High-resolution pressure sensor brings stair-track capability to Fitbit Ultra

April 18, 2013 // By Patrick Mannion
The Fitbit Ultra is the second generation of Fitbit Inc’s popular personal health-monitoring device, which tracks your movements and downloads the data wirelessly to your PC. The first Fitbit distinguished itself from a crowded field not just by its wireless connectivity but by the ability let the user upload the data online, where the tracking data could be integrated into a personalized online health and fitness program.

I tore down the original Fitbit shortly after its introduction in 2008, detailing the product’s genesis and the design issues that the Fitbit design team had encountered. Though the development effort ultimately was successful, the team had spent many hours trying to develop its own wireless interface before finally giving up and contacting Nordic Semiconductor . The rest, as they say, is history.

The first Fitbit had two limitations: It was easy to lose (I lost my first one, and apparently I wasn’t alone in doing so), and it didn’t have solid stair-tracking capability. The Ultra—which, like the original, sells for $99—sports a more secure physical design, with a better spring and clip holder, and adds a high-resolution (20-cm) barometric pressure sensor from MEAS Switzerland SA for monitoring changes in step height.

The Ultra swaps Texas Instruments’ MSP430F261T microcontroller for the newer, even lower-power F5419A. Much of the device’s layout and components remain, however, including Freescale Semiconductor’s MMA7341LC three-axis MEMS accelerometer. Together with the pressure sensor and proprietary algorithms from Fitbit, the parts keep the Ultra competitive in the low-power, high-performance arena.

It’s worth noting that Fitbit went to three different vendors for the microcontroller, MEMS accelerometer, and RF interface. For designers looking to experiment in this segment, Freescale has a full Activity Monitor reference design on its site.

1. The package base shows the spring on the left. Users of the first Fitbit complained that the unit was easy to lose, so the Ultra offers a stronger spring; a handy clip holder; and the ability to place the device in a pocket, rather than on a belt or the hip.

2. MEAS Switzerland’s MS5607-02BA barometric pressure sensor module is the big addition in the Ultra. Combined with Freescale’s MMA7341 and Fitbit’s proprietary algorithms, the sensor lets the Ultra track stair steps. It provides 20-cm altitude resolution and has SPI and I 2C interfaces; an internal oscillator; and an