AN3308 datasheet(14/52 Pages) STMICROELECTRONICS

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AN3308

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Doc ID 18198 Rev 1

3.2

Understanding acceleration data

The measured acceleration data are sent to OUTX_H, OUTX_L, OUTY_H, OUTY_L,

OUTZ_H, and OUTZ_L registers. These registers contain, respectively, the most significant

part and the least significant part of the acceleration signals acting on the X, Y, and Z axes.

The complete acceleration data for the X (Y, Z) channel is given by the concatenation

OUTX_H & OUTX_L (OUTY_H & OUTY_L, OUTZ_H & OUTZ_L) and it is expressed as a

2’s complement number.

3.2.1

Data alignment

Acceleration data are represented as 16-bit numbers and are left justified.

3.2.2

Big-little endian selection

The LIS3DH allows to swap the content of the lower and the upper part of the acceleration

registers (i.e. OUTX_H with OUTX_L), to be compliant with both little-endian and big-endian

data representations.

“Little Endian” means that the low-order byte of the number is stored in memory at the

lowest address, and the high-order byte at the highest address. (The little end comes first).

This mode corresponds to bit BLE in CTRL_REG4 reset to 0 (default configuration).

On the contrary, “Big Endian” means that the high-order byte of the number is stored in

memory at the lowest address, and the low-order byte at the highest address.

3.2.3

Example of acceleration data

Table 6 provides a few basic examples of the data that is read in the data-registers when the

device is subject to a given acceleration. The values listed in the table are given under the

hypothesis of perfect device calibration (i.e. no offset, no gain error,....) and practically show

the effect of the BLE bit.

Table 6.

Output data registers content vs. acceleration (FS = 2 g)

Acceleration

values

BLE = 0

BLE = 1

28h

29h

28h

29h

0 g

00h

350 mg

E0h

15h

E0h

1 g

00h

04h

00h

-350 mg

20h

EAh

20h

-1g

00h

C0h

00h

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