Teardown: Misfit Shine 2 – design win for sub-threshold silicon
The Misfit Shine 2 is thinner, stronger, and smarter than, well, the original Shine. As per the maker’s advertisements, it really is beautiful. Clad in T6061 T6 anodized aircraft-grade aluminium 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, above, 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 50m 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 theatre. 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 in January 2016, 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.[continues]
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 removing the centre black ring 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 SmartBond DA14581, an optimised 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 as in the Shine 2. The BLE stack resides in the ROM on the chip. “A4WP support only means that we fulfil 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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Apollo and the art of power management
As interesting as the DA14581 may be, it’s the Ambiq Micro Apollo MCU that forms the brains — and heart — of both the overall processing and power management. So much so, that on talking with Keith Odland, director of marketing at Ambiq, it became clear that the implementation of Apollo on the Shine 2 was a case study in intelligent power management.
Figure 5. Winner of the EEMBC ULPBench benchmark, the Apollo MCU with 512 kBytes of Flash was designed from the ground up for low power using Ambiq’s sub-threshold process technology.
Let’s start with the Apollo MCU itself. It comprises an ARM Cortex M4F 32-bit core with a floating point unit, wake-up interrupt controller and operating at up to 24 MHz. It’s surrounded by 512 kBytes of Flash, sensor peripherals, voltage monitoring, serial communications (GPIO, UART, I²C, and SPI master/slave) and an on-board temperature sensor accurate to ±2 degrees.
Though loaded with features, the Apollo weighs in with a power consumption of 34 µA/MHz while executing instructions from Flash, and a sleep-mode current draw of 140 nA.
I was familiar with its low power capabilities having attended a “private” demo of the Apollo MCU going through its ULPBench paces – and beating all other competitors, including STMicroelectronics and Texas Instruments, by more than 2x. The formal ULPBench results are shown here.
The Apollo uses Ambiq’s proprietary Sub-threshold Power Optimized Technology (SPOT) whereby the transistors are not fully “on” but instead operate at a point along the bias curve to minimise power consumption.
While this technique has been known for many years, the foundry process temperature stability required to maintain this operating point has only recently become viable for mass production. To this end, Ambiq partnered with TSMC to realise it in the Apollo. While the process is key, Ambiq still had to add its own temperature and voltage monitors and other proprietary techniques to ensure stability. And it didn’t happen overnight, it took at least 10 years.
However, according to Odland, the technology is now applicable beyond MCUs, applying to radios, analogue front ends, and sensors. This is critical, he said, as customers are no longer happy with incremental improvements of 15% in energy savings, “they need a complete step-function improvement to bring it to a whole new level.”
Figure 6. Ambiq’s Sub-threshold Power Optimized Technology operates transistors in the nether region along the curve between “off” and saturation to keep power down.
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.”
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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 Dialog Semiconductor to move in.
The original Shine was based on a Silicon Labs EFM32 “Gecko” MCU and a Texas Instruments CC2501 “SimpleLink” 2.45-GHz 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 Dialog 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.
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