Specifications

BMA180

Preliminary data sheet

Bosch Sensortec

Rev. 1.0 Page 49 / - proprietary information - 06 March 2009

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Specifications within this document are preliminary and subject to change without notice. Document is not intended for publication.

used as an indication signal to the µC for finalization of the offset regulation

procedure. Subsequently the EEPROM writing may occur.

Remarks:

• EEPROM writing has to be performed by the user.

• After completion of the offset regulation/storage process, offset fine-tuning bits have to be

reset to “00” to re-enable the low-interrupt function. There are different cases to be

distinguished, if low-g interrupt should be used and offset should be kept as regulated after

EEPROM writing (and thus before resetting offset fine-tuning).

o Coarse calibration (via DAC): after EEPROM writing offset_finetuning can be reset to

‘00’. Course offset is remaining as calibrated and low-g interrupt settings can be

changed to use low-g interrupt without influencing remaining offset.

o Fine tuning (changing of ADC output and not via DAC): after EEPROM writing

fine_offset_* image registers are written to the corresponding low-g-interrupt

registers. If offset_finetuning is reset to ‘00’, low-g interrupts can be used, but offset

is not “fine tuned” any more (fine_offset_* registers are overridden by low-g interrupt

settings in image register; cancellation is done in digital part using the low-g interrupt

image registers and not EEPROM registers). If coarse calibration has been

performed before, sensor stays at least within the coarse calibration values. Thus

working of offset fine-tuning and low-g interrupt functionality at the same time is not

possible.

o If sensor was fine-tuned and low-g interrupt functionality is not necessary any more,

very small offset could be achieved by update_image procedure (in this case

EEPROM content is copied into image register and thus fine_offset_* registers are

copied into image registers, if offset-finetuning is ’11’ or ‘01’).

• Calibration depends on bw. By changing the bw, an offset of 1 or 2 LSBs may be induced.

• Precondition for a minimized offset is the optimum position of the end consumer device

(including the sensor) with respect to the g-axis orientation. Any angular mismatch between

the sensor package and gravity vector (“0” degree for z-axis and “±90 degree for x- and y-

axis) are leading to offset errors, which are related to a position mismatch during calibration.

They are not due to a bad offset of the sensor itself.

• In order to regulate all offsets, set offset fine-tuning e. g. to “11” and then sequentially

enable en_offset_* bits. To optimize waiting time, offset_st_s could be checked. Another

solution is a frequent check using a software counter in the µC. Third possibility is waiting for

a certain time before enabling offset regulation of another axis.