ADXL345 with Free Fall detection

Here are some things to try.

• Check the arguments to accel.ff(600,10);
  The datasheet shows the 600 would be much smaller, Values between 300 mg and 600 mg
  (0x05 to 0x09) are recommended.
  
• Likewise the 10 Values between 100 ms
  and 350 ms (0x14 to 0x46) are recommended
  http://www.analog.com/media/en/technical­-documentation/data-sheets/ADXL345.pdf
  
  

page 26
Register 0x28—THRESH_FF (Read/Write)
The THRESH_FF register is eight bits and holds the threshold
value, in unsigned format, for free-fall detection. The acceleration on
all axes is compared with the value in THRESH_FF to determine if
a free-fall event occurred. The scale factor is 62.5 mg/LSB. Note
that a value of 0 mg may result in undesirable behavior if the freefall
interrupt is enabled. Values between 300 mg and 600 mg
(0x05 to 0x09) are recommended.
Register 0x29—TIME_FF (Read/Write)
The TIME_FF register is eight bits and stores an unsigned time
value representing the minimum time that the value of all axes
must be less than THRESH_FF to generate a free-fall interrupt.
The scale factor is 5 ms/LSB. A value of 0 may result in undesirable
behavior if the free-fall interrupt is enabled. Values between 100 ms
and 350 ms (0x14 to 0x46) are recommended.

• The interrupt polarity could be flipped.
  Register 0x31 bit D5
  INT_INVERT Bit
  A value of 0 in the INT_INVERT bit sets the interrupts to active
  high, and a value of 1 sets the interrupts to active low.