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TCA9536: status issue

Jeff YANG
Mastermind 9265 points

Part Number: TCA9536

Hi,

If VCC continues to supply 3.3V, but I2C_SDA/I2C_SCL become floating without being pulled high due to an internal power loss in the device, will P0 (output), P1 (input), and P2 (output) maintain their last states? Assuming the original states were P0=0, P1=0, P2=0 or P0=1, P1=1, P2=1.

Thanks!

Jeff

  • 0
    Guru 326790 points

    If the SCL/SDA lines are floating, they might, in theory, generate waveforms that look like an I²C write to the output or control register. But that would be extremely unlikely.

    (Can't you connect the pull-up resistors to the TCA9536's supply?)

  • 0 in reply to Clemens Ladisch
    Mastermind 9265 points

    Hi Clemens,

    Since our internal power will be cut off when P1 is pulled LOW, the SDA/SCL lines will be left floating.

    The functionality we require is:

    1. When P1 is pulled LOW, P0 and P2 must also be pulled LOW. (Will the TCA9536 handle this automatically, or does it require I2C commands?)

    2. The TCA9536 needs to report the 'P1 pulled LOW' signal back to the Controller via I2C.

    3. When P1 transitions from LOW (internal power off) to HIGH (internal power off), P0 and P2 must also be able to transition to HIGH.

    Given these conditions, does this mean the TCA9536 cannot meet our requirements?

    Thanks!

    Jeff

  • 0 in reply to Jeff YANG
    TI__Guru 64838 points

    Hi Jeff, 

    I will need more time to respond. 

  • 0 in reply to Jeff YANG
    TI__Guru 64838 points

    Hi Jeff,

    Jeff YANG said:
    When P1 is pulled LOW, P0 and P2 must also be pulled LOW. (Will the TCA9536 handle this automatically, or does it require I2C commands?)

    The TCA9536 can only change state of the Px GPIO's if an I2C command configures the device.

    Jeff YANG said:
    The TCA9536 needs to report the 'P1 pulled LOW' signal back to the Controller via I2C.

    This can be done through a read command. 

    Jeff YANG said:
    When P1 transitions from LOW (internal power off) to HIGH (internal power off), P0 and P2 must also be able to transition to HIGH.

    I am not sure I understand this use case. 

    From the original question: 

    Jeff YANG said:
    If VCC continues to supply 3.3V, but I2C_SDA/I2C_SCL become floating without being pulled high due to an internal power loss in the device, will P0 (output), P1 (input), and P2 (output) maintain their last states? Assuming the original states were P0=0, P1=0, P2=0 or P0=1, P1=1, P2=1.

    If SDA/SCL floats while VCC power is maintained to the I/O expander, the TCA9536 will retain its state, although the I2C may be stuck depending on if a start condition was recognized during the float period of the I2C bus. 

    To unstick the bus, you would need to conduct a recovery sequence on SCL. 

    Regards,

    Tyler