Park admitted that cost became less of an issue with the volumes that Fitbit was looking at. “We found that all of these vendors generally start to converge on similar prices at high volumes. On occasion there will be outliers, and we dropped those guys from the process.”
Fitbit had worked with Nordic in the past, and had a good rapport with their FAEs, noting that they understood the Fitbit business very well. Park notes, “It came down to the fact that their product met our requirements and from a relationship perspective we felt comfortable with them, and when you deal with a new radio, it’s important that the supplier you are working with has a responsive engineering organization in case you have questions or problems.”
The accelerometer on the Zip is a 3mm x 3mm part that can easily be mistaken for one of the passives.
Previous generations of the Fitbit devices were built with MSP430 microcontrollers from TI. But the Zip is designed with the ST Cortex M3. Beyond the price and performance benefits, the engineering team enjoyed the code density offered by the 32-bit part (as opposed to the 16-bit MSP430). A lightweight RTOS runs on the processor, mostly for providing some of the basic services. It’s a commercial cooperative multitasking OS.
Other vendors included in the processor “bake off” included TI, Atmel, Energy Micro, and a few others. At the time, ST offered Fitbit the best combination of price-performance, with the power consumption being a key consideration.
Park admits, “At a theoretical level, we knew what we wanted to achieve, but the devil is in the details, so there were a lot of question marks around whether we could get battery life that we wanted, especially since we switched processor architectures. Theoretically, we knew it was possible, but we weren’t sure in practice. And there are a lot of nuances in programming the M3 and getting it into the right power states. That took a lot of learning.”
A lot of the software for this model had to be written from scratch. That was because the team opted for both a new radio and a new processor. The team estimates that about 90 percent of the code had to be rewritten. Couple that with the fact that the team had very limited familiarity with the ARM architecture, at least not with this family.
Fitbit employs fairly distinct firmware and product design teams, with the latter consisting of electrical and mechanical engineers. But they all work very closely together. And the entire design was handled in-house.
The electro-mechanical integration was fairly challenging due to the device’s small size. In addition, it had to be water-sealed, to keep out water and perspiration. As you can see from the figure, there’s not a lot of wasted space. Packing everything in was a challenge.
Rich, it is interesting that you found the device so, how shall I say it, vanilla. This is becoming the trend in electronics design these days. The sensors typically have evolved to put out usable readings directly (rather than having to be processed by the CPU into a digital form). Rather than custom logic, it is much easier to program a microcontroller to perform the required function. I actually found this to be the case for student projects I have judged as an IEEE member.
The move from the 16-bit to 32-bit microcontroller is interesting. The ARM processor has a feature where it can use 16-bit instructions where that is useful. These can be used interchangably with 32-bit instructions. This aids in fitting code into a limited space. I have recently used the M4 version of this processor and it is very powerful.
The use of Bluetooth is very smart. This allows any Bluetooth device, including a PC or smartphone, to process the information. With the ubiquity of this interface this should make the Zip very usable. The new low power standard is, I think, very important. That it has been worked into the IEEE standards is a good sign.
I'm glad you had fun with your hammer. I once worked at a place where there was a senior engineer who would always take a device apart to see what was inside and how it worked. We would take the labels that said things like "do not disassemble" off and put the device on his desk. Sure enough, we would come back later and it would be all over his desk. Many of these were never reassembled.
I certainly have a hammer or two at home that I need to do a functional check on. I enjoy exercising my hammers all the time. For bigger devices there is a bigger challenge. Remember the goal is to unlock the goodies inside with the least amount of force and strokes of the hammer. We would not want to damage the hammer or the operator...
As for software the reason I went the Computer Science for my Masters is the demand for embedded.
Thanks, this is much better than a regular teardown, it's great to hear details on how design decisions were made and why particular components were chosen.
I agree with tekochip--kudos to Rich. It's enlightening to get input on the design of the torn-down (is that a word?) product. Not exactly the usual teardown style.
I, too, like the way this teardown was written. The more I delved into it, the more I realized how many hours must have gone into the product development. Ninety percent of the code had to be rewritten? The team had limited familiarity with the ARM architecture? Sometimes, we see these teardowns and don't realize how much went into the creation of the product. This serves as a reminder of how complex the design of these seemingly simple products can be.
i'm one of those too who can hardly resist seeing the insides of products. but being ever hopeful about retaining the devices functionality, the hammer is verboten. why not use a dremel tool with a saw or grinder attachment or a hacksaw so your exploratory surgery isn't invariably fatal to your subject device?
I certainly did enjoy the analysis and discussion that included the comments from the design group. And the thinking that goes into a small package is always educational.
But using a hammer is rather brutal. I routinely take apart devices that are not intended to be opened and serviced, and I am able to repair a good portion of them. If they have a welded seam in the plastic housing that is the first point of attack. A sharp "gerber" brand knife blade is often able to open a package in a manner that allows it to be re-sealed when the repairs are done. So while it does take a lot more effort, opening devices in a re-closeable manner has a lot going for it. Of course, it is a liitle bit like those diamond cutters that we see pictures of. That is, it does take a lot of practice and a good deal of examination.
Thanks for the comments. I appreciate that you can appreciate that it takes a lot more time to write the Tear Down article with insight form the actual design team. In my opinion, that makes it significantly more valuable. Anybody can take somethng apart and identify the components.
As to the hammer vs. a screwdriver or some other tool, that's generally a time saver. I will admit (but don't tell anyone) that I have used a screwdriver in the past. But sometimes these devices are so hard to get apart that frustration (and deadlines) set in, and the hammer becomes very appealing.
Yes, excellent article. Really enjoyed the write-up. Rich, do you have any "feel" for the accuracy of the device? I logged in to the Fitbit web site and definitely feel my wife and I should by at least one to share. We both exercise 3, 4, 5 times a week and this would be a great addition and allow tracking and calories burned. At $59.95 each, it's a great deal. Also, any difficulties with "syncing" with an i-phone or Android device? Again, many thanks for the information.
Hi Rich, nice article, well written and informative. There is one technical error that's worth correcting though. The article states "With peak currents as low as 12.5 mA and average currents down to 9 mA (for a 1-second connection interval), the nRF8001 enables a battery life ranging from months to years from a single coin cell". While the figure for the peak current is correct, the average current should read 12microamps (from the Nordic Semiconductor website which actually states "average currents as low as sub 12 microamps (for 1s connection intervals)"). Otherwise the battery life from a typical CR232 coin cell would be tens of hours rather than months.
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