AN4071
Sensors
Freescale Semiconductor, Inc. 3
2.1 Output Data, Sample Rates and Dynamic Ranges of all Three Products
2.1.1 MMA8451Q
1. 14-bit data
2g (4096 counts/g = 0.25 mg/LSB) 4g (2048 counts/g = 0.5 mg/LSB) 8g (1024 counts/g = 1 mg/LSB)
2. 8-bit data
2g (64 counts/g = 15.6 mg/LSB) 4g (32 counts/g = 31.25 mg/LSB) 8g (16 counts/g = 62.5 mg/LSB)
3. Embedded 32 sample FIFO (MMA8451Q)
2.1.2 MMA8452Q
1. 12-bit data
2g (1024 counts/g = 1 mg/LSB) 4g (512 counts/g = 2 mg/LSB) 8g (256 counts/g = 3.9 mg/LSB)
2. 8-bit data
2g (64 counts/g = 15.6 mg/LSB) 4g (32 counts/g = 31.25 mg/LSB) 8g (16 counts/g = 62.5 mg/LSB)
2.1.3 MMA8453Q Note: No HPF Data
1. 10-bit data
2g (256 counts/g = 3.9 mg/LSB) 4g (128 counts/g = 7.8 mg/LSB) 8g (64 counts/g = 15.6 mg/LSB)
2. 8-bit data
2g (64 counts/g = 15.6 mg/LSB) 4g (32 counts/g = 31.25 mg/LSB) 8g (16 counts/g = 62.5 mg/LSB)
3.0 Comparison of Motion Detection and Transient Detection
The transient function (with HPF enabled) is different from the motion detection embedded function in MMA8451, 2, 3Q. Refer
to Freescale application note AN4070 for details on the motion detection. The intended use of these functions is different.
A. The embedded motion function triggers an interrupt when any of the enabled axes has exceeded a set acceleration
threshold.
• Motion detection is useful when the device may be tilted in an orie
ntation and the interest is to know if the
acceleration in one axis has crossed a specified threshold amount measured from 0g.The motion detection
analyzes change in threshold considering both static and dynamic acceleration.
B. The transient detection embedded function compares whether any o
f the enabled axes has exceeded a set change
in acceleration disregarding the static acceleration. The data that is compared to the threshold to be exceeded is
high-pass filtered data.
• Transient detection is useful wh
en the device may be tilted in an orientation (i.e., there is some non-zero reading
on one or more of X, Y and Z) and the interest is more on the dynamic acceleration crossing a specified threshold
amount in a short amount of time (frequency being the reciprocal of time) regardless of what the current value is.
Example
If a change in acceleration of 0.5g in any axe
s was the target for an application, then it would be challenging to use the em-
bedded motion detection function for detecting this change. The reason for this can be better understood with an example:
Assume that the device was oriented such that X = 0.24g, Y = 0.44g and Z = 0.82g; it would be challenging to detect a 0.5g
change in acceleration in any of the axes using the motion detect function since the new readings have to be outside the range
X = -0.26g to 0.74g, Y = -0.06g to 0.94g and Z = 0.32g to 1.32g. Because only a single threshold setting can be chosen for each
axes when using the motion detect embedded function and because of the need to detect acceleration outside a range, it would,
therefore, be seen as impossible to choose a threshold that would detect a 0.5 change in acceleration for any of the axes.
3.1 Use Case 1
Use Case 1 Transient: Unknown Orientation with Specific Acceleration Change for Detection
A use case example for transient detection may be when the device
containing the accelerometer is in an environment of an
unknown orientation or changing orientation and a small amplitude, but quick disturbance must be detected, e.g., when the de-
vice is accelerating through 0g to +1g to -1g and a subtle impact force (change in acceleration of 0.5g) is used as an indication
to wake the device or as a motion signature of an event. The accelerometer reading combines the gravity and the linear
acceleration reading. It is much simpler to decouple the two by eliminating the effect of gravity. Using the example above, assume
that the device was oriented such that at given time T, X = 0.24g, Y = 0.44g and Z = 0.82g. If transient detection is enabled on
all
3 axes with a threshold of 0.5g and an ODR of 100 Hz, any time that X goes outside of the range -0.26g to 0.74g within
T + 10 ms,
a transient detect interrupt would be generated. Eliminating the static acceleration on the device simplifies the condi-
tion to focus on the change in acceleration only, since the orientation is of no consequence in this situation. Figure 2 demonstrates
that the sensor detects both the static and the dynamic acceleration. Decoupling the static from the dynamic acceleration can
simplify the analysis.