LMH1983: I2C Not Working

Hi Zareen,

Did you mean you have a quality issue? How many unit has this issue? 

As you know, there is nothing we can tweak on the I2C interface. Is there a schematic change in your new project? What is the ACK voltage in your other projects?

I assume you have the 27Mhz VCXO already attached on it, right?

Hi Noel,

Not sure if it is a quality issue. We have 3 prototypes of this project and in all the three boards we are facing the same issue. 

The change from the old to new project is the we are using a different processor. The ACK Voltage in other projects has a spike of 1V and then it goes down to 0V. 

Is there a possibility that LMH1983 requires clock stretching?

Yes, 27MHz VCXO is attached and a proper clock is going to that pin.

Hi Zareen,

Is the number of I2C device that sharing the same I2C bus different between new and old project? Would that help if you tweak the I2C pull-up resistors values?

Hi Noel,

The LMH1983 on the new project is not sharing the bus with anyone. I did tweak the pull-up resistors and it did not help.

I also want to know if the LMH1983 requires any clock stretching.

Hi Zareen,

No, clock stretching is not required. 

What are the pull-up resistors? I see some old E2E posts mentioned that 10kΩ works.

Hi Noel,

I currently have 4.7kΩ pull up resistors. I will try 10kΩ and see if it works.

I changed it to 10KΩ, it did not help. I captured the below response of LMH1983 from the other project on which it is working fine. 

Hi Zareen,

I don't have any explanation, would it be a good idea to swap the LMH1983 device between old and new project? See if the problem follows board or device.