MMA7455 Parallax, Inc., MMA7455 Datasheet - Page 4

• Manufacturer Part Number: MMA7455
• Description: Mma7455 3-axis Accelerometer Module
• Manufacturer: Parallax, Inc.

down through the Schottky diode to about 0.35 V. This is plenty low enough to be read as a logic low
(0) by the MMA7455L accelerometer device. When the CLK or CS pin is driven high or left floating, it
gets pulled up to the local V
higher external voltages damaging the input pin on the MMA7455L.
Measured Acceleration Output Values
The MMA7455L converts a detected acceleration into a number which can easily be read by an external
microcontroller. The number it generates depends on the sensitivity of the device which can be ±2g,
±4g, or ±8g.
1g ≈ 32.2ft/s
level. From a sensory standpoint, there is no detectable difference between forces observed under the
influence of acceleration and forces observed under the influence of gravity. Using units of g’s is simply a
convenient way of relating the magnitude of a measured acceleration to the familiar force of gravity.
The full measurement range is divided up into a number of counts which is determined by the number of
bits specified for the data; 8-bit data has 256 counts, and 10-bit data has 1,024 counts. The value of one
count, or least significant bit (LSB) of data, can be determined as follows:
So for a sensitivity range of ±2g (full range = 4g) in 8-bit mode (256 counts), the value of one LSB would
be approximately:
A few key numerical output values over each acceleration range for 8-bit data are shown below for
reference.
Copyright © Parallax Inc.
One
One
LSB
LSB
2
. Of course this value varies depending on the object’s latitude and distance from sea
Sensitivity range
=
=
One g represents the acceleration due to the earth’s gravity: 1g ≈ 9.8m/s
Number
256
±2g
±4g
±8g
Full
4g
counts
g
-
of
range
CC
counts
≈
by the 2.2kΩ resistor. Meanwhile, the diode prevents the possibility of
MMA7455 3-Axis Accelerometer Module (#28526)
0.016
Acceleration
g
count
+1g
+2g
+1g
+4g
+1g
+8g
-2g
-1g
-4g
-1g
-8g
-1g
0g
0g
0g
Output Value (hex/decimal)
$C1
$E1
$3F
$7F
$1F
$7F
$F1
$0F
$7F
$81
$00
$81
$00
$81
$00
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