原创 A different strategy for low power MCUs

One gauge of whether a high technology company has the ability to survive in a fast changing market is its ability to march to a different drummer occasionally; that is, to devise strategies that run counter to the accepted wisdom.

One such company is Silicon Labs, which this week introduced the EFM32 Happy Gecko family: nine low power "energy-friendly" 32-bit MCUs based on the ARM Cortex M0+.

New Cortex-M0+ -based processor from Silicon Labs has five USB-optimized energy optimizing modes.

 

With this new processor family, the company is marching to its own drummer in three ways:

- Rather than trying to compete with the likes of TI, Atmel and ST Micro in the race for the title of lowest power general purpose M0-based MCU, Silicon Labs has instead focused on bringing to market the lowest power ARM derivative aimed at USB applications.

- While other semiconductor vendors are developing small foot print architectures aimed at the emerging USB 3.0 Type C spec, Silicon Labs is going after the broader opportunities for older USB 2.0 and 3.0 connectors as well as the Type C...

- While other companies are rushing to incorporate the USB Implementer Forum's USB Power Delivery 2.0 spec, it Silicon Labs is implementing its own new Low Energy Mode (LEM) methodology, based on technologies developed in house as well as those that came with its acquisition of Energy Micro and its Gecko family of low power ARM processors. This does not mean that Silicon Labs is not ignoring the potential of the USB 3.0 Type C connector, nor the USB Power Delivery 2.0 specification. When the time is right they will extend their attention to both. The company is currently supporting USB 2.0 full speed through a Type C connector, but not the associated power spec.

Where a typical USB transceiver stays in “receive” mode when idle, wasting 3–5 mA, with LEM techniques it is kept in a low current mode (green) similar to suspend.

 

"Right now we are focused on bringing low power M0+ based MCUs to traditional USB 2.0 in such things as smart metering, home and building automation, alarm and security systems, smart accessories and wearable devices," said Øivind Loe, Senior Product Manager for 32-bit MCUs at Silicon Labs, in an interview with EETimes. "What designers are facing right now is the problem of how to improve the power efficiency of current USB connections."

Starter kit offers access to tools to optimize power in USB-based MCU designs.
(Source: Silicon Labs)

 

Loe said the company is taking advantage of a shift in the embedded IoT market away from traditional serial communications interfaces such as I2C to USB with its no-brainer plug-and-play functionality. But with traditional approaches USB does not come free where power is concerned, with most current implementations at least doubling application current consumption.

However, with a collection of power-savings technologies the company has built into its USB Low Energy Mode (LEM) framework, Loe claims the new family provides IoT developers lower USB power drain than competing MCU alternatives.

LEM is an advanced energy management system with five energy modes that allow designs based on the new EFM-32 based MCUS to remain in an energy-optimal state by spending as little time as possible in active mode. It does this without compromising response times.

In deep-sleep mode, the new MCUs have a stand-by current consumption of only 0.9 microamperes (with a 32.768 kHz RTC, RAM/CPU state retention, brown-out detector, and power-on-reset circuitry active). With real-world code, active-mode power consumption drops down to 130 µA/MHz at 24 MHz. The USB MCUs further reduce power consumption with a 2-microsecond wakeup time from standby mode.

One of the important elements in the LEM is incorporation of a Peripheral Reflex System (PRS). With six channels, the PRS monitors system-level events in a way that allows different peripherals to communicate with each other autonomously without CPU intervention. "It also watches for specific events to occur before waking the CPU, thereby keeping the Cortex-M0+ core in an energy-saving standby mode as long as possible," said Loe.

Complementing the LEMs are a number of low energy peripherals used in previous EFM-32 designs: an analog comparator, supply voltage comparator, on-chip temperature sensor, programmable current digital-to-analog converter (IDAC), and a 12-bit analog-to-digital converter (ADC) with 350 µA current consumption at a 1 MHz sample rate.

The new family also incorporates a number of improvements that allow the MCUs to be incorporated into space-saving QFN, QFP, and chip-scale package (CSP) options small enough for use in USB connectors and thin-form-factor wearable designs. These include crystal-less USB operation, integrated PHY, an on-chip regulator and resistors, integrated 5 V LDO, and dedicated RAM for endpoints. Some of these also have power-saving side effects. For example, by integrating the PHY on chip, a designer has much more control over the most power-hungry parts of the PHY, making possible a reduction in the MCU's suspend current from the typical 3 milliAmpere down to 3 microAmperes, including the draw current of the PHY.

To make it easy to implement their USB optimized MCUs into embedded IoT devices Silicon Labs is offering the SLSTK3400A starter kit. The kit provides two product development pathways. One is via the company's Simplicity Studio with its battery estimator and visual pin configurator tools as well as access to USB source code and software examples at no charge. The other is via the ARM mbed ecosystem, which incorporates new power management APIs developed by Silicon Labs and ARM.

Although it is somewhat like comparing apples and oranges, in a rough comparison of the Happy Gecko family with the general purpose ARM alternatives ranked by the EEMBC ULPBench tool, the Happy Gecko did well. Given the impressive track record the company’s engineers have racked up with previous Geckos, I will be interested to see what they do with the USB Implementer Forum's USB Power Delivery 2.0 specification.