Silicon Labs opts for security on M0+ low-power MCUs

By Chris Edwards |  No Comments  |  Posted: October 9, 2013
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Silicon Laboratories has launched the first Cortex M0+-based microcontroller to include a hardware AES encryption unit and which adds a current DAC that offers just 10nA of overhead that will run in the MCU’s lowest-power sleep mode.

The Zero Gecko family uses TSMC’s “extremely low leakage” (ELL) 180nm to help provide what Silicon Labs director of MCU marketing Daniel Cooley called a much flatter power per megahertz curve than competitors. At 1MHz and operating from a 3V supply, an active MCU consumes around 155µA. As the speed increases the power per meghertz drops to 110µA/MHz at 12MHz and on to the maximum of 24MHz.

Cooley said the power figures are obtained running code from flash and not by loading code into lower-power SRAM. “There is some smart caching going on but there is no loading and running code only from SRAM in these figures.”

The wakeup time from the lowest-power mode (EM3), which consumes around 900nA with an active above shutdown is 2µs. The wake-up time for MCUs from low-power modes can be lengthy because of the time it takes to reactivate the phase-locked loop (PLL) circuit from a cold start. Cooley said the company uses an onchip, silicon RC oscillator instead of a PLL. Although Silicon Labs has its own portfolio of silicon oscillators, the design was started at Energy Micro before the Scandinavian company was acquired, so the RC oscillator was Energy Micro’s own technology.

Onchip security

To support better security among sensor nodes in Internet of Things (IoT) applications, the company decided to implement its own AES engine with an entropy generator to support software-based key generation. Øyvind Janbu, director of strategic marketing for MCUs, said the company has had cracking techniques such as power analysis used to assess the circuitry’s protection against common hacking attacks.

Cooley said the current DAC or IDAC mirrors a periperal found on some members of the Silicon Labs Precision 32. “The killer feature here is that it is almost power free. It enables you to run in EM3 mode and still keep that current feeding other BOM [bill of materials/other components] in your system that need a current source. We have seen a lot of traction based on this block alone.”

In the shut-off mode, current consumption drops to 20nA. Although almost all of the chip is disabled in this mode, Cooley said it is still useful on some applications. He cited the example of an electronic door lock that uses encryption. Toggling a general-purpose I/O pin signals the device to wake up, a process that takes around 1ms. “If a door takes 1ms to wake up you are not going to notice it,” he said.

Once awake, the micro can run the decryption through the AES unit to determine whether it should unlock the door or not.

Power tools

To support software implementation the company provides an energy monitor and profiling tool. “You don’t need any other lab equipment to see what your current profile is,” said Cooley.

Although other vendors, such as Freescale Semiconductor have implemented M0+ designs with clock speeds up to 48MHz, Silicon Labs chose to pursue a design with a limit of 24MHz, Janbu said, for reasons of “power consumption and die area to some degree. It’s also fair to say that we allowed ourselves to trade off on that parameter believing that we could still meet requirements for a big part of the target audience.”

“Flash wait states are also a driver here,” said Cooley, as not having to use high wait-state numbers under high clock speeds with typical flash access times simplifies code development.

Following the acquisition of Energy Micro, the long-term strategy for Silicon Labs is to merge its MCU products under the Gecko name, which was used by Scandinavian company. “We will bring the best of the Precision 32 into our next-generation devices,” said Cooley.

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