AN3393 datasheet(14/103 Pages) STMICROELECTRONICS

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Startup sequence

AN3393

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DocID018750 Rev 4

setting the DR_EN bit of CTRL_REG3. Signal polarity is set through the IEA bit and signal

shape through the IEL bit of CTRL_REG3.

Figure 1 shows the behavior of the data-ready when the IEA bit is set to 1 in combination

with the setting of the IEL bit. The signal rises to 1 when a new set of acceleration data has

been generated and is available to be read.

Figure 1. Data-ready signal (IEA = 1)

2.1.3

Using the block data update (BDU) feature

If the reading of the acceleration data is particularly slow and cannot be synchronized (or it

is not required) with either the XYZDA bit in the STATUS register or with the DRDY signal, it

is strongly recommended to set the BDU (block data update) bit to 1 in the CTRL_REG4

This feature avoids the reading of values (most significant and least significant parts of the

acceleration data) related to different samples. In particular, when the BDU is activated, the

data registers related to each channel always contain the most recent acceleration data

produced by the device, but, if the reading of a given pair (i.e. OUT_X_H and OUT_X_L,

OUT_Y_H and OUT_Y_L, OUT_Z_H and OUT_Z_L) is initiated, the refresh for that pair is

blocked until both MSB and LSB parts of the data are read.

Note:

BDU only guarantees that OUT_X(Y, Z)_L and OUT_X(Y,Z)_H have been sampled at the

same time. For example, if the reading speed is too slow, it may read X and Y sampled at T1

and Z sampled at T2.

2.2

Understanding acceleration data

The measured acceleration data are sent to the OUT_X_H, OUT_X_L, OUT_Y_H,

OUT_Y_L, OUT_Z_H, and OUT_Z_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

OUT_X_H & OUT_X_L (OUT_Y_H & OUT_Y_L, OUT_Z_H & OUT_Z_L) and it is

expressed in two’s complement number.

IEL = 1

IEL = 0

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