MMA8451QR1(2010) 데이터 시트보기 (PDF) - Freescale Semiconductor
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MMA8451QR1 Datasheet PDF : 53 Pages
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5
Functionality
The MMA8451Q is a low-power, digital output 3-axis linear accelerometer with a I2C interface and embedded logic used to
detect events and notify an external microprocessor over interrupt lines. The functionality includes the following:
• 8-bit or 14-bit data, High Pass Filtered data, 8-bit or 14-bit configurable 32 sample FIFO
• 4 different oversampling options for compromising between resolution and current consumption based on application
requirements
• Additional Low Noise mode that functions independently of the Oversampling modes for higher resolution
• Low Power and Auto-WAKE/SLEEP for conservation of current consumption
• Single/Double tap with directional information 1 channel
• Motion detection with directional information or Freefall 1 channel
• Transient/Jolt detection based on a high pass filter and settable threshold for detecting the change in acceleration above
a threshold with directional information 1 channel
• Flexible user configurable portrait landscape detection algorithm addressing many use cases for screen orientation
All functionality is available in 2g, 4g or 8g dynamic ranges. There are many configuration settings for enabling all the different
functions. Separate application notes have been provided to help configure the device for each embedded functionality.
5.1 Device Calibration
The device interface is factory calibrated for sensitivity and Zero-g offset for each axis. The trim values are stored in Non
Volatile Memory (NVM). On power-up, the trim parameters are read from NVM and applied to the circuitry. In normal use, further
calibration in the end application is not necessary. However, the MMA8451Q allows the user to adjust the Zero-g offset for each
axis after power-up, changing the default offset values. The user offset adjustments are stored in 6 volatile registers. For more
information on device calibration, refer to Freescale application note, AN4069.
5.2 8-bit or 14-bit Data
The measured acceleration data is stored in the OUT_X_MSB, OUT_X_LSB, OUT_Y_MSB, OUT_Y_LSB, OUT_Z_MSB, and
OUT_Z_LSB registers as 2’s complement 14-bit numbers. The most significant 8-bits of each axis are stored in OUT_X (Y,
Z)_MSB, so applications needing only 8-bit results can use these 3 registers and ignore OUT_X,Y, Z_LSB. To do this, the
F_READ bit in CTRL_REG1 must be set. When the F_READ bit is cleared, the fast read mode is disabled.
When the full-scale is set to 2g, the measurement range is -2g to +1.99975g, and each count corresponds to 1g/4096
(0.25 mg) at 14-bits resolution. When the full-scale is set to 8g, the measurement range is -8g to +7.999g, and each count
corresponds to 1g/1024 (0.98 mg) at 14-bits resolution. The resolution is reduced by a factor of 64 if only the 8-bit results are
used. For more information on the data manipulation between data formats and modes, refer to Freescale application note,
AN4076. There is a device driver available that can be used with the Sensor Toolbox demo board (LFSTBEB8451, 2, 3Q) with
this application note.
5.3 Internal FIFO Data Buffer
MMA8451Q contains a 32 sample internal FIFO data buffer minimizing traffic across the I2C bus. The FIFO can also provide
power savings of the system by allowing the host processor/MCU to go into a SLEEP mode while the accelerometer independently
stores the data, up to 32 samples per axis. The FIFO can run at all output data rates. There is the option of accessing the full
14-bit data or for accessing only the 8-bit data. When access speed is more important than high resolution the 8-bit data read is a
better option.
The FIFO contains four modes (Fill Buffer Mode, Circular Buffer Mode, Trigger Mode, and Disabled Mode) described in the
F_SETUP Register 0x09. Fill Buffer Mode collects the first 32 samples and asserts the overflow flag when the buffer is full and
another sample arrives. It does not collect any more data until the buffer is read. This benefits data logging applications where all
samples must be collected. The Circular Buffer Mode allows the buffer to be filled and then new data replaces the oldest sample in
the buffer. The most recent 32 samples will be stored in the buffer. This benefits situations where the processor is waiting for an
specific interrupt to signal that the data must be flushed to analyze the event. The trigger mode will hold the last data up to the
point when the trigger occurs and can be set to keep a selectable number of samples after the event occurs.
The MMA8451Q FIFO Buffer has a configurable watermark, allowing the processor to be triggered after a configurable number
of samples has filled in the buffer (1 to 32).
For details on the configurations for the FIFO buffer as well as more specific examples and application benefits, refer to
Freescale application note, AN4073.
MMA8451Q
14
Sensors
Freescale Semiconductor
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