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.