ADXL345 accelerometer Tap detection

As I promised in my previous ADXL345 tutorial, today I am going to write about ADXL345 single and double tap functions. I already covered how to interface ADXL345 with STM32 and read the accelerations in different axes (i.e X, Y and Z), you can check that out by going to  http://controllerstech.com/adxl345-accelerometer-using-stm32/. Today we will be covering the TAP functionality of this device.

HOW TO

In order to program the single and double taps, we need to write the following registers:-

• POWER_CTL (0x2D) Register  for power saving features
• TAP AXES (0x2A) Register for Axis control for tap/double tap
• THRESH_TAP (0x1D) Register for Tap threshold
• DUR (0x21) Register for Tap duration
• LATENT (0x22) Register for Tap Latency
• WINDOW (0x23) Register for Tap window in order to detect Double tap
• INT_MAP (0x2F) Register for Interrupt mapping control
• INT_ENABLE (0x2E) Register for Interrupt enable control
• INT_SOURCE (0x30) Register for getting interrupt Source

After setting values to all registers, we will read the INT_SOURCE register and this will give us the source of interrupt i.e. whether it’s from single or double tap.

Some Insight into the CODE

void adxl_write (uint8_t reg, uint8_t value)
{
  uint8_t data[2];
  data[0] = reg;
  data[1] = value;
  HAL_I2C_Master_Transmit (&hi2c1, adxl_address, data, 2, 100);
}

uint8_t adxl_read(uint8_t reg)
{
  uint8_t data_rec;
  HAL_I2C_Mem_Read (&hi2c1, adxl_address, reg, 1, & data_rec, 1, 100);
  return data_rec;
}

These functions are from my previous tutorial and I am going to use these to write and read data to ADXL345.

Write the POWER_CTL register first to wake the ADXL345.

adxl_write (0x2d, 0x00);  // reset all bits
adxl_write (0x2d, 0x08);  // measure and wake up 8hz

Write the values to TAP AXES Register enabling only the tap detection in Z axis.

adxl_write (0x2A, 0x01);  // enable tap detection on Z axis

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.

adxl_write (0x1D, 40);  // set tap threshold 2.5g/.0625

Set the TAP DURATION to .02 sec and to do that we need to write 32 (.02/.000625) to the DUR Register. This value indicates maximum time that an event must be above the THRESH_TAP threshold to qualify as a tap event. 

adxl_write (0x21, 32);  // set tap duration .02 sec/.000625

Set the TAP LATENCY to .1 sec, write 80 (.1/.0125) to LATENT Register. This is the wait time from the detection of a tap event to the start of the time window (defined by the window register) during which a possible second tap event can be detected.

adxl_write (0x22, 80);  // double tap latency .1sec/.00125

Set the TAP WINDOW to .3 sec by writing 240 (.3/.00125) to WINDOW register. This value representing the amount of time after the expiration of the latency time (determined by the latent register) during which a second valid tap can begin.

adxl_write (0x23, 240);  // double tap window .3sec/.00125

Enable the INT1 pin by writing 0 to INT_MAP Register. 

 adxl_write (0x2F, 0x00);  // int map reg 0 means INT1

Next we will read the INT_SOURCE Register and analyse whether it was a single or double tap

data = adxl_read(0x30);
 if ((data>>5)& 0x01) 
  {
   HAL_UART_Transmit(&huart2, display_double, sizeof (display_double),  10);  // double tap detected
  }
 else if ((data>>6) & 0x01)
  {
   HAL_UART_Transmit(&huart2, display_single, sizeof (display_single),  10); // single tap
  }

RESULT :-

TO DOWNLOAD THE CODE, VISIT

http://controllerstech.com/adxl345-single-and-double-tap-functions/

CHECK OUT THE VIDEO BELOW