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AD5693 I2C pin fall time requirement

		
	
				

			

									
					    kopi
					
							

            
            			
				
									
											
			
			
				
    
			
			
			    	
			                    
                
				                on Dec 9, 2019 
				
			
			
		
    
		
		                     
				
        		
    
		    
		    		    
		    			
    
			    
    Hi.
    

    We are using DA-converter AD5694 in out design.
    

    I2C bus speed is close to 400kHz i.e it is used in fast mode. I2C specification state the same requirement for signal fall time as is stated in AD5694 data (table 4, page 4)
    

     
    

    Requirement for the fall time is 6.54ns when V_LOGIC is 1.8V.
    

    Fall (and rise) is measured between 30% and 70% level according to I2C specification
    

    https://www.nxp.com/docs/en/user-guide/UM10204.pdf 
    

    

    Questions :
    

    Signal fall time measured from design is close to 6ns, so it is a bit faster than required 6.54ns.
    

     - Is fall time too fast for the AD5693 I2C bus interface as stated in data (and I2C specification)?
    

     - Can you define component specific value for signal fall time which is safe to use without compromise functionality?
    

    Thank you for your reply in advance!
    

    

    

    

    
			
    
			
									
		                             
                        

    
    
                                            
                 Single-Channel Voltage Output D/A Converters
                
                                                    
                 ad5693
                
                                                    
                 i2c
                
                                                    
                 Voltage Output D/A Converters
                
                                                    
                 ad5694
                
                                        
                                            
                
                
                                
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      •  Analog Employees  
      
            
						
                            
            
            				
            
                on Dec 9, 2019 10:18 PM
            
            
						
			            		
      
		
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      Hi,
      

      Are you using pull-up resistors from SCL, SDA lines to the Vlogic supply? 
      

      You can basically increase the resistor value just a bit in order to meet the minimum requirement in the data sheet.
      

      Cheers,
      

      Ivan
      
			
      

		
			
	      

		
		
	
      
	
      
		
      
			
		
      
	
      
	
      
		
						
						
				
						
				
				
		
		
      
			
		
      
	
      

      

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                on Dec 10, 2019 11:50 PM
            
            
												
						in reply to isantos
					
										
			            		
        
		
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        Hi,
        

        Thank you for reply.
        

        SCL,SDA lines pull-up resistors are connected to Vlogic supply. Resistor value is 1k, same value for both lines. 
        

        I2C bus master has configurable drive strength and is set to minimum 2mA value. with 1k pull-up, fall time is still a bit too fast as specified for AD5693/I2C specification.
        

        But has other I2C nodes as well and they all have differences in drive strength. However there will be some margins to  increase pull-up strength to restrict fall time. 
        

        So answer to original questions "does AD5693 have component specific fall/rise time value" is no, and we have to follow I2C specification?
        

        Other I2C node ICs we are using will define component specific value for fall/rise time.     
        

        Br,
        

        Sami
        
			
        

		
			
	      

		
		
	
        
	
        
		
        
			
		
        
	
        
	
        
		
						
						
				
						
				
				
		
		
        
			
		
        
	
        

        

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        •  Analog Employees  
        
            
						
                            
            
            				
            
                on Dec 11, 2019 3:38 AM
            
            
												
						in reply to kopi
					
										
			            		
        
		
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        Hi ,
        

        Yes, no specific resistor value required as long as you meet the requirements in AD5693 timing specifications.
        

        Curious question, what's the i2c bus master you are using? looks like it has a built in pull-up resistor or constant current source, if this is the case, not sure why would you need additional 1k pull-up. is my understanding correct?
        

        Cheers,
        

        Ivan
        
			
        

		
			
	      

		
		
	
        
	
        
		
        
			
		
        
	
        
	
        
		
						
						
				
						
				
				
		
		
        
			
		
        
	
        

        

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