Datasheet

Data Sheet
BMA456
Page 27
BST-BMA456-DS000-01 | Version 1.1 | October 2017 Bosch Sensortec
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BOSCH and the symbol are registered trademarks of Robert Bosch GmbH, Germany.
Note: Specifications within this document are subject to change without notice.
Conditions and Details
Frame rates
The frame sampling rate of the FIFO is defined by the maximum output data rate of the sensors
enabled for FIFO sampling. The FIFO sampling configuration is set in register FIFO_CONFIG_0 to
FIFO_CONFIG_1. It is possible to select filtered or pre-filtered data as an input to the FIFO. If un-
filtered data are selected in register FIFO_DOWNS.acc_fifo_filt_data for the accelerometer, the
sample rate is 1600 Hz. The input data rate to the FIFO can be reduced by selecting a down-sampling
factor 2
k
in register FIFO_DOWNS.acc_fifo_downs, where k=[0,1..7].
FIFO Overflow
In the case of overflow the FIFO can either stop recording data or overwrite the oldest data. The
behavior is controlled by Register FIFO_CONFIG_0.fifo_stop_on_full. When
FIFO_CONFIG_0.fifo_stop_on_full =0b0, the FIFO logic may delete the oldest frames. If header
mode is enabled, the skip frame is the prepended at the next FIFO readout, when the free FIFO
space falls below the maximum size frame.
If FIFO_CONFIG_0.fifo_stop_on_full =0b1, the newest frame may be discarded, if the free FIFO
space falls below the maximum size frame. If header mode is enabled, a skip frame is prepended at
the next FIFO readout (which is not the position where the frame(s) have been discarded).
During a FIFO read operation of the host, no data at the FIFO tail may be dropped. If the host reads
the FIFO with a slower rate than it is filled, it may happen that the sensor needs to drop new data,
even when FIFO_CONFIG_0.fifo_stop_on_full =0b0. These events are recorded in the Register
ERR_REG.fifo_err.
Control frames
Control frames are only supported in header mode. There are a number of control frames defined
through the fh_parm field. These are shown in below.
A skip frame indicates the number of skipped frames after a FIFO overrun occurred, a sensortime
frame contains the sensortime when the last sampled frame stored in the FIFO is read, a FIFO input
config frames indicates a change in sensor configuration which affects the sensor data.
The FIFO fill level is contained in registers FIFO_LENGTH_1.fifo_byte_counter_13_8 and
FIFO_LENGTH_0.fifo_byte_counter_7_0 and includes the control frames, with the exception of the
sensortime frame.
fh_mode<3:0>
Definition
0x0
Skip Frame
0x1
Sensortime Frame
0x2
Fifo_Input_Config Frame
0x3
Reserved
0x4
Sample Drop Frame
0x5 – 0x7
Reserved