BMC156 datasheet(59/141 Pages) BOSCH

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Data sheet

eCompass BMC156

Page 59

BST-BMC156-DS000-01 | Revision 1.0 | July 2014

Bosch Sensortec

third parties. BOSCH and the symbol are registered trademarks of Robert Bosch GmbH, Germany.

Note: Specifications within this document are subject to change without notice. Not intended for publication.

5. FIFO Operation

5.1 FIFO Operating Modes

The IC of the accelerometer part of BMC156 features an integrated FIFO memory capable of

storing up to 32 frames. Conceptually each frame consists of three 16-bit words corresponding

to the x, y and z- axis, which are sampled at the same point in time. At the core of the FIFO is a

buffer memory, which can be configured to operate in the following modes:

 FIFO Mode: In FIFO mode the acceleration data of the selected axes are stored in the

buffer memory. If enabled, a watermark interrupt is triggered when the buffer has filled

up to a configurable level. The buffer will be continuously filled until the fill level reaches

32 frames. When it is full the data collection is stopped, and all additional samples are

ignored. Once the buffer is full, a FIFO-full interrupt is generated if it has been enabled.

 STREAM Mode: In STREAM mode the acceleration data of the selected axes are

stored in the buffer until it is full. The buffer has a depth of 31 frames. When the buffer is

full the data collection continues and oldest entry is discarded. If enabled, a watermark

interrupt is triggered when the buffer is filled to a configurable level. Once the buffer is

full, a FIFO-full interrupt is generated if it has been enabled.

 BYPASS Mode: In bypass mode, only the current sensor data can be read out from the

FIFO address. Essentially, the FIFO behaves like the STREAM mode with a depth of 1.

Compared to reading the data from the normal data registers, the advantage to the user

is that the packages X, Y, Z are from the same timestamp, while the data registers are

updated sequentially and hence mixing of data from different axes can occur.

The primary FIFO operating mode is selected with register (0x3E) fifo_mode according to ‘00b’

for BYPASS mode, ‘01b’ for FIFO mode, and ‘10b’ for STREAM mode. Writing to register

(0x3E) clears the buffer content and resets the FIFO-full and watermark interrupts. When

reading register (0x3E) fifo_mode always contains the current operating mode.

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