which — according to the folks at Atmel — is currently the highest publicly-recorded score for any Cortex-M-based processor or MCU in the world.
Atmel's SAM L21 family consumes less than 940nA with full 40KB SRAM retention, real-time clock and calendar and 200nA in the deepest sleep mode. One of the ways Atmel achieves these values is “thinking beyond the core.” This involves multiple power domains, the way in which the clocking system is implemented, the ways in which the regulators are implemented, and the use of smart peripherals that can make their own simple decisions without having to wake the main processor core.
The L21 goes much further than simply gating the clocks — it also gates the power, completely disconnecting the power rails from functions that are not currently in use. In the case of the smart peripherals, even when they are powered down, a small part of each peripheral keeps a “watchful eye” on what's happening in the outside world. If it sees something interesting, it can request clock and data services and — if the peripheral decides the situation justifies such an action — it can wake the main CPU.
Observe the PD0 power domain in the image above. This includes simple, low-power communications functions (I2C, SPI, UART), timers, peripheral touch controller (PTC), and so forth. In the case of the PTC, when the device is in sleep mode, all of the individual XY touch locations are made to look like “one big button,” which helps to dramatically reduce the power consumption (on wake-up, it's still possible to determine the individual location that was pressed). The resulting sub-2µA power consumption makes it possible to have always-on capacitive touch capability to wake up the system.
Also of interest is the CCL (custom configurable logic) block, which boasts four 3-input lookup tables (LUTs) that can implement a mix of combinatorial logic functions (AND, NAND, OR, NOR, XOR, XNOR, NOT) and sequential logic functions (gates D-type flip-flop, JK-type flip-flop, gated D-type latch, RS latch). These can be connected to the event system (including the peripherals), the interrupt system, and general-purpose input/outputs; also, they can be cascaded together. This makes it possible to implement sophisticated customized “wake-up” conditions for the various functional blocks.
Also available is Atmel's SAM L21 Xplained Pro — a professional evaluation board featuring auto-identification in Atmel Studio with an on-board debugger and standardized extension connectors.
The SAM L21 Xplained Pro is fully supported by Atmel Studio, a free IDE that features power profiling tools for the SAM L21 board, making it easy to pinpoint power peaks and calculate average power consumption. Using the Xplained Pro and Atmel Studio, developers can monitor real-time power consumption in conjunction with the program counter (PC); if a power consumption spike appears, the developer can loop back to see what's causing it in the code, and then modify the code accordingly.
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