Atmel Corp. (San Jose, Ca.) has begun sampling its SAML21 family in a bid to take the title for lowest power ARM Cortex-M0 MCU away from competitors such as Texas Instruments and ST Microelectronics.
Scheduled to be available in volume quantities in September 2015, the new MCU is fabricated with a 1.6 – 3.6 volt 110 nanometer CMOS that pushes power consumption down to 35 microAmps/MHz in the active mode and to 200nA in sleep mode with full 40KSRAM retention.
Like its competitors, Atmel is claiming leadership based on the benchmark measurements on the EEMBC ULPBench, which has become the industry arbiter for low power (see below). In an interview with EE Times, Oyvind Strom, Atmel’s senior director of Wireless MCUs, said the SAML21 achieved a score of 185, and claimed it was “the highest publicly recorded score for any Cortex-M based processor or MCU.”
He emphasized that no matter what the EEMBC benchmarks say, a far better measure is the range of power savings options that the new family of devices can provide a developer. “We have spent a lot of time in the wireless IoT space working with developers in consumer, industrial, medical, and other battery-powered applications, and from that we have made a range of improvements that will allow them to do better designs.”
Multiple power and clock domains
The prime example of this strategy is the SAML21's use of multiple clock domains to manage power (chart below), allowing different parts of the MCU, such as the core, the internal buses and particular peripherals, to be individually clocked to save power. If a designer does not need maximum throughput from any particular peripheral, the clock can be lowered. Not only that, individual clocks can used on different peripherals so they can be fully optimized with power consumption in mind.
“When used in combination with the SAML21's internal event management system, a developer can fine tune what to turn off and what to operate at a lower clock rate and thus create sophisticated customized wake-up conditions for the various functional blocks, either individually or together, through cascading.”
Company designers have not only expanded the number of peripheral logic direct memory access (DMA) channels on from 12 to 16, they have made use of a unique ‘sleepwalking’ feature. It allows the DMA to operate in a lower power standby mode and come back to active when needed. To give developers more fine-grained control over this feature, company engineers have incorporated additional priority levels on the DMA channels.
Strom said sleepwalking allows peripherals to request a clock without CPU intervention for situations where it is necessary to wake up from sleep modes and perform tasks without having to power up the CPU, flash, or other support systems. This feature can also be used to manage analog signal acquisition and measurement through integrated op-amps or ADCs while the CPU idles, greatly extending battery life for analog and digital sensor applications. In addition to DMA transfers, sleepwalking can be used for all of the MCU's serial communications channels, including I2C and UARTs.
To bring an additional level of configurability to power management, the SAML21 also incorporates a customizable logic block (CLB) with up to four programmable, three input look up tables (LUTs) capable of a range of combinatorial and sequential logic functions. They not only make power management of a design even more precise, but can be used to do such things as filtering out signals to prevent unnecessary CPU wake-ups. CLB outputs can be connected either to the IO pins or the internal peripheral event system. Also there is an optional synchronizer, filter, or edge detector available on each LUT output.
Strom said that in this iteration of the SAM architecture special attention has been given to power management of particular functions important to IoT and wearable developers. One such function is an ultra-low power capacitive touch sensing peripheral that can run in all operating modes and supports wake-up on touch. To achieve this, a touch accelerator was added to reduce CPU load and reduce overall power consumption. It allows the MCU to remain powered while the rest of the system is sleeping. “In many applications it is important for the application to become immediately active and responsive to the user, but not at the cost, in power, of keeping the entire device active.”
Because USB is so widely used in a variety of embedded IoT and consumer wearable apps, Atmel has added the ability to control the sleep modes in the peripheral logic controlling that function. “Specifically, the USB module can put the MCU in any sleep mode when the USB bus is idle and a suspend condition is given. And when bus activity resumes, the USB module can wake the MCU from any sleep mode.”
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