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PCA9306 SDA voltage level issue

Antony Lin
Genius 13480 points

Other Parts Discussed in Thread: PCA9306, TCA9517

Hi,

My customer uses PCA9306 to convert I2C from 3.3V(I2C2, master side) to 1.8V(I2C1, slave side), and found the strange SDA voltage level behavior as below .  (But this is not found in SCL.)

Their schematics is as below.  (They already removed R525, R545, and connected EN to VREF2, but with the same result.)  Is such waveform normal for SDA?

Thanks!

Antony

  • 0
    TI__Genius 16565 points
    Antony,

    Could you please supply some more information about your bus.

    The PCA9306 is unable to actually drive a low, by itself. Instead, it closes a MOSFET switch to allow a slave device on the other bus to pull both sides low.

    Your waveforms seem to suggest that whatever is pulling low on the bus, is either:
    1) only able to weakly pull it down
    2) Has a significant amount of series resistance resulting in a large VOL delta across the line
    3) Has a significant amount of current it must sink, resulting in a large VOL delta across the I2C line.

    I see two main options to debug this further
    1) Start probing inputs and outputs of any series resistors on teh bus in different locations to try and determine where the bulk of the VOL delta is coming from. THis means, compare the VOL at the slave/responding device, then measure the VOL of the slave at different points through the bus to the PCA9306, to see if you have a large difference.

    2) Use a buffered repeater to isolate portions of the bus to help prevent large IOL current issues from creating large VOLs, such as the TCA9517