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Prying Eyes: Power management wins in Misfit Shine 2

12 Feb 2016  | Patrick Mannion

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With SPOT, Ambiq is betting it will be the company to get them that step improvement, beyond MCUs. "We have a microcontroller product [among other devices] and we will have products in other families, but this is a technology platform more than a microcontroller platform."

Who got booted from original Shine?
To fully appreciate the art of power management that helped the Shine 2 get to 6 months of operating time despite monitoring 24/7, while also adding more functionality versus the original Shine, let's start by looking at who's no longer in the Shine design.

Figure 7: Texas Instrument's CC2501 Bluetooth IC and Silicon Labs' EFM32 "Gecko" MCU were in the original Shine, but added functionality required a rethink of the power budget allocations, opening the door to Ambiq and Dialogue Semiconductor to move in.

The original Shine was based on a Silicon Labs EFM32 "Gecko" MCU and a Texas Instruments CC2501 "SimpleLink" 2.45GHz Bluetooth Smart IC that also included a proprietary wireless link. However, for the Shine 2, Misfit wanted to add, among other features:

 • A 3-axis gyroscope (the original Shine only had a 3-axis accelerometer)
 • Vibration motor for haptic feedback
 • Capacitive touch control
 • Multicolour LEDs + driver IC

Of course, Misfit wanted to add all this, while still keeping within the same power budget – CR2032 coin cell for 6 months—and similar form factor.

This opened the door to Ambiq, which already had worked with Dialogue Semiconductor on optimising the Bluetooth stack and other interactions between the two chips for its Apollo EVB evaluation board and Apollo EVK (evaluation kits).

With that interaction nailed, Ambiq set about working with Misfit to ensure the full system was operating optimally. This need to do more for less power in the same form factor was a typical customer engagement path for Ambiq, said Odland. The solution is to look at the overall power budget, look at the power consumed by the new Ambiq MCU and allocate what's left over to the other functions.

However, allocating the power also meant ensure that every device on the board was operating optimally, said Odland, as the power is still limited. This translates to being aware of which sensor axis needs to be "on" and which ones don't, and also sampling periodically instead of continually, when there's no movement. Of course, then the sensor and MCU must wake up as quickly as possible when there is movement. Other techniques include taking advantage of the MCU's FIFO buffer by waiting until the sensors fill it before waking it up to process the data.

Complex simplicity
The elegance of the Misfit Shine 2 belies the complexity of the technology behind it, to the point that it's so often taken for granted. However, the overall design success depends as much on partnership between Misfit and its suppliers as it does on technology in a complex, interwoven relationship that is necessary to squeeze every microwatt out of each and every design, especially for wearables.

About the author
As Founder and CEO of ClariTek LLC, Patrick Mannion leads an independent content engineering firm specialising in technology analysis, editorial & media services.

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