AN3308 datasheet(13/52 Pages) STMICROELECTRONICS

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AN3308

Startup sequence

Doc ID 18198 Rev 1

13/52

The check performed at step 3 allows to understand whether the reading rate is adequate

compared to the data production rate. In case one or more acceleration samples have been

overwritten by new data, because of a too slow reading rate, the ZYXOR bit of

STATUS_REG is set to 1.

The overrun bits are automatically cleared when all the data present inside the device have

been read and new data have not been produced in the meantime.

3.1.2

Using the data-ready (DRY) signal

The device may be configured to have one HW signal to determinate when a new set of

measurement data is available for reading. This signal is represented by the XYZDA bit of

STATUS_REG. The signal can be driven to INT1 pin by setting the I1_DRDY1 bit of

CTRL_REG3 to 1 and its polarity set to active-low or active-high through the H_LACTIVE bit

of CTRL_REG6.

The data-ready signal rises to 1 when a new set of acceleration data has been generated

and it is available for reading.The interrupt is reset when the higher part of the data of all the

enabled channels has been read (29h, 2Bh, 2Dh).

Figure 1.

Data ready signal

3.1.3

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 present inside the STATUS_REG or with the RDY

signal, it is strongly recommended to set the BDU (block data update) bit in CTRL_REG4 to

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, in case 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(X,Z)_H have been sampled at the

same moment. For example, if the reading speed is too low, it may read X and Y sampled at

T1 and Z sampled at T2.

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