Hi Hithesh,
The sample rate depending in the .hex that you download to the MCU. The different .hex file can be found from the link below: http://www.analog.com/en/mems-sensors/mems-inertial-sensors/adxl345/products/EVAL-ADXL345Z-DB/eb.html. 1/128 sec is not the sample rate of the ADXL345.
BR
Neil
