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P82B715: SCL and SDA being pulled low

Katrina Barnes2
Intellectual 320 points
Part Number: P82B715


I currently have 2 boards using the P82B715 part to drive I2C over a cable (currently about 18").  The pull up on the "L" side is 330 ohms to +3.3V and on the "S" side is 1.8K to 3.3V.  One of the boards has an AC input to an on-board AC-DC power supply which ultimately generates the 3.3V.  The other board is powered by an off board AC-DC power supply that ultimately generates the 3.3V.  My problem is that I'm not seeing any data on my oscope at the slave board.  I measured the ground offset and it is about 3.2V--and when I connect the grounds from each board together, all the SCL lines (on both sides of each P82B715 device are pulled down to 109mV (master board), 180mV ("L" I2C line) and 235mV (slave board).  Both boards measure 3.3V at their pull-up power supply test points.  I feel like I am missing something stupid....

  • 0
    Intellectual 320 points

    Somehow the grounding issue disappeared--I hate when that happens.  Now my issue is that the SCL and SDA lines are always high when connected to board 2.  Board 2 is populated with the 330 ohm pull ups--could these be too strong?  When I disconnect board 2 and measure at board 2 end of the cable, the signals come across fine.

  • 0 in reply to Katrina Barnes2
    TI__Guru 74352 points

    Katrina,

    An engineer has been notified of your post and will respond accordingly.

    Regards,

    Eric Hackett 

  • 0 in reply to Katrina Barnes2
    TI__Guru* 77981 points

    Hi Katrina,

    I'm assuming you're enabling the correct channel from the 9546 device first before you try to communicate (double check). If not, then the signal will not reach to the L side of the P82B715 because the channel is HI-Z until it is programmed to be enabled.

    Usually if you disconnect a board and can communicate and then connect a board a see the communication stop, this is usually some kind of common ground issue. 

    Maybe double check your schematic and make sure the grounds you are connecting together are indeed ground (sometimes issues due to incorrect connections). Maybe you have a schematic of the second board you can share? (unless maybe its the second picture in your post?)

    330 ohms is pretty strong but if you are connecting it to another P82B715, then this shouldn't be a problem. 

    Can you share the scopeshot at the cable side when you disconnect the second board? I want to verify the signals look okay. 

    I'd also like to see what the scopeshot looks like when the second board is connect both on the same cable side and at the slave/target side.

    -Bobby

  • 0 in reply to BOBBY
    Intellectual 320 points

    Somehow the issue resolved itself and the 2 boards are communicating fine now.  Thank you for your help.