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Original question:

TCA9555: Interface forum

TCA9555: I2C Waveshape

David Lane
Prodigy 10 points
Part Number: TCA9555

Tool/software:

I am referencing the earlier thread as this is the same application. There are a number of photos in the previous thread. I'll repeat one here. In this application the TCA9555 interfaces to Microcontroller TM4C1292NCPDTI3. There is an Analog devices I2C isolator ADUM1250 between the two. The scope screen capture is taken between the controller and the isolator. The pink data trace shows zero levels that are sometimes about a diode drop above ground level. I'm not sure if this is a problem or normal.  The I2C lines have 4.7K pull ups to 3.3V on the controller side and 2.2K on the I2Cexpander side. Please comment.

-Dave Lane

  • 0
    Guru 316880 points

    Such a voltage offset is typical for most I²C buffers; the isolator is a buffer.
    The ADuM1250 datasheet specifies VSDA1OL and VSCL1OL as up to 900 mV.

  • 0
    TI__Mastermind 47668 points

    Hi Dave,

    I believe Clemens is correct here. The ADUM1250 has a set offset voltage on side 1 and looks to be at least 600mV - 900mV. Depending on which way the I2C communication is going either from side 1 to side 2, or side 2 to side 1, you may see the offset voltage at different times within the I2C frame. 

    In the first 18 clock cycles, it looks like a target device connected to side 2 is ACK'ing the bus (pulling LOW on side 2), which is why we see an offset voltage that is 600mV - 900mV on the scope every 9 clock cycles. 

    This looks like normal behavior of an I2C buffer/isolator with offset voltage. 

    Regards,

    Tyler

  • +1
    TI__Mastermind 47668 points

    Hi Dave,

    I believe Clemens is correct here. The ADUM1250 has a set offset voltage on side 1 and looks to be at least 600mV - 900mV. Depending on which way the I2C communication is going either from side 1 to side 2, or side 2 to side 1, you may see the offset voltage at different times within the I2C frame. 

    In the first 18 clock cycles, it looks like a target device connected to side 2 is ACK'ing the bus (pulling LOW on side 2), which is why we see an offset voltage that is 600mV - 900mV on the scope every 9 clock cycles. 

    This looks like normal behavior of an I2C buffer/isolator with offset voltage. 

    Regards,

    Tyler