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ADS7142-Q1: When VCC is not powered on my I2C bus is pulled low/corrupted

Connor McLaughlin
Prodigy 40 points
Part Number: ADS7142-Q1
Other Parts Discussed in Thread: ADS7138, ADS7142,

Tool/software:

Hello,

I have my ADS7142-Q1 hooked up to a I2C bus from a MCU and additional ADS7138. The 7142 is powered through wires to a different pcb than the MCU/7138, however I have noticed that if the VCC or GND is disconnected to the ADS7142-Q1 pcb then the I2C bus completely gets pulled down. Now the operating ADS7138 can't connect to the MCU at all and the entire I2C communication is lost. Is there a method to prevent this from happening? Like a buffer or switch to disconnect the I2C lines to the ADS7142 pcb if no VCC is present?

Bests,

Connor

  • 0
    TI__Genius 16710 points

    Hi Connor,

    Can you go into more detail about the method that the ADS7142 is disconnected? Is it during conversion, or while the bus is idle? Are you able to share an oscilloscope shot of what happens at the moment you disconnect the ADS7142 and how that is reflected in the SCL and SDA lines? If the I2C bus is pulled completely down when the ADS7142 is disconnected, then this points to an issue where the MCU is holding the I2C bus lines low.

    If this happens even if GND is disconnected from the ADS7142, then the ADS7142 has no method of even driving the I2C lines low to ground, since there is no path for current do flow.  

    Regards,
    Joel

  • 0 in reply to Joel Meraz
    Prodigy 40 points

    Hi Joel,

    It is just in idle, as well as in bootup. So the wiring scenario is:

    SDA -> connected

    SCL -> connected

    GND - > connected

    Vcc (3.3V) -> disconnected 

    It seems like the MCU is trying to pull the IC2 lines up but the ADS7142 chip is pulling those lines to GND. so the data is being stuck at a 1.7V range and looks corrupted.

  • 0 in reply to Connor McLaughlin
    Prodigy 40 points

    The thought process I have is that if the Vcc doesn't get disconnected I would like a circuit that also disconnected the I2C lines so the other ICs on the bus can still operate.

  • 0 in reply to Connor McLaughlin
    TI__Genius 16710 points

    Hi Connor,

    Let me discuss this with my team to see if they have encountered this or have solutions. As far as I know, the ADS7142 would have no method of pulling the I2C lines low if it is not being powered, since it needs to bias the transistors that pull these lines to ground. 

    If you remove the ADS7138 from the equation (disconnect it or remove it from the board), does removing power from the ADS7142 still cause the same issue?

    Regards,
    Joel