(Originally posted by Michael E Stanley in The Embedded Beat on Jun 18, 2013.)
I returned from Sensor’s Expo last week to find a new toy waiting for me. element14 has released a new board for the Freescale Freedom development platform featuring Xtrinsic sensors. For those of you who may have been hiding under a rock for the last year or two, these development boards duplicate the header pinout of the popular Arduino hobbyist boards, but are based upon industry standard ARM technology-based MCUs. The new sensor shield by element14 (pictured on top of the FRDM-KL25Z Freescale Freedom development platform) includes three Xtrinsic sensors:
I have blogged on the first two sensors before (Xtrinsic pressure sensor/altimeter: Part 1, Xtrinsic pressure sensor/altimeter: Part 2 and Magnetic sensor makes electronic compass design easy). I haven’t talked about the MMA8491, so it’s about time since it’s a really neat part. Unlike most other accelerometers, this little guy is designed to measure acceleration on demand, NOT based on some internal timer. The original target application was tamper detection. The device has 3 output pins which provide tilt detection at 45 degrees. Just toggle the EN enable line, and the X/Y/Z output signals are automatically updated. At 400nA per sample, this device has amazingly low power demands. And if your application does have a microcontroller, then just use the I2C port to read 14-bit sampled accelerometer values. That works out to be1mg/LSB sensitivity.
MMA8491 Block Diagram
element14 provides a 27-page user manual to take you through the paces of setting up and talking to your board. The board connects to your PC via a cable adapter from standard to mini USB. The board enumerates as a mass storage device, and element14 provides a precompiled command interpreter that can be downloaded to the Freedom development board via drag-and-drop from your PC desktop. The demo utilizes a virtual serial port over the USB line. You should be able to use almost any terminal emulator package to communicate to the board. My contact at element14 has used Termite and Teraterm Pro. I used RealTerm.
Demo app streaming altimeter data
The screen dump above shows a sequence of MPL3115 altitude and temperature readings. You will notice 3 “altitude levels” at approximately 398, 397 and 396 meters. These correspond to me holding the board above my head (while seated), on my desktop, and on the floor of my office. In other words, we’re seeing about two meters difference in height, as expected.
The MMA8491Q accelerometer can be sampled up to 800 times a second, and the MAG3110 has a max sample rate of 80Hz. So you’ve got enough bandwidth to implement an eCompass. But the combination of sensors on this board work out really nicely for tamper detection, smart grid and smart metering applications as well.
The kit includes both sensor shield and KL25Z Freescale Freedom development platform and cost only $26.99. You can order it now at www.element14.com/SensorEVK.