High-resolution pressure sensor brings stair-track capability to Fitbit Ultra
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 I2C interfaces; an internal oscillator; and an ultralow-power, 24-bit delta-sigma ADC.
3. The external contacts on the main power board are lifted up to reveal the general layout.
4. The 3.7V, 55-mAhr, Li-ion polymer battery lasts between five and seven days on a single charge.
5. Users can personalize the 1×0.25- in. blue organic LED display.
6. The MMA7341LC three-axis, ±3g capacitive MEMS accelerometer from Freescale is the same model used in the original Fitbit. The part features a sleep mode; signal conditioning; a one-pole low-pass filter; temperature compensation; and g-Select, which lets one device gauge multiple levels of acceleration.
7. TI’s MSP430F5419A 16-bit ultralow-power microcontroller replaces the MSP430F261T used in the original Fitbit and comes with 128 kbytes of flash, 16 kbytes of RAM, and a 12-bit ADC.
8. The nRF24AP2 2.4-GHz radio, released in July 2009, pairs a Nordic nRF24L01+ transceiver with the ANT protocol. The nRF24AP2 replaces the nRF24AP1 used in the first Fitbit. The newer radio reduces peak current to 17 mA and cuts average current by up to 75%, according to ANT Wireless and Nordic.
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