To get the design done, the team ended up leaning on Nordic, the supplier of the nRF24AP1 single-chip 2.4-GHz transceiver with embedded ANT protocol. According to Mike Paradis, sales manager at ANT Wireless, leaning on Nordic was one of the best decisions Park and his team could have made. “The initial [RF]design was very rudimentary: Nordic was very helpful once they got involved,” he said.
Nordic’s radio is proprietary, versus standards based, hence it is completely optimized for low power, according to David Day, sales manager at Nordic Semiconductor, ASA.
The importance of the component vendor for any design cannot be overstated, whether it be for the RF, main processor or other components. “That was one of the big decisions we made as a small company,” said Park. “Some vendors are great, and it’s not a function of size, as small or large can be responsive,” he said. “Others can be complete black holes.”
According to Paradis, the best thing a designer can do is take the help offered by the vendor. “Go to the people who are the leaders: they have all the use cases,” he said. “For example, Nordic has a complete reference design for a keyboard and mouse – just make your own form factor.” As a result, Nordic has design wins with Logitech (mice), Nike, and Polar. For its part, ANT has the complete low-power protocol and with the ANT Files System it has critical issues such as device association -- without a user interface – already figured out, he added.
When it came time to select the main processor upon which the algorithm would run, “a lot of it was educated guesses,” Park conceded. “We had only a vague notion of the power requirements of the software algorithm.” By this time, Park had divided the team up into separate groups: RF, firmware and I/O display, and algorithm development.
The team did know they had to minimize processing power consumption and so they narrowed the choice down to MCUs at that time (2007) from Texas Instruments and Atmel. As is so often the case, the final choice came down to a device they were familiar with, in this case a TI 16-MHz MSP430 MCU with 92 Kbytes of flash, 4 Kbytes of RAM and a 12-bit analog-to-digital converter. It also happened to have the exact amount of flash and RAM the system needed, said Park.
The recently published 2010 EETimes Embedded Market Study showed that 48 percent of the respondents stick with a chip they’re familiar with, though the ecosystem and tools are the prime reason behind that original choice. In this year’s study, TI was by far the favorite on that basis alone, at 17 percent. Freescale and Microchip came in second and third, at 10 and 9 percent, respectively.
Fig.2: Inside the FitBit, showing RF, MCU and MEMS choices.
3D MEMS: digital not necessarily better
The third and final piece of the component puzzle was the choice of 3-axis MEMS accelerometer. While designers have a tendency to go digital instead of analog, Park suggests that designers take a closer look at analog devices as they’re sufficient for most applications, without the added cost and complexity of a digital option.
For the FitBit, power consumption was a priority, along with noise levels, as the algorithm is particularly sensitive to noise, said Park. As a result, the team opted for the older-model MMA7341L 3-axis accelerometer from Freescale.
“We had worked with an ADI [Analog Devices Inc.] part, but chose this as it was cheaper, smaller and had lower noise,” said Park, adding that it’s half the size now than it was at the time.
A MUST READ ARTICLE FOR PRODUCT DESIGNERS
This is a beautiful article that has probably missed the eye of most of the users - this is apparent from the number of comments posted :)
Rather than discussing about the product, I'd like to lay stress on the issues that have been discussed in this article. These issues are not only related to the portable, low-power product category, but they throw light on what situations a team can face while making a product & releasing it in the market.
This article is thus, a must-read for all the prospective designers.
Let us take this further & exchange our experiences.
I'm impressed with the design, functionality, and usability of this product. It is compact and light yet does so much (Garmin tends to be larger). I agree with t. alex that integrating the product into other more common items should be added to their roadmap. For example, it could be an add-on (like OnStar). I personally use the Nike SportBand and find that helpful (except I believe there is no stopwatch function or heart rate monitor). I do have two tech savvy friends that have been using the fitbit for quite awhile and they've enjoyed being able to post their results on social media.
Join our online Radio Show on Friday 11th July starting at 2:00pm Eastern, when EETimes editor of all things fun and interesting, Max Maxfield, and embedded systems expert, Jack Ganssle, will debate as to just what is, and is not, and embedded system.