Freescale Semiconductor added a to its product line a new accelerometer, the MMA8450Q that gives equipment designers a choice of ±2, ±4, or ±8g ranges with 12-bits of resolution for each axis. But if you think, “Oh, just another sensor,” you’ll miss the opportunity to use what I call an acceleration”processor,” even though the sensor doesn’t include a CPU.
So, what makes this a “processor” rather than a bare sensor? To start, the MMA8450Q lets you enable or disable six built-in functions, Auto-Sleep, Motion/Freefall 1, Motion/ Freefall 2, Transient Detection, Orientation Detection, and Tap Detection. By enabling these functions individually, a processor can detect the listed events by checking event flags, or by sensing a change at two interrupt outputs. The two interrupts act like the outputs of two large OR gates that accepts an input from each event. Changes to internal register settings let developers determine which Event flags “connect” to each interrupt output. Code can program registers to set custom acceleration trip points and lock out the Z axis, among other things. Obtain a complete data sheet at: www.freescale.com/files/sensors/doc/data_sheet/MMA8450Q.pdf.
A built-in first-in first-out (FIFO) memory and a data-ready signal also can raise an event flag and contribute their own input signal to the interrupt controller. In my opinion, the FIFO offers designers at least two advantages because it can accumulate data while a host MCU handles other tasks or when the host MCU go into a sleep mode. The FIFO memory stores 32 samples of data from each axis and it can operate at all data output rates supported by the I2C interface. The host MCU can access the data either as 8- or 12-bit values. Developers can choose to operate this bank of memory in several modes such as a simple FIFO memory, as a circular buffer, or as a “high-watermark” FIFO that raises a flag when it holds X data entries.
In addition to its measuring capabilities, the accelerometer can save power because it operate between 1.71 and 1.89 volts with a draw of 10 μA in standby mode and from 27 μA (50-Hz output rate in low-power mode) to 250 μA (400-Hz oversampling in normal mode).
The MMA8450Q device contains many registers, but not to worry. Freescale offers a 40-minute web seminar, “Sensor Toolbox for Accelerometer, Pressure and Proximity Sensors” that explains how to use the free Sensor Toolbox GUI to set up and control sensors. For more seminar information, visit: www.freescale.com/webapp/sps/site/training_information.jsp?code=WBNR_VFTF09_AC116.
Freescale also will have the LFSTBEB8450 accelerometer development board available by the end of February 2010. This board requires the mating LFSTBUSB USB-communication board sold separately. A kit with both boards has a suggested price of $US 99. For more information, visit: www.freescale.com/sensortoolbox
Freescale also has many helpful application notes:
Embedded Orientation Detection Using the MMA8450Q (AN3915):
Low-Power Modes and Auto-Wake/Sleep Using the MMA8450Q (AN3921): www.freescale.com/files/sensors/doc/app_note/AN3921.pdf
Offset Calibration of the MMA8450Q (AN3916):
Motion and Freefall Detection Using the MMA8450Q (AN3917):
High Pass Filtered Data and Transient Detection Using the MMA8450Q (AN3918):
MMA8450Q Single/Double and Directional Tap Detection (AN3919):
Using the 32 Sample FIFO (First In First Out) in the MMA8450Q (AN3920):
Data Manipulation and Basic Settings of the MMA8450Q (AN3922):
MMA8450Q Design Checklist and Board Mounting Guidelines (AN3923):
By the way, Freescale is no latecomer to sensors. This year, the company celebrates its 30th anniversary in the sensor market and in 2009 it shipped its one-billionth sensors. –Jon Titus