MMA8451Q
Sensors
10
Freescale Semiconductor, Inc.
3 Terminology
3.1 Sensitivity
The sensitivity is represented in counts/g. In 2g mode the sensitivity is 4096
counts/g. In 4g mode the sensitivity is 2048 cou
nts/
g and in 8g mode the sensit
ivity is 1024 counts/g.
3.2 Zero-g Offset
Zero-g Offset (TyOff) describes the deviation of an actual output
signal from the ideal output signal if the sensor is stationa
ry. A
sensor stationary on a horizontal surface will measure 0g in X-ax
is and 0g in Y-axis whereas the Z-axis will measure 1g. The ou
tput
is ideally in the middle of the dynamic r
ange of the sensor (content of OUT Register
s 0x00, data expressed as 2's complement
number). A deviation from ideal value in this case is called Zero
-g offset. Offset is to some extent a result of stress on the
MEMS
sensor and therefore the offset can slightly change after mounting the sensor onto a printed circuit board or exposing it to
extensive mechanical stress.
3.3 Self-Test
Self-Test checks the transducer functionality
without external mechanical st
imulus. When Self-Test is activated, an electrostat
ic
actuation force is applied to the sensor, simulating a small a
cceleration. In this case, the sensor outputs will exhibit a chan
ge in
their DC levels which are related to the selected full scale throug
h the device sensitivity. When
Self-Test is activated, the d
evice
output level is given by the algebraic sum of the signals produced by the acceleration acting on the sensor and by the electros
tatic
test-force.
MMA8451Q
Sensors
Freescale Semiconductor, Inc.
11
4 Modes of Operation
Figure 6. MMA8451Q Mode
Transition Diagram
All register contents are preserved when transitioning from ACTIVE to STANDBY mode. Some registers are reset when
transitioning from STANDBY to ACTIVE.
These are all noted in the device memory
map register table. The SLEEP and WAKE
modes are ACTIVE modes. For more information on how to use the SLEEP and WAKE modes and how to transition between
these modes, please refer to the fu
nctionality section of this document.
Table 7. Mode of Operation Description
Mode
I
2
C Bus State
VDD
VDDIO
Function Description
OFF
Powered Down
<1.8V
VDDIO Can be > VDD
The device is powered off. Al
l analog and digital blocks
are shutdown. I
2
C bus inhibited.
STANDBY
I
2
C communication with
MMA8451Q is possible
ON
VDDIO = High
VDD = High
ACTIVE bit is cleared
Only digital blocks are enabled.
Analog subsystem is disabled.
Internal clocks disabled.
ACTIVE
(WAKE/SLEEP)
I
2
C communication with
MMA8451Q is possible
ON
VDDIO = High
VDD = High
ACTIVE bit is set
All blocks are enabled (digital, analog).
SLEEP
WAKE
STANDBY
OFF
ACTIVE
MMA8451Q
Sensors
12
Freescale Semiconductor, Inc.
5 Functionality
The MMA8451Q is a low-power, digital out
put 3-axis linear accelerometer with a I
2
C interface and embedded logic used to
detect events and notify an external microprocessor over in
terrupt 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
• Four different oversampling options for compromising bet
ween 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 modes 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 diffe
rent
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 fo
r 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 mo
re
information on device calibration, refer to Freescale application note, AN4069
.
Table 8. Features of the MMA845xQ devices
Feature List
MMA8451 MMA8452 MMA8453
Digital Resolution (Bits)
14
12
10
Digital Sensitivity (Counts/g)
4096
1024
256
Data-Ready Interrupt
Yes
Yes
Yes
Single-Pulse Interrupt
Yes
Yes
Yes
Double-Pulse Interrupt
Yes
Yes
Yes
Directional-Pulse Interrupt
Yes
Yes
Yes
Auto-WAKE
Yes
Yes
Yes
Auto-SLEEP
Yes
Yes
Yes
Freefall Interrupt
Yes
Yes
Yes
32 Level FIFO
Yes
No
No
High-Pass Filter
Yes
Yes
Yes
Low-Pass Filter
Yes
Yes
Yes
Orientation Detection Portrait/Landscape = 30°, Landscape to Portrait = 60°,
and Fixed 45° Threshold
Yes
Yes
Yes
Programmable Orientation Detection
Yes
No
No
Motion Interrupt with Direction
Yes
Yes
Yes
Transient Detection with
High-Pass Filter
Yes
Yes
Yes
Low Power Mode
Yes
Yes
Yes