Set the TAP THRESHOLD to 2.5g. To do that we need to write 40 (2.5/.0625) to the THRESH_TAP Register. This means that you need at least 2.5g acceleration in Z AXIS in order for the tap to qualify as TAP.
The ADXL345 is a complete 3-axis acceleration measurement system with a selectable measurement range of ±2 g, ±4 g, ±8 g, or ±16 g. It measures both dynamic acceleration resulting from motion or shock and static acceleration, such as gravity, which allows the device to be used as a tilt sensor. In this tutorial we are going to interface ADXL345 with STM32.