Copyright © Parallax Inc.

MMA7455 3-Axis Accelerometer Module (#28526)

v1.1 9/24/2009 Page 4 of 6

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 VCC by the 2.2kΩ resistor. Meanwhile, the diode prevents the possibility of

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.

One g represents the acceleration due to the earth’s gravity: 1g ≈ 9.8m/s2 or

1g ≈ 32.2ft/s2. Of course this value varies depending on the object’s latitude and distance from sea

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:

counts

of

Number

range

-

g

Full

LSB

One

=

So for a sensitivity range of ±2g (full range = 4g) in 8-bit mode (256 counts), the value of one LSB would

be approximately:

count

g

0.016

counts

256

4g

LSB

One

≈

=

A few key numerical output values over each acceleration range for 8-bit data are shown below for

reference.

Sensitivity range

Acceleration

Output Value (hex/decimal)

-2g

$81

-127

-1g

$C1

-63

0g

$00

0

+1g

$3F

63

±2g

+2g

$7F

127

-4g

$81

-127

-1g

$E1

-31

0g

$00

0

+1g

$1F

31

±4g

+4g

$7F

127

-8g

$81

-127

-1g

$F1

-15

0g

$00

0

+1g

$0F

15

±8g

+8g

$7F

127