Teardown: Misfit Shine 2 and the art of power management

-January 26, 2016

At DesignCon 2016 in Santa Clara, we put the Misfit Shine 2 Fitness + Sleep monitor under the knife, and exposed the latest in low-power technology, processes, and power management techniques.

The Misfit Shine 2 is thinner, stronger, and smarter than, well, the original Shine. As per the maker’s advertisements, it really is beautiful. Enshrined in T6061 T6 anodized aircraft-grade aluminum with glass-reinforced polycarbonate, the designers wisely opted for a minimalist aesthetic with a capacitive touch user-interface (UI) and visual feedback via 12 LEDs on the face. Tap the face, and the LEDs indicate your daily progress toward goal, followed by the time. You can set which comes first.

Photo courtesy of CNET.

The Shine 2 is 8-mm thin, 30.5 mm in diameter, and weighs 8.5 g (with battery installed). It’s water resistant up to 50 m and uses a 3-axis accelerometer and magnetometer to sense movement.

A vibration alarm provides status feedback and notifications/alerts, which is useful for phone calls and texts from the paired Apple (iOS 7 or above) or Android (Jelly Bean) phone whenever they’re in a bag or in mute mode. Audio feedback comes from an alarm that’s almost, well, pleasant to hear.

Connectivity to the phone is achieved using a Bluetooth 4.1 radio and motion sensing is accomplished using a three-axis accelerometer and magnetometer working together. Also, by downloading the Misfit Link app you can connect using IFTTT recipes to control your mobile device to play music, take a selfie, or tap the Shine to control one of a plethora of home connect services, called IFTTT “channels,” such as a WeMo switch, Nest thermostat, or even Twitter feeds.

Aside from aesthetics and ease of use, the Shine 2 stands out because the company addressed the common issue of battery life by opting for a coin cell that can last up to six months. This gets around the annoyance of having to recharge all too frequently and hoping it’s done before you leave the house. Having used the first iterations of the Fitbit for a period of time, this was definitely a drawback.

It turns out that its low power consumption was top of the list of reasons that Fossil Group, Inc. gave for buying Misfit when the deal was announced last November. However, sleek design was undermined in the Shine 2 by the poor design of the mechanism for holding the device firmly within the strap. I lost the first one when it got caught on my jacket cuff and went flying into space in a dark theater. Ironically, the Shine 2 was supposed to have addressed the well-documented “flying Shine” problem using the secure clip (Figure 1). The clip didn’t work, but that’s almost moot now, as Misfit addressed that with the Ray, a completely new design for a fitness monitor that it announced at CES earlier this month in Las Vegas.

Figure 1 It’s sleek design and ability to go 6 months on a single CR2032 3V coin cell battery made the Misfit Shine 2 an EDN teardown target at DesignCon.

As short as my time was with the Shine 2, I found it to be dead accurate, comfortable to wear, and easy to use, with great software and IFTTT support, and I actually miss the vibrating alerts on my wrist when texts and calls come in.

Anyway, let’s get inside and look at how the sleek design, low power consumption, and high functionality were achieved -- and who won and lost in the move from the original Shine to the Shine 2.

Figure 2 Misfit was kind enough to provide an exploded view of the construction. As the rear battery cover is turned into its locked position, it forms a tight seal around the rubber rings to keep the device watertight to 50m.

As sleek as the Shine 2 is, it was oddly lacking information in the skimpy manual. I had to turn to YouTube to figure out how to open the rear battery compartment. It’s actually easy, once you know how. The hard part, and this turned out to be really hard, was getting the center black ring off that holds the pc-board in place. It was fastened tight to the front plate using four tiny hex screws that were barely 1.5 mm in diameter. Nothing in my toolkit matched that. Misfit wasn’t keen to have this puppy opened.

So, a full Saturday afternoon was spent looking for a watch repair specialist. On my fourth try, I hit pay dirt. He didn’t have the right tool, but was just as stubborn as I (it’s a matter of pride, darnnit!) and he “hacked” it using the closest thing possible in his drawer of teeny tiny, almost invisible, tools. It’s always fun to see an artist at work. I should have taken a photo.

Figure 3 The hardest part was getting the ultra-small screws out, but once exposed, the brains behind the Shine 2 were clear: Ambiq Micro and Dialog Semiconductor.

Putting the board under a microscope, it quickly became clear what enabled the low power consumption. The Bluetooth 4.1 interface came courtesy of Dialog Semiconductor’s SmartBondTM DA14581, an optimized version of the DA14580 SoC.

The DA14581 was developed with A4WP wireless power and host controller interface (HCI) applications in mind. However, the Shine 2 doesn’t use a rechargeable battery, so the A4WP functionality is redundant.

Figure 4 Dialog Semiconductor’s DA14581 forms the Bluetooth 4.1 connectivity heart of the Shine 2. Loaded with functions, it still only consumes 4.9 mA peak, and consumes under 2 µA in extended sleep modes.

Given that it’s odd to have redundant functionality in such a size- and power-constrained application, I asked Tushar Rath, principal field application engineer with Dialog Semiconductor, why Misfit would have chosen the DA14581 versus a basic low-power Bluetooth-only option. He said it’s because, “the DA14581 still provides the lowest power consumption -- 4.9 mA peak at transmission and reception and extended sleep currents of less than 2 µA -- resulting in longest battery life.”

He added that it still wins out on size and integration, with only six external components, and integrated balun and power management functions. Also, the DA1458x family is designed for multiple applications, including radio-only applications like in the Shine 2. The BLE stack resides in the ROM on the chip. “A4WP support only means that we fulfill the A4WP requirements with faster boot time and more simultaneous connections while operating in central mode,” said Rath. Building an A4WP wireless charging solution would need other components: as it stands in the Shine 2, the A4WP capability lies dormant.

Dialog already has a family of derivative chips, with more “exciting new products” to come, said Rath.

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