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The ADXL345 is a small, thin, low power, 3-axis accelerometer with high resolution (13-bit) measurement at up to ±16g. Digital output data is formatted...
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ADXL345 on Analog.com

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ACTIVITY Threshold not working

		
	
				

			

									
					    jdecesare
					
							

            
            			
				
									
											
			
			
				
    
			
			
			    	
			                    
                
				                on Dec 17, 2025 
				
			
			
		
    
		
		                     
				
        		
    
		    
		    		    
    
		      		            		        
    Category: Software
    
    		        		            		    
    Product Number:  ADXL345  
    
    		        		    		    
    
		    		    
		    			
    
			    
    Hi,
    

    I have the ADXL345 in a custom design and I'm  settting it up so that the ACTIVITY only will interrupt my processor. It's really a very simple application in which I need to detect a 2g event in any axis and to have it interrupt my processor.  I think I just need to enable the ACTIVITY threshold only and enable only that interrupt which the interrupt is working.
    

    My code snippet is below.  I'm programming for 16g (not in the snippet but else where in my code), setting the threshold at 8 (0.5g) and enabling all 3 axis in DC mode. I think that is all I really have to do.
    

    The problem is that if I set the THRESH_ACT to 8 (62.5mg/bit or 0.5g) the interrupt sits high even without touching the board.  When I set the register to 254 (close to 16g’s!!), the interrupt is low but I can simply tap the board and the interrupt goes off. Much less sensitive but it’s NOT 16g’s. What am I doing wrong?
    

    I don't think I would need to calibrate or maybe I should use AC mode?
    

    Also, I'm assuming register 0x22 is for TAP only, is that correct?  
    

    

    Thanks so much!
    

    John
    

    

    

    // ---------- ADXL345 interrupt configuration for deep sleep ----------
    

    void configureAccelInterrupt() {
    

      // Configure ADXL345 for activity detection interrupt
    

    

      Wire.beginTransmission(ADXL345_ADDR);
    

      Wire.write(0x24);  // THRESH_ACT register
    

      Wire.write(ACCEL_ACTIVITY_THRESHOLD);
    

      Wire.endTransmission();
    

    

      Wire.beginTransmission(ADXL345_ADDR);
    

      Wire.write(0x27);  // ACT_INACT_CTL register
    

      Wire.write(0x70);  // DC-coupled, all axes enabled for activity
    

      Wire.endTransmission();
    

        
    

      // All interrupts that occur will be on the INT1 pin
    

      Wire.beginTransmission(ADXL345_ADDR);
    

      Wire.write(0x2F);  // INT_MAP register
    

      Wire.write(0x00);  // 0 = INT1 pin
    

      Wire.endTransmission();
    

      
    

      // Clear any existing interrupts
    

      clearAdxlIntLatch();
    

    

      // Enable the Activity Interrupt only
    

      Wire.beginTransmission(ADXL345_ADDR);
    

      Wire.write(0x2E);  // INT_ENABLE register
    

      Wire.write(0x10);  // Enable Activity bit (bit 4)
    

      Wire.endTransmission();
    

    }
    

    
			
    
			
									
		                             
                        

    
    
                                            
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