Datasheet

Data Sheet
BMA456
Page 22
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.
Sensor Time
The BMA456 supports the concept of sensortime. Its core element is a free running counter with a
width of 24 bits. It increments with a resolution of 39.0625us. The user can access the current state of
the counter by reading registers SENSORTIME_0 to SENSORTIME_2.
All sensor events e.g. updates of data registers are synchronous to this sensor time register as defined
in the table below. With every update of the data register or the FIFO, a bit m in the registers
SENSORTIME_0 to SENSORTIME_2 toggles where m depends on the output data rate for the data
register and the output data rate and the FIFO downsampling rate for the FIFO. The table below shows
which bit toggles for which update rate of data register and FIFO
Bit m in sensor_time
23
22
21
20
19
18
17
16
Resolution [s]
327.68
163.84
81.92
40.96
20.48
10.24
5.12
2.56
Update rate [Hz]
0.0031
0.0061
0.012
0.024
0.049
0.10
0.20
0.39
The sensortime is synchronized with the data capturing in the data register and the FIFO. Between
the data sampling and the data capturing there is a delay which depends on the settings in the Register
ACC_CONF. The sensortime supports multiple seconds of sample counting and a sub-microsecond
resolution, see Register SENSORTIME_0 for details.
Burst reads on the registers SENSORTIME_0 to SENSORTIME_2 deliver always consistent values,
i.e. the value of the register does not change during the burst read.
Configuration Changes
If accelerometer configuration settings in registers ACC_CONF, ACC_RANGE, or AUX_CONF are
changed while the accelerometer (PWR_CTRL.acc_en = 0b1) or auxiliary sensor
(PWR_CTRL.aux_en = 0b1) is enabled, the configuration changes are not immediately applied. The
configuration changes become effective if a sampling event for the currently active ODR coincides
with a sampling event for the newly requested ODR on the sensortime sampling grid. In the case
where the currently active ODR equals the newly requested ODR, the configuration changes become
effective at the next sampling event. See also following figure.
Bit m in sensor_time
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
Resolution [ms]
1280
640
320
160
80
40
20
10
5
2.5
1.250
0.625
0.3125
0.156
0.078
0.039
Update rate [Hz]
0.78
1.56
3.125
6.25
12.5
25
50
100
200
400
800
1600
3200